Big Bang (updated)

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Timeline of the metric expansion of space, where space (including hypothetical non-observable portions of the universe) is represented at each time by the circular sections. On the left, the dramatic expansion occurs in the inflationary epoch; and at the center, the expansion accelerates (artist's concept; not to scale).

 

The Big Bang theory is the prevailing cosmological model for the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from a very high-density and high-temperature state, and offers a comprehensive explanation for a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB), large scale structure and Hubble's law (the farther away galaxies are, the faster they are moving away from Earth). If the observed conditions are extrapolated backwards in time using the known laws of physics, the prediction is that just before a period of very high density there was a singularity which is typically associated with the Big Bang. Physicists are undecided whether this means the universe began from a singularity, or that current knowledge is insufficient to describe the universe at that time. Detailed measurements of the expansion rate of the universe place the Big Bang at around 13.8 billion years ago, which is thus considered the age of the universe. After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms. Giant clouds of these primordial elements (mostly hydrogen, with some helium and lithium) later coalesced through gravity, eventually forming early stars and galaxies, the descendants of which are visible today. Astronomers also observe the gravitational effects of dark matter surrounding galaxies. Though most of the mass in the universe seems to be in the form of dark matter, Big Bang theory and various observations seem to indicate that it is not made out of conventional baryonic matter (protons, neutrons, and electrons) but it is unclear exactly what it is made out of.

Since Georges Lemaître first noted in 1927 that an expanding universe could be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion. The scientific community was once divided between supporters of two different theories, the Big Bang and the Steady State theory, but a wide range of empirical evidence has strongly favored the Big Bang which is now universally accepted. In 1929, from analysis of galactic redshifts, Edwin Hubble concluded that galaxies are drifting apart; this is important observational evidence consistent with the hypothesis of an expanding universe. In 1964, the cosmic microwave background radiation was discovered, which was crucial evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the universe before it was discovered. More recently, measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy's existence. The known physical laws of nature can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature.

Overview

The Belgian astronomer and Catholic priest Georges Lemaître proposed on theoretical grounds that the universe is expanding, which was observationally confirmed soon afterwards by Edwin Hubble. In 1927 in the Annales de la Société Scientifique de Bruxelles (Annals of the Scientific Society of Brussels) under the title "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques" ("A homogeneous Universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae"), he presented his new idea that the universe is expanding and provided the first observational estimation of what is known as the Hubble constant. What later will be known as the "Big Bang theory" of the origin of the universe, he called his "hypothesis of the primeval atom" or the "Cosmic Egg".

American astronomer Edwin Hubble observed that the distances to faraway galaxies were strongly correlated with their redshifts. This was interpreted to mean that all distant galaxies and clusters are receding away from our vantage point with an apparent velocity proportional to their distance: that is, the farther they are, the faster they move away from us, regardless of direction. Assuming the Copernican principle (that the Earth is not the center of the universe), the only remaining interpretation is that all observable regions of the universe are receding from all others. Since we know that the distance between galaxies increases today, it must mean that in the past galaxies were closer together. The continuous expansion of the universe implies that the universe was denser and hotter in the past. 

Large particle accelerators can replicate the conditions that prevailed after the early moments of the universe, resulting in confirmation and refinement of the details of the Big Bang model. However, these accelerators can only probe so far into high energy regimes. Consequently, the state of the universe in the earliest instants of the Big Bang expansion is still poorly understood and an area of open investigation and speculation. 

The first subatomic particles to be formed included protons, neutrons, and electrons. Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed. The majority of atoms produced by the Big Bang were hydrogen, along with helium and traces of lithium. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies, and the heavier elements were synthesized either within stars or during supernovae. 

The Big Bang theory offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the CMB, large scale structure, and Hubble's Law. The framework for the Big Bang model relies on Albert Einstein's theory of general relativity and on simplifying assumptions such as homogeneity and isotropy of space. The governing equations were formulated by Alexander Friedmann, and similar solutions were worked on by Willem de Sitter. Since then, astrophysicists have incorporated 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. The Lambda-CDM model is the current "standard model" of Big Bang cosmology, consensus is that it is the simplest model that can account for the various measurements and observations relevant to cosmology.

Timeline

Singularity

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 indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone can not extrapolate toward the singularity beyond the end of the Planck epoch. 

This primordial singularity is itself sometimes called "the Big Bang", but the term can also refer to a more generic early hot, dense phase of the universe. In either case, "the Big Bang" as an event is also colloquially referred to as the "birth" of our universe since it represents the point in history where the universe can be verified to have entered into a regime where the laws of physics as we understand them (specifically general relativity and the standard model of particle physics) work. Based on measurements of the expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event — otherwise known as the "age of the universe" — is 13.799 ± 0.021 billion years. The agreement of independent measurements of this age supports the ΛCDM model that describes in detail the characteristics of the universe. 

Despite being extremely dense at this time—far denser than is usually required to form a black hole—the universe did not re-collapse into a black hole. This may be explained by considering that commonly-used calculations and limits for gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang.

Inflation and baryogenesis

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 a very 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 during which time density fluctuations that occurred because of the uncertainty principle were amplified into the seeds that would later form the large-scale structure of the universe. After inflation stopped, reheating occurred until the universe obtained the temperatures required for the production 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.

Cooling

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

 

The universe continued to decrease in density 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 accelerators. 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) 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. The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old.

Structure formation

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

 

Abell 2744 galaxy cluster – Hubble Frontier Fields view.

 

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 Wilkinson Microwave Anisotropy Probe (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" ${\displaystyle \Omega _{\text{b}}h^{2}}$ is estimated at about 0.023 (this is different from the 'baryon density' ${\displaystyle \Omega _{\text{b}}}$ 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 ${\displaystyle \Omega _{\text{c}}h^{2}}$ is about 0.11, the corresponding neutrino density ${\displaystyle \Omega _{\text{v}}h^{2}}$ is estimated to be less than 0.0062.

Cosmic acceleration

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 predominated, 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 through observation 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. 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.

Features of the model

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.

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.

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, are themselves 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 because of 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 co-moving 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. In other words, the Big Bang is not an explosion in space, but rather an expansion of space. 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

An important feature of the Big Bang spacetime is the presence of particle 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.

History

Etymology

English astronomer Fred Hoyle is credited with coining the term "Big Bang" during a 1949 BBC radio broadcast, saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past."

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

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 – 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 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, 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 (2.5 m) 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, 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 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 CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe. 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. 

In 1968 and 1970 Roger Penrose, Stephen Hawking, and George F. R. Ellis published papers where they showed that mathematical singularities were an inevitable initial condition of general relativistic models of the Big Bang. Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, Alan Guth made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang theory with the introduction of an epoch of rapid expansion in the early universe he called "inflation". Meanwhile, during these decades, two questions in observational cosmology that generated much discussion and disagreement were over the precise values of the Hubble Constant and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual fate of the universe).

In the mid-1990s, observations of certain globular clusters appeared to indicate that they were about 15 billion years old, which conflicted with most then-current estimates of the age of the universe (and indeed with the age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. While there still remain some questions as to how accurately the ages of the clusters are measured, globular clusters are of interest to cosmology as some of the oldest objects in the universe.

Significant progress in Big Bang cosmology has 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

[The] 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 observational evidence of the validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by Big Bang nucleosynthesis. More recent evidence includes observations of galaxy formation and evolution, and the distribution of large-scale cosmic structures, 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 include the cuspy halo problem and the dwarf galaxy problem of cold dark matter. 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. 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

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: ${\displaystyle v=H_{0}D}$ where

• ${\displaystyle v}$ is the recessional velocity of the galaxy or other distant object,
• ${\displaystyle D}$ is the comoving distance to the object, and
• ${\displaystyle H_{0}}$ 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 ${\displaystyle v=HD}$ to hold at all times, where ${\displaystyle D}$ is the comoving distance, v is the recessional velocity, and ${\displaystyle v}$, ${\displaystyle H}$, and ${\displaystyle D}$ vary as the universe expands (hence we write ${\displaystyle H_{0}}$ 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 ${\displaystyle v}$. 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 homogeneous, 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 CMB 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

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 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 big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s the radiation was found to be approximately consistent with a 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.

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

 

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), which made two major advances: in 1990, high-precision spectrum measurements showed that the CMB frequency spectrum is an almost perfect blackbody with no deviations at a level of 1 part in 10⁴, and measured a residual temperature of 2.726 K (more recent measurements have revised this figure down slightly to 2.7255 K); then in 1992, further COBE measurements discovered tiny fluctuations (anisotropies) in the CMB temperature across the sky, at a level of about one part in 10⁵. John C. Mather and George Smoot were awarded the 2006 Nobel Prize in Physics for their leadership in these results. 

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

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 ${\displaystyle {\ce {^4He/H}}}$, about 10⁻³ for ${\displaystyle {\ce {^2H/H}}}$, about 10⁻⁴ for ${\displaystyle {\ce {^3He/H}}}$ and about 10⁻⁹ for ${\displaystyle {\ce {^7Li/H}}}$.

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 ${\displaystyle {\ce {^4He}}}$, and off by a factor of two for ${\displaystyle {\ce {^7Li}}}$; 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 ${\displaystyle {\ce {^3He}}}$, and in constant ratios, too.

Galactic evolution and distribution

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

Focal plane of BICEP2 telescope under a microscope - used to search for polarization in the CMB.

 

In 2011, astronomers found what they believe to be pristine clouds of primordial gas by analyzing absorption lines in the spectra of distant quasars. Before this discovery, all other astronomical objects have been observed to contain heavy elements that are formed in stars. These two clouds of gas contain no elements heavier than hydrogen and deuterium. Since the clouds of gas have no heavy elements, they likely formed in the first few minutes after the Big Bang, during Big Bang nucleosynthesis.

Other lines of evidence

The age of the 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.

Future observations

Future gravitational waves observatories might be able to detect primordial gravitational waves, relics of the early universe, up to less than a second after the Big Bang.

Problems and related issues in physics

As with any theory, a number of mysteries and problems have arisen as a result of the development of the Big Bang theory. Some of these mysteries and problems have been resolved while others are still outstanding. Proposed solutions to some of the problems in the Big Bang model have revealed new mysteries of their own. For example, the horizon problem, the magnetic monopole problem, and the flatness problem are most commonly resolved with inflationary theory, but the details of the inflationary universe are still left unresolved and many, including some founders of the theory, say it has been disproven. What follows are a list of the mysterious aspects of the Big Bang theory still under intense investigation by cosmologists and astrophysicists.

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 effects are not strong enough to explain the present baryon asymmetry.

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. 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. 

The dark energy component of the universe has been explained by theorists using a variety of competing theories including Einstein's cosmological constant but also extending to more exotic forms of quintessence or other modified gravity schemes. A cosmological constant problem, sometimes called the "most embarrassing problem in physics", results from the apparent discrepancy between the measured energy density of dark energy, and the one naively predicted from Planck units.

Dark matter

Chart shows the proportion of different components of the universe  – about 95% is dark matter and dark energy.

 

During the 1970s and the 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.

Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the dwarf galaxy problem and the cuspy halo problem. Alternative theories have been proposed that do not require a large amount of undetected matter, but instead modify the laws of gravity established by Newton and Einstein; yet no alternative theory has been as successful as the cold dark matter proposal in explaining all extant observations.

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.

A related issue to the classic horizon problem arises because in most standard cosmological inflation models, inflation ceases well before electroweak symmetry breaking occurs, so inflation should not be able to prevent large-scale discontinuities in the electroweak vacuum since distant parts of the observable universe were causally separate when the electroweak epoch ended.

Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand unified 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 no monopoles have been found. 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.

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 (FLRW). 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 an explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the density of the universe must have been within one part in 10¹⁴ of its critical value, or it would not exist as it does today.

Cause

Physics may conclude that time did not exist before 'Big Bang', but 'started' with the Big Bang and hence there might be no 'beginning', 'before' or potentially 'cause' and instead always existed. Quantum fluctuations, or other laws of physics that may have existed at the start of the Big Bang could then create the conditions for matter to occur.

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.

Misconceptions

The following is a partial list of misconceptions about the Big Bang model: 

The Big Bang as the origin of the universe: One of the common misconceptions about the Big Bang model is the belief that it was the origin of the universe. However, the Big Bang model does not comment about how the universe came into being. Current conception of the Big Bang model assumes the existence of energy, time, and space, and does not comment about their origin or the cause of the dense and high temperature initial state of the universe.

The Big Bang was "small": It is misleading to visualize the Big Bang by comparing its size to everyday objects. When the size of the universe at Big Bang is described, it refers to the size of the observable universe, and not the entire universe.

Hubble's law violates the special theory of relativity: Hubble's law predicts that galaxies that are beyond Hubble Distance recede faster than the speed of light. However, special relativity does not apply beyond motion through space. Hubble's law describes velocity that results from expansion of space, rather than through space.

Doppler redshift vs cosmological red-shift: Astronomers often refer to the cosmological red-shift as a normal Doppler shift, which is a misconception. Although similar, the cosmological red-shift is not identical to the Doppler redshift. The Doppler redshift is based on special relativity, which does not consider the expansion of space. On the contrary, the cosmological red-shift is based on general relativity, in which the expansion of space is considered. Although they may appear identical for nearby galaxies, it may cause confusion if the behavior of distant galaxies is understood through the Doppler redshift.

Speculations

While the Big Bang model is well established in cosmology, it is likely to be refined. The Big Bang theory, built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time; this infinite energy density is regarded as impossible in physics. Still, it is known that the equations are not applicable before the time when the universe cooled down to the Planck temperature, and this conclusion depends on various assumptions, of which some could never be experimentally verified.

One proposed refinement to avoid this would-be singularity is to develop a correct treatment of quantum gravity.

It is not known what could have preceded the hot dense state of the early universe or how and why it originated, though speculation abounds in the field of cosmogony. 

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 any 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 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 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 interpretations

As a description of the origin of the universe, the Big Bang has significant bearing on religion and philosophy. As a result, it has become one of the liveliest areas in the discourse between science and religion. Some believe the Big Bang implies a creator, and some see its mention in their holy books, while others argue that Big Bang cosmology makes the notion of a creator superfluous.

End time

From Wikipedia, the free encyclopedia

The end time (also called end times, end of time, end of days, last days, final days, or eschaton) is a future time-period described variously in the eschatologies of several world religions (both Abrahamic and non-Abrahamic), which teach that world events will reach a final climax.

The Abrahamic faiths maintain a linear cosmology, with end-time scenarios containing themes of transformation and redemption. In Judaism, the term "end of days" makes reference to the Messianic Age and includes an in-gathering of the exiled Jewish diaspora, the coming of the Messiah, the resurrection of the righteous, and the world to come. Some sects of Christianity depict the end time as a period of tribulation that precedes the second coming of Christ, who will face the Antichrist along with his power structure and usher in the Kingdom of God.

In Islam, the Day of Judgement is preceded by the appearance of the al-Masih al-Dajjal, and followed by the descending of Isa (Jesus). Isa will triumph over the false messiah, or the Antichrist, which will lead to a sequence of events that will end with the sun rising from the west and the beginning of the Qiyamah (Judgment day).

Non-Abrahamic faiths tend to have more cyclical world-views, with end-time eschatologies characterized by decay, redemption, and rebirth. In Hinduism, the end time occurs when Kalki, the final incarnation of Vishnu, descends atop a white horse and brings an end to the current Kali Yuga. In Buddhism, the Buddha predicted that his teachings would be forgotten after 5,000 years, followed by turmoil. A bodhisattva named Maitreya will appear and rediscover the teaching of dharma. The ultimate destruction of the world will then come through seven suns.

Since the development of the concept of deep time in the 18th century and the calculation of the estimated age of the Earth, scientific discourse about end times has centered on the ultimate fate of the universe. Theories have included the Big Rip, Big Crunch, Big Bounce, and Big Freeze (heat death).

Cyclic cosmology

Indian religions

Buddhism

There are two major points of Buddhist eschatology, the appearance of Maitreya, followed by the Sermon of the Seven Suns.

Maitreya

Buddha described his teachings disappearing five thousand years from when he preached them, corresponding approximately to the year 2300. At this time, knowledge of dharma will be lost as well. The last of his relics will be gathered in Bodh Gaya and cremated. There will be a new era in which the next Buddha Maitreya will appear, but it will be preceded by the degeneration of human society. This will be a period of greed, lust, poverty, ill will, violence, murder, impiety, physical weakness, sexual depravity and societal collapse, and even the Buddha himself will be forgotten.

This will be followed by the coming of Maitreya when the teachings of dharma are forgotten. Maitreya was the first Bodhisattva around whom a cult developed, in approximately the 3rd century CE.

The earliest mention of Maitreya is in the Cakavatti, or Sihanada Sutta in Digha Nikaya 26 of the Pali Canon. In it, Gautama Buddha predicted that his teachings of dharma would be forgotten after 5,000 years.

At that period, brethren, there will arise in the world an Exalted One named Maitreya, Fully Awakened, abounding in wisdom and goodness, happy, with knowledge of the worlds, unsurpassed as a guide to mortals willing to be led, a teacher for gods and men, an Exalted One, a Buddha, even as I am now. He, by himself, will thoroughly know and see, as it were face to face, this universe, with Its worlds of the spirits, Its Brahmas and Its Maras, and Its world of recluses and Brahmins, of princes and peoples, even as I now, by myself, thoroughly know and see them

— Digha Nikaya, 26

Maitreya Buddha is then foretold to be born in the city of Ketumatī in present-day Benares, whose king will be the Cakkavattī Sankha. Sankha will live in the former palace of King Mahāpanadā, and will become a renunciate who follows Maitreya.

In Mahayana Buddhism, Maitreya will attain bodhi in seven days, the minimum period, by virtue of his many lifetimes of preparation. Once Buddha, he will rule over the Ketumati Pure Land, an earthly paradise sometimes associated with the Indian city of Varanasi or Benares in Uttar Pradesh. In Mahayana Buddhism, the Buddha presides over a land of purity. For example, Amitabha presides over Sukhavati, more popularly known as the 'Western Paradise'.

Bodhisattva Maitreya from the 2nd-century Gandharan art period

A notable teaching he will rediscover is that of the ten non-virtuous deeds—killing, stealing, sexual misconduct, lying, divisive speech, abusive speech, idle speech, covetousness, harmful intent and wrong views. These will be replaced by the ten virtuous deeds, which are the abandonment of each of these practices. He is described by Conze in his Buddhist Scriptures:

The Lord replied, 'Maitreya, the best of men, will then leave the Tuṣita heavens, and go for his last rebirth. As soon as he is born he will walk seven steps forward, and where he puts down his feet a jewel or a lotus will spring up. He will raise his eyes to the ten directions, and will speak these words: "This is my last birth. There will be no rebirth after this one. Never will I come back here, but, all pure, I shall win Nirvana."

— Buddhist Scriptures 

He currently resides in Tushita, but will come to Jambudvipa when needed most as successor to the historic Śākyamuni Buddha. Maitreya will achieve complete enlightenment during his lifetime, and following this reawakening, he will bring back the timeless teaching of dharma to this plane and rediscover enlightenment. The Arya Maitreya Mandala, founded by Lama Anagarika Govinda is based on the idea of Maitreya.

Maitreya eschatology forms the central canon of the White Lotus Society, a revolutionary movement during British colonial rule. It later branched into the Chinese underground criminal organization known as the Triad, which exists today as an international underground criminal network.

It is of note that description of Maitreya occurs in no other sutta in the canon, casting doubt as to authenticity of the scripture. In addition, sermons of the Buddha normally are in response to a question, or in a specific context, but this sutta has a beginning and an ending, and its content is quite different from the others. This has led some to conclude that the whole sutta is apocryphal, or tampered with.

Sermon of the Seven Suns

In his "Sermon of the Seven Suns" in the Pali Canon, the Buddha describes the ultimate fate of the world in an apocalypse that will be characterized by the consequent appearance of seven suns in the sky, each causing progressive ruin till the Earth is destroyed:

All things are impermanent, all aspects of existence are unstable and non-eternal. Beings will become so weary and disgusted with the constituent things that they will seek emancipation from them more quickly. There will come a season, O monks when, after hundreds of thousands of years, rains will cease. All seedlings, all vegetation, all plants, grasses and trees will dry up and cease to be...There comes another season after a great lapse of time when a second sun will appear. Now all brooks and ponds will dry up, vanish, cease to be.

— Aňguttara-Nikăya, VII, 6.2 Pali Canon

The canon goes on to describe the progressive destruction of each sun. A third sun will dry the mighty Ganges and other great rivers. A fourth will cause the great lakes to evaporate, and a fifth will dry the oceans. Finally:

Again after a vast period of time a sixth sun will appear, and it will bake the Earth even as a pot is baked by a potter. All the mountains will reek and send up clouds of smoke. After another great interval a seventh sun will appear and the Earth will blaze with fire until it becomes one mass of flame. The mountains will be consumed, a spark will be carried on the wind and go to the worlds of God....Thus, monks, all things will burn, perish and exist no more except those who have seen the path.

— Aňguttara-Nikăya, VII, 6.2 Pali Canon

The sermon completes with the planet engulfed by a vast inferno. The Pali Canon does not indicate when this will happen relative to Maitreya.

Hinduism

In Hindu eschatology, time is cyclic and consists of kalpas. Each lasts 4.1 – 8.2 billion years, which is a period of one full day and night for Brahma, who will be alive for 311 trillion, 40 billion years. Within a kalpa there are periods of creation, preservation and decline. After this larger cycle, all of creation will contract to a singularity and then again will expand from that single point, as the ages continue in a religious fractal pattern.

Within the current kalpa, there are four epochs that encompass the cycle. They progress from a beginning of complete purity to a descent into total corruption. The last of the four ages is Kali Yuga, our current time, during which will be characterized by impiety, violence and decay. The four pillars of dharma will be reduced to one, with truth being all that remains. As written in the Gita:

Yadā yadā hi dharmasya glānirbhavati Bhārata
Abhyutthānam adharmasya tadātmānam sṛjāmyaham

Whenever there is decay of righteousness O! Bharata
And a rise of unrighteousness then I manifest Myself!

At this time of chaos, the final avatar, Kalki, endowed with eight superhuman faculties will appear on a white horse. Kalki will amass an army to "establish righteousness upon the earth" and leave "the minds of the people as pure as crystal."

At the completion of Kali Yuga, the next cycle will begin with a new Satya Yuga, in which all will once again be righteous with the reestablishment of dharma. This, in turn, will be followed by epochs of Treta Yuga, Dwapara Yuga and again another Kali Yuga. This cycle will then repeat till the larger cycle of existence under Brahma returns to the singularity, and a new universe is born.

Norse religion

Odin fighting his old nemesis Fenrir

Ragnarök after Surtr has engulfed the world with fire

Norse mythology depicts the end of days as Ragnarök, an Old Norse term translatable as "twilight of the gods". It will be heralded by a devastation known as Fimbulvetr which will seize Midgard in cold and darkness. The sun and moon will disappear from the sky, and poison will fill the air. Dead will rise from the ground and there will be widespread despair.

There follows a battle between – on the one hand – the Gods with the Æsir, Vanir and Einherjar, led by Odin, and – on the other hand – forces of Chaos, including the fire giants and jötunn, led by Loki. In the fighting Odin will be swallowed whole by his old nemesis Fenrir. The god Freyr fights Surtr but loses. Víðarr, son of Odin, will then avenge his father by ripping Fenrir's jaws apart and stabbing the wolf in the heart with his spear. The serpent Jörmungandr will open its gaping maw and be met in combat by Thor. Thor, also a son of Odin, will defeat the serpent, only to take nine steps afterwards before collapsing to his own death.

After this people will flee their homes as the sun blackens and the earth sinks into the sea. The stars will vanish, steam will rise, and flames will touch the heavens. This conflict will result in the deaths of most of the major Gods and forces of Chaos. Finally, Surtr will fling fire across the nine worlds. The ocean will then completely submerge Midgard.

After the cataclysm the world will resurface new and fertile, and the surviving Gods will meet. Baldr, also a son of Odin will be reborn in the new world, according to Völuspá. The two human survivors, Líf and Lífþrasir, will then repopulate this new earth.

Linear cosmology

Abrahamic religions

Bahá'í Faith

The founder of the Bahá'í Faith, Bahá'u'lláh claimed that he was the return of Christ as well as prophetic expectations of other religions. The inception of the Bahá'í Faith coincides with Millerite prophesy, pointing to the year 1844. They also believe the Battle of Armageddon has passed and that the mass martyrdom anticipated during the End Times had already passed within the historical context of the Bahá'í Faith. Bahá'ís expect their faith to be eventually embraced by the masses of the world, ushering in a golden age.

Christianity

Date

Some first-century Christians believed Jesus would return during their lifetime. When the converts of Paul in Thessalonica were persecuted by the Roman Empire, they believed the end of days to be imminent.

While some who believe in the literal interpretation of the Bible insist that the prediction of dates or times is futile, others believe Jesus foretold signs of the end of days. The precise time, however, will come like a "thief in the night" (1 Thess. 5:2). They may also refer to Matthew 24:36 in which Jesus is quoted as saying:

But concerning that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father only.

Great Tribulation

In the New Testament, Jesus refers to this period preceding the end times as the "Great Tribulation" (Matthew 24:21), "Affliction"(Mark 13:19), and "days of vengeance"(Luke 21:22).

The Book of Matthew describes the devastation:

When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand). Then let them which be in Judaea flee into the mountains. Let him which is on the housetop not come down...Neither let him which is in the field return back to take his clothes, and woe unto them that are with child...For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened.

— Matthew 24:15–22

The resulting chaos will affect pregnancies, newborns, and a scourge will spread throughout the flesh, save for the elect. The vivid imagery of this section is repeated closely in Mark 13:14–20.

The Gospel of Luke describes a complete unraveling of the social fabric, with widespread calamity and war:

Then he said to them, “Nation will rise against nation, and kingdom against kingdom. There will be great earthquakes, and in various places famines and pestilences. And there will be terrors and great signs from heaven. But before all this they will lay their hands on you and persecute you, delivering you up to the synagogues and prisons, and you will be brought before kings and governors for my name's sake. This will be your opportunity to bear witness. Settle it therefore in your minds not to meditate beforehand how to answer, for I will give you a mouth and wisdom, which none of your adversaries will be able to withstand or contradict. You will be delivered up even by parents and brothers and relatives and friends, and some of you they will put to death. You will be hated by all for my name's sake. But not a hair of your head will perish. By your endurance you will gain your lives.
“But when you see Jerusalem surrounded by armies, then know that its desolation has come near. Then let those who are in Judea flee to the mountains, and let those who are inside the city depart, and let not those who are out in the country enter it, for these are days of vengeance, to fulfill all that is written. Alas for women who are pregnant and for those who are nursing infants in those days! For there will be great distress upon the earth and wrath against this people. They will fall by the edge of the sword and be led captive among all nations, and Jerusalem will be trampled underfoot by the Gentiles, until the times of the Gentiles are fulfilled.
“And there will be signs in sun and moon and stars, and on the earth distress of nations in perplexity because of the roaring of the sea and the waves, people fainting with fear and with foreboding of what is coming on the world. For the powers of the heavens will be shaken. And then they will see the Son of Man coming in a cloud with power and great glory. Now when these things begin to take place, straighten up and raise your heads, because your redemption is drawing near.”
And he told them a parable: “Look at the fig tree, and all the trees. As soon as they come out in leaf, you see for yourselves and know that the summer is already near. So also, when you see these things taking place, you know that the kingdom of God is near. Truly, I say to you, this generation will not pass away until all has taken place. Heaven and earth will pass away, but my words will not pass away.

— Luke 21:10–33

Catholicism

The Profession of Faith addresses Catholic beliefs concerning the Last Days. Catholicism adheres to the amillennial school of thought, promoted by Augustine of Hippo in his work The City of God.

Protestantism

The Antichrist, by Lucas Cranach the Elder (1521) Here the Antichrist is shown wearing the triple crown of the Roman papacy.

Protestants are divided between Millennialists and Amillennialists. Millennialists concentrate on the issue of whether the true believers will see the tribulation or be removed from it by what is referred to as a Pre-Tribulation Rapture. Amillennialists believe that the end times encompass the time from Christ's ascension to the Last day, and maintain that the mention of the "thousand years" in the Book of Revelation is meant to be taken metaphorically (i.e., not literally, or 'spiritually'), a view which continues to cause divisions within evangelical Christianity.

There is a range of eschatological belief in Protestant Christianity. Christian premillennialists who believe that the End Times are occurring now, are usually specific about timelines that climax in the end of the world. For some, Israel, the European Union, or the United Nations are seen as major players whose roles were foretold in scripture. Within dispensational premillennialist writing, there is the belief that Christians will be summoned to Heaven by Christ at the Rapture, occurring before a "Great Tribulation" prophesied in Matthew 24–25; Mark 13 and Luke 21. The Tribulation is described in the book of Revelation.

"End times" may also refer to the passing of an age or long period in the relationship between man and God. Adherents to this view cite the Second Epistle to Timothy and draw analogies to the late twentieth and early twenty-first centuries.

Post-Exilic Hebrew books of prophecy such as Daniel and Ezekiel are given new interpretations in this tradition, while in apocalyptic forecasts appear in the Judeo-Christian Sibylline Oracles which include the Book of Revelation ascribed to John, the apocryphal Apocalypse of Peter, and the Second Book Of Esdras.

Most fundamentalist Christians anticipate biblical prophecy to be literally fulfilled. They see current wars, natural disaster and famine as the birth pangs which Jesus described in Matthew 24:7–8 and Mark 13:8. They believe that mankind began in the garden of Eden, and point to the Valley of Megiddo as the place where the current world system will terminate, after which the Messiah will rule for 1,000 years.

Contemporary use of the term End Times has evolved from literal belief in Christian millennialism. In this tradition, Biblical apocalypse is believed to be imminent, with various current events as omens of impending Armageddon. These beliefs have been put forward by the Adventist movement (Millerites), Jehovah's Witnesses, and dispensational premillennialists. In 1918 a group of eight well known preachers produced the London Manifesto, warning of an imminent second coming of Christ shortly after the 1917 liberation of Jerusalem by the British.

Icon of the Second Coming. Greek, ca. 1700 A.D.

Religious movements which expect that the second coming of Christ as a cataclysmic event are generally called adventism. These have arisen throughout the Christian era, but were particularly common after the Protestant Reformation. Emanuel Swedenborg considered the second coming to be symbolic, and to have occurred in 1757. Along with others, he developed a religious system around the second coming of Christ, disclosed by new prophecy or special revelation not described in the Bible. The Millerites are diverse religious groups which similarly rely upon a special gift of interpretation for predicting the second coming.

The difference between the 19th-century Millerite and Adventist movements and contemporary prophecy is that William Miller and his followers, based on Biblical interpretation, predicted the time of the Second Coming to have occurred in 1844. Contemporary writing of end time has suggested that the timetable will be triggered by future wars and moral catastrophe, and that this time of tribulation is close at hand.

Seventh-day Adventists believe Biblical prophecy to foretell an end time scenario in which the United States works in conjunction with the Catholic Church to mandate worship on a day other than the true Sabbath, Saturday, as prescribed in the Ten Commandments (Exodus 20:8–11). This will bring about a situation where one must choose for or against the Bible as the will of God.

Preterism

Another view of the end times is preterism. It distinguishes the time of the end from the end of time. Preterists believe the term Last Days (or Time of the End) refers to, neither the last days of the Earth, nor the last days of humankind, but the end of the Old Covenant between God and Israel; which, according to preterism, took place when the Temple in Jerusalem which was destroyed in 70 CE.

Preterists believe that prophecies—such as the Second Coming, the desecration of the Jewish Temple, the destruction of Jerusalem, the rise of the Antichrist, the Great Tribulation, the advent of The Day of the Lord, and a Final Judgment—had been fulfilled when the Romans sacked Jerusalem and completely destroyed its Temple.

Proponents of full preterism do not believe in a coming resurrection of the dead. They place this event (as well as the Second Coming) in the year 70. Advocates of partial preterism do believe in a coming resurrection. Full preterists contend that partial preterists are merely futurists, since they believe the Second Coming, the Resurrection, the Rapture, and the Judgment are yet to come.

Many preterists believe that first-century Christians experienced the Rapture to rejoin the Christ.

According with Preterism's interpretation of end times, many "time passages" in the New Testament foretell a Second Coming of Christ, with Last Days to take place within the lifetimes of his disciples: Matt. 10:23, Matt. 16:28, Matt. 24:34, Matt. 26:64, Rom. 13:11–12, 1 Cor. 7:29–31, 1 Cor. 10:11, Phil. 4:5, James 5:8–9, 1 Pet. 4:7, 1 Jn. 2:18.

Dispensationalist prophecies

Dispensationalism is an evangelical futurist Bibilical interpretation that foresees a series of dispensations, or periods, in which God relates to human beings under different Biblical covenants. The belief system is primarily rooted in the writings of John Nelson Darby and is premillennial in content. The reestablishment of Israel in 1948 provided a major impetus to the dispensationalist belief system. The wars of Israel after 1948 with its Arab neighbors provided further support, according to John F. Walvoord. After the Six-Day War in 1967, and the Yom Kippur War in 1973, it seemed plausible to many Fundamentalist Christians in the 1970s that Middle East turmoil may well be leading up to the fulfillment of various Bible prophecies and to the Battle of Armageddon.

Members of the dispensationalist movement such as Hal Lindsey, J. Dwight Pentecost, John Walvoord, all of whom have Dallas Theological Seminary backgrounds, and some other writers, claimed further that the European Economic Community, which preceded the European Union, would become a United States of Europe, which would in turn become a Revived Roman Empire ruled by the Antichrist. The Revived Roman Empire also figured into the New Testament writers' vision of the future. The fact that in the early 1970s, there were (erroneously thought to be) seven nations in the European Economic Community was held to be significant; this aligned the Community with a seven-headed beast mentioned in Revelation. This specific prophecy has required revision, but the idea of a Revived Roman Empire remains.

The separate destinies of the Church and Israel, a belief which is inherent in dispensationalism, is a particular concern to some Jews and evangelical Christians. Evangelicals who reject dispensationalism, such as those who hold to a Post Tribulation Rapture, (or more accurately a Post Tribulation Resurrection-Rapture), see both the Church and Israel entering the crucible of the End Time together.

Dispensationalism, in contrast to the Millerite Adventist movement, had its beginning in the 19th century, when John Nelson Darby, founder of the Plymouth Brethren religious denomination, incorporated into his system of Biblical interpretation a system of organizing Biblical time into a number of discrete dispensations, each of which marks a separate covenant with God. Darby's beliefs were widely publicized in Cyrus I. Scofield's Scofield Reference Bible, an annotated Bible that became popular in the United States of America.

Since the majority of the Biblical prophets were writing at a time when the Temple in Jerusalem was still functioning, they wrote as if it would still be standing during the prophesied events. According to preterism, this was a fulfillment of the prophecies. However, according to Futurists, their destruction in AD 70 put the prophetic timetable on hold. Many such believers therefore anticipated the return of Jews to Israel and the reconstruction of the Temple before the Second Coming could occur.

Post-tribulation pre-millennialists

A view of the Second Coming of Christ as held by post-tribulational pre-millennialists holds that the Church of Christ will have to undergo great persecution by being present during the great tribulation.

Specific prophetic movements

William Miller predicted the end of the world in 1843, known as the Great Disappointment

In 1843, William Miller made the first of several predictions that the world would end in only a few months. As his predictions did not come true (referred to as the Great Disappointment), followers of Miller went on to found separate groups, the most successful of which is the Seventh-day Adventist Church.

Members of the Bahá'í Faith believe that Miller's interpretation of signs and dates of the coming of Jesus were, for the most part, correct. They believe that the fulfillment of biblical prophecies of the coming of Christ came through a forerunner of their own religion, the Báb. According to the Báb's words, 4 April 1844 was "the first day that the Spirit descended" into his heart. His subsequent declaration to Mullá Husayn-i Bushru'i that he was the "Promised One"—an event now commemorated by Bahá'ís as a major holy day—took place on 23 May 1844. It was in October of that year that the Báb embarked on a pilgrimage to Mecca, where he openly declared his claims to the Sharif of Mecca. The first news coverage of these events in the West was in 1845 by The Times, followed by others in 1850 in the United States. The first Bahá'í to come to America was in 1892. Several Bahá'í books and pamphlets make mention of the Millerites, the prophecies used by Miller and the Great Disappointment, most notably William Sears's Thief in the Night.

Restorationism (Christian primitivism)

End times theology is also significant to restorationist Christian religions, which consider themselves distinct from both Catholicism and Protestantism.

Jehovah's Witnesses

Former Watch Tower headquarters in Brooklyn. The society made a number of emphatic claims of impending last days and ensuing chaos between 1879–1924.

The eschatology of Jehovah's Witnesses is central to their religious beliefs. They believe that Jesus Christ has been ruling in heaven as king since 1914 (a date they believe was prophesied in Scripture), and that after that time a period of cleansing occurred, resulting in God's selection of the Bible Students associated with Charles Taze Russell to be his people in 1919. They also believe the destruction of those who reject their message and thus willfully refuse to obey God will shortly take place at Armageddon, ensuring that the beginning of the new earthly society will be composed of willing subjects of that kingdom.

The religion's doctrines surrounding 1914 are the legacy of a series of emphatic claims regarding the years 1799, 1874, 1878, 1914, 1918, and 1925 made in the Watch Tower Society's publications between 1879 and 1924. Claims about the significance of those years, including the presence of Jesus Christ, the beginning of the "last days", the destruction of worldly governments and the earthly resurrection of Jewish patriarchs, were successively abandoned. In 1922 the society's principal journal, The Watchtower, described its chronology as "no stronger than its weakest link", but also claimed the chronological relationships to be "of divine origin and divinely corroborated...in a class by itself, absolutely and unqualifiedly correct" and "indisputable facts", while repudiation of Russell's teachings was described as "equivalent to a repudiation of the Lord".

The Watch Tower Society has admitted its early leaders promoted "incomplete, even inaccurate concepts". The Governing Body of Jehovah's Witnesses says that, unlike Old Testament prophets, its interpretations of the Bible are not inspired or infallible. Witness publications say that Bible prophecies can be fully understood only after their fulfillment, citing examples of biblical figures who did not understand the meaning of prophecies they received. Watch Tower publications often cite Proverbs 4:18, "The path of the righteous ones is like the bright light that is getting lighter and lighter until the day is firmly established" (NWT) to support their view that there would be an increase in knowledge during "the time of the end", as mentioned in Daniel 12:4. Jehovah's Witnesses state that this increase in knowledge needs adjustments. Watch Tower publications also say that unfulfilled expectations are partly due to eagerness for God's Kingdom and that they do not call their core beliefs into question.

The Church of Jesus Christ of Latter-day Saints

Members of The Church of Jesus Christ of Latter-day Saints believe that there will be a Second Coming of Jesus to the earth sometime in the future. The LDS Church and its leaders do not make any predictions of the actual date of the Second Coming.

According to church doctrine, the true gospel will be taught in all parts of the world prior to the Second Coming. They also believe that there will be increasing war, earthquakes, hurricanes, and man-made disasters prior to the Second Coming. Disasters of all kind will happen before Christ comes. Upon the return of Jesus Christ, all people will be resurrected, the righteous in a first resurrection and the unrighteous in a second, later resurrection. Christ shall reign for a period of 1000 years, after which the Final Judgement will occur.

Islam

Muslims believe that there are three periods before the Day of Judgment, also known as ashratu's-sa'ah or alamatu qiyami's-sa'ah, with some debate as to whether the periods could overlap.

According to Harun Yahya, the first period is said to have begun with the death of Muhammad. The second began with the passing of all his Companions, and ended a thousand years later. Another event of the second period was the Tartar invasion, occurring 650 years after Muhammad. The Mongols, led by Hulagu Khan, grandson of Genghis Khan, attacked Baghdad in 1258 AD and brought the Abbasid caliphate to an end. They massacred millions of Muslims, and the water of the river Tigris turned red with blood. A traditional narration also predicted a fire at Madinah in the Hijaz near Busra in Syria, which Islamic scholars believe occurred in 654 AH. Following the second, the third and final period will be heralded by the appearance of the Mahdi.

Sunni

Sunnis believe that the dead will then stand in a grand assembly, awaiting a scroll detailing their righteous deeds, sinful acts and ultimate judgment. Muhammad will be the first to be resurrected. Punishments will include adhab, or severe pain and embarrassment, and khizy or shame. There will also be a punishment of the grave between death and the resurrection.

The signs of the coming end time are divided into major and minor signs:

Following the second period, the third is said to be marked by the ten major signs known as alamatu's-sa'ah al- kubra (The major signs of the end). They are as follows:

1. The false messiah (anti-Christ), Masih ad-Dajjal, shall appear with huge powers as a one eyed man with the right eye blind and deformed like a grape. He will claim to be God and to hold keys to heaven and hell and lead many astray, although believers will not be deceived. In reality, his heaven is the hell, and his hell is the heaven. The Dajjal would be followed by seventy thousand Jews of Isfahan wearing Persian shawls.
2. The return of Isa (Jesus), from the fourth sky to kill Dajjal.
3. Ya'jooj and Ma'jooj (Gog and Magog), Japhetic tribe of vicious beings which had been imprisoned by Dhul-Qarnayn will break out. They will ravage the earth, drink all the water of Lake Tiberias, and kill all believers in their way (or see). Isa, Imam Al-Mahdi, and the believers with them will go to the top of a mountain and pray for the destruction of Gog and Magog. God will eventually send disease and worms to wipe them out.
4. A huge black smoke cloud will cover the earth.
5. Dabbat al-ard, or the Beast that will come out of the ground to talk to people.
6. The sun will rise from the west.
7. Three sinking of the earth, one in the east,
8. One in the west,
9. And one in Arabia.
10. The second trumpet blow will be sounded, the dead will return to life and a fire will start come out of Yemen that shall gather all to Mahshar Al Qiy'amah (The Gathering for Judgment).

Shia

Concepts and terminology in Shia eschatology includes Mi'ad, The Occultation and Al-Yamani, Sufyani In Twelver Shia narrations about the last days, the literature largely revolves around Muhammad al-Mahdi, a messianic figure considered to be the twelfth appointed successor to Muhammad. Mahdi will help mankind against the deception by a man called Dajjal who will try to get people in to a new world religion which is called "the great deception".

Ahmadiyya

Ahmadiyya is considered distinct from mainstream Islam. In its writing, the present age has been witness to the evil of man and wrath of God, with war and natural disaster. Ghulam Ahmad is seen as the promised Messiah and the Mahdi, fulfilling Islamic and Biblical prophecies, as well as scriptures of other religions such as Hinduism. His teaching will establish spiritual reform and establish an age of peace. This will continue for a thousand years, and will unify mankind under one faith.

Ahmadis believe that despite harsh and strong opposition and discrimination they will eventually be triumphant and their message vindicated both by Muslims and non-Muslims alike. Ahmadis also incorporate the eschatological views from other religions into their doctrine and believe that Mirza Ghulam Ahmed falls into this sequence.

Judaism

Scroll of Book of Isaiah

In rabbinic literature, rabbis elaborated and explained the prophecies that were found in the Hebrew Bible, along with oral law and rabbinic traditions about its significance. The main tenets of Jewish eschatology, in no particular order, include:

• God will redeem Israel from the captivity that began during the Babylonian Exile in a new Exodus.
• God will return the Jewish people to the Land of Israel.
• God will restore the House of David and the Temple in Jerusalem.
• God will raise up a regent from the House of David, the Jewish Messiah, to lead the Jewish people and the world and to usher in an age of justice and peace.
• Nations will recognize that the God of Israel is the only true god.
• God will resurrect the dead.
• God will create a new heaven and earth.

The idea of a messianic age, an era of global peace and knowledge of the Creator, has a prominent place in Jewish thought, and is incorporated as part of the end of days. A well-known passage from the Book of Isaiah describes this future condition of the world: "They shall beat their swords into plowshares and their spears into pruning hooks; nation will not lift sword against nation and they will no longer study warfare" (2:4) Maimonides (1135–1204) further describes the Messianic Era in the Mishneh Torah: "And at that time there will be no hunger or war, no jealousy or rivalry. For the good will be plentiful, and all delicacies available as dust. The entire occupation of the world will be only to know God... the people Israel will be of great wisdom; they will perceive the esoteric truths and comprehend their Creator's wisdom as is the capacity of man. As it is written (Isaiah 11:9): 'For the earth shall be filled with the knowledge of God, as the waters cover the sea.'"

The Zohar maintains that the seven days of the week, based on the seven days of creation, correspond to the seven millennia of creation. The seventh day of the week, the Shabbat day of rest, corresponds to the seventh millennium, the age of universal rest, or the Messianic Era. The seventh millennium begins with the year 6000 AM, and is the latest time the Messiah can come. A number of early and late Jewish scholars have written in support of this, including the Ramban, Isaac Abrabanel, Abraham Ibn Ezra, Rabbeinu Bachya, the Vilna Gaon, the Lubavitcher Rebbe, the Ramchal, Aryeh Kaplan and Rebbetzin Esther Jungreis.

Rastafari movement

Haile Selassie I is viewed as god incarnate in Rastafarianism

Rastafarians have a unique interpretation of end times, based on the Old Testament and the Book of Revelation. They believe Ethiopian Emperor Haile Selassie I to be God incarnate, the King of kings and Lord of lords mentioned in Revelation 5:5. They saw the crowning of Selassie as the second coming, and the Second Italo-Ethiopian War as fulfillment of Revelation. There is also the expectation that Selassie will return for a day of judgment and bring home the lost children of Israel, which in Rastafarianism refer to those taken from Africa through the slave trade. There will then be an era of peace and harmony at Mount Zion in Africa.

Zoroastrianism

Zoroastrian eschatology is considered one of the oldest in recorded history. The birth of its founder, Zoroaster, is unknown, with scholarly dates ranging from 6th century BCE to 5,500 years earlier. Pliny the Elder even suggests there were two Zoroasters. However, with beliefs paralleling and possibly predating the framework of the major Abrahamic faiths, a fully developed concept of the end of the world was not established in Zoroastrianism until 500 BCE. The Bahman Yasht describes:

At the end of thy tenth hundredth winter, the sun is more unseen and more spotted; the year, month, and day are shorter; and the earth is more barren; and the crop will not yield the seed. And men become more deceitful and more given to vile practices. They will have no gratitude. Honorable wealth will proceed to those of perverted faith. And a dark cloud makes the whole sky night, and it will rain more noxious creatures than water.

A Manichaean battle between the righteous and wicked will be followed by the Frashokereti. On earth, the Saoshyant will arrive as the final savior of mankind, and bring about the resurrection of the dead. The yazatas Airyaman and Atar will melt the metal in the hills and mountains, which will flow as lava across the earth and all mankind, both the living and resurrected, will be required to wade through it. Ashavan will pass through the molten river as if it were warm milk, but the sinful will burn. It will then flow down to hell, where it will annihilate Angra Mainyu and the last vestiges of wickedness.

The righteous will partake of the parahaoma, which will confer immortality upon them. Humanity will become like the Amesha Spentas, living without food, hunger, thirst, weapons or injury. Bodies will become so light as to cast no shadow. All humanity will speak a single language, and belong to a single nation with no borders. All will share a single purpose and goal, joining with Ahura Mazda for a perpetual and divine exaltation.