Happiness economics

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Happiness_economics 

Richer countries tend to be happier than poorer countries (observations are lined up around an upward-sloping trend), and richer people within countries tend to be happier than poorer people in the same countries (arrows are consistently pointing northeast).

The economics of happiness or happiness economics is the theoretical, qualitative and quantitative study of happiness and quality of life, including positive and negative affects, well-being, life satisfaction and related concepts – typically tying economics more closely than usual with other social sciences, like sociology and psychology, as well as physical health. It typically treats subjective happiness-related measures, as well as more objective quality of life indices, rather than wealth, income or profit, as something to be maximized.

The field has grown substantially since the late 20th century, for example by the development of methods, surveys and indices to measure happiness and related concepts, as well as quality of life. Happiness findings have been described as a challenge to the theory and practice of economics. Nevertheless, furthering gross national happiness, as well as a specified Index to measure it, has been adopted explicitly in the Constitution of Bhutan in 2008, to guide its economic governance.

Subject classifications

The subject may be categorized in various ways, depending on specificity, intersection, and cross-classification. For example, within the Journal of Economic Literature classification codes, it has been categorized under:

• Welfare economics at JEL: D63 – Equity, Justice, Inequality, and Other Normative Criteria and Measurement
• Health, education, and welfare at JEL: I31 – General Welfare; Basic needs; Living standards; Quality of life; Happiness
• Demographic economics at JEL:J18 – Public policy.

Metrology

Given its very nature, reported happiness is subjective. It is difficult to compare one person's happiness with another's. It can be especially difficult to compare happiness across cultures. However, many happiness economists believe they have solved this comparison problem. Cross-sections of large data samples across nations and time demonstrate consistent patterns in the determinants of happiness.

Happiness is typically measured using subjective measures – e.g. self-reported surveys – and/or objective measures. One concern has always been the accuracy and reliability of people's responses to happiness surveys. Objective measures such as lifespan, income, and education are often used as well as or instead of subjectively reported happiness, though this assumes that they generally produce happiness, which while plausible may not necessarily be the case. The terms quality of life or well-being are often used to encompass these more objective measures.

Micro-econometric happiness equations have the standard form: ${\displaystyle W_{it}=\alpha +\beta x_{it}+{ϵ}_{it}}$. In this equation ${\displaystyle W}$ is the reported well-being of individual ${\displaystyle i}$ at time ${\displaystyle t}$, and ${\displaystyle x}$ is a vector of known variables, which include socio-demographic and socioeconomic characteristics.

Macro-econometric happiness has been gauged by some as Gross National Happiness, following Sicco Mansholt's 1972 introduction of the measure, and by others as a Genuine Wealth index. Anielski in 2008 wrote a reference definition on how to measure five types of capital: (1) human; (2) social; (3) natural; (4) built; and (5) financial.

Happiness, well-being, or satisfaction with life, was seen as unmeasurable in classical and neo-classical economics. Van Praag was the first person who organized large surveys in order to explicitly measure welfare derived from income. He did this with the Income Evaluation Question (IEQ). This approach is called the Leyden School. It is named after the Dutch university where this approach was developed. Other researchers included Arie Kapteyn and Aldi Hagenaars.

Some scientists claim that happiness can be measured both subjectively and objectively by observing the joy center of the brain lit up with advanced imaging, although this raises philosophical issues, for example about whether this can be treated as more reliable than reported subjective happiness.

Determinants

GDP and GNP

Typically national financial measures, such as gross domestic product (GDP) and gross national product (GNP), have been used as a measure of successful policy. There is a significant association between GDP and happiness, with citizens in wealthier nations being happier than those in poorer nations. In 2002, researchers argued that this relationship extends only to an average GDP per capita of about $15,000. In the 2000s, several studies have obtained the opposite result, so this Easterlin paradox is controversial.

Individual income

Historically, economists have said that well-being is a simple function of income. However, it has been found that once wealth reaches a subsistence level, its effectiveness as a generator of well-being is greatly diminished. Happiness economists hope to change the way governments view well-being and how to most effectively govern and allocate resources given this paradox.

In 2010, Daniel Kahneman and Angus Deaton found that higher earners generally reported better life satisfaction, but people's day-to-day emotional well-being only rose with earnings until a threshold annual household pre-tax income of $75,000. This particular study by Kahneman and Deaton showed the relationship between experienced happiness and the maximum amount of income at $75,000. Experienced happiness is the happiness received on a daily basis-"the frequency and intensity of experiences of joy, fascination, anxiety, sadness, anger, and affection that make one's life pleasant or unpleasant." The other finding from Kahneman and Deaton is there is no evidence supporting a maximum income to what is called reflective happiness. This data is supported by the use of the Cantrill Ladder, which revealed that there is a direct relationship between income and reflective happiness. This can conclude, to a point, that money does buy happiness.

Other factors have been suggested as making people happier than money. A short term course of psychological therapy is 32 times more cost effective at increasing happiness than simply increasing income.

Scholars at the University of Virginia, University of British Columbia and Harvard University released a study in 2011 after examining numerous academic papers in response to an apparent contradiction: "When asked to take stock of their lives, people with more money report being a good deal more satisfied. But when asked how happy they are at the moment, people with more money are barely different than those with less." The study included the following eight general recommendations:

• Spend money on "experiences" rather than goods.
• Donate money to others, including charities, rather than spending it solely on oneself.
• Spend small amounts of money on many small, temporary pleasures rather than less often on larger ones.
• Don't spend money on "extended warranties and other forms of overpriced insurance."
• Adjust one's mindset to "pay now, consume later," instead of "consume now, pay later."
• Exercise circumspection about the day-to-day consequences of a purchase beforehand.
• Rather than buying products that provide the "best deal," make purchases based on what will facilitate well-being.
• Seek out the opinions of other people who have prior experience of a product before purchasing it.

In their "Unhappy Cities" paper, Edward Glaeser, Joshua Gottlieb and Oren Ziv examined the self-reported subjective well-being of people living in American metropolitan areas, particularly in relation to the notion that "individuals make trade-offs among competing objectives, including but not limited to happiness." The researchers findings revealed that people living in metropolitan areas where lower levels of happiness are reported are receiving higher real wages, and they suggest in their conclusion that "humans are quite understandably willing to sacrifice both happiness and life satisfaction if the price is right."

Social security

Ruut Veenhoven claimed that social security payments do not seem to add to happiness. This may be due to the fact that non-self-earned income (e.g., from a lottery) does not add to happiness in general either. Happiness may be the mind's reward for a useful action. However, Johan Norberg of Centre for Independent Studies, a free enterprise economy think tank, presents a hypothesis that as people who think that they themselves control their lives are happier, paternalist institutions may decrease happiness.

An alternative perspective focuses on the role of the welfare state as an institution that improves quality of life not only by increasing the extent to which basic human needs are met, but also by promoting greater control of one's life by limiting the degree to which individuals find themselves at the mercy of impersonal market forces that are indifferent to the fate of individuals. This is the argument suggested by the U.S. political scientist Benjamin Radcliff, who has presented a series of papers in peer-reviewed scholarly journals demonstrating that a more generous welfare state contributes to higher levels of life satisfaction, and does so to rich and poor alike.

Employment

Generally, the well-being of those who are employed is higher than those who are unemployed. Employment itself may not increase subjective well-being, but facilitates activities that do (such as supporting a family, philanthropy, and education). While work does increase well-being through providing income, income level is not as indicative of subjective well-being as other benefits related to employment. Feelings of autonomy and mastery, found in higher levels in the employed than unemployed, are stronger predictors of subjective well-being than wealth.

When personal preference and the amount of time spent working do not align, both men and women experience a decrease in subjective well-being. The negative effect of working more or working less than preferred has been found across multiple studies, most finding that working more than preferred (over-employed) is more detrimental, but some found that working less (under-employed) is more detrimental. Most individuals' levels of subjective well-being returned to "normal" (level previous to time mismatch) within one year. Levels remained lower only when individuals worked more hours than preferred for a period of two years or more, which may indicate that it is more detrimental to be over-employed than under-employed in the long-term.

Employment status effects are not confined to the individual. Being unemployed can have detrimental effects on a spouse's subjective well-being, compared to being employed or not working (and not looking for work). Partner life satisfaction is inversely related to the number of hours their partner is underemployed. When both partners are underemployed, the life-satisfaction of men is more greatly diminished than women. However, just being in a relationship reduces the impact unemployment has on the subjective well-being of an individual. On a broad scale, high rates of unemployment negatively affect the subjective well-being of the employed.

Becoming self-employed can increase subjective well-being, given the right conditions. Those who leave work to become self-employed report greater life satisfaction than those who work for others or become self-employed after unemployment; this effect increases over time. Those who are self-employed and have employees of their own report higher life-satisfaction than those who are self-employed without employees, and women who are self-employed without employees report a higher life satisfaction than men in the same condition.

The effects of retirement on subjective well-being vary depending on personal and cultural factors. Subjective well-being can remain stable for those who retire from work voluntarily, but declines for those who are involuntarily retired. In countries with an average social norm to work, the well-being of men increases after retirement, and the well-being of retired women is at the same level as women who are homemakers or work outside the home. In countries with a strong social norm to work, retirement negatively impacts the well-being of men and women.

Relationships and children

In the 1970s, women typically reported higher subjective well-being than did men. By 2009, declines in reported female happiness had eroded a gender gap.

In rich societies, where a rise in income doesn't equate to an increase in levels of subjective well-being, personal relationships are the determining factors of happiness.

Glaeser, Gottlieb and Ziv suggest in their conclusion that the happiness trade-offs that individuals seem willing to make aligns with the tendency of parents to report less happiness, as they sacrifice their personal well-being for the "price" of having children.

Freedom and control

There is a significant correlation between feeling in control of one's own life and happiness levels.

A study conducted at the University of Zurich suggested that democracy and federalism bring well-being to individuals. It concluded that the more direct political participation possibilities available to citizens raises their subjective well-being. Two reasons were given for this finding. First, a more active role for citizens enables better monitoring of professional politicians by citizens, which leads to greater satisfaction with government output. Second, the ability for citizens to get involved in and have control over the political process, independently increases well-being.

American psychologist Barry Schwartz argues in his book The Paradox of Choice that too many consumer and lifestyle choices can produce anxiety and unhappiness due to analysis paralysis and raised expectations of satisfaction.

Religious diversity

National cross-sectional data suggest an inverse relationship between religious diversity and happiness, possibly by facilitating more bonding (and less bridging) social capital.

Happiness and leisure

See also: Happiness and Leisure

Much of the research regarding happiness and leisure relies on subjective well-being (SWB) as an appropriate measure of happiness. Research has demonstrated a wide variety of contributing and resulting factors in the relationship between leisure and happiness. These include psychological mechanisms, and the types and characteristics of leisure activities that result in the greatest levels of subjective happiness. Specifically, leisure may trigger five core psychological mechanisms including detachment-recovery from work, autonomy in leisure, mastery of leisure activities, meaning-making in leisure activities, and social affiliation in leisure (DRAMMA). Leisure activities that are physical, relational, and performed outdoors are correlated with greater feelings of satisfaction with free time. Research across 33 different countries shows that individuals who feel they strengthen social relationships and work on personal development during leisure time are happier than others. Furthermore, shopping, reading books, attending cultural events, getting together with relatives, listening to music and attending sporting events is associated with higher levels of happiness. Spending time on the internet or watching TV is not associated with higher levels of happiness as compared to these other activities.

Research has shown that culture influences how we measure happiness and leisure. While SWB is a commonly used measure of happiness in North America and Europe, this may not be the case internationally. Quality of life (QOL) may be a better measure of happiness and leisure in Asian countries, especially Korea. Countries such as China and Japan may require a different measurement of happiness, as societal differences may influence the concept of happiness (i.e. economic variables, cultural practices, and social networks) beyond what QOL is able to measure. There seem to be some differences in leisure preference cross-culturally. Within the Croatian culture, family related leisure activities may enhance SWB across a large spectrum of ages ranging from adolescent to older adults, in both women and men. Active socializing and visiting cultural events are also associated with high levels of SWB across varying age and gender. Italians seem to prefer social conceptions of leisure as opposed to individualistic conceptions. Although different groups of individuals may prefer varying types and amount of leisure activity, this variability is likely due to the differing motivations and goals that an individual intends to fulfill with their leisure time.

Research suggests that specific leisure interventions enhance feelings of SWB. This is both a top-down and bottom-up effect, in that leisure satisfaction causally affects SWB, and SWB causally affects leisure satisfaction. This bi-directional effect is stronger in retired individuals than in working individuals. Furthermore, it appears that satisfaction with our leisure at least partially explains the relationship between our engagement in leisure and our SWB. Broadly speaking, researchers classify leisure into active (e.g. volunteering, socializing, sports and fitness) and passive leisure (e.g. watching television and listening to the radio). Among older adults, passive leisure activities and personal leisure activities (e.g. sleeping, eating, and bathing) correlate with higher levels of SWB and feelings of relaxation than active leisure activities. Thus, although significant evidence has demonstrated that active leisure is associated with higher levels of SWB, or happiness, this may not be the case with older populations.

Both regular and irregular involvement in sports leisure can result in heightened SWB. Serious, or systematic involvement in certain leisure activities, such as taekwondo, correlates with personal growth and a sense of happiness. Additionally, more irregular (e.g. seasonal) sports activities, such as skiing, are also correlated with high SWB. Furthermore, the relationship between pleasure and skiing is thought to be caused in part by a sense of flow and involvement with the activity. Leisure activities, such as meeting with friends, participating in sports, and going on vacation trips, positively correlate with life satisfaction. It may also be true that going on a vacation makes our lives seem better, but does not necessarily make us happier in the long term. Research regarding vacationing or taking a holiday trip is mixed. Although the reported effects are mostly small, some evidence points to higher levels of SWB, or happiness, after taking a holiday.

Economic security

See also: Economic security

Poverty alleviation is associated with happier populations. According to the latest systematic review of the economic literature on life satisfaction: Volatile or high inflation is bad for a population's well-being, particularly those with a right-wing political orientation. That suggests the impact of disruptions to economic security are in part mediated or modified by beliefs about economic security.

Political stability

The Voxeu analysis of the economic determinants of happiness found that life satisfaction explains the largest share of an existing government's vote share, followed by economic growth, which itself explains six times as much as employment and twice as much as inflation.

Economic freedom

Individualistic societies have happier populations. Institutes of economic freedom are associated with increases wealth inequality but does not necessarily contribute to decreases in aggregate well-being or subjective well-being at the population level. In fact, income inequality enhances global well-being. There is some debate over whether living in poor neighbours make one happier. And, living among rich neighbours can dull the happiness that comes from wealth. This is purported to work by way of an upward or downward comparison effect (Keeping up with the Joneses). The balance of evidence is trending in favour of the hypothesis that living in poor neighbourhoods makes one less happy, and living in rich neighbourhoods actually makes one happier, in the United States. While social status matters, a balance of factors like amenities, safe areas, well maintained housing, turn the tide in favour of the argument that richer neighbours are happier neighbours.

Democracy

"The right to participate in the political process, measured by the extent of direct democratic rights across regions, is strongly correlated with subjective well-being (Frey and Stutzer, 2002) ... a potential mechanism that explains this relationship is the perception of procedural fairness and social mobility." Institutions and well-being, democracy and federalism are associated with a happier population. Correspondingly, political engagement and activism have associated health benefits. On the other hand, some non-democratic countries such as China and Saudi Arabia top the Ipsos list of countries where the citizenry is most happy with their government's direction. That suggests that voting preferences may not translate well into overall satisfaction with the government's direction. In any case, both of these factors revealed preference and domain specific satisfaction rather than overall subjective well being.

Economic development

Historically, economists thought economic growth was unrelated to population level well-being, a phenomenon labelled the Easterlin paradox. More robust research has identified that there is a link between economic development and the wellbeing of the population. A <2017 meta-analysis shows that the impact of infrastructure expenditure on economic growth varies considerably. So, one cannot assume an infrastructure project will yield welfare benefits. The paper doesn't investigate or elaborate on any modifiable variables that might predict the value of a project. However, government spending on roads and primary industries is the best value target for transport spending, according to a 2013 meta-analysis. 7%+/−3% per annum discount rates are typically applied as the discount rate on public infrastructure projects in Australia. Smaller real discount rates are used internationally to calculate the social return on investment by governments.

Alternative approach: economic consequences of happiness

While the mainstream happiness economics has focused on identifying the determinants of happiness, an alternative approach in the discipline examines instead what are the economic consequences of happiness. Happiness may act as a determinant of economic outcomes: it increases productivity, predicts one's future income and affects labour market performance. There is a growing number of studies justifying the so-called "happy-productive worker" thesis. The positive and causal impact of happiness on an individual's productivity has been established in experimental studies. Happiness can affect voting for the incumbent party. Research in political science and behavioral economics suggests that voters’ subjective well-being, or happiness, often influences their political decisions, including whether to support the incumbent party, independently from the effect mediated by the economic individual conditions.

Related studies

The Satisfaction with Life Index. Blue through red represent most to least happy respectively; grey areas have no reliable data available.

The Satisfaction with Life Index is an attempt to show the average self-reported happiness in different nations. This is an example of a recent trend to use direct measures of happiness, such as surveys asking people how happy they are, as an alternative to traditional measures of policy success such as GDP or GNP. Some studies suggest that happiness can be measured effectively. The Inter-American Development Bank (IDB), published in November 2008 a major study on happiness economics in Latin America and the Caribbean.

There are also several examples of measures that include self-reported happiness as one variable. Happy Life Years, a concept brought by Dutch sociologist Ruut Veenhoven, combines self-reported happiness with life expectancy. The Happy Planet Index combines it with life expectancy and ecological footprint.

Gross National Happiness (GNH) is a concept introduced by the King of Bhutan in 1972 as an alternative to GDP. Several countries have already developed or are in the process of developing such an index. Bhutan's index has led that country to limit the amount of deforestation it will allow and to require that all tourists to its nation must spend US$200.

After the military coup of 2006, Thailand also instituted an index. The stated promise of the new Prime Minister Surayud Chulanont is to make the Thai people not only richer but happier as well. Much like GDP results, Thailand releases monthly GNH data. The Thai GNH index is based on a 1–10 scale with 10 being the happiest. As of 13 May 2007, the Thai GNH measured 5.1 points. The index uses poll data from the population surveying various satisfaction factors such as security, public utilities, good governance, trade, social justice, allocation of resources, education and community problems.

Australia, China, France and the United Kingdom are also coming up with indexes to measure national happiness. The UK began to measure national wellbeing in 2012. North Korea also announced an international Happiness Index in 2011 through Korean Central Television. North Korea itself came in second, behind #1 China. Canada released the Canadian Index of Wellbeing (CIW) in 2011 to track changes in wellbeing. The CIW has adopted the following working definition of wellbeing: The presence of the highest possible quality of life in its full breadth of expression focused on but not necessarily exclusive to good living standards, robust health, a sustainable environment, vital communities, an educated populace, balanced time use, high levels of democratic participation, and access to and participation in leisure and culture

Ecuador's and Bolivia's new constitutions state the indigenous concept of "good life" ("buen vivir" in Spanish, "sumak kawsay" in Quichua, and "suma qamaña" in Aymara) as the goal of sustainable development.

Neoclassical economics

Neoclassical, as well as classical economics, are not subsumed under the term happiness economics although the original goal was to increase the happiness of the people. Classical and neoclassical economics are stages in the development of welfare economics and are characterized by mathematical modeling. Happiness economics represents a radical break with this tradition. The measurement of subjective happiness respectively life satisfaction by means of survey research across nations and time (in addition to objective measures like lifespan, wealth, security etc.) marks the beginning of happiness economics.

Criticism

Some have suggested that establishing happiness as a metric is only meant to serve political goals. Recently there has been concern that happiness research could be used to advance authoritarian aims. As a result, some participants at a happiness conference in Rome have suggested that happiness research should not be used as a matter of public policy but rather used to inform individuals.

Even on the individual level, there is discussion on how much effect external forces can have on happiness. Less than 3% of an individual's level of happiness comes from external sources such as employment, education level, marital status, and socioeconomic status. To go along with this, four of the Big Five Personality Traits are substantially associated with life satisfaction, openness to experience is not associated. Having high levels of internal locus of control leads to higher reported levels of happiness.

Even when happiness can be affected by external sources, it has high hedonic adaptation, some specific events such as an increase in income, disability, unemployment, and loss (bereavement) only have short-term (about a year) effects on a person's overall happiness and after a while happiness may return to levels similar to unaffected peers.

What has the most influence over happiness are internal factors such as genetics, personality traits, and internal locus of control. It is theorized that 50% of the variation in happiness levels is from genetic sources and is known as the genetic set point. The genetic set point is assumed to be stable over time, fixed, and immune to influence or control. This goes along with findings that well-being surveys have a naturally positive baseline.

With such strong internal forces on happiness, it is hard to have an effect on a person's happiness externally. This in turn lends itself back to the idea that establishing a happiness metric is only for political gain and has little other use. To support this even further it is believed that a country aggregate level of SWB can account for more variance in government vote share than standard macroeconomic variables, such as income and employment.

Technical issues

According to Bond and Lang (2018), the results are skewed due to the fact that the respondents have to "round" their true happiness to the scale of, e.g., 3 or 7 alternatives (e.g., very happy, pretty happy, not too happy). This "rounding error" may cause a less happy group seem happier, in the average. This would not be the case if the happiness of both groups would be normally distributed with the same variance, but that is usually not the case, based on their results. For some not-implausible log-normal assumptions on the scale, typical results can be reversed to the opposite results.

They also show that the "reporting function" seems to be different for different groups and even for the same individual at different times. For example, when a person becomes disabled, they soon start to lower their threshold for a given answer (e.g., "pretty happy"). That is, they give a higher answer than they would have given at the same happiness state before becoming disabled.

Chronology of the universe

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Chronology_of_the_universe 

The chronology of the universe describes the history and future of the universe according to Big Bang cosmology.

Research published in 2015 estimates the earliest stages of the universe's existence as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level.

Background

Expansion

Main article: Expansion of the universe

The current accepted model of the history of the universe is based on the concept of the Big Bang: the universe started hot and dense then expanded and cooled. Different particles interact during each major stage in the expansion; as the universe expands the density falls and some particle interactions cease to be important. The character of the universe changes. Moreover, the rate of the expansion itself depends upon the nature of the existing particles, creating an interplay between cosmology and particle physics.

Time

The lookback time of extragalactic observations by their cosmological redshift up to z=20.

Main article: Cosmic time

In cosmology, time and space are connected: space expands as time increases. Time at each point in space (for example a galaxy) can be uniquely defined in terms of an imaginary clock at that point. These clocks move with the point in space as the universe expands; they are synchronized to a single point in the distance past. Light from distant galaxies is emitted in the past then travels at the speed of light: knowledge about a distant galaxy is limited to one point in time called the lookback time. During the journey from a distant point, the universe continues to expand, stretching the wavelength of the light along the way, an effect called cosmological redshift. The redshift can be measured by comparing incoming light to known spectroscopic lines and the resulting value can be related to the comoving distance to the emitter. Consequently, experimental knowledge about the chronology of the universe is derived by observing distant light.

Overview

The NASA diagram shows the history of the universe from inflation until the present.

The chronology of the universe can be divided into five parts:

• Inflation, the first era supported by experimental evidence, a period of exponential expansion that ends with the conversion of energy into particles,
• Quark soup, the initial particles cool and coalesce, dark matter forms,
• Big bang nucleosynthesis, combining nucleons create the cores of the first atoms,
• Gravity builds cosmic structure, reduced density allows matter to dominate over radiation for control of expansion, photons decouple to form the cosmic background radiation, and gravitational attraction builds stars, galaxies, and clusters of galaxies.
• Cosmic acceleration, continued expansion allows dark energy to overcome gravitational force, inhibiting larger structures.

With these large subsections are many events and transitions. Older models divided the chronology differently, using different terminology or emphasis.

Tabular summary

Modern cosmological chronologies begin with inflation, the earliest time period supported by solid observational evidence. Anything earlier is considered non-standard cosmology, the subject of a great deal of as-yet-unconfirmed research.

Article subsection	Cosmic time	Redshift	Temperature: 72	Description
Inflation	unknown		not applicable	Cosmic inflation expands space by a factor of the order of 1026 over a time of the order of 10−36 to 10−32 seconds.
Reheating	unknown		unknown	Converts the energy in the inflation field into a thermal bath of Standard Model particles, initiating the Hot Big Bang. Many mechanisms have been proposed.
Baryogenesis	unknown		unknown	Matter and antimatter are created with one extra particle of matter for every 1010 pairs. The pairs annihilate producing photons and leaving the matter particles. Many mechanisms have been proposed but no observations select one.
Electroweak phase transition	20×10−12 s	20×1015	> 1015 K (150 GeV/kB)	The strong interaction becomes distinct from the electroweak interaction. Matter particles have mass.  The sphere of space that will become the observable universe is approximately 300 light-seconds (~0.6 au) in radius at this time.
Quantum chromodynamics phase transition	20×10−6 s	1012	1015 K – 1012 K (150 GeV/kB – 150 MeV/kB)	The quark–gluon plasma of matter particles coalesce into hadrons: mostly protons, neutrons, and pions.
Neutrino decoupling	1 s	6×109	1010 K (1 MeV/kB)	Neutrinos cease interacting with baryonic matter, and form cosmic neutrino background.  The sphere of space that will become the observable universe is approximately 10 light-years in radius at this time.
Electron-positron annihilation	6 s	2×109	1010 K – 109 K (1 MeV/kB – 100 keV/kB)	As the temperature falls, photons no longer have sufficient energy to produce electron/positron pairs. Electrons and positrons annihilate, leaving photons.
Big Bang nucleosynthesis	10 s – 1000 s	4×108	109 K – 107 K (0.1 MeV/kB – 1 keV/kB)	Protons and neutrons are bound into primordial atomic nuclei: hydrogen and helium-4. Trace amounts of deuterium, helium-3, and lithium-7 also form. At the end of this epoch, the spherical volume of space which will become the observable universe is about 300 light-years in radius, baryonic matter density is on the order of 4 grams per m3 (about 0.3% of sea level air density)—however, most energy at this time is in electromagnetic radiation.
Recombination	290 ka – 370 ka	1090 – 1270	4000 K (0.4 eV/kB)	Electrons and atomic nuclei first become bound to form neutral atoms. Photons are no longer in thermal equilibrium with matter and the universe first becomes transparent. Recombination lasts for about 100 ka, during which the universe is becoming more and more transparent to photons. The photons of the cosmic microwave background radiation originate at this time. The spherical volume of space that will become the observable universe is 42 million light-years in radius at this time. The baryonic matter density at this time is about 500 million hydrogen and helium atoms per cubic metre, approximately a billion times higher than today. This density corresponds to pressure on the order of 10−17 atm.
Dark Ages	370 ka – 150 Ma? (Only fully ends by about 1 Ga)	1100 – 20	4000 K – 60 K	The time between recombination and the formation of the first stars. During this time, the only source of photons was hydrogen emitting radio waves at hydrogen line. Freely propagating CMB photons quickly (within about 3 million years) red-shifted to infrared, and the universe was devoid of visible light.
Star and galaxy formation and evolution	Earliest galaxies: from about 300–400 Ma? (first stars: similar or earlier) Modern galaxies: 1 Ga – 10 Ga (exact timings being researched)	From about 20	From about 60 K	The earliest known galaxies existed by about 280 Ma. Galaxies coalesce into "proto-clusters" from about 1 Ga (redshift z = 6) and into galaxy clusters beginning at 3 Ga (z = 2.1), and into superclusters from about 5 Ga (z = 1.2). See: list of galaxy groups and clusters, list of superclusters.
Reionization	200 Ma – 1 Ga (exact timings being researched)	20 – 6	60 K – 19 K	The most distant astronomical objects observable with telescopes date to this period; as of June 2025, the most remote galaxy observed is MoM-z14, at a redshift of 14.44. The earliest "modern" Population I stars are formed in this period.
Present time	13.8 Ga	0	2.7 K	Farthest observable photons at this moment are CMB photons. They arrive from a sphere with a radius of 46 billion light-years. The spherical volume inside it is commonly referred to as the observable universe.
Alternative subdivisions of the chronology (overlapping several of the above periods)
Radiation-dominated era	From inflation (~ 10−32 sec) – 47 ka	> 3600	> 104 K	During this time, the energy density of massless and near-massless relativistic components such as photons and neutrinos, which move at or close to the speed of light, dominate both matter density and dark energy.
Matter-dominated era	47 ka – 9.8 Ga	3600 – 0.4	104 K – 4 K	During this time, the energy density of matter dominates both radiation density and dark energy, resulting in a decelerated expansion of the universe.
Dark-energy-dominated era	> 9.8 Ga	< 0.4	< 4 K	Matter density falls below dark energy density (vacuum energy), and expansion of space begins to accelerate. This time happens to correspond roughly to the time of the formation of the Solar System and the evolutionary history of life.
Stelliferous Era	150 Ma – 100 Ta	20 – −0.99	60 K – 0.03 K	The time between the first formation of Population III stars until the cessation of star formation, leaving all stars in the form of degenerate remnants.
Far future	> 100 Ta	< −0.99	< 0.1 K	The stelliferous era will end as stars eventually die and fewer are born to replace them, leading to a darkening universe. Various theories suggest a number of subsequent possibilities. Assuming proton decay, the matter may eventually evaporate into a Dark Era (heat death). Alternatively, the universe may collapse in a Big Crunch. Other suggested ends include a false vacuum catastrophe or a Big Rip as possible ends to the universe.

Inflation

Before c. 10⁻³² seconds after the Big Bang

Main articles: Inflation (cosmology) and Expansion of the universe

At this point of the very early universe, the universe is thought to have expanded by at least a factor of 10²⁶ in time on the order of 10⁻³⁶ seconds. All of the mass-energy in all of the galaxies currently visible started in a sphere with a radius around 4×10⁻²⁹ m, then grew to a sphere with a radius around 0.09m by the end of inflation. This phase of the cosmic expansion history is known as inflation or sometimes as the inflationary epoch.

Inflation explains how today's universe has concentrations of matter, like galaxies and clusters of galaxies, rather than having matter spatially uniform through the universe. Tiny quantum fluctuations in the universe, amplified by inflation, are believed to be the basis of large-scale structures that formed much later.

The mechanism that drove inflation remains unknown, although many models have been put forward. In several of the more prominent models, it is thought to have been triggered by the separation of the strong and electroweak interactions which ended the grand unification epoch. One of the theoretical products of this phase transition was a scalar field called the inflaton field. As this field settled into its lowest-energy state throughout the universe, it generated an enormous repulsive force that led to a rapid expansion of the universe.

The rapid expansion meant that any potential particles (or other "unwanted" artifacts, such as topological defects) remaining from the time before inflation were now distributed very thinly across the universe.

Reheating

It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10⁻³³ and 10⁻³² seconds after the Big Bang. The rapid expansion of space meant that any elementary particles remaining from the grand unification epoch were now distributed very thinly across the universe to the point where there is no physical temperature that can be associated with them. However, the large potential energy of the inflaton field was released at the end of the inflationary epoch, as the inflaton field decayed into other particles, known as reheating. This heating effect led to the universe being repopulated with a dense, hot mixture of Standard Model particles.

After inflation ended, the universe continued to expand. A region the size of a melon at that time has since grown to be our entire observable universe.

Hot Big Bang

The physical model for the chronology of the universe with strong observational and theoretical support is called the hot Big Bang model. The concept includes an early state of extreme temperature and density followed by expansion of the universe continuing to this day. A high-precision version of the Big Bang model using conventional physics, known as Lambda-CDM, agrees with a wide array of astrophysical observations. The concept is not extrapolated back to zero time. Within the standard model of cosmology the initial state is set by a process called inflation. The relative timeline for the earliest phenomena is unclear. Speculation on processes occurring before inflation involves physics considered outside of standard cosmology.

Electroweak phase transition

Main article: Electroweak symmetry breaking

10⁻¹² seconds after the Big Bang

As the universe's temperature continued to fall below 159.5±1.5 GeV/k_B, electroweak symmetry breaking happened. So far as we know, it was the penultimate symmetry breaking event in the formation of the universe, the final one being chiral symmetry breaking in the quark sector. This has two related effects:

1. Via the Higgs mechanism, all elementary particles interacting with the Higgs field became massive, having been massless at higher energy levels.
2. As a side-effect, the weak nuclear force and electromagnetic force, and their respective bosons (the W and Z bosons and photon) began to manifest differently in the present universe. Before electroweak symmetry breaking, these bosons were all massless particles and interacted over long distances, but at this point the W and Z bosons abruptly became massive particles only interacting over distances smaller than the size of an atom, while the photon remained massless and remained a long-distance interaction.

After electroweak symmetry breaking, the fundamental interactions we know of—gravitation, electromagnetic, weak and strong interactions—all took their present forms, and fundamental particles had their expected masses, but the temperature of the universe was still too high to allow the stable formation of many of the particles we now see in the universe, so there were no protons or neutrons, and therefore no atoms, atomic nuclei, or molecules. (More precisely, any composite particles that formed by chance almost immediately broke up again due to the extreme energies.)

Quantum chromodynamics phase transition

Between 10⁻¹² seconds and 10⁻⁵ seconds after the Big Bang

Main article: Quark epoch

After cosmic inflation ended, the universe was filled with a hot quark–gluon plasma, the remains of reheating. From this point onwards the physics of the early universe is much better understood, and the energies involved in the quark epoch are directly accessible in particle physics experiments and other detectors.

The quark epoch began approximately 10⁻¹² seconds after the Big Bang. This was the period in the evolution of the early universe immediately after electroweak symmetry breaking when the fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction had taken their present forms, but the temperature of the universe was still too high to allow quarks to bind together to form hadrons. The quark epoch ended when the universe was about 10⁻⁵ seconds old; two non-equilibrium events must have occurred next, formation of baryons and of dark matter.

Neutrino decoupling and cosmic neutrino background (CνB)

Around 1 second after the Big Bang

Main articles: Neutrino decoupling and Cosmic neutrino background

At approximately 1 second after the Big Bang neutrinos decouple and begin travelling freely through space. As neutrinos rarely interact with matter, these neutrinos still exist today, analogous to the much later cosmic microwave background emitted during recombination, around 370,000 years after the Big Bang. The neutrinos from this event have a very low energy, around 10⁻¹⁰ times the amount of those observable with present-day direct detection. Even high-energy neutrinos are notoriously difficult to detect, so this cosmic neutrino background (CνB) may not be directly observed in detail for many years, if at all.

However, Big Bang cosmology makes many predictions about the CνB, and there is very strong indirect evidence that the CνB exists, both from Big Bang nucleosynthesis predictions of the helium abundance, and from anisotropies in the cosmic microwave background (CMB). One of these predictions is that neutrinos will have left a subtle imprint on the CMB. It is well known that the CMB has irregularities. Some of the CMB fluctuations were roughly regularly spaced, because of the effect of baryonic acoustic oscillations. In theory, the decoupled neutrinos should have had a very slight effect on the phase of the various CMB fluctuations.

In 2015, it was reported that such shifts had been detected in the CMB. Moreover, the fluctuations corresponded to neutrinos of almost exactly the temperature predicted by Big Bang theory (1.96±0.02 K compared to a prediction of 1.95 K), and exactly three types of neutrino, the same number of neutrino flavors predicted by the Standard Model.

Cosmological models of this early time remain unsettled. The Standard Model of particle physics is only tested up to temperatures of order 10¹⁷K (10 TeV) in particle colliders, such as the Large Hadron Collider. Moreover, new physical phenomena not yet covered by the Standard Model could have been important before the time of neutrino decoupling, when the temperature of the universe was about 10¹⁰K (1 MeV).

Electron-positron annihilation

Between 1 second and 10 seconds after the Big Bang

Main article: Lepton epoch

The majority of hadrons and anti-hadrons annihilate each other leaving leptons (such as the electron, muons and certain neutrinos) and antileptons, dominating the mass of the universe. Initially leptons and antileptons are produced in pairs. About 10 seconds after the Big Bang the temperature of the universe falls to the point at which new lepton–antilepton pairs are no longer created and most remaining leptons and antileptons quickly annihilated each other, giving rise to pairs of high-energy photons, and leaving a small residue of non-annihilated leptons. After most leptons and antileptons are annihilated, most of the mass–energy in the universe is left in the form of photons.

Baryogenesis

Around 3 minutes after the Big Bang

Main article: Baryogenesis

Baryons are subatomic particles such as protons and neutrons that are composed of three quarks. It would be expected that both baryons, and particles known as antibaryons would have formed in equal numbers. However, almost no antibaryons are observed in nature. It is not clear how this came about. Any explanation for this phenomenon must allow the Sakharov conditions related to baryogenesis to have been satisfied at some time after the end of cosmological inflation. Current particle physics suggests asymmetries under which these conditions would be met, but these asymmetries appear to be too small to account for the observed baryon-antibaryon asymmetry of the universe.

Theory predicts that about 1 neutron remained for every 6 protons, with the ratio falling to 1:7 over time due to neutron decay. This is believed to be correct because, at a later stage, the neutrons and some of the protons fused, leaving hydrogen, a hydrogen isotope called deuterium, helium and other elements, which can be measured. A 1:7 ratio of hadrons would indeed produce the observed element ratios in the early and current universe.

Nucleosynthesis of light elements

Between 3 minutes and 20 minutes after the Big Bang

Main article: Big Bang nucleosynthesis

Between about 3 and 20 minutes after the Big Bang nuclear fusion reactions convert a 1:7 mixture of neutrons and protons in to a mix of protons, deuterium (a proton fused with a neutron), ³He, ⁴He, with trace amounts of ⁷Li and ⁷Be. These reactions end when the temperature falls below the 0.07MeV needed for nuclear fusion. The final mixture depends upon the reaction rates, the temperature, and the density of the components. The reaction rates can be measured in nuclear physics laboratories while the temperature and densities can be calculated from models of the expansion of the universe.

About 25% of the protons, and all the neutrons fuse to form deuterium, a hydrogen isotope, and almost all of the deuterium quickly fuses to form helium-4. Helium-4 has much higher binding energy than nuclei with 5 to 8 nucleons so only trace amounts of those nuclei are created. Heavier nuclei produced in stars do not appear because they require the combination of three Helium-4 nuclei and the density of Helium-4 is too low for many three way collisions to occur before the expansion cools the universe below the fusion temperature. Small amounts of tritium (another hydrogen isotope) and beryllium-7 and -8 are formed, but these are unstable and quickly decay. A small amount of deuterium is left unfused.

The amounts of each light element in the early universe can be estimated from old galaxies, and is strong evidence for the Big Bang. For example, the Big Bang should produce about 1 neutron for every 7 protons, allowing for 25% of all nucleons to be fused into helium-4 (2 protons and 2 neutrons out of every 16 nucleons), and this is the amount we find today, and far more than can be explained by production in stars. Similarly, deuterium fuses extremely easily; any alternative explanation must also explain how conditions existed for deuterium to form, but also left some of that deuterium unfused and not immediately fused again into helium. Any alternative must also explain the proportions of the various light elements and their isotopes. A few isotopes, such as lithium-7, were found to be present in amounts that differed from theory.

Matter-radiation equality

47,000 years after the Big Bang

Main articles: Matter-dominated era and Structure formation

Until now, the universe's large-scale dynamics and behavior have been determined mainly by radiation—meaning, those constituents that move relativistically (at or near the speed of light), such as photons and neutrinos. As the universe cools, from around 47,000 years (redshift z = 3600), the universe's large-scale behavior becomes dominated by matter instead. This occurs because the energy density of matter begins to exceed both the energy density of radiation and the vacuum energy density Around or shortly after 47,000 years, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) become equal, the Jeans length, which determines the smallest structures that can form (due to competition between gravitational attraction and pressure effects), begins to fall and perturbations, instead of being wiped out by free streaming radiation, can begin to grow in amplitude.

According to the Lambda-CDM model, by this stage, the matter in the universe is around 84.5% cold dark matter and 15.5% "ordinary" matter. There is overwhelming evidence that dark matter exists and dominates the universe, but since the exact nature of dark matter is still not understood, the Big Bang theory does not presently cover any stages in its formation.

From this point on, and for several billion years to come, the presence of dark matter accelerates the formation of structure in the universe. In the early universe, dark matter gradually gathers in huge filaments under the effects of gravity, collapsing faster than ordinary (baryonic) matter because its collapse is not slowed by radiation pressure. This amplifies the tiny inhomogeneities (irregularities) in the density of the universe which were left by cosmic inflation. Over time, slightly denser regions become denser and slightly rarefied (emptier) regions become more rarefied. Ordinary matter eventually gathers together faster than it would otherwise do, because of the presence of these concentrations of dark matter.

The properties of dark matter that allow it to collapse quickly without radiation pressure also mean that it cannot lose energy by radiation. Losing energy is necessary for particles to collapse into dense structures beyond a certain point. Therefore, dark matter collapses into huge but diffuse filaments and haloes, and not into stars or planets. Ordinary matter, which can lose energy by radiation, forms dense objects and also gas clouds when it collapses.

Recombination, photon decoupling, and the cosmic microwave background (CMB)

Main articles: Recombination (cosmology) and decoupling (cosmology)

9-year WMAP image of the cosmic microwave background radiation (2012). The radiation is isotropic to roughly one part in 100,000.

About 370,000 years after the Big Bang, two connected events occurred: the ending of recombination and photon decoupling. Recombination describes the ionized particles combining to form the first neutral atoms, and decoupling refers to the photons released ("decoupled") as the newly formed atoms settle into more stable energy states.

Just before recombination, the baryonic matter in the universe was at a temperature where it formed a hot ionized plasma. Most of the photons in the universe interacted with electrons and protons, and could not travel significant distances without interacting with ionized particles. As a result, the universe was opaque or "foggy". Although there was light, it was not possible to see, nor can we observe that light through telescopes.

Starting around 18,000 years, the universe has cooled to a point where free electrons can combine with helium nuclei to form He⁺
atoms. After around 50,000 years, as the universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule. Much later, hydrogen and helium hydride react to form molecular hydrogen (H₂), the fuel needed for the first stars. At about 370,000 years, neutral hydrogen atoms finish forming ("recombination" of hydrogen ions and electrons), greatly reducing the Thomson scattering of photons. No longer scattered by free electrons, the photons were "decoupled" from the earlier plasma and propagated freely. The majority of these photons still exist as the cosmic microwave background (CMB). This is the oldest era of the universe that we can directly observe today.

Directly combining in a low energy state (ground state) is less efficient, so these hydrogen atoms generally form with the electrons still in a high-energy state, and once combined, the electrons quickly release energy in the form of one or more photons as they transition to a low energy state. This release of photons is known as photon decoupling. Some of these decoupled photons are captured by other hydrogen atoms, the remainder remain free. By the end of recombination, most of the protons in the universe have formed neutral atoms. This change from charged to neutral particles means that the mean free path photons can travel before capture in effect becomes infinite, so any decoupled photons that have not been captured can travel freely over long distances (see Thomson scattering). The universe has become transparent to visible light, radio waves and other electromagnetic radiation for the first time in its history.

The photons released by these newly formed hydrogen atoms initially had a temperature/energy of around ~ 4000 K. This would have been visible to the eye as a pale yellow/orange tinted, or "soft", white color. Over billions of years since decoupling, as the universe has expanded, the photons have been red-shifted from visible light to radio waves (microwave radiation corresponding to a temperature of about 2.7 K). Red shifting describes the photons acquiring longer wavelengths and lower frequencies as the universe expanded over billions of years, so that they gradually changed from visible light to radio waves. These same photons can still be detected as radio waves today. They form the cosmic microwave background, and they provide crucial evidence of the early universe and how it developed.

Around the same time as recombination, existing pressure waves within the electron-baryon plasma—known as baryon acoustic oscillations—became embedded in the distribution of matter as it condensed, giving rise to a very slight preference in distribution of large-scale objects. Therefore, the cosmic microwave background is a picture of the universe at the end of this epoch including the tiny fluctuations generated during inflation (see 9-year WMAP image), and the spread of objects such as galaxies in the universe is an indication of the scale and size of the universe as it developed over time.

Gravity builds cosmic structure

370 thousand to about 1 billion years after the Big Bang

See also: Hydrogen line and List of the most distant astronomical objects

Even before recombination and decoupling, matter began to accumulate around clumps of dark matter. Clouds of hydrogen collapsed very slowly to form stars and galaxies.

Duration: 50 seconds.0:50

Hubble Space Telescope—Ultra Deep Field galaxies to Legacy Field zoom out (video 00:50; 2 May 2019)

Dark Ages

See also: 21 centimeter radiation

After recombination and decoupling, the universe was transparent and had cooled enough to allow light to travel long distances, but there were no light-producing structures such as stars and galaxies. Stars and galaxies are formed when dense regions of gas form due to the action of gravity, and this takes a long time within a near-uniform density of gas and on the scale required, so it is estimated that stars did not exist for perhaps hundreds of millions of years after recombination.

This period, known as the Dark Ages, began at photon decoupling around 370,000 years after the Big Bang and ends over a long period of time called reionization. During the Dark Ages, the temperature of the universe cooled from some 4000 K to about 60 K (3727 °C to about −213 °C), and only two sources of photons existed: the photons released during recombination/decoupling (as neutral hydrogen atoms formed), which we can still detect today as the cosmic microwave background (CMB), and photons occasionally released by neutral hydrogen atoms, known as the 21 cm spin line of neutral hydrogen. The hydrogen spin line is in the microwave range of frequencies, and within 3 million years, the CMB photons had redshifted out of visible light to infrared; from that time until the first stars, there were no visible light photons. Other than perhaps some rare statistical anomalies, the universe was truly dark.

The first generation of stars, known as Population III stars, formed within a few hundred million years after the Big Bang. These stars were the first source of visible light in the universe after recombination. Structures may have begun to emerge from around 150 million years, and early galaxies emerged from around 180 to 700 million years. As they emerged, the Dark Ages gradually ended. Because this process was gradual, the Dark Ages only ended fully at around 1 billion years, as the universe took on its present appearance.

Artist's impression of the first stars, 400 million years after the Big Bang

Oldest observations of stars and galaxies

Main articles: Hubble Space Telescope, James Webb Space Telescope, and List of the most distant astronomical objects

At present, the oldest observations of stars and galaxies are from shortly after the start of reionization, with galaxies such as GN-z11 (Hubble Space Telescope, 2016) at about z≈11.1 (about 400 million years cosmic time). Hubble's successor, the James Webb Space Telescope, launched December 2021, is designed to detect objects up to 100 times fainter than Hubble, and much earlier in the history of the universe, back to redshift z≈20 (about 180 million years cosmic time). This is believed to be earlier than the first galaxies, and around the era of the first stars.

There is also an observational effort underway to detect the faint 21 cm spin line radiation, as it is in principle an even more powerful tool than the cosmic microwave background for studying the early universe.

Earliest structures and stars emerge

Around 150 million to 1 billion years after the Big Bang

See also: Stellar formation, Dwarf galaxy, Baryon acoustic oscillations, Large-scale structure, Structure formation, and Stelliferous Era

The Hubble Ultra Deep Fields often feature galaxies that are examples of what the early Stelliferous Era was like.

Another Hubble image shows an infant galaxy forming nearby, which means this happened very recently on the cosmological timescale. This shows that new galaxy formation in the universe is still occurring.

The matter in the universe is around 84.5% cold dark matter and 15.5% "ordinary" matter. Since the start of the matter-dominated era, dark matter has gradually been gathering in huge spread-out (diffuse) filaments under the effects of gravity. Ordinary matter eventually gathers together faster than it would otherwise, because of the presence of these concentrations of dark matter. It is also slightly more dense at regular distances due to early baryon acoustic oscillations (BAO) which became embedded into the distribution of matter when photons decoupled. Unlike dark matter, ordinary matter can lose energy by many routes, which means that as it collapses, it can lose the energy which would otherwise hold it apart, and collapse more quickly, and into denser forms. Ordinary matter gathers where dark matter is denser, and in those places it collapses into clouds of mainly hydrogen gas. The first stars and galaxies form from these clouds. Where numerous galaxies have formed, galaxy clusters and superclusters will eventually arise. Large voids with few stars will develop between them, marking where dark matter became less common.

The exact timings of the first stars, galaxies, supermassive black holes, and quasars, and the start and end timings and progression of the period known as reionization, are still being actively researched, with new findings published periodically. As of 2019: the earliest confirmed galaxies (for example GN-z11) date from around 380–400 million years, suggesting surprisingly fast gas cloud condensation and stellar birth rates; and observations of the Lyman-alpha forest, and of other changes to the light from ancient objects, allow the timing for reionization and its eventual end to be narrowed down.

Structure formation in the Big Bang model proceeds hierarchically, due to gravitational collapse, with smaller structures forming before larger ones. The earliest structures to form are the first stars (known as Population III stars), dwarf galaxies, and quasars (which are thought to be bright, early active galaxies containing a supermassive black hole surrounded by an inward-spiraling accretion disk of gas). Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory: that is, all structures could be understood as small deviations from a perfect homogeneous universe. This is computationally relatively easy to study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, N-body simulations with billions of particles. The Bolshoi cosmological simulation is a high precision simulation of this era.

These Population III stars are also responsible for turning the few light elements that were formed in the Big Bang (hydrogen, helium and small amounts of lithium) into many heavier elements. They can be huge as well as perhaps small—and non-metallic (no elements except hydrogen and helium). The larger stars have very short lifetimes compared to most Main Sequence stars we see today, so they commonly finish burning their hydrogen fuel and explode as supernovae after mere millions of years, seeding the universe with heavier elements over repeated generations. They mark the start of the Stelliferous Era.

As yet, no Population III stars have been found, so the understanding of them is based on computational models of their formation and evolution. Fortunately, observations of the cosmic microwave background radiation can be used to date when star formation began in earnest. Analysis of such observations made by the Planck microwave space telescope in 2016 concluded that the first generation of stars may have formed from around 300 million years after the Big Bang.

Quasars provide some additional evidence of early structure formation. Their light shows evidence of elements such as carbon, magnesium, iron and oxygen. This is evidence that by the time quasars formed, a massive phase of star formation had already taken place, including sufficient generations of Population III stars to give rise to these elements.

Reionization

See also: Reionization, Dwarf galaxy, and Quasar

Phases of the reionization

As the first stars, dwarf galaxies and quasars gradually form, the intense radiation they emit reionizes much of the surrounding universe; splitting the neutral hydrogen atoms back into a plasma of free electrons and protons for the first time since recombination and decoupling.

Reionization is evidenced from observations of quasars. Quasars are a form of active galaxy, and the most luminous objects observed in the universe. Electrons in neutral hydrogen have specific patterns of absorbing ultraviolet photons, related to electron energy levels and called the Lyman series. Ionized hydrogen does not have electron energy levels of this kind. Therefore, light travelling through ionized hydrogen and neutral hydrogen shows different absorption lines. Ionized hydrogen in the intergalactic medium (particularly electrons) can scatter light through Thomson scattering as it did before recombination, but the expansion of the universe and clumping of gas into galaxies resulted in a concentration too low to make the universe fully opaque by the time of reionization. Because of the immense distance travelled by light (billions of light years) to reach Earth from structures existing during reionization, any absorption by neutral hydrogen is redshifted by various amounts, rather than by one specific amount, indicating when the absorption of then-ultraviolet light happened. These features make it possible to study the state of ionization at many different times in the past.

Reionization began as "bubbles" of ionized hydrogen which became larger over time until the entire intergalactic medium was ionized, when the absorption lines by neutral hydrogen become rare. The absorption was due to the general state of the universe (the intergalactic medium) and not due to passing through galaxies or other dense areas. Reionization might have started to happen as early as z = 16 (250 million years of cosmic time) and was mostly complete by around z = 9 or 10 (500 million years), with the remaining neutral hydrogen becoming fully ionized z = 5 or 6 (1 billion years), when Gunn-Peterson troughs that show the presence of large amounts of neutral hydrogen disappear. The intergalactic medium remains predominantly ionized to the present day, the exception being some remaining neutral hydrogen clouds, which cause Lyman-alpha forests to appear in spectra.

These observations have narrowed down the period of time during which reionization took place, but the source of the photons that caused reionization is still not completely certain. To ionize neutral hydrogen, an energy larger than 13.6 eV is required, which corresponds to ultraviolet photons with a wavelength of 91.2 nm or shorter, implying that the sources must have produced significant amounts of ultraviolet and higher energy. Protons and electrons will recombine if energy is not continuously provided to keep them apart, which also sets limits on how numerous the sources were and their longevity. With these constraints, it is expected that quasars and first generation stars and galaxies were the main sources of energy. The current leading candidates from most to least significant are currently believed to be Population III stars (the earliest stars; possibly 70%), dwarf galaxies (very early small high-energy galaxies; possibly 30%), and a contribution from quasars (a class of active galactic nuclei).

However, by this time, matter had become far more spread out due to the ongoing expansion of the universe. Although the neutral hydrogen atoms were again ionized, the plasma was much more thin and diffuse, and photons were much less likely to be scattered. Despite being reionized, the universe remained largely transparent during reionization due how sparse the intergalactic medium was. Reionization gradually ended as the intergalactic medium became virtually completely ionized, although some regions of neutral hydrogen do exist, creating Lyman-alpha forests.

In August 2023, images of black holes and related matter in the very early universe by the James Webb Space Telescope were reported and discussed.

Galaxies, clusters and superclusters

Computer simulated view of the large-scale structure of a part of the universe about 50 million light-years across

See also: Galaxy formation and evolution

Matter continues to draw together under the influence of gravity, to form galaxies. The stars from this time period, known as Population II stars, are formed early on in this process, with more recent Population I stars formed later. Gravitational attraction also gradually pulls galaxies towards each other to form groups, clusters and superclusters. Hubble Ultra Deep Field observations has identified a number of small galaxies merging to form larger ones, at 800 million years of cosmic time (13 billion years ago). (This age estimate is now believed to be slightly overstated).

Present and future

Further information: Timeline of natural history, Geologic time scale, Timeline of the evolutionary history of life, and Timeline of the far future

Nature timeline
−13 — – −12 — – −11 — – −10 — – −9 — – −8 — – −7 — – −6 — – −5 — – −4 — – −3 — – −2 — – −1 — – 0 —	Dark Ages Reionization Matter-dominated era Water on Earth Life Multicellular life Vertebrates	← Earliest stars ← Earliest galaxy ← Earliest quasar / black hole ← Omega Centauri ← Andromeda Galaxy ← Milky Way spirals ← NGC 188 star cluster ← Alpha Centauri ← Accelerated expansion ← Earth / Solar System ← Earliest known life ← Atmospheric oxygen ← Sexual reproduction ← Earliest fungi ← Earliest land plants ← Cambrian explosion ← Earliest mammals ← Earliest apes / humans
(billion years ago)

Universe as it appears today

From 1 billion years, and for about 12.8 billion years, the universe has looked much as it does today and it will continue to appear very similar for many billions of years into the future. The thin disk of the Milky Way began to form when the universe was about 5 billion years old or 9 ± 2 Gya. The Solar System formed at about 9.2 billion years (4.6 Gya); the oldest organic matter consistent with life processes dates back 4 billion years.

The thinning of matter over time reduces the ability of the matter to gravitationally decelerate the expansion of the universe; in contrast, dark energy is a constant factor tending to accelerate the expansion of the universe. The universe's expansion passed an inflection point about five or six billion years ago when the universe entered the modern "dark-energy-dominated era" where the universe's expansion is now accelerating rather than decelerating. The present-day universe is quite well understood, but beyond about 100 billion years of cosmic time (about 86 billion years in the future), scientists are less sure which path the universe will take.

Dark energy-dominated era

From about 9.8 billion years after the Big Bang

Main articles: Dark energy and Scale factor (cosmology)

From about 9.8 billion years of cosmic time, the universe's large-scale behavior is believed to have gradually changed for the third time in its history. Its behavior had originally been dominated by radiation (relativistic constituents such as photons and neutrinos) for the first 47,000 years, and since about 370,000 years of cosmic time, its behavior had been dominated by matter. During its matter-dominated era, the expansion of the universe had begun to slow down, as gravity reined in the initial outward expansion. But from about 9.8 billion years of cosmic time, observations show that the expansion of the universe slowly stops decelerating, and gradually begins to accelerate again, instead.

While the precise cause is not known, the observation is accepted as correct by the cosmologist community. By far the most accepted understanding is that this is due to an unknown form of energy which has been given the name "dark energy". "Dark" in this context means that it is not directly observed, but its existence can be deduced by examining the gravitational effect it has on the universe. Research is ongoing to understand this dark energy. Dark energy is now believed to be the single largest component of the universe, as it constitutes about 68.3% of the entire mass–energy of the physical universe.

Dark energy is believed to act like a cosmological constant—a scalar field that exists throughout space. Unlike gravity, the effects of such a field do not diminish (or only diminish slowly) as the universe grows. While matter and gravity have a greater effect initially, their effect quickly diminishes as the universe continues to expand. Objects in the universe, which are initially seen to be moving apart as the universe expands, continue to move apart, but their outward motion gradually slows down. This slowing effect becomes smaller as the universe becomes more spread out. Eventually, the outward and repulsive effect of dark energy begins to dominate over the inward pull of gravity. Instead of slowing down and perhaps beginning to move inward under the influence of gravity, from about 9.8 billion years of cosmic time, the expansion of space starts to slowly accelerate outward at a gradually increasing rate.

Beyond standard cosmology

Cosmogenesis

Main articles: Cosmogenesis and Origin of the universe

Cosmological models extrapolated back to 10⁻⁴³ seconds combined with particle physics models both with and beyond the Standard Model allow well-informed speculation on the character and properties of the early universe.

Singularity

Main article: Initial singularity

Approaching infinite temperature, a scale factor of zero, or time at zero is known to be outside of our physical models. Speculating about an initial gravitational singularity is not sensible: the conditions are outside of the range of the theory.

Planck epoch

Times within 10⁻⁴³ seconds of the Big Bang

See also: Planck units § In cosmology

Since the standard model of cosmology predicts expansion of the universe from a very hot time in the distant past, it can be followed back to smaller and smaller scales. However, it cannot be followed back to zero space. Below distance known as a Planck length, the basis for the equations breaks down. The energy of particles in this time is so large that quantum effects take over from classical equations for gravity. The Planck time, 10⁻⁴³ seconds, is therefore the beginning time for the Big Bang model of cosmology.

Grand unification epoch

Main article: Grand unification epoch

Between 10⁻⁴³ seconds and 10⁻³⁶ seconds after the Big Bang

After the Planck era, the universe could, in principle, be modeled by extensions of the Standard Model of particle physics, for example, those called grand unified theories. Many such theories have proposed, but none have been successful in producing quantitative agreement with modern astrophysical observations. Nevertheless, the time between 10⁻⁴³ and 10⁻³⁶ seconds has been called the grand unification epoch.

Before the GUT epoch, the temperature of the universe exceeded 10¹⁵ GeV. As the universe expanded and cooled, it may have crossed a cosmological phase transition, which may have resulted in the large ratio of matter to antimatter we observe today. This phase transition is a thermodynamic effect similar to condensation of a gas or freezing of a liquid. While the transition in the GUT epoch is speculative, electroweak and quark-hadron transitions which happen later are supported by theoretical models with some successful predictions.

Electroweak epoch

Main article: Electroweak epoch

Starting anywhere between 10⁻²² and 10⁻¹⁵ seconds after the Big Bang, until 10⁻¹² seconds after the Big Bang

Sometime after inflation, the created particles went through thermalization, where mutual interactions lead to thermal equilibrium. Before the electroweak symmetry breaking, at a temperature of around 10¹⁵ K, approximately 10⁻¹⁵ seconds after the Big Bang, the electromagnetic and weak interaction had not yet separated, and the gauge bosons and fermions had not yet gained mass through the Higgs mechanism. This epoch ended with electroweak symmetry breaking, potentially through a phase transition. In some extensions of the Standard Model of particle physics, baryogenesis also happened at this stage, creating an imbalance between matter and antimatter (though in extensions to this model, this may have happened earlier). Little is known about the details of these processes.

Far future and ultimate fate

Main articles: Ultimate fate of the universe and Timeline of the far future

Further information: Future of an expanding universe and Heat death of the universe

There are several competing scenarios for the long-term evolution of the universe. Which of them will happen, if any, depends on the precise values of physical constants such as the cosmological constant, the possibility of proton decay, the energy of the vacuum (meaning, the energy of "empty" space itself), and the natural laws beyond the Standard Model.

If the expansion of the universe continues and it stays in its present form, eventually all but the nearest galaxies will be carried away from us by the expansion of space at such a velocity that the observable universe will be limited to our own gravitationally bound local galaxy cluster. In the very long term (after many trillions—thousands of billions—of years, cosmic time), the Stelliferous Era will end, as stars cease to be born and even the longest-lived stars gradually die. Beyond this, all objects in the universe will cool and (with the possible exception of protons) gradually decompose back to their constituent particles and then into subatomic particles and very low-level photons and other fundamental particles, by a variety of possible processes.

The following scenarios have been proposed for the ultimate fate of the universe:

Scenario		Description
Heat death	As expansion continues, the universe becomes larger, colder, and more dilute; in time, all structures eventually decompose to subatomic particles and photons.	In the case of indefinitely continuing cosmic expansion, the energy density in the universe will decrease until, after an estimated time of 101000 years, it reaches thermodynamic equilibrium and no more structure will be possible. This will happen only after an extremely long time because first, some (less than 0.1%) matter will collapse into black holes, which will then evaporate extremely slowly via Hawking radiation. The universe in this scenario will cease to be able to support life much earlier than this, after some 1014 years or so, when star formation ceases. In some Grand Unified Theories, proton decay after at least 1034 years will convert the remaining interstellar gas and stellar remnants into leptons (such as positrons and electrons) and photons. Some positrons and electrons will then recombine into photons. In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known, however, whether it eventually achieves thermodynamic equilibrium., The hypothesis of a universal heat death stems from the 1850s ideas of William Thomson (Lord Kelvin), who extrapolated the classical theory of heat and irreversibility (as embodied in the first two laws of thermodynamics) to the universe as a whole.
Big Rip	Expansion of space accelerates and at some point becomes so extreme that even subatomic particles and the fabric of spacetime are pulled apart and unable to exist.	For any value of the dark energy content of the universe where the negative pressure ratio is less than −1, the expansion rate of the universe will continue to increase without limit. Gravitationally bound systems, such as clusters of galaxies, galaxies, and ultimately the Solar System will be torn apart. Eventually the expansion will be so rapid as to overcome the electromagnetic forces holding molecules and atoms together. Even atomic nuclei will be torn apart. Finally, forces and interactions even on the Planck scale—the smallest size for which the notion of "space" currently has a meaning—will no longer be able to occur as the fabric of spacetime itself is pulled apart and the universe as we know it will end in an unusual kind of singularity.
Big Crunch	Expansion eventually slows and halts, then reverses as all matter accelerates towards its common centre. Currently considered to be likely incorrect.	In the opposite of the "Big Rip" scenario, the expansion of the universe would at some point be reversed and the universe would contract towards a hot, dense state. This is a required element of oscillatory universe scenarios, such as the cyclic model, although a Big Crunch does not necessarily imply an oscillatory universe. Current observations suggest that this model of the universe is unlikely to be correct, and the expansion will continue or even accelerate.
Vacuum instability	Collapse of the quantum fields that underpin all forces, particles and structures, to a different form.	Cosmology traditionally has assumed a stable or at least metastable universe, but the possibility of a false vacuum in quantum field theory implies that the universe at any point in spacetime might spontaneously collapse into a lower-energy state (see Bubble nucleation), a more stable or "true vacuum", which would then expand outward from that point with the speed of light.

In this kind of protracted timescale, extremely rare quantum phenomena may also occur that are unlikely to be seen on a timescale smaller than trillions of years. These may also lead to unpredictable changes to the state of the universe which would not be likely to be significant on any smaller timescale. For example, on a timescale of millions of trillions of years, black holes might appear to evaporate almost instantly, uncommon quantum tunnelling phenomena would appear to be common, and quantum (or other) phenomena so unlikely that they might occur just once in a trillion years may occur many times.