Civilization

Civilization (or civilisation) is a sometimes controversial term that has been used in several related ways. Primarily, the term has been used to refer to the material and instrumental side of human cultures that are complex in terms of technology, science, and division of labor. Such civilizations are generally hierarchical and urbanized. In a classical context, people were called "civilized" to set them apart from barbarians, savages, and primitive peoples while in a modern-day context, "civilized peoples" have been contrasted with indigenous peoples or tribal societies.

There is a tendency to use the term in a less strict way, to mean approximately the same thing as "culture" and therefore, the term can more broadly refer to any important and clearly defined human society. Still, even when used in this second sense, the word is often restricted to apply only to societies that have attained a particular level of advancement—especially the founding of cities.

The level of advancement of a civilization is often measured by its progress in agriculture, long-distance trade, occupational specialization, a special governing class, and urbanism. Aside from these core elements, a civilization is often marked by any combination of a number of secondary elements, including a developed transportation system, writing, standardized measurement, currency, contractual and tort-based legal systems, characteristic art and architecture, mathematics, enhanced scientific understanding, metallurgy, political structures, and organized religion.

Huntington's map of major civilizations. What constitutes Western civilization in his view is coloured dark blue.

Etymology

The word civilization comes from the Latin civilis, meaning civil, related to the Latin civis, meaning citizen, and civitas, meaning city or city-state.

In the sixth century, the Byzantine Emperor Justinian oversaw the consolidation of Roman civil law. The resulting collection is called the Corpus Juris Civilis. In the 11th century, professors at the University of Bologna, Western Europe's first university, rediscovered the Corpus Juris Civilis, and its influence began to be felt across Europe. In 1388, the word civil appeared in English meaning "of or related to citizens." In 1704, civilization was used to mean "a law which makes a criminal process into a civil case." Civilization was not used in its modern sense to mean "the opposite of barbarism"—as contrasted to civility, meaning politeness or civil virtue—until the second half of the 18th century.

According to Emile Benveniste (1954), the earliest written occurrence in English of civilisation in its modern sense may be found in Adam Ferguson's An Essay on the History of Civil Society (Edinburgh, 1767 – p. 2): "Not only the individual advances from infancy to manhood, but the species itself from rudeness to civilisation."

It should be noted that this usage incorporates the concept of superiority and maturity of "civilized" existence, as contrasted to "rudeness", which is used to denote coarseness, as in a lack of refinement or "civility."

Before Benveniste's inquiries, the New English Dictionary quoted James Boswell's conversation with Samuel Johnson concerning the inclusion of Civilization in Johnson's dictionary:

On Monday, March 23 (1772), I found him busy, preparing a fourth edition of his folio Dictionary... He would not admit civilization, but only civility. With great deference to him I thought civilization, from to civilize, better in the sense opposed to barbarity than civility, as it is better to have a distinct word for each sense, than one word with two senses, which civility is, in his way of using it.

Benveniste demonstrated that previous occurrences could be found, which explained the quick adoption of Johnson's definition. In 1775 the dictionary of Ast defined civilization as "the state of being civilized; the act of civilizing", and the term was frequently used by Adam Smith in An Inquiry into the Nature and Causes of the Wealth of Nations (1776). Beside Smith and Ferguson, John Millar also used it in 1771 in his Observations concerning the distinction of ranks in society.

The history of the word in English appears to be connected with the parallel development in French, which may be the original source. As the first occurrence of civilization in French was found by Benveniste in the Marquis de Mirabeau's L'Ami des hommes ou traité de la population (written in 1756 but published in 1757), Benveniste's query was to know if the English word derived from the French, or if both evolved independently — a question which needed more research. According to him, the word civilization may in fact have been used by Ferguson as soon as 1759.

Furthermore, Benveniste notes that, contrasted to civility, a static term, civilization conveys a sense of dynamism. He thus writes that:

It was not only a historical view of society; it was also an optimist and resolutely non theological interpretation of its evolution which asserted itself, sometimes at the insu of those who proclaimed it, and even if some of them, and first of all Mirabeau, still counted religion as the first factor of 'civilization.

In the late 1700s and early 1800s, both during the French revolution, and in English, "civilization" was referred to in the singular, never the plural, because it referred to the progress of humanity as a whole. This is still the case in French. More recently "civilizations" is sometimes used as a synonym for the broader term "cultures" in both popular and academic circles. However, the concepts of civilization and culture are not always considered interchangeable. For example, a small nomadic tribe may be judged not to have a civilization, but it would surely be judged to have a culture (defined as "the arts, customs, habits... beliefs, values, behavior and material habits that constitute a people's way of life").

Civilization is not always seen as an improvement. One historically important distinction between culture and civilization stems from the writings of Rousseau, and particularly his work concerning education, Emile. In this perspective, civilization, being more rational and socially driven, is not fully in accordance with human nature, and "human wholeness is achievable only through the recovery of or approximation to an original prediscursive or prerational natural unity". (See noble savage.) From this notion, a new approach was developed especially in Germany, first by Johann Gottfried Herder, and later by philosophers such as Kierkegaard and Nietzsche. This sees cultures (plural) as natural organisms which are not defined by "conscious, rational, deliberative acts" but rather a kind of pre-rational "folk spirit". Civilization, in contrast, though more rational and more successful concerning material progress, is seen as un-natural, and leads to "vices of social life" such as guile, hypocrisy, envy, and avarice. During World War II, Leo Strauss, having fled Germany, argued in New York that this approach to civilization was behind Nazism and German militarism and nihilism.

In his book The Philosophy of Civilization, Albert Schweitzer outlined the idea that there are dual opinions within society: one regarding civilization as purely material and another regarding civilization as both ethical and material. He stated that the current world crisis was, then in 1923, due to a humanity having lost the ethical conception of civilization. In this same work, he defined civilization, saying that it "is the sum total of all progress made by man in every sphere of action and from every point of view in so far as the progress helps towards the spiritual perfecting of individuals as the progress of all progress."

Characteristics

26th century BCE Sumerian cuneiform script in Sumerian language, listing gifts to the high priestess of Adab on the occasion of her election. One of the earliest examples of human writing.

Social scientists such as V. Gordon Childe have named a number of traits that distinguish a civilization from other kinds of society. Civilizations have been distinguished by their means of subsistence, types of livelihood, settlement patterns, forms of government, social stratification, economic systems, literacy, and other cultural traits.

All civilizations have depended on agriculture for subsistence. Growing food on farms results in a surplus of food, particularly when people use intensive agricultural techniques such as irrigation and crop rotation. Grain surpluses have been especially important because they can be stored for a long time. A surplus of food permits some people to do things besides produce food for a living: early civilizations included artisans, priests and priestesses, and other people with specialized careers. A surplus of food results in a division of labor and a more diverse range of human activity, a defining trait of civilizations. However, in some places hunter-gatherers have had access to food surpluses, such as among some of the indigenous peoples of the Pacific Northwest and perhaps during the Mesolithic Natufian culture. It is possible that food surpluses and relatively large scale social organization and division of labor predates plant and animal domestication.

Civilizations have distinctly different settlement patterns from other societies. The word civilization is sometimes simply defined as "'living in cities'". Non-farmers tend to gather in cities to work and to trade.

"No one in the history of civilization has shaped our understanding of science and natural philosophy more than the great Greek philosopher and scientist Aristotle (384–322 BCE), who exerted a profound and pervasive influence for more than two thousand years" —Gary B. Ferngren

Compared with other societies, civilizations have a more complex political structure, namely the state. State societies are more stratified than other societies; there is a greater difference among the social classes. The ruling class, normally concentrated in the cities, has control over much of the surplus and exercises its will through the actions of a government or bureaucracy. Morton Fried, a conflict theorist, and Elman Service, an integration theorist, have classified human cultures based on political systems and social inequality. This system of classification contains four categories:

• Hunter-gatherer bands, which are generally egalitarian.
• Horticultural/pastoral societies in which there are generally two inherited social classes; chief and commoner.
• Highly stratified structures, or chiefdoms, with several inherited social classes: king, noble, freemen, serf and slave.
• Civilizations, with complex social hierarchies and organized, institutional governments.

Economically, civilizations display more complex patterns of ownership and exchange than less organized societies. Living in one place allows people to accumulate more personal possessions than nomadic people. Some people also acquire landed property, or private ownership of the land. Because a percentage of people in civilizations do not grow their own food, they must trade their goods and services for food in a market system, or receive food through the levy of tribute, redistributive taxation, tariffs or tithes from the food producing segment of the population. Early civilizations developed money as a medium of exchange for these increasingly complex transactions. To oversimplify, in a village the potter makes a pot for the brewer and the brewer compensates the potter by giving him a certain amount of beer. In a city, the potter may need a new roof, the roofer may need new shoes, the cobbler may need new horseshoes, the blacksmith may need a new coat, and the tanner may need a new pot. These people may not be personally acquainted with one another and their needs may not occur all at the same time. A monetary system is a way of organizing these obligations to ensure that they are fulfilled fairly.

Writing, developed first by people in Sumer, is considered a hallmark of civilization and "appears to accompany the rise of complex administrative bureaucracies or the conquest state." Traders and bureaucrats relied on writing to keep accurate records. Like money, writing was necessitated by the size of the population of a city and the complexity of its commerce among people who are not all personally acquainted with each other. However, writing is not always necessary for civilization. The Inca civilization of the Andes did not use writing at all, yet it still functioned as a society.

Aided by their division of labor and central government planning, civilizations have developed many other diverse cultural traits. These include organized religion, development in the arts, and countless new advances in science and technology.

Through history, successful civilizations have spread, taking over more and more territory, and assimilating more and more previously-uncivilized people. Nevertheless, some tribes or people remain uncivilized even to this day. These cultures are called by some "primitive," a term that is regarded by others as pejorative. "Primitive" implies in some way that a culture is "first" (Latin = primus), that it has not changed since the dawn of humanity, though this has been demonstrated not to be true. Specifically, as all of today's cultures are contemporaries, today's so-called primitive cultures are in no way antecedent to those we consider civilized. Many anthropologists use the term "non-literate" to describe these peoples.

Civilization has been spread by colonization, invasion, religious conversion, the extension of bureaucratic control and trade, and by introducing agriculture and writing to non-literate peoples. Some non-civilized people may willingly adapt to civilized behaviour. But civilization is also spread by the technical, material and social dominance that civilization engenders.

Cultural identity

"Civilization" can also refer to the culture of a complex society, not just the society itself. Every society, civilization or not, has a specific set of ideas and customs, and a certain set of manufactures and arts that make it unique. Civilizations tend to develop intricate cultures, including literature, professional art, architecture, organized religion, and complex customs associated with the elite.

The intricate culture associated with civilization has a tendency to spread to and influence other cultures, sometimes assimilating them into the civilization (a classic example being Chinese civilization and its influence on nearby civilizations such as Korea, Japan and Vietnam). Many civilizations are actually large cultural spheres containing many nations and regions. The civilization in which someone lives is that person's broadest cultural identity.

Many historians have focused on these broad cultural spheres and have treated civilizations as discrete units. Early twentieth-century philosopher Oswald Spengler, uses the German word "Kultur," "culture," for what many call a "civilization". Spengler believes a civilization's coherence is based on a single primary cultural symbol. Cultures experience cycles of birth, life, decline, and death, often supplanted by a potent new culture, formed around a compelling new cultural symbol. Spengler states civilization is the beginning of the decline of a culture as, "...the most external and artificial states of which a species of developed humanity is capable."

This "unified culture" concept of civilization also influenced the theories of historian Arnold J. Toynbee in the mid-twentieth century. Toynbee explored civilization processes in his multi-volume A Study of History, which traced the rise and, in most cases, the decline of 21 civilizations and five "arrested civilizations." Civilizations generally declined and fell, according to Toynbee, because of the failure of a "creative minority", through moral or religious decline, to meet some important challenge, rather than mere economic or environmental causes.

Samuel P. Huntington defines civilization as "the highest cultural grouping of people and the broadest level of cultural identity people have short of that which distinguishes humans from other species." Huntington's theories about civilizations are discussed below.

Complex systems

Another group of theorists, making use of systems theory, looks at a civilization as a complex system, i.e., a framework by which a group of objects can be analyzed that work in concert to produce some result. Civilizations can be seen as networks of cities that emerge from pre-urban cultures, and are defined by the economic, political, military, diplomatic, social, and cultural interactions among them. Any organization is a complex social system, and a civilization is a large organization. Systems theory helps guard against superficial but misleading analogies in the study and description of civilizations.

Systems theorists look at many types of relations between cities, including economic relations, cultural exchanges, and political/diplomatic/military relations. These spheres often occur on different scales. For example, trade networks were, until the nineteenth century, much larger than either cultural spheres or political spheres. Extensive trade routes, including the Silk Road through Central Asia and Indian Ocean sea routes linking the Roman Empire, Persian Empire, India, and China, were well established 2000 years ago, when these civilizations scarcely shared any political, diplomatic, military, or cultural relations. The first evidence of such long distance trade is in the ancient world. During the Uruk period Guillermo Algaze has argued that trade relations connected Egypt, Mesopotamia, Iran and Afghanistan. Resin found later in the Royal Tombs of Ur it is suggested was traded northwards from Mozambique.

Many theorists argue that the entire world has already become integrated into a single "world system", a process known as globalization. Different civilizations and societies all over the globe are economically, politically, and even culturally interdependent in many ways. There is debate over when this integration began, and what sort of integration – cultural, technological, economic, political, or military-diplomatic – is the key indicator in determining the extent of a civilization. David Wilkinson has proposed that economic and military-diplomatic integration of the Mesopotamian and Egyptian civilizations resulted in the creation of what he calls the "Central Civilization" around 1500 BCE. Central Civilization later expanded to include the entire Middle East and Europe, and then expanded to a global scale with European colonization, integrating the Americas, Australia, China and Japan by the nineteenth century. According to Wilkinson, civilizations can be culturally heterogeneous, like the Central Civilization, or homogeneous, like the Japanese civilization. What Huntington calls the "clash of civilizations" might be characterized by Wilkinson as a clash of cultural spheres within a single global civilization. Others point to the Crusades as the first step in globalization. The more conventional viewpoint is that networks of societies have expanded and shrunk since ancient times, and that the current globalized economy and culture is a product of recent European colonialism.

Future

See also: Risks to civilization, humans and planet Earth

Political scientist Samuel Huntington has argued that the defining characteristic of the 21st century will be a clash of civilizations. According to Huntington, conflicts between civilizations will supplant the conflicts between nation-states and ideologies that characterized the 19th and 20th centuries. These views have been strongly challenged by others like Edward Said, Muhammed Asadi and Amartya Sen. Ronald Inglehart and Pippa Norris have argued that the "true clash of civilizations" between the Muslim world and the West is caused by the Muslim rejection of the West's more liberal sexual values, rather than a difference in political ideology, although they note that this lack of tolerance is likely to lead to an eventual rejection of (true) democracy. In Identity and Violence Sen questions if people should be divided along the lines of a supposed 'civilization', defined by religion and culture only. He argues that this ignores the many others identities that make up people and leads to a focus on differences.

Some environmental scientists see the world entering a Planetary Phase of Civilization, characterized by a shift away from independent, disconnected nation-states to a world of increased global connectivity with worldwide institutions, environmental challenges, economic systems, and consciousness. In an attempt to better understand what a Planetary Phase of Civilization might look like in the current context of declining natural resources and increasing consumption, the Global scenario group used scenario analysis to arrive at three archetypal futures: Barbarization, in which increasing conflicts result in either a fortress world or complete societal breakdown; Conventional Worlds, in which market forces or Policy reform slowly precipitate more sustainable practices; and a Great Transition, in which either the sum of fragmented Eco-Communalism movements add up to a sustainable world or globally coordinated efforts and initiatives result in a new sustainability paradigm.

Author Derrick Jensen argues that modern civilization is intrinsically directed towards the domination of the environment and humanity itself in a harmful and destructive fashion.

The Kardashev scale classifies civilizations based on their level of technological advancement, specifically measured by the amount of energy a civilization is able to harness. The Kardashev scale makes provisions for civilizations far more technologically advanced than any currently known to exist (see also: Civilizations and the Future, Space civilization).

Fall of civilizations

Main article: Societal collapse

There have been many explanations put forward for the collapse of civilization. Some focus on historical examples, and others on general theory.

• Ibn Khaldūn's Muqaddimah influenced theories of the analysis, growth and decline of the Islamic civilization. He suggested repeated invasions from nomadic peoples limited development and led to social collapse.
• Edward Gibbon's work The Decline and Fall of the Roman Empire was a well-known and detailed analysis of the fall of Roman civilization. Gibbon suggested the final act of the collapse of Rome was the fall of Constantinople to the Ottoman Turks in 1453 CE. For Gibbon:

  The decline of Rome was the natural and inevitable effect of immoderate greatness. Prosperity ripened the principle of decay; the cause of the destruction multiplied with the extent of conquest; and, as soon as time or accident had removed the artificial supports, the stupendous fabric yielded to the pressure of its own weight. The story of the ruin is simple and obvious; and instead of inquiring why the Roman Empire was destroyed, we should rather be surprised that it has subsisted for so long.[Gibbon, Decline and Fall of the Roman Empire, 2nd ed., vol. 4, ed. by J. B. Bury (London, 1909), pp. 173–174.-Chapter XXXVIII: Reign Of Clovis.--Part VI. General Observations On The Fall Of The Roman Empire In The West.]
  

• Theodor Mommsen in his "History of Rome (Mommsen)", suggested Rome collapsed with the collapse of the Western Roman Empire in 476 CE and he also tended towards a biological analogy of "genesis," "growth," "senescence," "collapse" and "decay."
• Oswald Spengler, in his "Decline of the West" rejected Petrarch's chronological division, and suggested that there had been only eight "mature civilizations." Growing cultures, he argued, tend to develop into imperialistic civilizations which expand and ultimately collapse, with democratic forms of government ushering in plutocracy and ultimately imperialism.
• Arnold J. Toynbee in his "A Study of History" suggested that there had been a much larger number of civilizations, including a small number of arrested civilizations, and that all civilizations tended to go through the cycle identified by Mommsen. The cause of the fall of a civilization occurred when a cultural elite became a parasitic elite, leading to the rise of internal and external proletariats.
• Joseph Tainter in "The Collapse of Complex Societies" suggested that there were diminishing returns to complexity, due to which, as states achieved a maximum permissible complexity, they would decline when further increases actually produced a negative return. Tainter suggested that Rome achieved this figure in the 2nd century CE.
• Jared Diamond in his 2005 book "Collapse: How Societies Choose to Fail or Succeed" suggests five major reasons for the collapse of 41 studied cultures: environmental damage, such as deforestation and soil erosion; climate change; dependence upon long-distance trade for needed resources; increasing levels of internal and external violence, such as war or invasion; and societal responses to internal and environmental problems.
• Peter Turchin in his Historical Dynamics and Andrey Korotayev et al. in their Introduction to Social Macrodynamics, Secular Cycles, and Millennial Trends suggest a number of mathematical models describing collapse of agrarian civilizations. For example, the basic logic of Turchin's "fiscal-demographic" model can be outlined as follows: during the initial phase of a sociodemographic cycle we observe relatively high levels of per capita production and consumption, which leads not only to relatively high population growth rates, but also to relatively high rates of surplus production. As a result, during this phase the population can afford to pay taxes without great problems, the taxes are quite easily collectible, and the population growth is accompanied by the growth of state revenues. During the intermediate phase, the increasing overpopulation leads to the decrease of per capita production and consumption levels, it becomes more and more difficult to collect taxes, and state revenues stop growing, whereas the state expenditures grow due to the growth of the population controlled by the state. As a result, during this phase the state starts experiencing considerable fiscal problems. During the final pre-collapse phases the overpopulation leads to further decrease of per capita production, the surplus production further decreases, state revenues shrink, but the state needs more and more resources to control the growing (though with lower and lower rates) population. Eventually this leads to famines, epidemics, state breakdown, and demographic and civilization collapse (Peter Turchin. Historical Dynamics. Princeton University Press, 2003:121–127).
• Peter Heather argues in his book The Fall of the Roman Empire: a New History of Rome and the Barbarians that this civilization did not end for moral or economic reasons, but because centuries of contact with barbarians across the frontier generated its own nemesis by making them a much more sophisticated and dangerous adversary. The fact that Rome needed to generate ever greater revenues to equip and re-equip armies that were for the first time repeatedly defeated in the field, led to the dismemberment of the Empire. Although this argument is specific to Rome, it can also be applied to the Asiatic Empire of the Egyptians, to the Han and Tang dynasties of China, to the Muslim Abbasid Caliphate, and others.
• Bryan Ward-Perkins, in his book The Fall of Rome and the End of Civilization, shows the real horrors associated with the collapse of a civilization for the people who suffer its effects, unlike many revisionist historians who downplay this. The collapse of complex society meant that even basic plumbing disappeared from the continent for 1,000 years. Similar Dark Age collapses are seen with the Late Bronze Age collapse in the Eastern Mediterranean, the collapse of the Maya, on Easter Island and elsewhere.
• Arthur Demarest argues in Ancient Maya: The Rise and Fall of a Rainforest Civilization, using a holistic perspective to the most recent evidence from archaeology, paleoecology, and epigraphy, that no one explanation is sufficient but that a series of erratic, complex events, including loss of soil fertility, drought and rising levels of internal and external violence led to the disintegration of the courts of Mayan kingdoms which began a spiral of decline and decay. He argues that the collapse of the Maya has lessons for civilization today.
• Jeffrey A. McNeely has recently suggested that "A review of historical evidence shows that past civilizations have tended to over-exploit their forests, and that such abuse of important resources has been a significant factor in the decline of the over-exploiting society."
• Thomas Homer-Dixon in "The Upside of Down: Catastrophe, Creativity, and the Renewal of Civilization", considers that the fall in the energy return on investments; the energy expended to energy yield ratio, is central to limiting the survival of civilizations. The degree of social complexity is associated strongly, he suggests, with the amount of disposable energy environmental, economic and technological systems allow. When this amount decreases civilizations either have to access new energy sources or they will collapse....

History

Early civilizations

Further information: Prehistory and Cradle of Civilization

Map of the world showing approximate centers of origin of agriculture and its spread in prehistory: the Fertile Crescent (11,000 BP), the Yangtze and Yellow River basins (9000 BP) and the New Guinea Highlands (9000–6000 BP), Central Mexico (5000–4000 BP), Northern South America (5000–4000 BP), sub-Saharan Africa (5000–4000 BP, exact location unknown), eastern USA (4000–3000 BP).

The process of sedentarization is first thought to have occurred around 12,000 BCE in the Levant region of southwest Asia though other regions around the world soon followed. The emergence of civilization is generally associated with the Neolithic, or Agricultural Revolution, which occurred in various locations between 8,000 and 5,000 BCE, specifically in southwestern/southern Asia, northern/central Africa and Central America. This revolution marked the beginning of stable agriculture and animal domestication which enabled economies and cities to develop.

The following articles discuss the development of major early civilizations.

The Big Bang theory is the prevailing cosmological model that describes the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the Universe to cool and resulted in its present continuously expanding state. According to the most recent measurements and observations, the Big Bang occurred approximately 13.75 billion years ago, which is thus considered the age of the Universe. After its initial expansion from a singularity, the Universe cooled sufficiently to allow energy to be converted into various subatomic particles, including protons, neutrons, and electrons. While protons and neutrons combined to form the first atomic nuclei only a few minutes after the Big Bang, it would take thousands of years for electrons to combine with them and create electrically neutral atoms. The first element produced was hydrogen, along with traces of helium and lithium. Giant clouds of these primordial elements would coalesce through gravity to form stars and galaxies, and the heavier elements would be synthesized either within stars or during supernovae.
The Big Bang is a well-tested scientific theory and is widely accepted within the scientific community. It offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background, large scale structure, and the Hubble diagram for Type Ia supernovae.^[6] The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, and the formation of galaxies—are derived from these and other observations that are independent of any cosmological model. As the distance between galaxy clusters is increasing today, it can be inferred that everything was closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures, and large particle accelerators have been built to experiment in such conditions, resulting in further development of the model. On the other hand, these accelerators have limited capabilities to probe into such high energy regimes. There is little evidence regarding the absolute earliest instant of the expansion. Thus, the Big Bang theory cannot and does not provide any explanation for such an initial condition; rather, it describes and explains the general evolution of the universe going forward from that point on.
Georges Lemaître first proposed what would become the Big Bang theory in what he called his "hypothesis of the primeval atom." Over time, scientists would build on his initial ideas to form the modern synthesis. The framework for the Big Bang model relies on Albert Einstein's general relativity and on simplifying assumptions such as homogeneity and isotropy of space. The governing equations had been formulated by Alexander Friedmann. In 1929, Edwin Hubble discovered that the distances to far away galaxies were generally proportional to their redshifts—an idea originally suggested by Lemaître in 1927. Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.
While the scientific community was once divided between supporters of the Big Bang and those of the Steady State theory, most scientists became convinced that some version of the Big Bang scenario best fit observations after the discovery of the cosmic microwave background radiation in 1964, and especially when its spectrum (i.e., the amount of radiation measured at each wavelength) was found to match that of thermal radiation from a black body. Since then, astrophysicists have incorporated a wide range of observational and theoretical additions into the Big Bang model, and its parametrization as the Lambda-CDM model serves as the framework for current investigations of theoretical cosmology.

Overview

Timeline of the Big Bang

Main article: Timeline of the Big Bang

A graphical timeline is available at Graphical timeline of the Big Bang

Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly no closer than the end of the Planck epoch. This singularity is sometimes called "the Big Bang", but the term can also refer to the early hot, dense phase itself, which can be considered the "birth" of our Universe. Based on measurements of the expansion using Type Ia supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the Universe has a calculated age of 13.75 ± 0.11 billion years The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents of the Universe.
The earliest phases of the Big Bang are subject to much speculation. In the most common models the Universe was filled homogeneously and isotropically with an incredibly high energy density and huge temperatures and pressures and was very rapidly expanding and cooling. Approximately 10⁻³⁷ seconds into the expansion, a phase transition caused a cosmic inflation, during which the Universe grew exponentially.After inflation stopped, the Universe consisted of a quark–gluon plasma, as well as all other elementary particles. Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present Universe.

Hubble eXtreme Deep Field (XDF)

XDF size compared to the size of the moon - several thousand galaxies, each consisting of billions of stars, are in this small view.

XDF (2012) view - each light speck is a galaxy - some of these are as old as 13.2 billion years^[20] - the universe is estimated to contain 200 billion galaxies.

XDF image shows fully mature galaxies in the foreground plane - nearly mature galaxies from 5 to 9 billion years ago - protogalaxies, blazing with young stars, beyond 9 billion years.

The Universe continued to grow in size and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form. After about 10⁻¹¹ seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. At about 10⁻⁶ seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton–antiproton pairs (similarly for neutrons–antineutrons), so a mass annihilation immediately followed, leaving just one in 10¹⁰ of the original protons and neutrons, and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the Universe was dominated by photons (with a minor contribution from neutrinos).
A few minutes into the expansion, when the temperature was about a billion (one thousand million; 10⁹; SI prefix giga-) kelvin and the density was about that of air, neutrons combined with protons to form the Universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. Most protons remained uncombined as hydrogen nuclei. As the Universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation.
Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the Universe. The four possible types of matter are known as cold dark matter, warm dark matter, hot dark matter, and baryonic matter. The best measurements available (from WMAP) show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be cold (warm dark matter is ruled out by early reionization), and is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. In an "extended model" which includes hot dark matter in the form of neutrinos, then if the "physical baryon density" Ω_bh² is estimated at about 0.023 (this is different from the 'baryon density' Ω_b expressed as a fraction of the total matter/energy density, which as noted above is about 0.046), and the corresponding cold dark matter density Ω_ch² is about 0.11, the corresponding neutrino density Ω_vh² is estimated to be less than 0.0062.
Independent lines of evidence from Type Ia supernovae and the CMB imply that the Universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 73% of the total energy density of today's Universe is in this form. When the Universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity had the upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the Universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.
All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity. As noted above, there is no well-supported model describing the action prior to 10⁻¹⁵ seconds or so. Apparently a new unified theory of quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the Universe is currently one of the greatest unsolved problems in physics.

Underlying assumptions

The Big Bang theory depends on two major assumptions: the universality of physical laws and the cosmological principle. The cosmological principle states that on large scales the Universe is homogeneous and isotropic.
These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the universe is of order 10⁻⁵. Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.
If the large-scale Universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler Copernican principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10⁻⁵ via observations of the CMB. The Universe has been measured to be homogeneous on the largest scales at the 10% level.

FLRW metric

Main articles: Friedmann–Lemaître–Robertson–Walker metric and Metric expansion of space

General relativity describes spacetime by a metric, which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, themselves are specified using a coordinate chart or "grid" that is laid down over all spacetime. The cosmological principle implies that the metric should be homogeneous and isotropic on large scales, which uniquely singles out the Friedmann–Lemaître–Robertson–Walker metric (FLRW metric). This metric contains a scale factor, which describes how the size of the Universe changes with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates. In this coordinate system the grid expands along with the Universe, and objects that are moving only due to the expansion of the Universe remain at fixed points on the grid. While their coordinate distance (comoving distance) remains constant, the physical distance between two such comoving points expands proportionally with the scale factor of the Universe.
The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distance between two comoving points. Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our Universe only on large scales—local concentrations of matter such as our galaxy are gravitationally bound and as such do not experience the large-scale expansion of space.

Horizons

Main article: Cosmological horizon

An important feature of the Big Bang spacetime is the presence of horizons. Since the Universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our Universe. Our understanding of the Universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the Universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the Universe continues to accelerate, there is a future horizon as well.

Etymology

Fred Hoyle is credited with coining the term Big Bang during a 1949 radio broadcast. It is popularly reported that Hoyle, who favored an alternative "steady state" cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.

Development

Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big Bang

The Big Bang theory developed from observations of the structure of the Universe and from theoretical considerations. In 1912 Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way. Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einstein's equations of general relativity, showing that the Universe might be expanding in contrast to the static Universe model advocated by Einstein at that time. In 1924 Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, proposed that the inferred recession of the nebulae was due to the expansion of the Universe.
In 1931 Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the Universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence.
Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2,500 mm) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929 Hubble discovered a correlation between distance and recession velocity—now known as Hubble's law. Lemaître had already shown that this was expected, given the Cosmological Principle.
In the 1920s and 1930s almost every major cosmologist preferred an eternal steady state Universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the steady state theory. This perception was enhanced by the fact that the originator of the Big Bang theory, Monsignor Georges Lemaître, was a Roman Catholic priest. Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz., that matter is eternal. A beginning in time was "repugnant" to him. Lemaître, however, thought that

If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time.

During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model, the oscillatory Universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard Tolman) and Fritz Zwicky's tired light hypothesis.
After World War II, two distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the Universe seemed to expand. In this model the Universe is roughly the same at any point in time. The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis (BBN) and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background radiation (CMB). Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast in March 1949. For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor Big Bang over Steady State. The discovery and confirmation of the cosmic microwave background radiation in 1964 secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the Universe at earlier and earlier times, and reconciling observations with the basic theory.
Significant progress in Big Bang cosmology have been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as COBE, the Hubble Space Telescope and WMAP. Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the Universe appears to be accelerating.

Observational evidence

"[T]he big bang picture is too firmly grounded in data from every area to be proved invalid in its general features."

 Lawrence Krauss

The earliest and most direct kinds of observational evidence are the Hubble-type expansion seen in the redshifts of galaxies, the detailed measurements of the cosmic microwave background, the relative abundances of light elements produced by Big Bang nucleosynthesis, and today also the large scale distribution and apparent evolution of galaxies predicted to occur due to gravitational growth of structure in the standard theory. These are sometimes called "the four pillars of the Big Bang theory".
Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently subjected to the most active laboratory investigations. Remaining issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not fatal to the dark matter explanation as solutions to such problems exist which involve only further refinements of the theory. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible. Inflation and baryogenesis remain more speculative features of current Big Bang models: they explain important features of the early universe, but could be replaced by alternative ideas without affecting the rest of the theory. Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved problems in physics.

Hubble's law and the expansion of space

Main articles: Hubble's law and Metric expansion of space

See also: Distance measures (cosmology) and Scale factor (universe)

Observations of distant galaxies and quasars show that these objects are redshifted—the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission lines or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed:

v = H₀D,

where

• v is the recessional velocity of the galaxy or other distant object,
• D is the comoving distance to the object, and
• H₀ is Hubble's constant, measured to be 70.4 +1.3
  −1.4 km/s/Mpc by the WMAP probe.

Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable given the Copernican principle—or the Universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Alexander Friedmann in 1922 and Georges Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson, and Walker.
The theory requires the relation v = HD to hold at all times, where D is the comoving distance, v is the recessional velocity, and v, H, and D vary as the Universe expands (hence we write H₀ to denote the present-day Hubble "constant"). For distances much smaller than the size of the observable Universe, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity v. However, the redshift is not a true Doppler shift, but rather the result of the expansion of the Universe between the time the light was emitted and the time that it was detected.
That space is undergoing metric expansion is shown by direct observational evidence of the Cosmological principle and the Copernican principle, which together with Hubble's law have no other explanation. Astronomical redshifts are extremely isotropic and homogenous, supporting the Cosmological principle that the Universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.
Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican principle, that, on a cosmological scale, the Earth is not in a central position. Radiation from the Big Bang was demonstrably warmer at earlier times throughout the Universe. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the Universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

Cosmic microwave background radiation

Main article: Cosmic microwave background radiation

WMAP image of the cosmic microwave background radiation. The radiation is isotropic to roughly one part in 100,000.

In 1964 Arno Penzias and Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the microwave band. Their discovery provided substantial confirmation of the general CMB predictions: the radiation was found to be consistent with an almost perfect black body spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded a Nobel Prize in 1978.
The surface of last scattering corresponding to emission of the CMB occurs shortly after recombination, the epoch when neutral hydrogen becomes stable. Prior to this, the universe comprised a hot dense photon-baryon plasma sea where photons were quickly scattered from free charged particles. Peaking at around 372±14 kyr, the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent.

The cosmic microwave background spectrum measured by the FIRAS instrument on the COBE satellite is the most-precisely measured black body spectrum in nature. The data points and error bars on this graph are obscured by the theoretical curve.

In 1989 NASA launched the Cosmic Background Explorer satellite (COBE). Its findings were consistent with predictions regarding the CMB, finding a residual temperature of 2.726 K (more recent measurements have revised this figure down slightly to 2.725 K) and providing the first evidence for fluctuations (anisotropies) in the CMB, at a level of about one part in 10⁵. John C. Mather and George Smoot were awarded the Nobel Prize for their leadership in this work. During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001 several experiments, most notably BOOMERanG, found the shape of the Universe to be spatially almost flat by measuring the typical angular size (the size on the sky) of the anisotropies.
In early 2003 the first results of the Wilkinson Microwave Anisotropy Probe (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. The results disproved several specific cosmic inflation models, but are consistent with the inflation theory in general. The Planck space probe was launched in May 2009. Other ground and balloon based cosmic microwave background experiments are ongoing.

Abundance of primordial elements

Main article: Big Bang nucleosynthesis

Using the Big Bang model it is possible to calculate the concentration of helium-4, helium-3, deuterium, and lithium-7 in the Universe as ratios to the amount of ordinary hydrogen. The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for 4He/H, about 10⁻³ for 2H/H, about 10⁻⁴ for 3He/H and about 10⁻⁹ for 7Li/H.
The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for 4He, and off by a factor of two 7Li; in the latter two cases there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by Big Bang nucleosynthesis is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium. Indeed there is no obvious reason outside of the Big Bang that, for example, the young Universe (i.e., before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than 3He, and in constant ratios, too.

Galactic evolution and distribution

Main articles: Galaxy formation and evolution, Large-scale structure of the cosmos, and Structure formation

This panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The galaxies are color coded by redshift.

Detailed observations of the morphology and distribution of galaxies and quasars are in agreement with the current state of the Big Bang theory. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as galaxy clusters and superclusters. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early Universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions and larger structures agree well with Big Bang simulations of the formation of structure in the Universe and are helping to complete details of the theory.

Primordial gas clouds

In 2011 astronomers have found pristine clouds of the primordial gas that formed in the first few minutes after the Big Bang. The composition of the gas matches theoretical predictions, providing direct evidence in support of the modern cosmological explanation for the origins of elements in the universe. The researchers discovered the two clouds of pristine gas by analyzing the light from distant quasars, using the HIRES spectrometer on the Keck I Telescope at the W. M. Keck Observatory in Hawaii. They saw absorption lines in the spectrum where the light was absorbed by the gas, and that allows them to measure the composition of the gas.

Other lines of evidence

The age of Universe as estimated from the Hubble expansion and the CMB is now in good agreement with other estimates using the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars.
The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of very low temperature absorption lines in gas clouds at high redshift. This prediction also implies that the amplitude of the Sunyaev–Zel'dovich effect in clusters of galaxies does not depend directly on redshift. Observations have found this to be roughly true, but this effect depends on cluster properties that do change with cosmic time, making precise measurements difficult.

Related issues in physics

Baryon asymmetry

Main article: Baryon asymmetry

It is not yet understood why the Universe has more matter than antimatter. It is generally assumed that when the Universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and antibaryons. However, observations suggest that the Universe, including its most distant parts, is made almost entirely of matter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the Universe depart from thermodynamic equilibrium. All these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry.

Dark energy

Main article: Dark energy

Measurements of the redshift–magnitude relation for type Ia supernovae indicate that the expansion of the Universe has been accelerating since the Universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the Universe consists of a component with large negative pressure, dubbed "dark energy". Dark energy, though speculative, solves numerous problems. Measurements of the cosmic microwave background indicate that the Universe is very nearly spatially flat, and therefore according to general relativity the Universe must have almost exactly the critical density of mass/energy. But the mass density of the Universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy also helps to explain two geometrical measures of the overall curvature of the Universe, one using the frequency of gravitational lenses, and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.
Negative pressure is believed to be a property of vacuum energy, but the exact nature and existence of dark energy remains one of the great mysteries of the Big Bang. Possible candidates include a cosmological constant and quintessence. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos. According to theory, the energy density in matter decreases with the expansion of the Universe, but the dark energy density remains constant (or nearly so) as the Universe expands. Therefore matter made up a larger fraction of the total energy of the Universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.

Dark matter

Main article: Dark matter

A pie chart indicating the proportional composition of different energy-density components of the Universe, according to the best ΛCDM model fits – roughly 95% is in the exotic forms of dark matter and dark energy

During the 1970s and 1980s, various observations showed that there is not sufficient visible matter in the Universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the Universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the Universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the Universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter has always been controversial, it is inferred by various observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters.
Indirect evidence for dark matter comes from its gravitational influence on other matter, as no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.

Globular cluster age

In the mid-1990s observations of globular clusters appeared to be inconsistent with the Big Bang theory. Computer simulations that matched the observations of the stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13.7 billion year age of the Universe. This issue was partially resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. There remain some questions as to how accurately the ages of the clusters are measured, but it is clear that observations of globular clusters no longer appear inconsistent with the Big Bang theory.

Problems

There are generally considered to be three outstanding problems with the Big Bang theory: the horizon problem, the flatness problem, and the magnetic monopole problem. The most common answer to these problems is inflationary theory; however, since this creates new problems, other options have been proposed, such as the Weyl curvature hypothesis.

Horizon problem

Main article: Horizon problem

The horizon problem results from the premise that information cannot travel faster than light. In a Universe of finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in causal contact. The observed isotropy of the CMB is problematic in this regard: if the Universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature.
A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the Universe at some very early period (before baryogenesis). During inflation, the Universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable Universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.
Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the Universe. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB.
If inflation occurred, exponential expansion would push large regions of space well beyond our observable horizon.

Flatness problem

Main article: Flatness problem

The overall geometry of the Universe is determined by whether the Omega cosmological parameter is less than, equal to or greater than 1. Shown from top to bottom are a closed Universe with positive curvature, a hyperbolic Universe with negative curvature and a flat Universe with zero curvature.

The flatness problem (also known as the oldness problem) is an observational problem associated with a Friedmann–Lemaître–Robertson–Walker metric. The Universe may have positive, negative, or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density, positive if greater, and zero at the critical density, in which case space is said to be flat. The problem is that any small departure from the critical density grows with time, and yet the Universe today remains very close to flat. Given that a natural timescale for departure from flatness might be the Planck time, 10⁻⁴³ seconds, the fact that the Universe has reached neither a heat death nor a Big Crunch after billions of years requires some explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the Universe density must have been within one part in 10¹⁴ of its critical value, or it would not exist as it does today.
A resolution to this problem is offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that its curvature would have been smoothed out. Thus, it is theorized that inflation drove the Universe to a very nearly spatially flat state, with almost exactly the critical density.

Magnetic monopoles

Main article: Magnetic monopole

The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted topological defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the hot early Universe, resulting in a density much higher than is consistent with observations, given that searches have never found any monopoles. This problem is also resolved by cosmic inflation, which removes all point defects from the observable Universe in the same way that it drives the geometry to flatness.

The future according to the Big Bang theory

Main article: Ultimate fate of the universe

Before observations of dark energy, cosmologists considered two scenarios for the future of the Universe. If the mass density of the Universe were greater than the critical density, then the Universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started—a Big Crunch. Alternatively, if the density in the Universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out leaving white dwarfs, neutron stars, and black holes. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the Universe would asymptotically approach absolute zero—a Big Freeze. Moreover, if the proton were unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the Universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.
Modern observations of accelerating expansion imply that more and more of the currently visible Universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the Universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death as the Universe expands and cools. Other explanations of dark energy, called phantom energy theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.

Speculative physics beyond Big Bang theory

This is an artist's concept of the Universe expansion, where space (including hypothetical non-observable portions of the Universe) is represented at each time by the circular sections. Note on the left the dramatic expansion (not to scale) occurring in the inflationary epoch, and at the center the expansion acceleration. The scheme is decorated with WMAP images on the left and with the representation of stars at the appropriate level of development.

While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the Universe's history. The equations of classical general relativity indicate a singularity at the origin of cosmic time, although this conclusion depends on several assumptions. Moreover, general relativity must break down before the Universe reaches the Planck temperature, and a correct treatment of quantum gravity may avoid the would-be singularity.
Some proposals, each of which entails untested hypotheses, are:

• Models including the Hartle–Hawking no-boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.

• Big Bang lattice model states that the Universe at the moment of the Big Bang consists of an infinite lattice of fermions which is smeared over the fundamental domain so it has both rotational, translational, and gauge symmetry. The symmetry is the largest symmetry possible and hence the lowest entropy of any state.

• Brane cosmology models in which inflation is due to the movement of branes in string theory; the pre-Big Bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically. In the latter model the Big Bang was preceded by a Big Crunch and the Universe endlessly cycles from one process to the other.

• Eternal inflation, in which universal inflation ends locally here and there in a random fashion, each end-point leading to a bubble universe expanding from its own big bang.

Proposals in the last two categories see the Big Bang as an event in either a much larger and older Universe, or in a multiverse.

Religious and philosophical implications

Main article: Religious interpretations of the Big Bang theory

As a theory relevant to the origin of the universe, the Big Bang has significant bearing on religion and philosophy.^[94][95] As a result, it has become one of the liveliest areas in the discourse between science and religion.^[96] Some believe the Big Bang implies a creator, while others argue that Big Bang cosmology makes the notion of a creator superfluous.^[95][98]

Notes

1. ^ There is no consensus about how long the Big Bang phase lasted. For some writers this denotes only the initial singularity, for others the whole history of the Universe. Usually, at least the first few minutes (during which helium is synthesized) are said to occur "during the Big Bang".^[15] (see also Big Bang nucleosynthesis)
2. ^ Detailed information of and references for tests of general relativity are given in the article tests of general relativity.
3. ^ This ignores the dipole anisotropy at a level of 0.1% due to the peculiar velocity of the solar system through the radiation field.
4. ^ It is commonly reported that Hoyle intended this to be pejorative. However, Hoyle later denied that, saying that it was just a striking image meant to emphasize the difference between the two theories for radio listeners.^[51]
5. ^ If inflation is true, baryogenesis must have occurred, but not vice versa.
6. ^ Strictly, dark energy in the form of a cosmological constant drives the Universe towards a flat state; however, our Universe remained close to flat for several billion years, before the dark energy density became significant.