Timeline of the Big Bang

Physical cosmology
Universe · Big Bang Age of the universe Timeline of the Big Bang Ultimate fate of the universe
Early universe Inflation · Nucleosynthesis GWB · Neutrino background Cosmic microwave background
Expanding universe Redshift · Hubble's law Metric expansion of space Friedmann equations FLRW metric
Structure formation Shape of the universe Structure formation Reionization Galaxy formation Large-scale structure Galaxy filament
Components Lambda-CDM model Dark energy · Dark matter Dark fluid · Dark flow
Timeline Timeline of cosmological theories Future of an expanding universe
Experiments Observational cosmology 2dF · SDSS COBE · BOOMERanG · WMAP · Planck
Scientists Isaac Newton · Einstein · Hawking · Friedman · Lemaître · Hubble · Penzias · Wilson · Gamow · Dicke · Zel'dovich · Mather · Rubin · Smoot · others
v · d · e

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

This timeline of the Big Bang describes the history of the universe according to the prevailing scientific theory of how the universe came into being, using the cosmological time parameter of comoving coordinates. The instant in which the universe is thought to have begun rapidly expanding from an extremely high energy density is known as the Big Bang.

The best available measurements as of 2011 suggest that the initial conditions occurred about 13.7 billion years ago.^[1][2] It is convenient to divide the evolution of the universe since then into three phases. The very early universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth.

Following this period, in the early universe, the evolution of the universe proceeded in accordance with the tenets of high-energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted.

Matter then continued to aggregate into the first stars and ultimately galaxies, quasars, clusters of galaxies and superclusters formed. There are several theories about the ultimate fate of the universe.

Very early universe

All ideas concerning the very early universe (cosmogony) are speculative. No accelerator experiments have yet probed energies of sufficient magnitude to provide any experimental insight into the behavior of matter at the energy levels that prevailed during this period. Proposed scenarios differ radically. Some examples are the Hartle–Hawking initial state, string landscape, brane inflation, string gas cosmology, and the ekpyrotic universe. Some of these are mutually compatible, while others are not.

Planck epoch

Up to 10^–43 seconds after the Big Bang

Main article: Planck epoch

The Planck epoch is an era in traditional (non-inflationary) big bang cosmology in which the temperature is high enough that the four fundamental forces—electromagnetism, gravitation, weak nuclear interaction, and strong nuclear interaction—are all unified in one fundamental force. Little is understood about physics at this temperature, and different theories propose different scenarios. Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory is based on general relativity and is expected to break down due to quantum effects. Physicists hope that proposed theories of quantum gravitation, such as string theory, loop quantum gravity, and causal sets, will eventually lead to a better understanding of this epoch.^{[citation needed]} In inflationary cosmology, times prior to the end of inflation (roughly 10⁻³² seconds after the Big Bang) do not follow the traditional big bang timeline. The universe before the end of inflation is a near-vacuum with a very low temperature, and persists for much longer than 10⁻³² second. Times from the end of inflation are based on the big bang time of the non-inflationary big bang model, not on the actual age of the universe at that time, which cannot be determined in inflationary cosmology. Thus, in inflationary cosmology there is no Planck epoch in the traditional sense, though similar conditions may have prevailed in a pre-inflationary era of the universe.

Grand unification epoch

Between 10^–43 seconds and 10^–36 seconds after the Big Bang^[3]

Main article: Grand unification epoch

As the universe expands and cools, it crosses transition temperatures at which forces separate from each other. These are phase transitions much like condensation and freezing. The grand unification epoch begins when gravitation separates from the other forces of nature, which are collectively known as gauge forces. The non-gravitational physics in this epoch would be described by a so-called grand unified theory (GUT). The grand unification epoch ends when the GUT forces further separate into the strong and electroweak forces. This transition should produce magnetic monopoles in large quantities, which are not observed. The lack of magnetic monopoles was one problem solved by the introduction of inflation.

In modern inflationary cosmology, the traditional grand unification epoch, like the Planck epoch, does not exist, though similar conditions likely would have existed in the universe prior to inflation.

Electroweak epoch

Between 10^–36 seconds (or the end of inflation) and 10^–12 seconds after the Big Bang^[3]

Main article: Electroweak epoch

In traditional big bang cosmology, the Electroweak epoch begins 10^–36 seconds after the Big Bang, when the temperature of the universe is low enough (10²⁸ K) to separate the strong force from the electroweak force (the name for the unified forces of electromagnetism and the weak interaction). In inflationary cosmology, the electroweak epoch begins when the inflationary epoch ends, at roughly 10^–32 seconds.

Inflationary epoch

Unknown duration, ending 10^–32(?) seconds after the Big Bang

Main article: Inflationary epoch

Cosmic inflation is an era of accelerating expansion produced by a hypothesized field called the inflaton, which would have properties similar to the Higgs field and dark energy. While decelerating expansion magnifies deviations from homogeneity, making the universe more chaotic, accelerating expansion makes the universe more homogeneous. A sufficiently long period of inflationary expansion in our past could explain the high degree of homogeneity that is observed in the universe today at large scales, even if the state of the universe before inflation was highly disordered.

Inflation ends when the inflaton field decays into ordinary particles in a process called "reheating", at which point ordinary Big Bang expansion begins. The time of reheating is usually quoted as a time "after the Big Bang". This refers to the time that would have passed in traditional (non-inflationary) cosmology between the Big Bang singularity and the universe dropping to the same temperature that was produced by reheating, even though, in inflationary cosmology, the traditional Big Bang did not occur.

According to the simplest inflationary models, inflation ended at a temperature corresponding to roughly 10^–32 seconds after the Big Bang. As explained above, this does not imply that the inflationary era lasted less than 10^–32 seconds. In fact, in order to explain the observed homogeneity of the universe, the duration must be longer than 10^–32 seconds, and it can even be infinite (eternal inflation). In inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation.

Baryogenesis

Main article: Baryogenesis

There is currently insufficient observational evidence to explain why the universe contains far more baryons than antibaryons. A candidate explanation for this phenomenon must allow the Sakharov conditions to be satisfied at some time after the end of cosmological inflation. While particle physics suggests asymmetries under which these conditions are met, these asymmetries are too small empirically to account for the observed baryon-antibaryon asymmetry of the universe.

Early universe

Cosmic History

After cosmic inflation ends, the universe is filled with a quark–gluon plasma. From this point onwards the physics of the early universe is better understood, and less speculative.

Supersymmetry breaking

Main article: Supersymmetry breaking

If supersymmetry is a property of our universe, then it must be broken at an energy that is no lower than 1 TeV, the electroweak symmetry scale. The masses of particles and their superpartners would then no longer be equal, which could explain why no superpartners of known particles have ever been observed.

Quark epoch

Between 10^–12 seconds and 10^–6 seconds after the Big Bang

Main article: Quark epoch

In electroweak symmetry breaking, at the end of the electroweak epoch, all the fundamental particles are believed to acquire a mass via the Higgs mechanism in which the Higgs boson acquires a vacuum expectation value. The fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction have now taken their present forms, but the temperature of the universe is still too high to allow quarks to bind together to form hadrons.

Hadron epoch

Between 10^–6 seconds and 1 second after the Big Bang

Main article: Hadron epoch

The quark-gluon plasma that composes the universe cools until hadrons, including baryons such as protons and neutrons, can form. At approximately 1 second after the Big Bang neutrinos decouple and begin traveling freely through space. This cosmic neutrino background, while unlikely to ever be observed in detail, is analogous to the cosmic microwave background that was emitted much later. (See above regarding the quark-gluon plasma, under the String Theory epoch)

Lepton epoch

Between 1 second and 10 seconds after the Big Bang

Main article: Lepton epoch

The majority of hadrons and anti-hadrons annihilate each other at the end of the hadron epoch, leaving leptons and anti-leptons dominating the mass of the universe. Approximately 10 seconds after the Big Bang the temperature of the universe falls to the point at which new lepton/anti-lepton pairs are no longer created and most leptons and anti-leptons are eliminated in annihilation reactions, leaving a small residue of leptons.^[4]

Photon epoch

Between 10 seconds and 380,000 years after the Big Bang

Main article: Photon epoch

After most leptons and anti-leptons are annihilated at the end of the lepton epoch the energy of the universe is dominated by photons. These photons are still interacting frequently with charged protons, electrons and (eventually) nuclei, and continue to do so for the next 380,000 years.

Nucleosynthesis

Between 3 minutes and 20 minutes after the Big Bang^[5]

Main article: Big Bang nucleosynthesis

During the photon epoch the temperature of the universe falls to the point where atomic nuclei can begin to form. Protons (hydrogen ions) and neutrons begin to combine into atomic nuclei in the process of nuclear fusion. Free neutrons combine with protons to form deuterium. Deuterium rapidly fuses into helium-4. Nucleosynthesis only lasts for about seventeen minutes, since the temperature and density of the universe has fallen to the point where nuclear fusion cannot continue. By this time, all neutrons have been incorporated into helium nuclei. This leaves about three times more hydrogen than helium-4 (by mass) and only trace quantities of other nuclei.

Matter domination: 70,000 years

At this time, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) are equal. The Jeans length, which determines the smallest structures that can form (due to competition between gravitational attraction and pressure effects), begins to fall and perturbations, instead of being wiped out by free-streaming radiation, can begin to grow in amplitude.

According to ΛCDM, at this stage, cold dark matter dominates, paving the way for gravitational collapse to amplify the tiny inhomogeneities left by cosmic inflation, making dense regions denser and rarefied regions more rarefied. However, because present theories as to the nature of dark matter are inconclusive, there is as yet no consensus as to its origin at earlier times, as currently exist for baryonic matter.

Recombination: ca 377,000 years

Main article: Recombination (cosmology)

WMAP data shows the microwave background radiation variations throughout the Universe from our perspective, though the actual variations are much smoother than the diagram suggests

Hydrogen and helium atoms begin to form as the density of the universe falls. This is thought to have occurred about 377,000 years after the Big Bang.^[6] Hydrogen and helium are at the beginning ionized, i.e., no electrons are bound to the nuclei, which (containing positively charged protons) are therefore electrically charged (+1 and +2 respectively). As the universe cools down, the electrons get captured by the ions, forming electrically neutral atoms. This process is relatively fast (actually faster for the helium than for the hydrogen) and is known as recombination.^[7] At the end of recombination, most of the protons in the universe are bound up in neutral atoms. Therefore, the photons can now travel freely (see Thomson scattering): the universe has become transparent. This cosmic event is usually referred to as decoupling. The photons present at the time of decoupling can now travel undisturbed (the photons' mean free path becomes effectively infinite) and are the same photons that we see in the cosmic microwave background (CMB) radiation, after being greatly cooled by the expansion of the Universe. Therefore the CMB is a picture of the universe at the end of this epoch including the tiny fluctuations generated during inflation (see diagram).

Dark ages

See also: Hydrogen line

Before decoupling occurs most of the photons in the universe are interacting with electrons and protons in the photon–baryon fluid. The universe is opaque or "foggy" as a result. There is light but not light we could observe through telescopes. The baryonic matter in the universe consisted of ionized plasma, and it only became neutral when it gained free electrons during "recombination," thereby releasing the photons creating the CMB. When the photons were released (or decoupled) the universe became transparent. At this point the only radiation emitted is the 21 cm spin line of neutral hydrogen. There is currently an observational effort underway to detect this faint radiation, as it is in principle an even more powerful tool than the cosmic microwave background for studying the early universe. The Dark Ages are currently thought to have lasted between 150 million to 800 million years after the Big Bang. The recent (October 2010) discovery of UDFy-38135539, the first observed galaxy to have existed during the following reionization epoch, gives us a window into these times. There was a report in January 2011 of yet another more than 13 billion years old that existed a mere 480 million years after the Big Bang.

Structure formation

See also: Large-scale structure of the cosmos and Structure formation

The Hubble Ultra Deep Fields often showcase galaxies from an ancient era that tell us what the early Stelliferous Age was like.

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

Structure formation in the big bang model proceeds hierarchically, with smaller structures forming before larger ones. The first structures to form are quasars, which are thought to be bright, early active galaxies, and population III stars. Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory: that is, all structures could be understood as small deviations from a perfect homogeneous universe. This is computationally relatively easy to study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, N-body simulations with billions of particles.

Reionization: 150 million to 1 billion years

See also: Reionization and 21 centimeter radiation

The first stars and quasars form from gravitational collapse. The intense radiation they emit reionizes the surrounding universe. From this point on, most of the universe is composed of plasma.

Formation of stars

See also: Star formation

The first stars, most likely Population III stars, form and start the process of turning the light elements that were formed in the Big Bang (hydrogen, helium and lithium) into heavier elements. However, as of yet there have been no observed Population III stars, and understanding of them is currently based on computational models of their formation and evolution.^[8]

Formation of galaxies

See also: Galaxy formation and evolution

Large volumes of matter collapse to form a galaxy. Population II stars are formed early on in this process, with Population I stars formed later.

Johannes Schedler's project has identified a quasar CFHQS 1641+3755 at 12.7 billion light-years away,^[9] when the Universe was just 7% of its present age.

On July 11, 2007, using the 10-metre Keck II telescope on Mauna Kea, Richard Ellis of the California Institute of Technology at Pasadena and his team found six star forming galaxies about 13.2 billion light years away and therefore created when the universe was only 500 million years old.^[10] Only about 10 of these extremely early objects are currently known.^[11]

The Hubble Ultra Deep Field shows a number of small galaxies merging to form larger ones, at 13 billion light years, when the Universe was only 5% its current age.^[12]

Based upon the emerging science of nucleocosmochronology, the Galactic thin disk of the Milky Way is estimated to have been formed 8.8 ± 1.7 billion years ago.^[13]

Formation of groups, clusters and superclusters

See also: Large-scale structure of the cosmos

Gravitational attraction pulls galaxies towards each other to form groups, clusters and superclusters.

Formation of our solar system: 8 billion years

Main article: Formation and evolution of the Solar System

See also: Solar system

Finally, objects on the scale of our solar system form. Our sun is a late-generation star, incorporating the debris from several generations of earlier stars, and formed about 4.56 billion years ago, or roughly 8 to 9 billion years after the big bang.

Today: 13.7 billion years

The best current data estimate the age of the universe today as 13.75 ± 0.11 billion years since the big bang. Since the expansion of the universe appears to be accelerating, superclusters are likely to be the largest structures that will ever form in the universe. The present accelerated expansion prevents any more inflationary structures entering the horizon and prevents new gravitationally bound structures from forming.

Ultimate fate of the universe

Main article: Ultimate fate of the universe

As with interpretations of what happened in the very early universe, advances in fundamental physics are required before it will be possible to know the ultimate fate of the universe with any certainty. Below are some of the main possibilities.

Big freeze: 10¹⁴ years and beyond

Main articles: Future of an expanding universe and Heat death of the universe

This scenario is generally considered to be the most likely^{[citation needed]}, as it occurs if the universe continues expanding as it has been. Over a time scale on the order of 10¹⁴ years or less, existing stars burn out, stars cease to be created, and the universe goes dark.^{[14], §IID.} Over a much longer time scale in the eras following this, the galaxy evaporates as the stellar remnants comprising it escape into space, and black holes evaporate via Hawking radiation.^{[14], §III, §IVG.} In some grand unified theories, proton decay after at least 10³⁴ years will convert the remaining interstellar gas and stellar remnants into leptons (such as positrons and electrons) and photons. Some positrons and electrons will then recombine into photons.^{[14], §IV, §VF.} In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known however whether it eventually achieves thermodynamic equilibrium.^{[14], §VIB, VID.}

Big Crunch: 100+ billion years from now

See also: Big Crunch

If the energy density of dark energy were negative or the universe were closed, then it would be possible that the expansion of the universe would reverse and the universe would contract towards a hot, dense state. This is a required element of oscillatory universe scenarios, such as the cyclic model, although a Big Crunch does not necessarily imply an oscillatory Universe. Current observations suggest that this model of the universe is unlikely to be correct, and the expansion will continue or even accelerate.

Big Rip: 20+ billion years from now

See also: Big Rip

This scenario is possible only if the energy density of dark energy actually increases without limit over time^{[citation needed]}. Such dark energy is called phantom energy and is unlike any known kind of energy. In this case, the expansion rate of the universe will increase without limit. Gravitationally bound systems, such as clusters of galaxies, galaxies, and ultimately the solar system will be torn apart. Eventually the expansion will be so rapid as to overcome the electromagnetic forces holding molecules and atoms together. Finally even atomic nuclei will be torn apart and the universe as we know it will end in an unusual kind of gravitational singularity. At the time of this singularity, the expansion rate of the universe will reach infinity, so that any and all forces (no matter how strong) that hold composite objects together (no matter how closely) will be overcome by this expansion, literally tearing everything apart.

Vacuum metastability event

See also: False vacuum

If our universe is in a very long-lived false vacuum, it is possible that a small region of the universe will tunnel into a lower energy state. If this happens, all structures within will be destroyed instantaneously and the region will expand at near light speed, bringing destruction without any forewarning.

Heat death: 10¹⁵⁰+ years from now

See also: Heat death

The heat death is a possible final state of the universe, estimated at after 10¹⁵⁰ years, in which it has "run down" to a state of no thermodynamic free energy to sustain motion or life. In physical terms, it has reached maximum entropy (because of this, the term "entropy" has often been confused with Heat Death, to the point of entropy being labelled as the "force killing the universe"). The hypothesis of a universal heat death stems from the 1850s ideas of William Thomson (Lord Kelvin)^[15] who extrapolated the theory of heat views of mechanical energy loss in nature, as embodied in the first two laws of thermodynamics, to universal operation.

References

1. ^ Komatsu, E.; Dunkley, J.; Nolta, M. R.; Bennett, C. L.; Gold, B.; Hinshaw, G.; Jarosik, N.; Larson, D. et al. (2009). "Five-Year Wilkinson Microwave Anisotropy Probe Observations: Cosmological Interpretation". Astrophysical Journal Supplement 180 (2): 330. Bibcode 2009ApJS..180..330K. doi:10.1088/0067-0049/180/2/330. 
2. ^ Menegoni, Eloisa et al. (2009). "New constraints on variations of the fine structure constant from CMB anisotropies". Physical Review D 80 (8): 087302. arXiv:0909.3584. Bibcode 2009PhRvD..80h7302M. doi:10.1103/PhysRevD.80.087302 
3. ^ ^{a b} Ryden B: "Introduction to Cosmology", pg. 196 Addison-Wesley 2003
4. ^ The Timescale of Creation
5. ^ Detailed timeline of Big Bang nucleosynthesis processes
6. ^ Hinshaw, G.; et al. (2009). "Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results" (PDF). Astrophysical Journal Supplement 180 (2): 225–245. arXiv:0803.0732. Bibcode 2009ApJS..180..225H. doi:10.1088/0067-0049/180/2/225. http://lambda.gsfc.nasa.gov/product/map/dr3/pub_papers/fiveyear/basic_results/wmap5basic.pdf. 
7. ^ Mukhanov, V: "Physical foundations of Cosmology", pg. 120, Cambridge 2005
8. ^ Ferreting Out The First Stars; physorg.com
9. ^ APOD: 2007 September 6 - Time Tunnel
10. ^ "New Scientist" 14th July 2007
11. ^ HET Helps Astronomers Learn Secrets of One of Universe's Most Distant Objects
12. ^ APOD: 2004 March 9 – The Hubble Ultra Deep Field
13. ^ Eduardo F. del Peloso a1a, Licio da Silva a1, Gustavo F. Porto de Mello and Lilia I. Arany-Prado (2005), "The age of the Galactic thin disk from Th/Eu nucleocosmochronology: extended sample" (Proceedings of the International Astronomical Union (2005), 1: 485-486 Cambridge University Press)
14. ^ ^{a b c d} A dying universe: the long-term fate and evolution of astrophysical objects, Fred C. Adams and Gregory Laughlin, Reviews of Modern Physics 69, #2 (April 1997), pp. 337–372. Bibcode: 1997RvMP...69..337A. doi:10.1103/RevModPhys.69.337.
15. ^ Thomson, William. (1851). "On the Dynamical Theory of Heat, with numerical results deduced from Mr Joule’s equivalent of a Thermal Unit, and M. Regnault’s Observations on Steam." Excerpts. [§§1-14 & §§99-100], Transactions of the Royal Society of Edinburgh, March, 1851; and Philosophical Magazine IV. 1852, [from Mathematical and Physical Papers, vol. i, art. XLVIII, pp. 174]

External links

• PBS Online (2000). From the Big Bang to the End of the Universe – The Mysteries of Deep Space Timeline. Retrieved March 24, 2005.
• Schulman, Eric (1997). The History of the Universe in 200 Words or Less. Retrieved March 24, 2005.
• Space Telescope Science Institute Office of Public Outreach (2005). Home of the Hubble Space Telescope. Retrieved March 24, 2005.
• Fermilab graphics (see "Energy time line from the Big Bang to the present" and "History of the Universe Poster")
• Exploring Time from Planck time to the lifespan of the universe
• Astronomers' first detailed hint of what was going on less than a trillionth of a second after time began
• The Universe Adventure
• Cosmology FAQ, Professor Edward L. Wright, UCLA

• Sean Carroll on the arrow of time (Part 1), The origin of the universe and the arrow of time, Sean Carroll, video, CHAST 2009, Templeton, Faculty of science, University of Sydney, November 2009, TED.com
• A Universe From Nothing, video, Lawrence Krauss, AAI 2009, YouTube.com