The Big Bang is a
physical theory that describes how the
universe expanded from an initial state of high
density and
temperature.[1] The notion of an expanding universe was first scientifically originated by
physicistAlexander Friedmann in 1922 with the mathematical derivation of the
Friedmann equations.[2][3][4][5] The earliest empirical observation of the notion of an expanding universe is known as
Hubble's Law, published in work by physicist
Edwin Hubble in 1929, which discerned that galaxies are moving away from Earth at a rate that accelerates proportionally with distance.
Independent of Friedmann's work, and independent of Hubble's observations, physicist
Georges Lemaître proposed that the universe emerged from a "primeval
atom" in 1931, introducing the modern notion of the Big Bang.
Extrapolating this cosmic expansion backward in time using the known
laws of physics, the models describe an increasingly concentrated cosmos preceded by
a singularity in which
space and time lose meaning (typically named "the Big Bang singularity").[9] Physics lacks a widely accepted theory of
quantum gravity that can model the earliest conditions of the Big Bang. In 1964 the CMB was discovered, which convinced many cosmologists that the competing
steady-state model of cosmic evolution was
falsified, since the Big Bang models predict a uniform background radiation caused by high temperatures and densities in the distant past.[10] A wide range of empirical evidence strongly favors the Big Bang event, which is now essentially universally accepted.[11] Detailed measurements of the expansion rate of the
universe place the Big Bang singularity at an estimated 13.787±0.020billion years ago, which is considered the
age of the universe.[12]
There remain aspects of the observed universe that are not yet adequately explained by the Big Bang models. After its initial expansion, the universe cooled sufficiently to allow the formation of
subatomic particles, and later
atoms. The unequal abundances of matter and
antimatter that allowed this to occur is an unexplained effect known as
baryon asymmetry. These primordial elements—mostly
hydrogen, with some
helium and
lithium—later coalesced through
gravity, forming early
stars and galaxies. Astronomers observe the gravitational effects of an unknown
dark matter surrounding galaxies. Most of the
gravitational potential in the universe seems to be in this form, and the Big Bang models and various observations indicate that this excess gravitational potential is not created by
baryonic matter, such as normal atoms. Measurements of the redshifts of
supernovae indicate that the
expansion of the universe is accelerating, an observation attributed to an unexplained phenomenon known as
dark energy.[13]
Features of the models
The Big Bang models offer a comprehensive explanation for a broad range of observed phenomena, including the abundances of the
light elements, the
CMB,
large-scale structure, and
Hubble's law.[14] The models depend on two major assumptions: the universality of physical laws and the
cosmological principle. The universality of physical laws is one of the underlying principles of the
theory of relativity. The cosmological principle states that on large scales the
universe is
homogeneous and
isotropic—appearing the same in all directions regardless of location.[15]
These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that the largest possible deviation of the
fine-structure constant over much of the age of the universe is of order 10−5.[16] Also,
general relativity has passed stringent
tests on the scale of the
Solar System and
binary stars.[17][18][notes 1]
The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, 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 temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound
on the order of 10% inhomogeneity, as of 1995.[19]
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 yet had time to reach earth. 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
Friedmann–Lemaître–Robertson–Walker (FLRW) metric that describes the expansion of the universe.[20]
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.[20]
Thermalization
Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate
thermodynamic equilibrium. Others were fast enough to reach
thermalization. The parameter usually used to find out whether a process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the
Hubble parameter. The larger the ratio, the more time particles had to thermalize before they were too far away from each other.[21]
In the absence of a
perfect cosmological principle, extrapolation of the expansion of the universe backwards in time using general relativity yields an
infinitedensity and
temperature at a finite time in the past.[22] This irregular behavior, known as the
gravitational singularity, indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone cannot fully extrapolate toward the singularity.[9] In some proposals, such as the
emergent Universe models, the singularity is replaced by another cosmological epoch. A different approach identifies the
initial singularity as a
singularity predicted by some models of the Big Bang theory to have existed before the Big Bang event.[23][clarification needed]
This primordial singularity is itself sometimes called "the Big Bang",[24] but the term can also refer to a more generic early hot, dense phase[25][notes 2] 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—known as the "
age of the universe"—is 13.8 billion years.[26]
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 singularity. Commonly used calculations and limits for explaining
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. Since the early universe did not immediately collapse into a multitude of black holes, matter at that time must have been very evenly distributed with a negligible
density gradient.[27]
The earliest phases of the Big Bang are subject to much speculation, given the lack of available data. 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. The period up to 10−43 seconds into the expansion, the
Planck epoch, was a phase in which the four
fundamental forces—the
electromagnetic force, the
strong nuclear force, the
weak nuclear force, and the
gravitational force, were unified as one.[28] In this stage, the
characteristic scale length of the universe was the
Planck length, 1.6×10−35 m, and consequently had a temperature of approximately 1032 degrees Celsius. Even the very concept of a particle breaks down in these conditions. A proper understanding of this period awaits the development of a theory of
quantum gravity.[29][30] The Planck epoch was succeeded by the
grand unification epoch beginning at 10−43 seconds, where gravitation separated from the other forces as the universe's temperature fell.[28]
Inflation stopped locally at around 10−33 to 10−32 seconds, with the observable universe's volume having increased by a factor of at least 1078. Reheating followed as the
inflaton field decayed, until the universe obtained the temperatures required for the
production of a
quark–gluon plasma as well as all other
elementary particles.[33][34] 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.[1] 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.[35]
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, with the electromagnetic force and weak nuclear force separating at about 10−12 seconds.[32][36]
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 no longer high enough to create either new proton–antiproton or neutron–antineutron pairs. A mass
annihilation immediately followed, leaving just one in 108 of the original matter particles and none of their
antiparticles.[37] 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
kelvin and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's
deuterium and
heliumnuclei in a process called
Big Bang nucleosynthesis (BBN).[38] Most protons remained uncombined as hydrogen nuclei.[39]
As the universe cooled, the
rest energy density of matter came to gravitationally dominate that of the photon
radiation. The
recombination epoch began after about 379,000 years, when the electrons and nuclei combined into
atoms (mostly
hydrogen), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background.[39]
After the recombination epoch, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today.[1] 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 (CDM),
warm dark matter,
hot dark matter, and
baryonic matter. The best measurements available, from the
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.)[41] This CDM is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%.[42]
In an "extended model" which includes hot dark matter in the form of neutrinos,[43] then the "physical baryon density" is estimated at 0.023. (This is different from the 'baryon density' expressed as a fraction of the total matter/energy density, which is about 0.046.) The corresponding cold dark matter density is about 0.11, and the corresponding neutrino density is estimated to be less than 0.0062.[42]
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 appears to homogeneously permeate all of space. Observations suggest that 73% of the total energy density of the present day universe is in this form. When the universe was very young it was likely infused with dark energy, but with everything closer together,
gravity predominated, braking the expansion. Eventually, after billions of years of expansion, the declining density of matter relative to the density of dark energy allowed the expansion of the universe to begin to accelerate.[13]
Dark energy in its simplest formulation is modeled by a
cosmological constant term in
Einstein field equations of general relativity, but its composition and mechanism are unknown. 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 theory.[13]
All of this cosmic evolution after the
inflationary epoch can be rigorously described and modeled by the lambda-CDM model of cosmology, which uses the independent frameworks of
quantum mechanics and general relativity. There are no easily testable models that would describe the situation prior to approximately 10−15 seconds.[44] Understanding this earliest of eras in the history of the universe is one of the greatest
unsolved problems in physics.
EnglishastronomerFred Hoyle is credited with coining the term "Big Bang" during a talk for a March 1949
BBC Radio broadcast,[45] 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."[46][47] However, it did not catch on until the 1970s.[47]
It is popularly reported that Hoyle, who favored an alternative "
steady-state" cosmological model, intended this to be pejorative,[48][49][50] but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.[51][52][54]Helge Kragh writes that the evidence for the claim that it was meant as a pejorative is "unconvincing", and mentions a number of indications that it was not a pejorative.[47]
The term itself has been argued to be a misnomer because it evokes an explosion.[47][55] The argument is that whereas an explosion suggests expansion into a surrounding space, the Big Bang only describes the intrinsic expansion of the contents of the universe.[56][57] Another issue pointed out by Santhosh Mathew is that bang implies sound, which is not an important feature of the model.[49] An attempt to find a more suitable alternative was not successful.[47][50]
XDF size compared to the size of the
Moon (XDF is the small box to the left of, and nearly below, 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[59] – 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 models 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.[60][61] Ten years later,
Alexander Friedmann, a
Russiancosmologist and
mathematician, derived the
Friedmann equations from the Einstein field equations, showing that the universe might be expanding in contrast to the
static universe model advocated by
Albert Einstein at that time.[62]
In 1924,
American astronomer
Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, 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
recessional velocity—now known as Hubble's law.[63][64]
Independently deriving Friedmann's equations in 1927,
Georges Lemaître, a
Belgianphysicist and
Roman Catholic priest, proposed that the recession of the nebulae was due to the expansion of the universe.[65] He inferred the relation that Hubble would later observe, given the cosmological principle.[13] 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.[66]
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.[67] This perception was enhanced by the fact that the originator of the Big Bang concept, Lemaître, was a Roman Catholic priest.[68]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.[69][70] Lemaître, however, disagreed:
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.[71]
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.[75] The other was Lemaître's Big Bang theory, advocated and developed by
George Gamow, who introduced BBN[76] and whose associates,
Ralph Alpher and
Robert Herman, predicted the CMB.[77] 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.[52][47][notes 3] 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.[78]
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 relativistic models of the Big Bang.[79][80] 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 models with the introduction of an epoch of rapid expansion in the early universe he called "inflation".[81] Meanwhile, during these decades, two questions in
observational cosmology that generated much discussion and disagreement were over the precise values of the Hubble Constant[82] 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).[83]
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.[84]
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 the
Cosmic Background Explorer (COBE),[85] the
Hubble Space Telescope and WMAP.[86] 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.[87][88]
Observational evidence
"[The] big bang picture is too firmly grounded in data from every area to be proved invalid in its general features."
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 (BBN). More recent evidence includes observations of
galaxy formation and evolution, and the distribution of
large-scale cosmic structures.[90] These are sometimes called the "four pillars" of the Big Bang models.[91]
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 the subject of most active laboratory investigations.[92] Remaining issues include the
cuspy halo problem[93] and the
dwarf galaxy problem[94] 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.[95] Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are unsolved problems in physics.
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 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:[63]
where
is the recessional velocity of the galaxy or other distant object,
Hubble's law implies that the universe is uniformly expanding everywhere. This cosmic expansion was predicted from general relativity by Friedmann in 1922[62] and Lemaître in 1927,[65] well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang model as developed by Friedmann, Lemaître, Robertson, and Walker.
The theory requires the relation to hold at all times, where is the proper distance, is the recessional velocity, and , , and vary as the universe expands (hence we write 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 . For distances comparable to the size of the observable universe, the attribution of the cosmological redshift becomes more ambiguous, although its interpretation as a kinematic Doppler shift remains the most natural one.[96]
An unexplained discrepancy with the determination of the Hubble constant is known as
Hubble tension. Techniques based on observation of the CMB suggest a lower value of this constant compared to the quantity derived from measurements based on the cosmic distance ladder.[97]
In 1964,
Arno Penzias and
Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the
microwave band.[78] 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
blackbody 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 the 1978
Nobel Prize in Physics.
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,[41] the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent.
In 1989,
NASA launched 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.[85]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.[102][103][104]
In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe 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.[86] The Planck space probe was launched in May 2009. Other ground and balloon-based
cosmic microwave background experiments are ongoing.
Using Big Bang models, it is possible to calculate the expected concentration of the isotopes
helium-4 (4He),
helium-3 (3He), deuterium (2H), and
lithium-7 (7Li) in the universe as ratios to the amount of ordinary hydrogen.[38] 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 abundance) are about 0.25 for 4He:H, about 10−3 for 2H:H, about 10−4 for 3He:H, and about 10−9 for 7Li:H.[38]
The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for 4He, and off by a factor of two for 7Li (this anomaly is known as the
cosmological lithium problem); in the latter two cases, there are substantial
systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN 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.[105] Indeed, there is no obvious reason outside of the Big Bang that, for example, the young universe before
star formation, as determined by studying matter supposedly free of
stellar nucleosynthesis products, should have more helium than deuterium or more deuterium than 3He, and in constant ratios, too.[106]: 182–185
Detailed observations of the
morphology and distribution of galaxies and
quasars are in agreement with the current Big Bang models. A combination of observations and theory suggest that the first quasars and galaxies formed within a billion years after the Big Bang,[107] and since then, larger structures have been forming, such as
galaxy clusters and
superclusters.[108]
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.[108][109]
Primordial gas clouds
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. Despite being sensitive to carbon, oxygen, and silicon, these three elements were not detected in these two clouds.[114][115] Since the clouds of gas have no detectable levels of heavy elements, they likely formed in the first few minutes after the Big Bang, during BBN.
Other lines of evidence
The age of the universe as estimated from the Hubble expansion and the CMB is now in 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.[116] It is also in agreement with age estimates based on measurements of the expansion using
Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background.[26] The agreement of independent measurements of this age supports the
Lambda-CDM (ΛCDM) model, since the model is used to relate some of the measurements to an age estimate, and all estimates turn agree. Still, some observations of objects from the relatively early universe (in particular quasar
APM 08279+5255) raise concern as to whether these objects had enough time to form so early in the ΛCDM model.[117][118]
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.[119] 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.[120][121]
As with any theory, a number of mysteries and problems have arisen as a result of the development of the Big Bang models. 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 inflation 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.[124][125][126][127] What follows are a list of the mysterious aspects of the Big Bang concept still under intense investigation by cosmologists and
astrophysicists.
It is not yet understood why the universe has more matter than antimatter.[35] 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 normal matter, rather than antimatter. 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.[128] All these conditions occur in the Standard Model, but the effects are not strong enough to explain the present baryon asymmetry.
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".[13]
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.[13] 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,[129] and the other using the characteristic pattern of the large-scale structure--
baryon acoustic oscillations--as a cosmic ruler.[130][131]
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.[42] 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.[citation needed]
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.[132] 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.[133]
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 clustervelocity dispersions, large-scale structure distributions,
gravitational lensing studies, and
X-ray measurements of galaxy clusters.[134]
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.[135]
Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the
dwarf galaxy problem[94] and the
cuspy halo problem.[93] 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.[136]
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.[137] 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.[106]: 191–202
A resolution to this apparent inconsistency is offered by inflation 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.[31]: 180–186
Heisenberg's uncertainty principle predicts that during the inflationary phase there would be
quantum thermal fluctuations, which would be magnified to a cosmic scale. These fluctuations served as the seeds for all the current structures in the universe.[106]: 207 Inflation predicts that the primordial fluctuations are nearly
scale invariant and
Gaussian, which has been confirmed by measurements of the CMB.[86]: sec 6
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.[138]
Magnetic monopoles
The magnetic monopole objection was raised in the late 1970s.
Grand unified theories (GUTs) 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 resolved by cosmic inflation, which removes all point defects from the observable universe, in the same way that it drives the geometry to flatness.[137]
Flatness problem
The flatness problem (also known as the oldness problem) is an observational problem associated with a FLRW.[137] 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. Observations indicate the universe is consistent with being flat.[139][140]
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,[1] 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.[141]
Misconceptions
One of the common misconceptions about the Big Bang model is that it fully explains the
origin of the universe. However, the Big Bang model does not describe how energy, time, and space were caused, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state.[142] 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.[143]
Another common misconception is that the Big Bang must be understood as the expansion of space and not in terms of the contents of space exploding apart. In fact, either description can be accurate. The expansion of space (implied by the FLRW metric) is only a mathematical convention, corresponding to a choice of
coordinates on spacetime. There is no
generally covariant sense in which space expands.[144]
The recession speeds associated with Hubble's law are not velocities in a relativistic sense (for example, they are not related to the spatial components of
4-velocities). Therefore, it is not remarkable that according to Hubble's law, galaxies farther than the Hubble distance recede faster than the speed of light. Such recession speeds do not correspond to
faster-than-light travel.
Many popular accounts attribute the cosmological redshift to the expansion of space. This can be misleading because the expansion of space is only a coordinate choice. The most natural interpretation of the cosmological redshift is that it is a
Doppler shift.[96]
Implications
Given current understanding, scientific extrapolations about the future of the universe are only possible for finite durations, albeit for much longer periods than the current age of the universe. Anything beyond that becomes increasingly speculative. Likewise, at present, a proper understanding of the origin of the universe can only be subject to conjecture.[145]
Pre–Big Bang cosmology
The Big Bang explains the evolution of the universe from a starting density and temperature that is well beyond humanity's capability to replicate, so extrapolations to the most extreme conditions and earliest times are necessarily more speculative. Lemaître called this initial state the "primeval atom" while Gamow called the material "ylem". How the initial state of the universe originated is still an open question, but the Big Bang model does constrain some of its characteristics. For example, if specific
laws of nature were to come to existence in a random way, inflation models show, some combinations of these are far more probable,[146] partly explaining why our Universe is rather stable. Another possible explanation for the stability of the Universe could be a hypothetical multiverse, which assumes every possible universe to exist, and thinking species could only emerge in those stable enough.[147] A flat universe implies a balance between
gravitational potential energy and other energy forms, requiring no additional energy to be created.[139][140]
The Big Bang theory, built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time, and such an infinite energy density may be a physical impossibility. However, the physical theories of general relativity and quantum mechanics as currently realized are not applicable before the Planck epoch, and correcting this will require the development of a correct treatment of quantum gravity.[22] Certain quantum gravity treatments, such as the
Wheeler–DeWitt equation, imply that time itself could be an
emergent property.[148] As such, physics may conclude that
time did not exist before the Big Bang.[149][150]
While it is not known what could have preceded the hot dense state of the early universe or how and why it originated, or even whether such questions are sensible, speculation abounds on the subject of "
cosmogony".
Some speculative proposals in this regard, each of which entails untested hypotheses, are:
The simplest models, in which the Big Bang was caused by
quantum fluctuations. That scenario had very little chance of happening, but, according to the
totalitarian principle, even the most improbable event will eventually happen. It took place instantly, in our perspective, due to the absence of perceived time before the Big Bang.[151][152][153][154]
Emergent Universe models, which feature a low-activity past-eternal era before the Big Bang, resembling ancient ideas of a
cosmic egg and birth of the world out of
primordial chaos.
Models in which the whole of spacetime is finite, including the
Hartle–Hawking no-boundary condition. For these cases, the Big Bang does represent the limit of time but without a singularity.[155] In such a case, the universe is self-sufficient.[156]
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.[157][158][159][160]
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.[161][162] This is sometimes referred to as pre-big bang inflation.[163]
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.
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.[20]
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. Collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would very gradually asymptotically approach
absolute zero—a
Big Freeze.[164] Moreover, if protons are
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.[165]
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.[166]
As a description of the origin of the universe, the Big Bang has significant bearing on religion and philosophy.[167][168] As a result, it has become one of the liveliest areas in the discourse between
science and religion.[169] Some believe the Big Bang implies a creator,[170][171] while others argue that Big Bang cosmology makes the notion of a creator superfluous.[168][172]
Steady-state model – Model of the universe – alternative to the Big Bang model, a discredited theory that denied the Big Bang and posited that the universe always existed
Notes
^Further information of, and references for, tests of general relativity are given in the article
tests of general relativity.
^There is no consensus about how long the Big Bang phase lasted. For some writers, this denotes only the initial singularity, for others the whole history of the universe. Usually, at least the first few minutes (during which helium is synthesized) are said to occur "during the Big Bang".
^It is commonly reported that Hoyle intended this to be pejorative. However, Hoyle later denied that, saying that it was just a striking image meant to emphasize the difference between the two theories for radio listeners.[51]
^Strictly, dark energy in the form of a cosmological constant drives the universe towards a flat state; however, our universe remained close to flat for several billion years before the dark energy density became significant.
^Singh 2004, p. 560. Book limited to 532 pages. Correct source page requested.
^NASA/WMAP Science Team (6 June 2011).
"Cosmology: The Study of the Universe". Universe 101: Big Bang Theory. Washington, D.C.:
NASA.
Archived from the original on 29 June 2011. Retrieved 18 December 2019. The second section discusses the classic tests of the Big Bang theory that make it so compelling as the most likely valid and accurate description of our universe.
^Ivanchik, Alexandre V.; Potekhin, Alexander Y.; Varshalovich, Dmitry A. (March 1999). "The fine-structure constant: a new observational limit on its cosmological variation and some theoretical consequences". Astronomy & Astrophysics. 343 (2): 439–445.
arXiv:astro-ph/9810166.
Bibcode:
1999A&A...343..439I.
^Schewe, Phillip F.; Stein, Ben P. (20 April 2005).
"An Ocean of Quarks". Physics News Update. Vol. 728, no. 1. Archived from
the original on 23 April 2005. Retrieved 30 November 2019.
^
abTimothy Ferris writes:
"The term "big bang" was coined with derisive intent by Fred Hoyle, and its endurance testifies to Sir Fred's creativity and wit. Indeed, the term survived an international competition in which three judges — the television science reporter Hugh Downs, the astronomer Carl Sagan, and myself — sifted through 13,099 entries from 41 countries and concluded that none was apt enough to replace it. No winner was declared, and like it or not, we are stuck with "big bang.""[58]
^
abMitton 2011, p.
129: "To create a picture in the mind of the listener, Hoyle had likened the explosive theory of the universe's origin to a 'big bang'."
^Hoyle stated:
"I was constantly striving over the radio – where I had no visual aids, nothing except the spoken word – for visual images. And that seemed to be one way of distinguishing between the steady-state and the explosive big bang. And so that was the language I used."[47][53]
^
Emam, Moataz (2021).
Covariant Physics: From Classical Mechanics to General Relativity and Beyond. Oxford University Press. pp. 208–246.
ISBN978-0-19-886489-9. The term "Big Bang" is an unfortunate misnomer. It implies an "explosion," and explosions are events that happen in space. This is incorrect; the term describes the first instant in the expansion of space itself. Some would even interpret it as the very beginning of the universe, evolving from "nothing." It is hard to imagine exactly what it was, but an explosion it most definitely wasn't.
^Reiss, Adam G.; Filippenko, Alexei V.; Challis, Peter; Clocchiatti, Alejandro; Diercks, Alan; Garnavich, Peter M.; Gilliland, Ron L.; Hogan, Craig J.; Jha, Saurabh; Kirshner, Robert P.; Leibundgut, B.; Phillips, M. M.; Reiss, David; Schmidt, Brian P.; Schommer, Robert A.; Smith, R. Chris; Spyromilio, J.; Stubbs, Christopher; Suntzeff, Nicholas B.; Tonry, John (1998). "Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant". The Astronomical Journal. 116 (3): 1009–1038.
arXiv:astro-ph/9805201.
Bibcode:
1998AJ....116.1009R.
doi:
10.1086/300499.
S2CID15640044.
From retired website: Shellard, Paul; et al., eds. (2006).
"The Four Pillars of the Standard Cosmology". Cambridge Relativity and Cosmology. Cambridge, UK: University of Cambridge. Archived from
the original on 28 January 1998. Retrieved 6 December 2019.
^Di Valentino, Eleonora; Mena, Olga; Pan, Supriya; Visinelli, Luca; Yang, Weiqiang; Melchiorri, Alessandro; Mota, David F.; Riess, Adam G.; Silk, Joseph (2021). "In the realm of the Hubble tension—a review of solutions". Classical and Quantum Gravity. 38 (15): 153001.
arXiv:2103.01183.
Bibcode:
2021CQGra..38o3001D.
doi:
10.1088/1361-6382/ac086d.
S2CID232092525.
^Overbye, Dennis (17 March 2014). "Space Ripples Reveal Big Bang's Smoking Gun". Space & Cosmos. The New York Times. New York.
ISSN0362-4331.
Archived from the original on 17 March 2014. Retrieved 11 December 2019. "A version of this article appears in print on March 18, 2014, Section A, Page 1 of the New York edition with the headline: Space Ripples Reveal Big Bang's Smoking Gun." The online version of this article was originally titled "Detection of Waves in Space Buttresses Landmark Theory of Big Bang".
^Overbye, Dennis (24 March 2014). "Ripples From the Big Bang". Out There. The New York Times. New York.
ISSN0362-4331.
Archived from the original on 25 March 2014. Retrieved 24 March 2014. "A version of this article appears in print on March 25, 2014, Section D, Page 1 of the New York edition with the headline: Ripples From the Big Bang."
^Yang, R. J., & Zhang, S. N. (2010). The age problem in the ΛCDM model. Monthly Notices of the Royal Astronomical Society, 407(3), pp. 1835–1841.
^Yu, H., & Wang, F. Y. (2014). Reconciling the cosmic age problem in the $$ R_\mathrm {h}= ct $$ universe. The European Physical Journal C, 74(10), 3090.
^
Alam, Shadab; et al. (April 2021). "Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Cosmological implications from two decades of spectroscopic surveys at the Apache Point Observatory". Physical Review D. 103 (8): 083533.
arXiv:2007.08991.
Bibcode:
2021PhRvD.103h3533A.
doi:
10.1103/PhysRevD.103.083533.
^Tanabashi, M. 2018, pp.
406–413, chpt. 27: "Dark Energy" (Revised September 2017) by David H. Weinberg and Martin White.
^Keel, William C. (October 2009) [Last changes: February 2015].
"Dark Matter". Bill Keel's Lecture Notes – Galaxies and the Universe.
Archived from the original on 3 May 2019. Retrieved 15 December 2019.
^Tanabashi, M. 2018, pp.
396–405, chpt. 26: "Dark Matter" (Revised September 2017) by Manuel Drees and Gilles Gerbier.
Yao, W.-M. 2006, pp.
233–237, chpt. 22: "Dark Matter" (September 2003) by Manuel Drees and Gilles Gerbier.
Block, David L.; Puerari, Ivânio; Stockton, Alan; et al., eds. (2000). Toward a New Millennium in Galaxy Morphology: Proceedings of an International Conference 'Toward a New Millennium in Galaxy Morphology: from z=0 to the Lyman Break, held at the Eskom Conference Centre, Midrand, South Africa, September 13–18, 1999. Dordrecht:
Kluwer Academic Publishers.
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ISBN978-94-010-5801-8.
LCCN00042415.
OCLC851369444. "Reprinted from Astrophysics and Space Science Volumes 269–270, Nos. 1–4, 1999".