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Complexity

Complexity characterizes the behavior of a system or model whose components interact in multiple ways and follow local rules, leading to non-linearity, randomness, collective dynamics, hierarchy, and emergence.

The term is generally used to characterize something with many parts where those parts interact with each other in multiple ways, culminating in a higher order of emergence greater than the sum of its parts. The study of these complex linkages at various scales is the main goal of complex systems theory.

The intuitive criterion of complexity can be formulated as follows: a system would be more complex if more parts could be distinguished, and if more connections between them existed.

, a number of approaches to characterizing complexity have been used in science; Zayed et al. reflect many of these. Neil Johnson states that "even among scientists, there is no unique definition of complexity – and the scientific notion has traditionally been conveyed using particular examples..." Ultimately Johnson adopts the definition of "complexity science" as "the study of the phenomena which emerge from a collection of interacting objects".

Overview
Definitions of complexity often depend on the concept of a "system" – a set of parts or elements that have relationships among them differentiated from relationships with other elements outside the relational regime. Many definitions tend to postulate or assume that complexity expresses a condition of numerous elements in a system and numerous forms of relationships among the elements. However, what one sees as complex and what one sees as simple is relative and changes with time.

Warren Weaver posited in 1948 two forms of complexity: disorganized complexity, and organized complexity. Phenomena of 'disorganized complexity' are treated using probability theory and statistical mechanics, while 'organized complexity' deals with phenomena that escape such approaches and confront "dealing simultaneously with a sizable number of factors which are interrelated into an organic whole". Weaver's 1948 paper has influenced subsequent thinking about complexity.

The approaches that embody concepts of systems, multiple elements, multiple relational regimes, and state spaces might be summarized as implying that complexity arises from the number of distinguishable relational regimes (and their associated state spaces) in a defined system.

Some definitions relate to the algorithmic basis for the expression of a complex phenomenon or model or mathematical expression, as later set out herein.

Disorganized vs. organized
One of the problems in addressing complexity issues has been formalizing the intuitive conceptual distinction between the large number of variances in relationships extant in random collections, and the sometimes large, but smaller, number of relationships between elements in systems where constraints (related to correlation of otherwise independent elements) simultaneously reduce the variations from element independence and create distinguishable regimes of more-uniform, or correlated, relationships, or interactions.

Weaver perceived and addressed this problem, in at least a preliminary way, in drawing a distinction between "disorganized complexity" and "organized complexity".

In Weaver's view, disorganized complexity results from the particular system having a very large number of parts, say millions of parts, or many more. Though the interactions of the parts in a "disorganized complexity" situation can be seen as largely random, the properties of the system as a whole can be understood by using probability and statistical methods.

A prime example of disorganized complexity is a gas in a container, with the gas molecules as the parts. Some would suggest that a system of disorganized complexity may be compared with the (relative) simplicity of planetary orbits – the latter can be predicted by applying Newton's laws of motion. Of course, most real-world systems, including planetary orbits, eventually become theoretically unpredictable even using Newtonian dynamics; as discovered by modern chaos theory.

Organized complexity, in Weaver's view, resides in nothing else than the non-random, or correlated, interaction between the parts. These correlated relationships create a differentiated structure that can, as a system, interact with other systems. The coordinated system manifests properties not carried or dictated by individual parts. The organized aspect of this form of complexity in regards to other systems, rather than the subject system, can be said to "emerge," without any "guiding hand".

The number of parts does not have to be very large for a particular system to have emergent properties. A system of organized complexity may be understood in its properties (behavior among the properties) through modeling and simulation, particularly modeling and simulation with computers. An example of organized complexity is a city neighborhood as a living mechanism, with the neighborhood people among the system's parts.

Sources and factors
There are generally rules which can be invoked to explain the origin of complexity in a given system.

The source of disorganized complexity is the large number of parts in the system of interest, and the lack of correlation between elements in the system.

In the case of self-organizing living systems, usefully organized complexity comes from beneficially mutated organisms being selected to survive by their environment for their differential reproductive ability or at least success over inanimate matter or less organized complex organisms. See e.g. Robert Ulanowicz's treatment of ecosystems.

Complexity of an object or system is a relative property. For instance, for many functions (problems), such a computational complexity as time of computation is smaller when multitape Turing machines are used than when Turing machines with one tape are used. Random Access Machines allow one to even more decrease time complexity (Greenlaw and Hoover 1998: 226), while inductive Turing machines can decrease even the complexity class of a function, language or set (Burgin 2005). This shows that tools of activity can be an important factor of complexity.

Varied meanings
In several scientific fields, "complexity" has a precise meaning:

 * In computational complexity theory, the amounts of resources required for the execution of algorithms is studied. The most popular types of computational complexity are the time complexity of a problem equal to the number of steps that it takes to solve an instance of the problem as a function of the size of the input (usually measured in bits), using the most efficient algorithm, and the space complexity of a problem equal to the volume of the memory used by the algorithm (e.g., cells of the tape) that it takes to solve an instance of the problem as a function of the size of the input (usually measured in bits), using the most efficient algorithm. This allows classification of computational problems by complexity class (such as P, NP, etc.). An axiomatic approach to computational complexity was developed by Manuel Blum. It allows one to deduce many properties of concrete computational complexity measures, such as time complexity or space complexity, from properties of axiomatically defined measures.
 * In algorithmic information theory, the Kolmogorov complexity (also called descriptive complexity, algorithmic complexity or algorithmic entropy) of a string is the length of the shortest binary program that outputs that string. Minimum message length is a practical application of this approach. Different kinds of Kolmogorov complexity are studied: the uniform complexity, prefix complexity, monotone complexity, time-bounded Kolmogorov complexity, and space-bounded Kolmogorov complexity. An axiomatic approach to Kolmogorov complexity based on Blum axioms (Blum 1967) was introduced by Mark Burgin in the paper presented for publication by Andrey Kolmogorov. The axiomatic approach encompasses other approaches to Kolmogorov complexity. It is possible to treat different kinds of Kolmogorov complexity as particular cases of axiomatically defined generalized Kolmogorov complexity. Instead of proving similar theorems, such as the basic invariance theorem, for each particular measure, it is possible to easily deduce all such results from one corresponding theorem proved in the axiomatic setting. This is a general advantage of the axiomatic approach in mathematics. The axiomatic approach to Kolmogorov complexity was further developed in the book (Burgin 2005) and applied to software metrics (Burgin and Debnath, 2003; Debnath and Burgin, 2003).
 * In information theory, information fluctuation complexity is the fluctuation of information about information entropy. It is derivable from fluctuations in the predominance of order and chaos in a dynamic system and has been used as a measure of complexity in many diverse fields.
 * In information processing, complexity is a measure of the total number of properties transmitted by an object and detected by an observer. Such a collection of properties is often referred to as a state.
 * In physical systems, complexity is a measure of the probability of the state vector of the system. This should not be confused with entropy; it is a distinct mathematical measure, one in which two distinct states are never conflated and considered equal, as is done for the notion of entropy in statistical mechanics.
 * In dynamical systems, statistical complexity measures the size of the minimum program able to statistically reproduce the patterns (configurations) contained in the data set (sequence). While the algorithmic complexity implies a deterministic description of an object (it measures the information content of an individual sequence), the statistical complexity, like forecasting complexity, implies a statistical description, and refers to an ensemble of sequences generated by a certain source. Formally, the statistical complexity reconstructs a minimal model comprising the collection of all histories sharing a similar probabilistic future, and measures the entropy of the probability distribution of the states within this model. It is a computable and observer-independent measure based only on the internal dynamics of the system, and has been used in studies of emergence and self-organization.
 * In mathematics, Krohn–Rhodes complexity is an important topic in the study of finite semigroups and automata.
 * In Network theory complexity is the product of richness in the connections between components of a system, and defined by a very unequal distribution of certain measures (some elements being highly connected and some very few, see complex network).
 * In software engineering, programming complexity is a measure of the interactions of the various elements of the software. This differs from the computational complexity described above in that it is a measure of the design of the software.

Other fields introduce less precisely defined notions of complexity:

 * A complex adaptive system has some or all of the following attributes:
 * The number of parts (and types of parts) in the system and the number of relations between the parts is non-trivial – however, there is no general rule to separate "trivial" from "non-trivial";
 * The system has memory or includes feedback;
 * The system can adapt itself according to its history or feedback;
 * The relations between the system and its environment are non-trivial or non-linear;
 * The system can be influenced by, or can adapt itself to, its environment;
 * The system is highly sensitive to initial conditions.

Study
Complexity has always been a part of our environment, and therefore many scientific fields have dealt with complex systems and phenomena. From one perspective, that which is somehow complex – displaying variation without being random – is most worthy of interest given the rewards found in the depths of exploration.

The use of the term complex is often confused with the term complicated. In today's systems, this is the difference between myriad connecting "stovepipes" and effective "integrated" solutions. This means that complex is the opposite of independent, while complicated is the opposite of simple.

While this has led some fields to come up with specific definitions of complexity, there is a more recent movement to regroup observations from different fields to study complexity in itself, whether it appears in anthills, human brains or social systems. One such interdisciplinary group of fields is relational order theories.

Behaviour
The behavior of a complex system is often said to be due to emergence and self-organization. Chaos theory has investigated the sensitivity of systems to variations in initial conditions as one cause of complex behaviour.

Mechanisms
Recent developments in artificial life, evolutionary computation and genetic algorithms have led to an increasing emphasis on complexity and complex adaptive systems.

Simulations
In social science, the study on the emergence of macro-properties from the micro-properties, also known as macro-micro view in sociology. The topic is commonly recognized as social complexity that is often related to the use of computer simulation in social science, i.e. computational sociology.

Systems
Systems theory has long been concerned with the study of complex systems (in recent times, complexity theory and complex systems have also been used as names of the field). These systems are present in the research of a variety disciplines, including biology, economics, social studies and technology. Recently, complexity has become a natural domain of interest of real world socio-cognitive systems and emerging systemics research. Complex systems tend to be high-dimensional, non-linear, and difficult to model. In specific circumstances, they may exhibit low-dimensional behaviour.

Data
In information theory, algorithmic information theory is concerned with the complexity of strings of data.

Complex strings are harder to compress. While intuition tells us that this may depend on the codec used to compress a string (a codec could be theoretically created in any arbitrary language, including one in which the very small command "X" could cause the computer to output a very complicated string like "18995316"), any two Turing-complete languages can be implemented in each other, meaning that the length of two encodings in different languages will vary by at most the length of the "translation" language – which will end up being negligible for sufficiently large data strings.

These algorithmic measures of complexity tend to assign high values to random noise. However, under a certain understanding of complexity, arguably the most intuitive one, random noise is meaningless and so not complex at all.

Information entropy is also sometimes used in information theory as indicative of complexity, but entropy is also high for randomness. In the case of complex systems, information fluctuation complexity was designed so as not to measure randomness as complex and has been useful in many applications. More recently, a complexity metric was developed for images that can avoid measuring noise as complex by using the minimum description length principle.

Classification Problems
There has also been interest in measuring the complexity of classification problems in supervised machine learning. This can be useful in meta-learning to determine for which data sets filtering (or removing suspected noisy instances from the training set) is the most beneficial and could be expanded to other areas. For binary classification, such measures can consider the overlaps in feature values from differing classes, the separability of the classes, and measures of geometry, topology, and density of manifolds.

For non-binary classification problems, instance hardness is a bottom-up approach that first seeks to identify instances that are likely to be misclassified (assumed to be the most complex). The characteristics of such instances are then measured using supervised measures such as the number of disagreeing neighbors or the likelihood of the assigned class label given the input features.

In molecular recognition
A recent study based on molecular simulations and compliance constants describes molecular recognition as a phenomenon of organisation. Even for small molecules like carbohydrates, the recognition process can not be predicted or designed even assuming that each individual hydrogen bond's strength is exactly known.

The law of requisite complexity
Driving from the law of requisite variety, Boisot and McKelvey formulated the ‘Law of Requisite Complexity’, that holds that, in order to be efficaciously adaptive, the internal complexity of a system must match the external complexity it confronts.

Positive, appropriate and negative complexity
The application in project management of the Law of Requisite Complexity, as proposed by Stefan Morcov, is the analysis of positive, appropriate and negative complexity.

In project management
Project complexity is the property of a project which makes it difficult to understand, foresee, and keep under control its overall behavior, even when given reasonably complete information about the project system.

In systems engineering
Maik Maurer considers complexity as a reality in engineering. He proposed a methodology for managing complexity in systems engineering :

1.           Define the system.

2.           Identify the type of complexity.

3.           Determine the strategy.

4.           Determine the method.

5.           Model the system.

6.           Implement the method.

Applications
Computational complexity theory is the study of the complexity of problems – that is, the difficulty of solving them. Problems can be classified by complexity class according to the time it takes for an algorithm – usually a computer program – to solve them as a function of the problem size. Some problems are difficult to solve, while others are easy. For example, some difficult problems need algorithms that take an exponential amount of time in terms of the size of the problem to solve. Take the travelling salesman problem, for example. It can be solved, as denoted in Big O notation, in time $$O(n^2 2^n)$$ (where n is the size of the network to visit – the number of cities the travelling salesman must visit exactly once). As the size of the network of cities grows, the time needed to find the route grows (more than) exponentially.

Even though a problem may be computationally solvable in principle, in actual practice it may not be that simple. These problems might require large amounts of time or an inordinate amount of space. Computational complexity may be approached from many different aspects. Computational complexity can be investigated on the basis of time, memory or other resources used to solve the problem. Time and space are two of the most important and popular considerations when problems of complexity are analyzed.

There exist a certain class of problems that although they are solvable in principle they require so much time or space that it is not practical to attempt to solve them. These problems are called intractable.

There is another form of complexity called hierarchical complexity. It is orthogonal to the forms of complexity discussed so far, which are called horizontal complexity.

Emerging applications in other fields
The concept of complexity is being increasingly used in the study of cosmology, big history, and cultural evolution with increasing granularity, as well as increasing quantification.

Application in cosmology
Eric Chaisson has advanced a cosmoglogical complexity metric which he terms Energy Rate Density. This approach has been expanded in various works, most recently applied to measuring evolving complexity of nation-states and their growing cities.

title: Complexity


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Chastity

Chastity, also known as purity, is a virtue related to temperance. Someone who is chaste refrains either from sexual activity that is considered immoral or from any sexual activity, according to their state of life. In some contexts, for example when making a vow of chastity, chastity means celibacy.

Etymology
The words chaste and chastity stem from the Latin adjective castus ("cut off", "separated", "pure"). The words entered the English language around the middle of the 13th century. Chaste meant "virtuous", "pure from unlawful sexual intercourse" or (from the early 14th century on) as a noun, a virgin, while chastity meant "(sexual) purity".

Thomas Aquinas links castus (chastity) to the Latin verb castigo ("chastise, reprimand, correct"), with a reference to Aristotle's Nicomachean Ethics: "Chastity takes its name from the fact that reason 'chastises' concupiscence, which, like a child, needs curbing, as the Philosopher states".

In Abrahamic religions
For many Jews, Christians, and Muslims, people should restrict their acts of a sexual nature to the context of marriage. For unmarried people, chastity is equivalent to sexual abstinence. Sexual acts outside of or apart from marriage, such as adultery, fornication, masturbation, and prostitution, are considered immoral due to lust.

Traditions
In many Christian traditions, chastity is synonymous with purity. The Catholic Church teaches that chastity involves, in the words of cardinal bishop Alfonso López Trujillo, "the successful integration of sexuality within the person and thus the inner unity of man in his bodily and spiritual being", which according to one's marital status requires either having no sexual relationship, or only having sexual relations with one's spouse. In Western Christian morality, chastity is placed opposite the deadly sin of lust, and is classified as one of seven virtues. The moderation of sexual desires is also required to be virtuous. Reason, will, and desire can harmoniously work together to do what is good.

As an emblem of inward chastity, some Christians choose to wear a cord, girdle or a cincture of one of the several Confraternities of the Cord or a purity ring. The cord is worn as a symbol of chastity in honour of a chaste saint whom the bearer asks for intercession. The purity ring is worn before holy matrimony by those who marry or for the rest of their lives by those who stay single.

Marital chastity
In marriage, the spouses commit to a lifelong relationship that excludes sexual intimacy with other persons. A third form of chastity, often called "vidual chastity", is expected by the society for a period after the woman's husband dies. For example, Anglican Bishop Jeremy Taylor defined five rules in Holy Living (1650), including abstaining from marrying "so long as she is with child by her former husband" and "within the year of mourning".

Celibacy
In the Roman Catholic Church, members of the consecrated life vow or promise celibacy as one of the evangelical counsels. In 306, the Synod of Elvira proscribed clergy from marrying. This was unevenly enforced until the Second Lateran Council in 1139 when it found its way into canon law. Unmarried deacons promise celibacy to their local bishop when ordained.

Eastern Catholic priests are permitted to marry, provided they do so before ordination and outside monastic life.

Vows of chastity
Vows of chastity can be taken either as part of an organised religious life (such as Roman Catholic Beguines and Beghards in the past) or on an individual basis: as a voluntary act of devotion, or as part of an ascetic lifestyle (often devoted to contemplation), or both. Some Protestant religious communities, such as the Bruderhof, take vows of chastity as part of the church membership process.

Catholicism
Chastity is a central and pivotal concept in Roman Catholic praxis. Roman Catholic teaching regards chastity as essential in maintaining and cultivating the unity of body with spirit and thus the integrity of the human being. It is also fundamental to the practise of the Catholic life because it involves an apprenticeship in self-mastery. By attaining mastery over one's passions, reason, will, and desire can harmoniously work together to do what is good.

Lutheranism
The theology of the body of the Lutheran Churches emphasizes the role of the Holy Spirit, who sanctified the bodies of Christians to be God's temple.

Many Lutheran monks and Lutheran nuns practice celibacy, though in some Lutheran religious orders it is not compulsory.

The Church of Jesus Christ of Latter-Day Saints
In the Church of Jesus Christ of Latter-day Saints chastity is very important:

Physical intimacy between husband and wife is a beautiful and sacred part of God's plan for His children. It is an expression of love within marriage and allows husband and wife to participate in the creation of life. God has commanded that this sacred power be expressed only between a man and a woman who are legally married. The law of chastity applies to both men and women. It includes strict abstinence from sexual relations before marriage and complete fidelity and loyalty to one's spouse after marriage.

The law of chastity requires that sexual relations be reserved for marriage between a man and a woman.

In addition to reserving sexual intimacy for marriage, we obey the law of chastity by controlling our thoughts, words, and actions. Jesus Christ taught, "Ye have heard that it was said by them of old time, Thou shalt not commit adultery: but I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart" ."

Teachings of the Church of Jesus Christ of Latter-Day Saints also include that sexual expression within marriage is an important dimension of spousal bonding apart from, but not necessarily avoiding, its procreative result.

Quran
The most famous personal example of chastity in the Quran is the Virgin Mary (Mariam):

"And ˹remember˺ the one who guarded her chastity, so We breathed into her through Our angel, ˹Gabriel,˺ making her and her son a sign for all peoples."

"screening herself off from them. Then We sent to her Our angel, ˹Gabriel,˺ appearing before her as a man, perfectly formed. She appealed, “I truly seek refuge in the Most Compassionate from you! ˹So leave me alone˺ if you are God-fearing.” He responded, “I am only a messenger from your Lord, ˹sent˺ to bless you with a pure son.” She wondered, “How can I have a son when no man has ever touched me, nor am I unchaste?”"

Extramarital sex is forbidden. The Quran says:

"Do not go near adultery. It is truly a shameful deed and an evil way."

"˹They are˺ those who do not invoke any other god besides Allah, nor take a ˹human˺ life—made sacred by Allah—except with ˹legal˺ right, nor commit fornication. And whoever does ˹any of˺ this will face the penalty. Their punishment will be multiplied on the Day of Judgment, and they will remain in it forever, in disgrace. As for those who repent, believe, and do good deeds, they are the ones whose evil deeds Allah will change into good deeds. For Allah is All-Forgiving, Most Merciful."

In a list of commendable deeds the Quran says:

"Surely ˹for˺ Muslim men and women, believing men and women, devout men and women, truthful men and women, patient men and women, humble men and women, charitable men and women, fasting men and women, men and women who guard their chastity, and men and women who remember Allah often—for ˹all of˺ them Allah has prepared forgiveness and a great reward."

Because the sex desire is usually attained before a man is financially capable of marriage, the love to God and mindfulness of Him should be sufficient motive for chastity:

"And let those who do not have the means to marry keep themselves chaste until Allah enriches them out of His bounty. And if any of those ˹bondspeople˺ in your possession desires a deed of emancipation, make it possible for them, if you find goodness in them. And give them some of Allah’s wealth which He has granted you. Do not force your ˹slave˺ girls into prostitution for your own worldly gains while they wish to remain chaste. And if someone coerces them, then after such a coercion Allah is certainly All-Forgiving, Most Merciful ˹to them˺."

Sharia (Law)
Chastity is mandatory in Islam. Sex outside legitimacy is prohibited, for both men and women, whether married or unmarried. The injunctions and forbiddings in Islam apply equally to men and women. The legal punishment for adultery is equal for men and women.

The prophet's prescription to the youth was: "'O young people! Whoever among you can marry, should marry, because it helps him lower his gaze and guard his modesty (i.e. his private parts from committing illegal sexual intercourse etc.), and whoever is not able to marry, should fast, as fasting diminishes his sexual power.'"

Chastity is an attitude and a way of life. In Islam it is both a personal and a social value. A Muslim society should not condone relations entailing or conducive to sexual license. Social patterns and practices calculated to inflame sexual desire are frowned upon by Islam, such incitements to immorality including permissive ideologies, titillating works of art, and the failure to inculcate sound moral principles in the young. At the heart of such a view of human sexuality lies the conviction that the notion of personal freedom should never be misconstrued as the freedom to flout God's laws by overstepping the bounds which, in His infinite wisdom, He has set upon the relations of the sexes.

Baháʼí Faith
Chastity is highly prized in the Baháʼí Faith. Similar to other Abrahamic religions, Baháʼí teachings call for the restriction of sexual activity to that between a wife and husband in Baháʼí marriage, and discourage members from using pornography or engaging in sexually explicit recreational activities. The concept of chastity is extended to include avoidance of alcohol and mind-altering drugs, profanity, and gaudy or immodest attire.

Hinduism
Hinduism's view on premarital sex is rooted in its concept of or the stages of life. The first of these stages, known as, roughly translates as chastity. Celibacy and chastity are considered the appropriate behavior for both male and female students during this stage, which precedes the stage of the married householder. and Hindu monks or are also celibate as part of their ascetic discipline.

Sikhism
In Sikhism, premarital or extramarital sex is strictly forbidden. However, it is encouraged to marry and live as a family unit to provide and nurture children for the perpetual benefit of creation (as opposed to or living as a monk, which was, and remains, a common spiritual practice in India). A Sikh is encouraged not to live as a recluse, beggar, monk, nun, celibate, or in any similar vein.

Jainism
The Jain ethical code contains the vow of (meaning "pure conduct"), which prescribes the expectations for Jains concerning sexual activity. is one of the five major and minor vows of Jainism, prescribing slightly different expectations for ascetics and laypeople, respectively.

Complete celibacy is expected only of Jain ascetics (who are also referred to as monks and nuns). For laypeople, chastity is expected, with extramarital sex and adultery being prohibited.

Buddhism
The teachings of Buddhism include the Noble Eightfold Path, comprising a division called right action. Under the Five Precepts ethical code, and  lay followers should abstain from sexual misconduct, while  and  monastics should practice strict chastity.

Taoism
The Five Precepts of the Taoist religion include "no sexual misconduct", which is interpreted as prohibiting extramarital sex for lay practitioners and marriage or sexual intercourse for monks and nuns.

Government
In Iran, women are required to wear hijabs as part of that society's efforts to enforce chastity. In 2023 the Ministry of Culture and Islamic Guidance announced a new bill titled the Protection of the Culture of Chastity and Hijab Law, expanding its former sections from 15 to 70.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Chastity


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Talk:Copyleft

Advertising under "See Also"?
Why is Creative Commons listed under "See Also"? This does not directly relate to copyleft at all, and is more like an advertisement for the Creative Commons than anything else (and it certainly does not fit in with the other links there). If you object feel free to change it back, but I'm going to take a bushwhacker to it right now.

And now that I look at it, the same goes for "Magnatune". Let's keep the advertising out, please. -- mjk

 * I put CC and Magnatune back in. They are both orgs that promote and use copyleft licenses, and are in the external links section. MikeCapone (talk) 08:39, 28 Mar 2005 (UTC)

 * Just because they promote and use copyleft licenses does not make them relevent to the discussion of what copyleft is (the whole point of the article). If we're putting them under the "See also" links, I suppose that means that we should be putting links to every organization that promotes and uses copyleft licenses under the "See also" links as well, right? --mjk — Preceding unsigned comment added by 69.139.5.241 (talk) 21:38, 28 March 2005 (UTC)

 * No, just the really really famous ones — like Creative Commons. They've become nearly as well-known as the FSF of late. Magnatune is less well-known, but more remarkable here for being the best-known company applying copyleft ideas to a the medium of music. Take a look at these Google News searches:
 * http://news.google.com/news?hl=en&lr=&tab=wn&ie=UTF-8&q=creative+commons&btnG=Search+News
 * http://news.google.com/news?hl=en&lr=&tab=wn&ie=UTF-8&q=magnatune&btnG=Search+News
 * I think they should stay. Deco 06:23, 29 Mar 2005 (UTC)
 * Agree.SR - RE (talk) —Preceding undated comment added 23:16, 21 December 2005 (UTC)

CC use images similar to the copyleft. Alfa-ketosav (talk) 20:50, 12 September 2019 (UTC)

Word play
Can anyone explain or show me a external link explain what word plays in Copyleft? I don't use English as mother tongue and guess that is the double-meaning of "right" in Copyright, right? Anyway, Wikipedia should explain this in the article. Vinhtantran (talk) 10:02, 12 November 2008 (UTC)

Right versus left of course. And the implication that copyleft is "leftist" -- i.e. left wing, politically. Xardox (talk) 20:17, 23 November 2013 (UTC)

Just that copyright and left is opposite of right sum to copyleft. Alfa-ketosav (talk) 20:49, 12 September 2019 (UTC)

Missing elements
-Inowen (nlfte) 19:53, 23 October 2018 (UTC)
 * Section on the various specific copyleft licenses.
 * Link to a list of copyleft licenses
 * Article on list of copyleft licenses

Classification of CC BY
The classification of CC BY as a Permissive licence is clearly wrong (modern variants of the OGL would probably be a better example). While often presented as an "attribution only" licence this is not carried by the actual licence text nor by the intent of CC. In CC BY 4.0 the relevant terms are in 2(a)(5)(B), earlier versions contain terms that work in a similar manner. This restricts the form and terms on which "adapted material", that is derivatives of the original, can be distributed, so "copyleft" like restrictions. SimonPoole (talk) 08:30, 19 March 2020 (UTC)

Merger proposal
I propose merging Design Science Licence into Copyleft. Design Science Licence is a very short article that overlaps with the coverage of the Design Science Licence in the CopyLeft article. Hmee2 (talk) 20:31, 8 January 2023 (UTC)

Self-Redirect
The supposed 'Main article' for Viral license (linked at the top of the 'Viral license' section) redirects back to this article. Littlecat456💬(here be shitty edits) 17:30, 28 August 2023 (UTC)

Missing history
I would like to assert that the history section of this article misattributes the origin of the copy left concept and name. It describes both as being conceived in the 1980s, but the first use of the concept was in the 1960s in the Principia Discordia by Greg Hill and Kerry Thornley, albeit spelled differently and using a different symbol: the relevant page said "KOPYLEFT - ALL RITES REVERSED" and was accompanied with a circled K. That history IS included in the Wikipedia article for "All Rights Reversed", but not in this article as it should be. 2001:56A:FCD0:BE00:ADBB:2B9D:30EE:40E2 (talk) 07:07, 24 January 2024 (UTC)

Wiki Education assignment: IFS213-Hacking and Open Source Culture
— Assignment last updated by KAN2035117 (talk) 22:49, 3 April 2024 (UTC)

Cctv hack
CCtv hack

Xxx GN 2409:40E5:104C:257F:8000:0:0:0 (talk) 09:07, 4 July 2024 (UTC)

title: Talk:Copyleft


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Talk:Causes of sexual orientation

 * Talk:Causes of sexual orientation/Archive 1 (moved from Talk:Homosexuality Causal theories)
 * Talk:Causes of sexual orientation/Archive 2 (moved from Causes of sexual orientation/Debate)
 * Talk:Causes of sexual orientation/Archive 3 (from this page, refers to older versions of text now in subarticles)

Total re-write
Much of the material on this page needed to be merged into the full discussion on the genetics and environment/choice pages. I'm in the process of moving material into those articles from several places, so if you're looking for material deleted from this article to appear in one of those two, you might want to wait a few hours for me to finish.

This article was also getting rather clunky from repeated patching by different people, and some of the statements weren't really saying much, or were making controversial conclusions best handled by letting other articles discuss the situation in detail (like the stuff about religion). So, instead of just cutting and moving, I re-wrote this article to be what I hope is a proper introduction to the topic. -- Beland 03:00, 21 Oct 2004 (UTC)

Rename this article?
So in these nature vs. nurture arguments, there are some finer points of terminology. To my understanding, the causes of sexual orientation include lots of far-ranging things like "what makes people sexually attracted to anyone?"; "cause" refers to the mechanisms that make something go. Determinants are the factors that determine whether an organism will express phenotype A vs. phenotype B. It's the difference between "what makes some people have blue eyes and others brown" (a single gene, in many cases, I guess; anyway, this would be a determinants question) and "what makes people have eyes (blue, brown, or otherwise)?" - that would be a causes question, I think. Anyway, this is a long-winded way of asking whether this article wouldn't be more properly named "Determinants of sexual orientation", because I think that's what was originally meant. (Certainly, that's what it's about right now.) "Etiology of sexual orientation" might also be a candidate, though I am less familiar with that term, so I'm not sure it's exactly appropiate. -- Beland 03:07, 21 Oct 2004 (UTC)

 * I think some people would object to "etiology" because it suggests/implies a diseased state. Technically, the word refers to the logos or description of an aitia or cause. So it should work except for the connotation that most people would attach to it.

 * The idea of "cause" is murky in common language. We ask, "What caused the water to boil?" Somebody says, "It boiled because I lit the gas flame under it." But the second speaker has mentioned only one of several factors involved, the factor that s/he changed. To know whether water will boil you have to know the air pressure, whether it's pure water or has some percentage of salt in it, and the temperature to which the water is to be raised. Temperature, pressure, and the nature of the liquid are all causal factors that determine whether it boils or not. (Actually there is at least one more factor that can determine the exact time when the water starts boiling, but let's keep it simple.)

 * "Cause" can be defined as "anything that is a factor in producing an effect or result." By that definition, which allele or alleles one has of the two copies of the gene that determines eye color is a "cause." You could call it a determinant too, but the dictionary says that "determinant" is a synonym for "cause".  If there is a distinction to be made between "cause" and "determinant," it probably ties in with our human idea of actors and our human idea of relatively immutable things in the environment that the actors have to deal with, move around, etc.  A gene is a kind of template or jig according to which other components in the cell of some organism fabricates amino acids. It doesn't do anything itself. It is like a pile of plastic fruit on a tray that sits there while art student after art student comes in and draws it.  But the gene determines the structure of the amino acid that is associated with it. If the gene gets mutated by a cosmic ray or something, it may become the template for an amino acid that doesn't do its job well or even doesn't do its job well enough to allow the organism to live.  So by your use of language, the genes might be determinants and the ribosomes and other cell components might be causes.

 * What happens to make a human being a 0 or a 10 on the homosexual -- heterosexual continuum is not at all clear. It is possible that there may be a single gene that has more than one allele, and one or more of these allels may determine whether the human is homosexual or heterosexual. If there is such a gene, then one could speak of its being a "determinant" in the way you seem to want to use that word. But all of the other explanations with which I am familiar seem to me to be more of the "actor" sort. The mother is suffering from a cancer and has lots of tissue that produces male hormones, so the baby she is carrying, although female, is pushed in the direction of masculinity. If that push happens, it "causes" the baby's brain to be masculinized. Actually, once you get beyond the proposed "gay gene" (which should be a "gay/straight gene"), I can't think of anything that serves as an immutable factor that the body just has to deal with in the way that the growing body just has to deal with the eye color gene.  I also can't think of any way in which homosexuality/heterosexuality can be reduced to the kind of "on-off" choice found in the "blue-eyed or not-blue-eyed" choice.  Homosexuality/heterosexuality more nearly parallels the shades of brown that turn as black as the pupil at one extreme. 金 (Kim) 04:22, 21 Oct 2004 (UTC)

 * I don't see any problem with both articles, if there is enough information regarding the supposed determinants of gayness that would not need to be repeated on the causes page. Hyacinth 04:35, 21 Oct 2004 (UTC)

 * According to Wikipedia, etiology "is the study of ccausation." Causation, along with cause (which may be called reason), redirects to causality. Also according to Wikipedia a determinant "is a function that associates a scalar det(A) to every square matrix A."
 * Ignoring the mathematical definition, it does seem that determinates are not (only) about causes, but (also) about effects, and thus do not belong in a article about "causes" per se.
 * Any wikipedia article is required to be about "the study of X", rather than "X", which is to say the article is not about the causes of sexuality, but the study of the causes of sexuality.
 * Hyacinth 04:48, 21 Oct 2004 (UTC)

After some more hemming and hawing, and looking again at how short this article is, I'm thinking maybe the best thing to do is just to make it a section of Sexual orientation. Or is it needed as a handy destination for these and other fine articles?

Reorganize sub-articles?
As I was adding to the Environment and Genetics articles, I found myself thinking that if the Causes article needs to be split into subarticles, maybe this isn't the best breakdown. The passages that discuss anatomical and psychological differences of indeterminate origin really don't belong entirely in either Environment or Genetics. Perhaps it would be better to have a subarticle focusing on scientific research, broken into multiple articles based on the type of research, if necessary; and another subarticle on the choice debate (which is related to the nature/nurture controversy, but it's a particular angle that has more to do with morality and politics than any of the scientific stuff). -- Beland 06:35, 21 Oct 2004 (UTC)

What to do with this page?
Examining this article, I can't help but feel that most of the material is repeated in other articles. This page exists only as a link page to other articles on causes of sexual orientation and seeks to summarise these but does so poorly and incoherently. Either arguments and content common to both articles (environment and genetics) need to be merged here and this section cleared up, or this article should just become a link page, perhaps even a category? For example, the political controversy sections from both could be moved here. Alternatively, is there an argument for deleting this page altogether? Thoughts? --Axon 12:05, 12 Nov 2004 (UTC)

 * Yeah, I think my earlier suggestion to merge this page with Sexual orientation was a good idea. This page could be converted into a coherent section there, more or less. -- Beland 14:18, 1 Mar 2005 (UTC)

title: Talk:Causes of sexual orientation


text:
Talk:Causes of sexual orientation/Archive 2

Isn't this entire dicussion (below) about the main entry itself? This isn't a off-topic debate; it is a discussion about what text we should write in the main entry. Why was it moved out of the "Talk" section? User:RK

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The following statement in the main entry is very wrong, and needs to be changed. This is largely because heterosexuality was considered the norm and homosexuality an aberration, although this view has been contested and weakened by the results of the research. Heterosexuality is most certainly the norm; there isn't a sane scientist in the world who disputes this. If it were not, the our species would face a demographic disaster that could end in extinction. Being liberal on homosexuality (as I am) does not mean rewriting basic facts about the nature of reality. Homosexuality is an abnormality, and science is begining to uncover the basic causes of it. That does not mean that homosexual people are evil, nor perverted, not "sick". It does not justify hating or murdering homosexuals. It means, in point of fact, nothing beyond the biological facts in of themselves. We don't need to rewrite nature to be tolerant and accepting of people with different sexual orientations. User:RK

Consider: albinos are certainly abnormal...yet this doesn't imply that they are perverts, or need to be "cured". It just calls a spade, a spade; it just discusses the biological facts. I get the idea that some people think that if something occurs in nature, then it is "normal". If so, then this is a definition of the world "normal" that I was previously unaware of. Having six fingers naturally occurs. Is this also normal? Of course not. It is an exception to the normal rule; i.e. an abnormality. Not evil, not sinful, not sick...but technically abnormal. Having five fingers is the norm. Does that make me overyly judgemental?User:RK

 * Just because the majority of a species has a trait, that doesn't make what the minority have to be "abnormal". If most people have a certain eye color, does that make the other eye colors "abnormal"?  The fact that the species needs heterosexuality in order to survive doesn't necessarily imply that homosexuality doesn't serve some evolutionary or other purpose.

 * I agree that people with blue eyes are not abnormal. But color blindness is abnormal. You are mixing up the types of variations that exist by an order of magnitude!  Having longer fingers than other people isn't really all that abnormal. But having six fingers is abnormal. User:RK

 * So some kinds of human diversity are normal, and others abnormal. Under that reasoning, one could argue that a person without any sexuality whatsoever would be akin to a person having color blindness.  You are making an assumption that homosexuality falls into the category of "abnormal" variations rather than "normal" human variation.  What are your criteria for this distinction?  You haven't stated what they are.

I would strongly argue in favor of this article making no assumptions whatsoever about what is "normal" or "abnormal", but instead should address the question of what causes humans to have any particular sexuality, be it homo- or hetero- or bi- or whatever. It should make no assumptions about any type of sexuality being the default.

 * I disagree for the reasons stated above. Such arguments are politically driven, not scientifically driven. I think this is an example of political-correctness.User:RK

 * Nonsense, and this is a classic example of why the term "political correctness" is such an offensive term and should not be used in discourse, since its purpose is simply to discredit and smear the other person rather than address the issues at hand. You are assuming a political agenda behind my statements without any basis for that charge.

A friend of mine was born with some fingers missing from one hand. I consider that hand "abnormal". But I don't consider his hand "sinful". I just think he'd rather have all five fingers. User:Ed Poor

 * Wow, you think that's something - I actually know some people who use their left hands rather than their right for doing many things. I don't think they'd rather have been born right-hand-dominant rather than left-hand-dominant, but I think sometimes they wish the right-hand-dominant majority wouldn't gratutously assume their convenience and happiness is of lesser importance.

Completely aside from the association between "abnormal" and "perverted" or "sinful" -- an important issue that the article must acknowledge, but still one that we can bracket for a moment -- I think that in order to have a meaningful discussion on this topic we need to distinguish between three uses of "norm" and "normal."

First is the statistical norm, or average. I think everyone would agree that in this sense heterosexuality is normal in US society, although (given that most American do not often think of "normal" instatistical terms, or that they identify statistical norms with moral norms, to say this without a lot of qualification is inflammatory and very vulnerable to misinterpretation).

Second is the moral norm, by which I do not mean what a minority of people consider write or wrong, but rather what the social consensus is (so if you prefer you can call this a social norm). I mean what almost every one would consider ideal behavior, behavior that society should encourage and value, even if a lot of people do not match this ideal. I think most people would agree that there was a time in the United States when heterosexuality was the social norm, and that today there are still communities where homosexuality is the social norm, but that in the past thirty years the United States has developed a more pluralistic attitude towards sexuality, based on the notion that sexuality is private and that sexual discrimination in public places is wrong, and as a result it is increasingly difficult to identify anything (including heterosexuality) as a social norm in this country. (But because statistically most Americans are heterosexual, there is still a general presumption that people are heterosexual).

The third use of "normal" is biological; it is based on a notion of what a healthy genetic human should be like. But this kind of "normal" is really hard to talk about because people often rely on the first kind as evidence, or use this kind to legitimize the second kind. In other words, although (or perhaps because it is) austensibly scientific it is very prone to politicization. In any event, I think there is genuine dispute among scientists as to whether heterosexuality is normal in this sense. One of Kinsey's inferences, and one shared my many anthropologists from the 30s on, is that for whatever reasons (genetic or social) it is normal for a minority of people to be homosexuality and normal for a majority to be heterosexual, in the same sense that it is normal for most human beings to have dark hare and normal for some human beings to have blond hair. Unlike color-blindness, being gay or straight does not involve any kind of organic disfunction or deficit. But I think this view (which was based on statistical observations) is still actively debated. One reason is that the whole notion of sexual orientation has been parsed a good deal. For example, what may ge genetic are certain biases to be sexually by aroused not by "men" or "women" but by something more discrete (a specific image or odor or something) that it generally associated with one sex but may be associated with some members of the other sex. But the main point I think is that one can be "naturally" gay without in anyway being sick or disabled i.e. being gay (well, or being strtaight) cannot be compared to colorblindness or sickle-cell anemia or diabetes (type I). SR

 * See also : Causes of sexual orientation


text:
Cosmic microwave background

The cosmic microwave background (CMB or CMBR) is microwave radiation that fills all space in the observable universe. It is a remnant that provides an important source of data on the primordial universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive radio telescope detects a faint background glow that is almost uniform and is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum. The accidental discovery of the CMB in 1965 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s.

The CMB is landmark evidence of the Big Bang theory for the origin of the universe. In the Big Bang cosmological models, during the earliest periods, the universe was filled with an opaque fog of dense, hot plasma of sub-atomic particles. As the universe expanded, this plasma cooled to the point where protons and electrons combined to form neutral atoms of mostly hydrogen. Unlike the plasma, these atoms could not scatter thermal radiation by Thomson scattering, and so the universe became transparent. Known as the recombination epoch, this decoupling event released photons to travel freely through space – sometimes referred to as relic radiation. However, the photons have grown less energetic due to the cosmological redshift associated with the expansion of the universe. The surface of last scattering refers to a shell at the right distance in space so photons are now received that were originally emitted at the time of decoupling.

The CMB is not completely smooth and uniform, showing a faint anisotropy that can be mapped by sensitive detectors. Ground and space-based experiments such as COBE, WMAP and Planck have been used to measure these temperature inhomogeneities. The anisotropy structure is determined by various interactions of matter and photons up to the point of decoupling, which results in a characteristic lumpy pattern that varies with angular scale. The distribution of the anisotropy across the sky has frequency components that can be represented by a power spectrum displaying a sequence of peaks and valleys. The peak values of this spectrum hold important information about the physical properties of the early universe: the first peak determines the overall curvature of the universe, while the second and third peak detail the density of normal matter and so-called dark matter, respectively. Extracting fine details from the CMB data can be challenging, since the emission has undergone modification by foreground features such as galaxy clusters.

Features
The cosmic microwave background radiation is an emission of uniform black body thermal energy coming from all directions. Intensity of the CMB is expressed in kelvin (K), the SI unit of temperature. The CMB has a thermal black body spectrum at a temperature of $2.725 K$. Variations in intensity are expressed as variations in temperature. The blackbody temperature uniquely characterizes the intensity of the radiation at all wavelengths; a measured brightness temperature at any wavelength can be converted to a blackbody temperature.

The radiation is remarkably uniform across the sky, very unlike the almost point-like structure of stars or clumps of stars in galaxies. The radiation is isotropic to roughly one part in 25,000: the root mean square variations are just over 100 μK, after subtracting out a dipole anisotropy from the Doppler shift of the background radiation. The latter is caused by the peculiar velocity of the Sun relative to the comoving cosmic rest frame as it moves at 369.82 ± 0.11 km/s towards the constellation Crater near its boundary with the constellation Leo The CMB dipole and aberration at higher multipoles have been measured, consistent with galactic motion. Despite the very small degree of anisotropy in the CMB, many aspects can be measured with high precision and such measurements are critical for cosmological theories.

In addition to temperature anisotropy, the CMB should have an angular variation in polarization. The polarization at each direction in the sky has an orientation described in terms of E-mode and B-mode polarization. The E-mode signal is a factor of 10 less strong than the temperature anisotropy; it supplements the temperature data as they are correlated. The B-mode signal is even weaker but may contain additional cosmological data.

The anisotropy is related to physical origin of the polarization. Excitation of an electron by linear polarized light generates polarized light at 90 degrees to the incident direction. If the incoming radiation is isotropic, different incoming directions create polarizations that cancel out. If the incoming radiation has quadrupole anisotropy, residual polarization will be seen.

Other than the temperature and polarization anisotropy, the CMB frequency spectrum is expected to feature tiny departures from the black-body law known as spectral distortions. These are also at the focus of an active research effort with the hope of a first measurement within the forthcoming decades, as they contain a wealth of information about the primordial universe and the formation of structures at late time.

The CMB contains the vast majority of photons in the universe by a factor of 400 to 1; the number density of photons in the CMB is one billion times  (109) the number density of matter in the universe. Without the expansion of the universe to cause the cooling of the CMB, the night sky would shine as brightly as the Sun. The energy density of the CMB is 0.260 eV/cm3, about 411 photons/cm3.

Early speculations
In 1931, Georges Lemaître speculated that remnants of the early universe may be observable as radiation, but his candidate was cosmic rays. Richard C. Tolman showed in 1934 that expansion of the universe would cool blackbody radiation while maintaining a thermal spectrum. The cosmic microwave background was first predicted in 1948 by Ralph Alpher and Robert Herman, in a correction they prepared for a paper by Alpher's PhD advisor George Gamow. Alpher and Herman were able to estimate the temperature of the cosmic microwave background to be 5 K.

Discovery

The first published recognition of the CMB radiation as a detectable phenomenon appeared in a brief paper by Soviet astrophysicists A. G. Doroshkevich and Igor Novikov, in the spring of 1964. In 1964, David Todd Wilkinson and Peter Roll, Dicke's colleagues at Princeton University, began constructing a Dicke radiometer to measure the cosmic microwave background. In 1964, Arno Penzias and Robert Woodrow Wilson at the Crawford Hill location of Bell Telephone Laboratories in nearby Holmdel Township, New Jersey had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments. The antenna was constructed in 1959 to support Project Echo—the National Aeronautics and Space Administration's passive communications satellites, which used large earth orbiting aluminized plastic balloons as reflectors to bounce radio signals from one point on the Earth to another. On 20 May 1964 they made their first measurement clearly showing the presence of the microwave background, with their instrument having an excess 4.2K antenna temperature which they could not account for. After receiving a telephone call from Crawford Hill, Dicke said "Boys, we've been scooped." A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was indeed due to the microwave background. Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery.

Cosmic origin
The interpretation of the cosmic microwave background was a controversial issue in the late 1960s. Alternative explanations included energy from within the solar system, from galaxies, from intergalactic plasma, from multiple extragalactic radio sources. Two requirements would show that the microwave radiation was truly "cosmic". First the intensity vs frequency or spectrum needed to be shown to match a thermal or blackbody source. This was accomplished by 1968 in a series of measurements of the radiation temperature at higher and lower wavelengths. Second the radiation needed be shown to be isotropic, the same from all directions. This was also accomplished by 1970, demonstrating that this radiation was truly cosmic in origin.

Progress on theory
In the 1970s numerous studies showed that tiny deviations from isotropy in the CMB could result from events in the early universe. Harrison, Peebles and Yu, and Zel'dovich realized that the early universe would require quantum inhomogeneities that would result in temperature anisotropy at the level of 10−4 or 10−5. Rashid Sunyaev calculated the observable imprint that these inhomogeneities would have on the cosmic microwave background.

COBE
After a lull in the 1970s caused in part by the many experimental difficulties in measuring CMB at high precision, increasingly stringent limits on the anisotropy of the cosmic microwave background were set by ground-based experiments during the 1980s. RELIKT-1, a Soviet cosmic microwave background anisotropy experiment on board the Prognoz 9 satellite (launched 1 July 1983), gave the first upper limits on the large-scale anisotropy.

The other key event in the 1980s was the proposal by Alan Guth for cosmic inflation. This theory of rapid spatial expansion gave an explanation for large-scale isotropy by allowing causal connection just before the epoch of last scattering. With this and similar theories, detailed prediction encouraged larger and more ambitious experiments.

The NASA Cosmic Background Explorer (COBE) satellite orbited Earth in 1989–1996 detected and quantified the large scale anisotropies at the limit of its detection capabilities. The NASA COBE mission clearly confirmed the primary anisotropy with the Differential Microwave Radiometer instrument, publishing their findings in 1992. The team received the Nobel Prize in physics for 2006 for this discovery.

Precision cosmology
Inspired by the COBE results, a series of ground and balloon-based experiments measured cosmic microwave background anisotropies on smaller angular scales over the two decades. The sensitivity of the new experiments improved dramatically, with a reduction in internal noise by three orders of magnitude. The primary goal of these experiments was to measure the scale of the first acoustic peak, which COBE did not have sufficient resolution to resolve. This peak corresponds to large scale density variations in the early universe that are created by gravitational instabilities, resulting in acoustical oscillations in the plasma. The first peak in the anisotropy was tentatively detected by the MAT/TOCO experiment and the result was confirmed by the BOOMERanG and MAXIMA experiments. These measurements demonstrated that the geometry of the universe is approximately flat, rather than curved. They ruled out cosmic strings as a major component of cosmic structure formation and suggested cosmic inflation was the right theory of structure formation.

Observations after COBE
Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground- and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. These measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflation was the right theory.

During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctuations occur at scales of approximately one degree. Together with other cosmological data, these results implied that the geometry of the universe is flat. A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array, Degree Angular Scale Interferometer (DASI), and the Cosmic Background Imager (CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum.

Wilkinson Microwave Anisotropy Probe
In June 2001, NASA launched a second CMB space mission, WMAP, to make much more precise measurements of the large scale anisotropies over the full sky. WMAP used symmetric, rapid-multi-modulated scanning, rapid switching radiometers at five frequencies to minimize non-sky signal noise. The data from the mission was released in five installments, the last being the nine year summary. The results are broadly consistent Lambda CDM models based on 6 free parameters and fitting in to Big Bang cosmology with cosmic inflation.

Planck Surveyor
A third space mission, the ESA (European Space Agency) Planck Surveyor, was launched in May 2009 and performed an even more detailed investigation until it was shut down in October 2013. Planck employed both HEMT radiometers and bolometer technology and measured the CMB at a smaller scale than WMAP. Its detectors were trialled in the Antarctic Viper telescope as ACBAR (Arcminute Cosmology Bolometer Array Receiver) experiment—which has produced the most precise measurements at small angular scales to date—and in the Archeops balloon telescope.

On 21 March 2013, the European-led research team behind the Planck cosmology probe released the mission's all-sky map (565x318 jpeg, 3600x1800 jpeg) of the cosmic microwave background. The map suggests the universe is slightly older than researchers expected. According to the map, subtle fluctuations in temperature were imprinted on the deep sky when the cosmos was about $370,000$ years old. The imprint reflects ripples that arose as early, in the existence of the universe, as the first nonillionth (10-30) of a second. Apparently, these ripples gave rise to the present vast cosmic web of galaxy clusters and dark matter. Based on the 2013 data, the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. On 5 February 2015, new data was released by the Planck mission, according to which the age of the universe is $13.799$ billion years old and the Hubble constant was measured to be $67.74 (km/s)/Mpc$.

Theoretical models
The cosmic microwave background radiation and the cosmological redshift-distance relation are together regarded as the best available evidence for the Big Bang event. Measurements of the CMB have made the inflationary Big Bang model the Standard Cosmological Model. The discovery of the CMB in the mid-1960s curtailed interest in alternatives such as the steady state theory.

In the Big Bang model for the formation of the universe, inflationary cosmology predicts that after about 10−37 seconds the nascent universe underwent exponential growth that smoothed out nearly all irregularities. The remaining irregularities were caused by quantum fluctuations in the inflaton field that caused the inflation event. Long before the formation of stars and planets, the early universe was smaller, much hotter and, starting 10−6 seconds after the Big Bang, filled with a uniform glow from its white-hot fog of interacting plasma of photons, electrons, and baryons.

As the universe expanded, adiabatic cooling caused the energy density of the plasma to decrease until it became favorable for electrons to combine with protons, forming hydrogen atoms. This recombination event happened when the temperature was around 3000 K or when the universe was approximately 379,000 years old. As photons did not interact with these electrically neutral atoms, the former began to travel freely through space, resulting in the decoupling of matter and radiation.

The color temperature of the ensemble of decoupled photons has continued to diminish ever since; now down to $2.726 K$, it will continue to drop as the universe expands. The intensity of the radiation corresponds to black-body radiation at 2.726 K because red-shifted black-body radiation is just like black-body radiation at a lower temperature. According to the Big Bang model, the radiation from the sky we measure today comes from a spherical surface called the surface of last scattering. This represents the set of locations in space at which the decoupling event is estimated to have occurred and at a point in time such that the photons from that distance have just reached observers. Most of the radiation energy in the universe is in the cosmic microwave background, making up a fraction of roughly $6$ of the total density of the universe.

Two of the greatest successes of the Big Bang theory are its prediction of the almost perfect black body spectrum and its detailed prediction of the anisotropies in the cosmic microwave background. The CMB spectrum has become the most precisely measured black body spectrum in nature.

Predictions based on the Big Bang model
In the late 1940s Alpher and Herman reasoned that if there was a Big Bang, the expansion of the universe would have stretched the high-energy radiation of the very early universe into the microwave region of the electromagnetic spectrum, and down to a temperature of about 5 K. They were slightly off with their estimate, but they had the right idea. They predicted the CMB. It took another 15 years for Penzias and Wilson to discover that the microwave background was actually there.

According to standard cosmology, the CMB gives a snapshot of the hot early universe at the point in time when the temperature dropped enough to allow electrons and protons to form hydrogen atoms. This event made the universe nearly transparent to radiation because light was no longer being scattered off free electrons. When this occurred some 380,000 years after the Big Bang, the temperature of the universe was about 3,000 K. This corresponds to an ambient energy of about $0.26 eV$, which is much less than the $13.6 eV$ ionization energy of hydrogen. This epoch is generally known as the "time of last scattering" or the period of recombination or decoupling.

Since decoupling, the color temperature of the background radiation has dropped by an average factor of 1,089 due to the expansion of the universe. As the universe expands, the CMB photons are redshifted, causing them to decrease in energy. The color temperature of this radiation stays inversely proportional to a parameter that describes the relative expansion of the universe over time, known as the scale length. The color temperature Tr of the CMB as a function of redshift, z, can be shown to be proportional to the color temperature of the CMB as observed in the present day (2.725 K or 0.2348 meV):
 * Tr = 2.725 K × (1 + z)

The high degree of uniformity throughout the observable universe and its faint but measured anisotropy lend strong support for the Big Bang model in general and the ΛCDM ("Lambda Cold Dark Matter") model in particular. Moreover, the fluctuations are coherent on angular scales that are larger than the apparent cosmological horizon at recombination. Either such coherence is acausally fine-tuned, or cosmic inflation occurred.

Primary anisotropy
The anisotropy, or directional dependency, of the cosmic microwave background is divided into two types: primary anisotropy, due to effects that occur at the surface of last scattering and before; and secondary anisotropy, due to effects such as interactions of the background radiation with intervening hot gas or gravitational potentials, which occur between the last scattering surface and the observer.

The structure of the cosmic microwave background anisotropies is principally determined by two effects: acoustic oscillations and diffusion damping (also called collisionless damping or Silk damping). The acoustic oscillations arise because of a conflict in the photon–baryon plasma in the early universe. The pressure of the photons tends to erase anisotropies, whereas the gravitational attraction of the baryons, moving at speeds much slower than light, makes them tend to collapse to form overdensities. These two effects compete to create acoustic oscillations, which give the microwave background its characteristic peak structure. The peaks correspond, roughly, to resonances in which the photons decouple when a particular mode is at its peak amplitude.

The peaks contain interesting physical signatures. The angular scale of the first peak determines the curvature of the universe (but not the topology of the universe). The next peak—ratio of the odd peaks to the even peaks—determines the reduced baryon density. The third peak can be used to get information about the dark-matter density.

The locations of the peaks give important information about the nature of the primordial density perturbations. There are two fundamental types of density perturbations called adiabatic and isocurvature. A general density perturbation is a mixture of both, and different theories that purport to explain the primordial density perturbation spectrum predict different mixtures.
 * Adiabatic density perturbations:In an adiabatic density perturbation, the fractional additional number density of each type of particle (baryons, photons, etc.) is the same. That is, if at one place there is a 1% higher number density of baryons than average, then at that place there is a 1% higher number density of photons (and a 1% higher number density in neutrinos) than average. Cosmic inflation predicts that the primordial perturbations are adiabatic.
 * Isocurvature density perturbations:In an isocurvature density perturbation, the sum (over different types of particle) of the fractional additional densities is zero. That is, a perturbation where at some spot there is 1% more energy in baryons than average, 1% more energy in photons than average, and 2% energy in neutrinos than average, would be a pure isocurvature perturbation. Hypothetical cosmic strings would produce mostly isocurvature primordial perturbations.

The CMB spectrum can distinguish between these two because these two types of perturbations produce different peak locations. Isocurvature density perturbations produce a series of peaks whose angular scales (ℓ values of the peaks) are roughly in the ratio 1 : 3 : 5 : ..., while adiabatic density perturbations produce peaks whose locations are in the ratio 1 : 2 : 3 : ... Observations are consistent with the primordial density perturbations being entirely adiabatic, providing key support for inflation, and ruling out many models of structure formation involving, for example, cosmic strings.

Collisionless damping is caused by two effects, when the treatment of the primordial plasma as fluid begins to break down: These effects contribute about equally to the suppression of anisotropies at small scales and give rise to the characteristic exponential damping tail seen in the very small angular scale anisotropies.
 * the increasing mean free path of the photons as the primordial plasma becomes increasingly rarefied in an expanding universe,
 * the finite depth of the last scattering surface (LSS), which causes the mean free path to increase rapidly during decoupling, even while some Compton scattering is still occurring.

The depth of the LSS refers to the fact that the decoupling of the photons and baryons does not happen instantaneously, but instead requires an appreciable fraction of the age of the universe up to that era. One method of quantifying how long this process took uses the photon visibility function (PVF). This function is defined so that, denoting the PVF by P(t), the probability that a CMB photon last scattered between time t and t + dt is given by P(t) dt.

The maximum of the PVF (the time when it is most likely that a given CMB photon last scattered) is known quite precisely. The first-year WMAP results put the time at which P(t) has a maximum as 372,000 years. This is often taken as the "time" at which the CMB formed. However, to figure out how it took the photons and baryons to decouple, we need a measure of the width of the PVF. The WMAP team finds that the PVF is greater than half of its maximal value (the "full width at half maximum", or FWHM) over an interval of 115,000 years. By this measure, decoupling took place over roughly 115,000 years, and when it was complete, the universe was roughly 487,000 years old.

Late time anisotropy
Since the CMB came into existence, it has apparently been modified by several subsequent physical processes, which are collectively referred to as late-time anisotropy, or secondary anisotropy. When the CMB photons became free to travel unimpeded, ordinary matter in the universe was mostly in the form of neutral hydrogen and helium atoms. However, observations of galaxies today seem to indicate that most of the volume of the intergalactic medium (IGM) consists of ionized material (since there are few absorption lines due to hydrogen atoms). This implies a period of reionization during which some of the material of the universe was broken into hydrogen ions.

The CMB photons are scattered by free charges such as electrons that are not bound in atoms. In an ionized universe, such charged particles have been liberated from neutral atoms by ionizing (ultraviolet) radiation. Today these free charges are at sufficiently low density in most of the volume of the universe that they do not measurably affect the CMB. However, if the IGM was ionized at very early times when the universe was still denser, then there are two main effects on the CMB:
 * 1) Small scale anisotropies are erased. (Just as when looking at an object through fog, details of the object appear fuzzy.)
 * 2) The physics of how photons are scattered by free electrons (Thomson scattering) induces polarization anisotropies on large angular scales. This broad angle polarization is correlated with the broad angle temperature perturbation.

Both of these effects have been observed by the WMAP spacecraft, providing evidence that the universe was ionized at very early times, at a redshift more than 17. The detailed provenance of this early ionizing radiation is still a matter of scientific debate. It may have included starlight from the very first population of stars (population III stars), supernovae when these first stars reached the end of their lives, or the ionizing radiation produced by the accretion disks of massive black holes.

The time following the emission of the cosmic microwave background—and before the observation of the first stars—is semi-humorously referred to by cosmologists as the Dark Age, and is a period which is under intense study by astronomers (see 21 centimeter radiation).

Two other effects which occurred between reionization and our observations of the cosmic microwave background, and which appear to cause anisotropies, are the Sunyaev–Zeldovich effect, where a cloud of high-energy electrons scatters the radiation, transferring some of its energy to the CMB photons, and the Sachs–Wolfe effect, which causes photons from the Cosmic Microwave Background to be gravitationally redshifted or blueshifted due to changing gravitational fields.

Alternative theories
The standard cosmology that includes the Big Bang "enjoys considerable popularity among the practicing cosmologists" However there are challenges to the standard big bang framework for explaining CMB data. In particular standard cosmology requires fine-tuning of some free parameters, with different values supported by different experimental data. As an example of the fine-tuning issue, standard cosmology cannot predict the present temperature of the relic radiation, $$T_0$$. This value of $$T_0$$ is one of the best results of experimental cosmology and the steady state model can predict it. However, alternative models have their own set of problems and they have only made post-facto explanations of existing observations. Nevertheless these alternatives have played an important historic role in providing ideas for and challenges to the standard explanation.

Polarization

The cosmic microwave background is polarized at the level of a few microkelvin. There are two types of polarization, called E-mode (or gradient-mode) and B-mode (or curl mode). This is in analogy to electrostatics, in which the electric field (E-field) has a vanishing curl and the magnetic field (B-field) has a vanishing divergence.

E-modes
The E-modes arise from Thomson scattering in a heterogeneous plasma. E-modes were first seen in 2002 by the Degree Angular Scale Interferometer (DASI).

B-modes
B-modes are expected to be an order of magnitude weaker than the E-modes. The former are not produced by standard scalar type perturbations, but are generated by gravitational waves during cosmic inflation shortly after the big bang. However, gravitational lensing of the stronger E-modes can also produce B-mode polarization. Detecting the original B-modes signal requires analysis of the contamination caused by lensing of the relatively strong E-mode signal.

Primordial gravitational waves
Models of "slow-roll" cosmic inflation in the early universe predicts primordial gravitational waves that would impact the polarisation of the cosmic microwave background, creating a specific pattern of B-mode polarization. Detection of this pattern would support the theory of inflation and their strength can confirm and exclude different models of inflation. Claims that this characteristic pattern of B-mode polarization had been measured by BICEP2 instrument were later attributed to cosmic dust due to new results of the Planck experiment.

Gravitational lensing
The second type of B-modes was discovered in 2013 using the South Pole Telescope with help from the Herschel Space Observatory. In October 2014, a measurement of the B-mode polarization at 150 GHz was published by the POLARBEAR experiment. Compared to BICEP2, POLARBEAR focuses on a smaller patch of the sky and is less susceptible to dust effects. The team reported that POLARBEAR's measured B-mode polarization was of cosmological origin (and not just due to dust) at a 97.2% confidence level.

Multipole analysis
The CMB angular anisotropies are usually presented in terms of power per multipole. The angular the map of temperature across the sky, $$T(\theta,\varphi),$$ is written as coefficients of spherical harmonics, $$T(\theta,\varphi) = \sum_{\ell m} a_{\ell m} Y_{\ell m}(\theta,\varphi)$$ where the $$a_{\ell m}$$ term measures the strength of the angular oscillation in $$Y_{\ell m}(\theta,\varphi)$$, and ℓ is the multipole number while m is the azimuthal number. The azimuthal variation is not significant and is removed by applying the angular correlation function, giving power spectrum term $$C_{\ell}\equiv \langle |a_{\ell m}|^2 \rangle.$$ Increasing values of ℓ correspond to higher multipole moments of CMB, meaning more rapid variation with angle.

CMBR monopole term (ℓ = 0)
The monopole term, ℓ = 0, is the constant isotropic mean temperature of the CMB, Tγ = $2.726 K$ with one standard deviation confidence. This term must be measured with absolute temperature devices, such as the FIRAS instrument on the COBE satellite.

CMBR dipole anisotropy (ℓ = 1)
CMB dipole represents the largest anisotropy, which is in the first spherical harmonic (ℓ = 1), a cosine function. The amplitude of CMB dipole is around $3.362 mK$. The CMB dipole moment is interpreted as the peculiar motion of the Earth relative to the CMB. Its amplitude depends on the time due to the Earth's orbit about the barycenter of the solar system. This enables us to add a time-dependent term to the dipole expression. The modulation of this term is 1 year, which fits the observation done by COBE FIRAS. The dipole moment does not encode any primordial information.

From the CMB data, it is seen that the Sun appears to be moving at $369.82 km/s$ relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB). The Local Group — the galaxy group that includes our own Milky Way galaxy — appears to be moving at $620 km/s$ in the direction of galactic longitude ℓ = $271.9 °$, b = $30 °$. The dipole is now used to calibrate mapping studies.

Multipole (ℓ ≥ 2)
The temperature variation in the CMB temperature maps at higher multipoles, or ℓ ≥ 2, is considered to be the result of perturbations of the density in the early Universe, before the recombination epoch at a redshift of around z ⋍ 1100. Before recombination, the Universe consisted of a hot, dense plasma of electrons and baryons. In such a hot dense environment, electrons and protons could not form any neutral atoms. The baryons in such early Universe remained highly ionized and so were tightly coupled with photons through the effect of Thompson scattering. These phenomena caused the pressure and gravitational effects to act against each other, and triggered fluctuations in the photon-baryon plasma. Quickly after the recombination epoch, the rapid expansion of the universe caused the plasma to cool down and these fluctuations are "frozen into" the CMB maps we observe today.

Data analysis challenges
Raw CMBR data, even from space vehicles such as WMAP or Planck, contain foreground effects that completely obscure the fine-scale structure of the cosmic microwave background. The fine-scale structure is superimposed on the raw CMBR data but is too small to be seen at the scale of the raw data. The most prominent of the foreground effects is the dipole anisotropy caused by the Sun's motion relative to the CMBR background. The dipole anisotropy and others due to Earth's annual motion relative to the Sun and numerous microwave sources in the galactic plane and elsewhere must be subtracted out to reveal the extremely tiny variations characterizing the fine-scale structure of the CMBR background. The detailed analysis of CMBR data to produce maps, an angular power spectrum, and ultimately cosmological parameters is a complicated, computationally difficult problem.

In practice it is hard to take the effects of noise and foreground sources into account. In particular, these foregrounds are dominated by galactic emissions such as Bremsstrahlung, synchrotron, and dust that emit in the microwave band; in practice, the galaxy has to be removed, resulting in a CMB map that is not a full-sky map. In addition, point sources like galaxies and clusters represent another source of foreground which must be removed so as not to distort the short scale structure of the CMB power spectrum.

Constraints on many cosmological parameters can be obtained from their effects on the power spectrum, and results are often calculated using Markov chain Monte Carlo sampling techniques.

Anomalies
With the increasingly precise data provided by WMAP, there have been a number of claims that the CMB exhibits anomalies, such as very large scale anisotropies, anomalous alignments, and non-Gaussian distributions. The most longstanding of these is the low-ℓ multipole controversy. Even in the COBE map, it was observed that the quadrupole (ℓ = 2, spherical harmonic) has a low amplitude compared to the predictions of the Big Bang. In particular, the quadrupole and octupole (ℓ = 3) modes appear to have an unexplained alignment with each other and with both the ecliptic plane and equinoxes. A number of groups have suggested that this could be the signature of new physics at the greatest observable scales; other groups suspect systematic errors in the data.

Ultimately, due to the foregrounds and the cosmic variance problem, the greatest modes will never be as well measured as the small angular scale modes. The analyses were performed on two maps that have had the foregrounds removed as far as possible: the "internal linear combination" map of the WMAP collaboration and a similar map prepared by Max Tegmark and others. Later analyses have pointed out that these are the modes most susceptible to foreground contamination from synchrotron, dust, and Bremsstrahlung emission, and from experimental uncertainty in the monopole and dipole.

A full Bayesian analysis of the WMAP power spectrum demonstrates that the quadrupole prediction of Lambda-CDM cosmology is consistent with the data at the 10% level and that the observed octupole is not remarkable. Carefully accounting for the procedure used to remove the foregrounds from the full sky map further reduces the significance of the alignment by ~5%. Recent observations with the Planck telescope, which is very much more sensitive than WMAP and has a larger angular resolution, record the same anomaly, and so instrumental error (but not foreground contamination) appears to be ruled out. Coincidence is a possible explanation, chief scientist from WMAP, Charles L. Bennett suggested coincidence and human psychology were involved, "I do think there is a bit of a psychological effect; people want to find unusual things."

Measurements of the density of quasars based on Wide-field Infrared Survey Explorer data finds a dipole significantly different from the one extracted from the CMB anisotropy. This difference is conflict with the cosmological principle.

Future evolution
Assuming the universe keeps expanding and it does not suffer a Big Crunch, a Big Rip, or another similar fate, the cosmic microwave background will continue redshifting until it will no longer be detectable, and will be superseded first by the one produced by starlight, and perhaps, later by the background radiation fields of processes that may take place in the far future of the universe such as proton decay, evaporation of black holes, and positronium decay.

Thermal (non-microwave background) temperature predictions

 * 1896 – Charles Édouard Guillaume estimates the "radiation of the stars" to be 5–6 K.
 * 1926 – Sir Arthur Eddington estimates the non-thermal radiation of starlight in the galaxy "... by the formula E = σT4 the effective temperature corresponding to this density is 3.18° absolute ... black body".
 * 1930s – Cosmologist Erich Regener calculates that the non-thermal spectrum of cosmic rays in the galaxy has an effective temperature of 2.8 K.
 * 1931 – Term microwave first used in print: "When trials with wavelengths as low as 18 cm. were made known, there was undisguised surprise+that the problem of the micro-wave had been solved so soon." Telegraph & Telephone Journal XVII. 179/1
 * 1934 – Richard Tolman shows that black-body radiation in an expanding universe cools but remains thermal.
 * 1946 – Robert Dicke predicts "... radiation from cosmic matter" at < 20 K, but did not refer to background radiation.
 * 1946 – George Gamow calculates a temperature of 50 K (assuming a 3-billion year old universe), commenting it "... is in reasonable agreement with the actual temperature of interstellar space", but does not mention background radiation.
 * 1953 – Erwin Finlay-Freundlich in support of his tired light theory, derives a blackbody temperature for intergalactic space of 2.3 K and in the following year values of 1.9K and 6.0K.

Microwave background radiation predictions and measurements

 * 1941 – Andrew McKellar detected a "rotational" temperature of 2.3 K for the interstellar medium by comparing the population of CN doublet lines measured by W. S. Adams in a B star.
 * 1948 – Ralph Alpher and Robert Herman estimate "the temperature in the universe" at 5 K. Although they do not specifically mention microwave background radiation, it may be inferred.
 * 1953 – George Gamow estimates 7 K based on a model that does not rely on a free parameter
 * 1955 – Émile Le Roux of the Nançay Radio Observatory, in a sky survey at λ = 33 cm, initially reported a near-isotropic background radiation of 3 kelvins, plus or minus 2; he did not recognize the cosmological significance  and later revised the error bars to 20K.
 * 1957 – Tigran Shmaonov reports that "the absolute effective temperature of the radioemission background ... is 4±3 K". with radiation intensity was independent of either time or direction of observation. Although Shamonov did not recognize it at the time, it is now clear that Shmaonov did observe the cosmic microwave background at a wavelength of 3.2 cm
 * 1964 – A. G. Doroshkevich and Igor Dmitrievich Novikov publish a brief paper suggesting microwave searches for the black-body radiation predicted by Gamow, Alpher, and Herman, where they name the CMB radiation phenomenon as detectable.
 * 1964–65 – Arno Penzias and Robert Woodrow Wilson measure the temperature to be approximately 3 K. Robert Dicke, James Peebles, P. G. Roll, and D. T. Wilkinson interpret this radiation as a signature of the Big Bang.
 * 1966 – Rainer K. Sachs and Arthur M. Wolfe theoretically predict microwave background fluctuation amplitudes created by gravitational potential variations between observers and the last scattering surface (see Sachs–Wolfe effect).
 * 1968 – Martin Rees and Dennis Sciama theoretically predict microwave background fluctuation amplitudes created by photons traversing time-dependent wells of potential.
 * 1969 – R. A. Sunyaev and Yakov Zel'dovich study the inverse Compton scattering of microwave background photons by hot electrons (see Sunyaev–Zel'dovich effect).
 * 1983 – Researchers from the Cambridge Radio Astronomy Group and the Owens Valley Radio Observatory first detect the Sunyaev–Zel'dovich effect from clusters of galaxies.
 * 1983 – RELIKT-1 Soviet CMB anisotropy experiment was launched.
 * 1990 – FIRAS on the Cosmic Background Explorer (COBE) satellite measures the black body form of the CMB spectrum with exquisite precision, and shows that the microwave background has a nearly perfect black-body spectrum with T = 2.73 K and thereby strongly constrains the density of the intergalactic medium.
 * January 1992 – Scientists that analysed data from the RELIKT-1 report the discovery of anisotropy in the cosmic microwave background at the Moscow astrophysical seminar.
 * 1992 – Scientists that analysed data from COBE DMR report the discovery of anisotropy in the cosmic microwave background.
 * 1995 – The Cosmic Anisotropy Telescope performs the first high resolution observations of the cosmic microwave background.
 * 1999 – First measurements of acoustic oscillations in the CMB anisotropy angular power spectrum from the MAT/TOCO, BOOMERANG, and Maxima Experiments. The BOOMERanG experiment makes higher quality maps at intermediate resolution, and confirms that the universe is "flat".
 * 2002 – Polarization discovered by DASI.
 * 2003 – E-mode polarization spectrum obtained by the CBI. The CBI and the Very Small Array produces yet higher quality maps at high resolution (covering small areas of the sky).
 * 2003 – The Wilkinson Microwave Anisotropy Probe spacecraft produces an even higher quality map at low and intermediate resolution of the whole sky (WMAP provides high-resolution data, but improves on the intermediate resolution maps from BOOMERanG).
 * 2004 – E-mode polarization spectrum obtained by the CBI.
 * 2004 – The Arcminute Cosmology Bolometer Array Receiver produces a higher quality map of the high resolution structure not mapped by WMAP.
 * 2005 – The Arcminute Microkelvin Imager and the Sunyaev–Zel'dovich Array begin the first surveys for very high redshift clusters of galaxies using the Sunyaev–Zel'dovich effect.
 * 2005 – Ralph A. Alpher is awarded the National Medal of Science for his groundbreaking work in nucleosynthesis and prediction that the universe expansion leaves behind background radiation, thus providing a model for the Big Bang theory.
 * 2006 – The long-awaited three-year WMAP results are released, confirming previous analysis, correcting several points, and including polarization data.
 * 2006 – Two of COBE's principal investigators, George Smoot and John Mather, received the Nobel Prize in Physics in 2006 for their work on precision measurement of the CMBR.
 * 2006–2011 – Improved measurements from WMAP, new supernova surveys ESSENCE and SNLS, and baryon acoustic oscillations from SDSS and WiggleZ, continue to be consistent with the standard Lambda-CDM model.
 * 2010 – The first all-sky map from the Planck telescope is released.
 * 2013 – An improved all-sky map from the Planck telescope is released, improving the measurements of WMAP and extending them to much smaller scales.
 * 2014 – On March 17, 2014, astrophysicists of the BICEP2 collaboration announced the detection of inflationary gravitational waves in the B-mode power spectrum, which if confirmed, would provide clear experimental evidence for the theory of inflation.   However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings was reported.
 * 2015 – On January 30, 2015, the same team of astronomers from BICEP2 withdrew the claim made on the previous year. Based on the combined data of BICEP2 and Planck, the European Space Agency announced that the signal can be entirely attributed to dust in the Milky Way.
 * 2018 – The final data and maps from the Planck telescope is released, with improved measurements of the polarization on large scales.
 * 2019 – Planck telescope analyses of their final 2018 data continue to be released.

In popular culture

 * In the Stargate Universe TV series (2009–2011), an ancient spaceship, Destiny, was built to study patterns in the CMBR which is a sentient message left over from the beginning of time.
 * In Wheelers, a novel (2000) by Ian Stewart & Jack Cohen, CMBR is explained as the encrypted transmissions of an ancient civilization. This allows the Jovian "blimps" to have a society older than the currently-observed age of the universe.
 * In The Three-Body Problem, a 2008 novel by Liu Cixin, a probe from an alien civilization compromises instruments monitoring the CMBR in order to deceive a character into believing the civilization has the power to manipulate the CMBR itself.
 * The 2017 issue of the Swiss 20 francs bill lists several astronomical objects with their distances – the CMB is mentioned with 430 · 1015 light-seconds.
 * In the 2021 Marvel series WandaVision, a mysterious television broadcast is discovered within the Cosmic Microwave Background.

title: Cosmic microwave background