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Big Bang
"Big Bang theory" redirects here. For the American TV sitcom, see The Big Bang Theory. For other uses, see Big Bang (disambiguation) and Big Bang Theory (disambiguation).
The Big Bang theory is the prevailingcosmological model for the universe from theearliest known periods through its subsequent large-scale evolution.[1][2][3] The model accounts for the fact that the universeexpanded from a very high density and high temperature state,[4][5] and offers a comprehensive explanation for a broad range of phenomena, including the abundance oflight elements, the cosmic microwave background, large scale structure andHubble's Law.[6] If the known laws of physics are extrapolated to the highest density regime, the result is a singularity which is typically associated with the Big Bang. Detailed measurements of the expansion rate of the universe place this moment at approximately 13.8 billion years ago, which is thus considered the age of the universe.[7]After the initial expansion, the universe cooled sufficiently to allow the formation ofsubatomic particles, and later simple atoms. Giant clouds of these primordial elements later coalesced through gravity in halos ofdark matter, eventually forming the stars andgalaxies visible today.
Timeline of themetric 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 theinflationary epoch, and at the center the expansionaccelerates(artist's concept; not to scale).
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. While the scientific community was once divided between supporters of two different expanding universe theories, the Big Bang and the Steady State theory, empirical evidence provides strong support for the former.[8] In 1929, from analysis of galacticredshifts, Edwin Hubble concluded that galaxies are drifting apart; this is important observational evidence consistent with the hypothesis of an expanding universe. In 1965 the cosmic microwave background radiationwas discovered, which was crucial evidence in favor of the Big Bang model,[9] 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.[10] 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 densityand temperature.[11]
Overview
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Earliest universe (-13.8)
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Milky Way Galaxy
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Axis scale: billions of years.
also see {{Human timeline}} and {{Life timeline}}
A graphical timeline is available at
Graphical timeline of the Big Bang
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.[12] 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 firstelectrically 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 starsor during supernovae.
The Big Bang theory offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background,large scale structure, and Hubble's Law.[6] The framework for the Big Bang model relies onAlbert Einstein's theory of general relativityand on simplifying assumptions such ashomogeneity and isotropy of space. The governing equations were formulated byAlexander 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 parametrizationas 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
Main article: Chronology of the universe
Singularity
See also: Gravitational singularity and Planck epoch
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.[13] This singularityindicates that general relativity is not an adequate description of the laws of physics in this regime. How closely models based on general relativity alone can be used to extrapolate toward the singularity is debated—certainly no closer than the end of the Planck epoch.
This primordial singularity is itself sometimes called "the Big Bang",[14] but the term can also refer to a more generic early hot, dense phase[15][notes 1] 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 aregime 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.[16] The agreement of independent measurements of this age supports the ΛCDM model that describes in detail the characteristics of the universe.
Inflation and baryogenesis
Main articles: Cosmic inflation and baryogenesis
The earliest phases of the Big Bang are subject to much speculation. In the most common models the universe was filledhomogeneously 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 theuncertainty principle were amplified into the seeds that would later form the large-scale structure of the universe.[17] After inflation stopped, reheating occurred until the universe obtained the temperatures required for theproduction of a quark–gluon plasma as well as all other elementary particles.[18]Temperatures were so high that the random motions of particles were at relativisticspeeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions.[4] At some point, an unknown reaction called baryogenesisviolated the conservation of baryon number, leading to a very small excess of quarks andleptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter overantimatter in the present universe.[19]
Cooling
Main articles: Big Bang nucleosynthesis and cosmic microwave background radiation
Panoramic view of the entirenear-infraredsky reveals the distribution of galaxies beyond the Milky Way. Galaxies are color-coded byredshift.
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 thefundamental forces of physics and the parameters of elementary particles into their present form.[20] After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton–antiproton pairs (similarly for neutrons–antineutrons), so a mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons, and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos).
A few minutes into the expansion, when the temperature was about a billion (one thousand million; 109; SI prefix giga-) kelvinand 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.[21]Most protons remained uncombined as hydrogen nuclei. As the universe cooled, therest mass energy density of matter came to gravitationally dominate that of the photonradiation. 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.[22] The chemistry of lifemay 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.[23][24]
Structure formation
Main article: Structure formation
Abell 2744galaxy cluster-Hubble Frontier Fields view.[25]
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.[4]The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter, warm dark matter, hot dark matter, and baryonic matter. The best measurements available (from WMAP) show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be cold (warm dark matter is ruled out by earlyreionization),[26] and is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%.[27] In an "extended model" which includes hot dark matter in the form ofneutrinos, then if the "physical baryon density" Ωbh2 is estimated at about 0.023 (this is different from the 'baryon density' Ωbexpressed as a fraction of the total matter/energy density, which as noted above is about 0.046), and the corresponding cold dark matter density Ωch2 is about 0.11, the corresponding neutrino density Ωvh2 is estimated to be less than 0.0062.[27]
Cosmic acceleration
Main article: Accelerating universe
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 theexpansion of the universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of thecosmological 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.[10]
All of this cosmic evolution after theinflationary 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 greatestunsolved problems in physics.
Features of the model
The Big Bang theory depends on two major assumptions: the universality of physical lawsand the cosmological principle. The cosmological principle states that on large scales the universe is homogeneous andisotropic.
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 universeis of order 10−5.[28] Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars.[notes 2]
If the large-scale universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simplerCopernican 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 10−5via observations of the CMB. The universe has been measured to be homogeneous on the largest scales at the 10% level.[29]
Expansion of space
Main articles: Friedmann–Lemaître–Robertson–Walker metric and Metric expansion of space
General relativity describes spacetime by ametric, which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, themselves are specified using a coordinate chart or "grid" that is laid down over allspacetime. The cosmological principle implies that the metric should behomogeneous 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 theircoordinate distance (comoving distance) remains constant, the physical distance between two such comoving points expands proportionally with the scale factor of the universe.[30]
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 explosionin space, but rather an expansion of space.[4]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.[31]
Horizons
Main article: Cosmological horizon
An important feature of the Big Bang spacetime is the presence of horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe. Our understanding of the universe back to very early times suggeststhat 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.[32]
History
Observational evidence
Problems and related issues in physics
See also: List of unsolved problems 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, thehorizon 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.[94][95][96][97] What follows are a list of the mysterious aspects of the Big Bang theory still under intense investigation by cosmologists and astrophysicists.
Baryon asymmetry
Main article: Baryon asymmetry
It is not yet understood why the universe has more matter than antimatter.[98] 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.[99] All these conditions occur in the Standard Model, but the effects are not strong enough to explain the present baryon asymmetry.
Dark energy
Main article: Dark energy
Measurements of the redshift–magnituderelation for type Ia supernovae indicate that the expansion of the universe has beenaccelerating 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".[10] 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.[10] 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 ofgravitational 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.[27] 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 ofquintessence or other modified gravity schemes.[100] 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.[101]
Dark matter
Main article: Dark matter
Chartshows the proportion of different components of the universe – about 95% isdark matteranddark energy.
During the 1970s and 80s, 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 normalbaryonic 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 lensingstudies, and X-ray measurements of galaxy clusters.[102]
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 physicscandidates for dark matter have been proposed, and several projects to detect them directly are underway.[103]
Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the dwarf galaxy problem[104] and the cuspy halo problem.[105] 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, but no alternative theory as been as successful as the cold dark matter proposal in explaining all extant observations.[106]
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 causalcontact.[107] The observed isotropy of theCMB 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.[79]:191–202
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.[17]:180–186
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.[79]:207 Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB.[108]:sec 6
If inflation occurred, exponential expansion would push large regions of space well beyond our observable horizon.[17]:180–186
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 epochended.[109]
Magnetic monopoles
The magnetic monopole objection was raised in the late 1970s. Grand unified theoriespredicted 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 bycosmic inflation, which removes all point defects from the observable universe, in the same way that it drives the geometry to flatness.[107]
Flatness problem
The overallgeometry of the universeis determined by whether theOmega cosmological parameteris less than, equal to or greater than 1. Shown from top to bottom are aclosed universewith positive curvature, ahyperbolic universewith negative curvature and aflat universewith zero curvature.
The flatness problem (also known as the oldness problem) is an observational problem associated with a Friedmann–Lemaître–Robertson–Walker metric.[107] The universe may have positive, negative, or zero spatialcurvature 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.[notes 4] Given that a natural timescale for departure from flatness might be the Planck time, 10−43 seconds,[4] 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 universe density must have been within one part in 1014 of its critical value, or it would not exist as it does today.[110]
Ultimate fate of the universe
Main article: Ultimate fate of the universe
Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass densityof 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.[32] 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.[111] Moreover, if the proton wereunstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. Theentropy 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.[112]:sec VI.D
Modern observations of accelerating expansion imply that more and more of the currently visible universe will pass beyond ourevent 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, calledphantom 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.[113]
Speculations
Main article: Cosmogony
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 asingularity at the origin of cosmic time; thisinfinite 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 thePlanck temperature, and this conclusion depends on various assumptions, of which some could never be experimentally verified.(Also see Planck epoch.)
One proposed refinement to avoid this would-be singularity is to develop a correct treatment of quantum gravity.[114]
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.[115]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.[116]Brane cosmology models, in which inflation is due to the movement of branes in string theory; the pre-Big Bang model; the ekpyroticmodel, 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.[117][118][119][120]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.[121][122]
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
Main article: Religious interpretations of the Big Bang theory
As a description of the origin of the universe, the Big Bang has significant bearing on religion and philosophy.[123][124] As a result, it has become one of the liveliest areas in the discourse between science and religion.[125]Some believe the Big Bang implies a creator,[126][127] and some see its mention in their holy books,[128] while others argue that Big Bang cosmology makes the notion of a creator superfluous.[124][129]
See also
Notes
References
Further reading
External links
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