(Translated in: )
In 1931
Lemaître went further and suggested that the evident expansion in forward time required that the Universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the Universe into a single point, a "primeval
atom", at a point in time before which time and space did not exist. As such, at this point, the fabric of time and space had not yet come into existence.
Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the
cosmic distance ladder, using the Hooker telescope at
Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose
redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recession velocity—now known as
Hubble's law.
had already shown that this was expected, given the
Cosmological Principle.
satellite gathering data to help scientists understand the Big Bang
During the 1930s other ideas were proposed as
non-standard cosmologies to explain Hubble's observations, including the
Milne model, the
oscillatory Universe (originally suggested by Friedmann, but advocated by
Albert Einstein and
Richard Tolman) and
Fritz Zwicky's
tired light hypothesis.
After
World War II, two distinct possibilities emerged. One was
Fred Hoyle's
steady state model, whereby new matter would be created as the Universe seemed to expand. In this model, the Universe is roughly the same at any point in time. The other was
Lemaître's Big Bang theory, advocated and developed by
George Gamow, who introduced
big bang nucleosynthesis (BBN) and whose associates,
Ralph Alpher and
Robert Herman, predicted the
cosmic microwave background radiation (CMB). Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it derisively as "this
big bang idea" during a
BBC Radio broadcast in March 1949.
[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. See chapter 9 of The Alchemy of the Heavens by Ken Croswell, Anchor Books, 1995.]For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio
source counts, began to favor the latter. The discovery and confirmation of the cosmic microwave background radiation in 1964
secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the Universe at earlier and earlier times, and reconciling observations with the basic theory.
Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances in
telescope technology as well as the analysis of copious data from satellites such as
COBE,
the
Hubble Space Telescope and
WMAP.
Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the Universe appears to be accelerating.
Overview
Timeline of the Big Bang
Extrapolation of the expansion of the Universe backwards in time using
general relativity yields an infinite
density and
temperature at a finite time in the past. This
singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the
Planck epoch. The early hot, dense phase is itself referred to as "the Big Bang",
[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".] and is considered the "birth" of our Universe. Based on measurements of the expansion using
Type Ia supernovae, measurements of temperature fluctuations in the
cosmic microwave background, and measurements of the
correlation function of galaxies, the Universe has a calculated age of 13.73 ± 0.12 billion years.
The agreement of these three independent measurements strongly supports the
ΛCDM model that describes in detail the contents of the Universe.
The earliest phases of the Big Bang are subject to much speculation. In the most common models, the Universe was filled
homogeneously and
isotropically with an incredibly high
energy density, 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.
After inflation stopped, the Universe consisted of a
quark–gluon plasma, as well as all other
elementary particles. Temperatures were so high that the random motions of particles were at
relativistic speeds, and
particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point an unknown reaction called
baryogenesis violated the conservation of
baryon number, leading to a very small excess of
quarks and
leptons over antiquarks and antileptons—of the order of 1 part in 30 million. This resulted in the predominance of
matter over
antimatter in the present Universe.
[Kolb and Turner (1988), chapter 6]The Universe continued to grow in size 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.
[Kolb and Turner (1988), chapter 7] 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 10
10 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; 10
9; SI prefix
giga-)
kelvins and the density was about that of air, neutrons combined with protons to form the Universe's
deuterium and
helium nuclei in a process called
Big Bang nucleosynthesis.
Most protons remained uncombined as
hydrogen nuclei. As the Universe cooled, the
rest mass energy density of matter came to
gravitationally dominate that of the photon
radiation. After about 379,000 years the electrons and nuclei combined into atoms (mostly
hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the
cosmic microwave background radiation.
[Peacock (1999), chapter 9]thumb|left|The Hubble Ultra Deep Field showcases galaxies from an ancient era when the Universe was younger, denser, and warmer according to the Big Bang theory.
Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds,
stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the Universe. The three possible types of matter are known as
cold dark matter,
hot dark matter and
baryonic matter. The best measurements available (from
WMAP) show that the dominant form of matter in the Universe is cold dark matter. The other two types of matter make up less than 18% of the matter in the Universe.
Independent lines of evidence from
Type Ia supernovae and the
CMB imply the Universe today is dominated by a mysterious form of energy known as
dark energy, which apparently permeates all of space. The observations suggest 72% 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 had the upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the
expansion of the Universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the
cosmological constant term in
Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its
equation of state and relationship with the
Standard Model of particle physics continue to be investigated both observationally and theoretically.
All of this cosmic evolution after the
inflationary epoch can be rigorously described and modeled by the
ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity. As noted above, there is no well-supported model describing the action prior to 10
−15 seconds or so. Apparently a new unified theory of
quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the Universe is currently one of the greatest
unsolved problems in physics.
Underlying assumptions
The Big Bang theory depends on two major assumptions: the universality of
physical laws, and the
Cosmological Principle. The cosmological principle states that on large scales the Universe is
homogeneous and
isotropic.
These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the
fine structure constant over much of the
age of the Universe is of order 10
−5. Also,
General Relativity has passed stringent
tests on the scale of the solar system and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.
[Detailed information of and references for tests of general relativity are given at Tests of general relativity.]If the large-scale Universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler
Copernican Principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10
−5 via observations of the CMB.
[This ignores the dipole anisotropy at a level of 0.1% due to the peculiar velocity of the solar system through the radiation field.] The Universe has been measured to be homogeneous on the largest scales at the 10% level.
FLRW metric
General relativity describes spacetime by a
metric, which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, themselves are specified using a
coordinate chart or "grid" that is laid down over all
spacetime. The cosmological principle implies that the metric should be
homogeneous and
isotropic on large scales, which uniquely singles out the
Friedmann–Lemaître–Robertson–Walker metric (FLRW metric). This metric contains a
scale factor, which describes how the size of the Universe changes with time. This enables a convenient choice of a
coordinate system to be made, called
comoving coordinates. In this coordinate system, the grid expands along with the Universe, and objects that are moving only due to the expansion of the Universe remain at fixed points on the grid. While their
coordinate distance (
comoving distance) remains constant, the
physical distance between two such comoving points expands proportionally with the
scale factor of the Universe.
The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead,
space itself expands with time everywhere and increases the physical distance between two comoving points. Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our Universe only on large scales—local concentrations of matter such as our galaxy are gravitationally bound and as such do not experience the large-scale expansion of space.
Horizons
An important feature of the Big Bang spacetime is the presence of
horizons. Since the Universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a
past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a
future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our Universe. Our understanding of the Universe back to very early times
suggests that there is a past horizon, though in practice our view is also limited by the opacity of the Universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the Universe continues to
accelerate, there is a future horizon as well.
[Kolb and Turner (1988), chapter 3]Observational evidence
The earliest and most direct kinds of observational evidence are the
Hubble-type expansion seen in the
redshifts of galaxies, the detailed measurements of the
cosmic microwave background, the abundance of light elements (see
Big Bang nucleosynthesis), and today also the
large scale distribution and apparent
evolution of galaxies which are predicted to occur due to gravitational growth of structure in the standard theory. These are sometimes called
"the four pillars of the Big Bang theory".
Hubble's law and the expansion of space
Observations of distant galaxies and
quasars show that these objects are
redshifted—the
light emitted from them has been shifted to longer wavelengths. This can be seen by taking a
frequency spectrum of an object and matching the
spectroscopic pattern of
emission lines or
absorption lines corresponding to
atoms of the
chemical elements interacting with the light. These redshifts are
uniformly isotropic, distributed evenly among the observed objects in all directions. If the
redshift is interpreted as a
Doppler shift, the recessional
velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the
cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as
Hubble's law is observed:
v = H_0 D \,
where
:
Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable given the
Copernican Principle—or the Universe is
uniformly expanding everywhere. This universal expansion was predicted from
general relativity by
Alexander Friedman in 1922
and
Georges Lemaître in 1927,
well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by
Friedmann, Lemaître, Robertson and Walker.
The theory requires the relation
to hold at all times, where
is the
proper distance,
is the
recessional velocity, and
,
, and
varying 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
. However, the redshift is not a true Doppler shift, but rather the result of the expansion of the Universe between the time the light was emitted and the time that it was detected.
[Peacock (1999), chapter 3]That
space is undergoing metric expansion is shown by direct observational evidence of the
Cosmological Principle and the Copernican Principle, which together with Hubble's law have no other explanation. Astronomical
redshifts are extremely
isotropic and
homogenous,
supporting the Cosmological Principle that the Universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.
Measurements of the effects of the
cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the
Copernican Principle, that the Earth is not in a central position, on a cosmological scale.
[Astronomers reported their measurement in a paper published in the December 2000 issue of Nature titled which can be read . A from the European Southern Observatory explains the findings to the public.] Radiation from the Big Bang was demonstrably warmer at earlier times throughout the Universe. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the Universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.
Cosmic microwave background radiation
thumb|WMAP image of the cosmic microwave background radiation
During the first few days of the Universe, the Universe was in full
thermal equilibrium, with photons being continually emitted and absorbed, giving the radiation a
blackbody spectrum. As the Universe expanded, it cooled to a temperature at which photons could no longer be created or destroyed. The temperature was still high enough for electrons and nuclei to remain unbound, however, and photons were constantly "reflected" from these free electrons through a process called
Thomson scattering. Because of this repeated scattering, the early Universe was opaque to light.
When the temperature fell to a few thousand
Kelvin, electrons and nuclei began to combine to form atoms, a process known as
recombination. Since photons scatter infrequently from neutral atoms, radiation decoupled from matter when nearly all the electrons had recombined, at the
epoch of last scattering, 379,000 years after the Big Bang. These photons make up the CMB that is observed today, and the observed pattern of fluctuations in the CMB is a direct picture of the Universe at this early epoch. The energy of photons was subsequently redshifted by the expansion of the Universe, which preserved the blackbody spectrum but caused its temperature to fall, meaning that the photons now fall into the
microwave region of the
electromagnetic spectrum. The radiation is thought to be observable at every point in the Universe, and comes from all directions with (almost) the same intensity.
In 1964,
Arno Penzias and
Robert Wilson accidentally discovered the cosmic background radiation while conducting diagnostic observations using a new
microwave receiver owned by
Bell Laboratories.
Their discovery provided substantial confirmation of the general CMB predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K—and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded a
Nobel Prize for their discovery.
In 1989,
NASA launched the
Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the Big Bang's predictions regarding the CMB. COBE found a residual temperature of 2.726 K and in 1992 detected for the first time the fluctuations (anisotropies) in the CMB, at a level of about one part in 10
5.
and
George Smoot were awarded Nobels for their leadership in this work. 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 Universe to be almost spatially flat by measuring the typical angular size (the size on the sky) of the anisotropies. (See
shape of the Universe.)
In early 2003, the first results of the
Wilkinson Microwave Anisotropy satellite (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. This satellite also disproved several specific
cosmic inflation models, but the results were consistent with the inflation theory in general,
it confirms too that a sea of
cosmic neutrinos permeates the Universe, a clear evidence that the first stars took more than a half-billion years to create a cosmic fog. Another satellite like it, scheduled for launch in April 2009, the
Planck Surveyor, will provide even more accurate measurements of the CMB anisotropies. Many other ground- and balloon-based experiments are also currently running; see
Cosmic microwave background experiments.
The background radiation is exceptionally smooth, which presented a problem in that conventional expansion would mean that photons coming from opposite directions in the sky were coming from regions that had never been in contact with each other. The leading explanation for this far reaching equilibrium is that the Universe had a brief period of rapid exponential expansion, called
inflation. This would have the effect of driving apart regions that had been in
equilibrium, so that all the observable Universe was from the same equilibrated region.
Abundance of primordial elements
Using the Big Bang model it is possible to calculate the concentration of
helium-4,
helium-3,
deuterium and
lithium-7 in the Universe as ratios to the amount of ordinary hydrogen, H.
All the abundances depend on a single parameter, the ratio of
photons to
baryons, which itself can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) 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.
[Kolb and Turner (1988), chapter 4]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 a factor of two off for
7Li; 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. Indeed there is no obvious reason outside of the Big Bang that, for example, the young Universe (i.e., before star formation, as determined by studying matter supposedly free of
stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than ³He, and in constant ratios, too.
Galactic evolution and distribution
right|thumb|This panoramic view of the entire [[near-infrared sky reveals the distribution of galaxies beyond the
Milky Way. The galaxies are color coded by
redshift.]]
Detailed observations of the
morphology and
distribution of galaxies and
quasars provide strong evidence for the Big Bang. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as
galaxy clusters and
superclusters. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early Universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of
star formation, galaxy and quasar distributions and larger structures agree well with Big Bang simulations of the formation of structure in the Universe and are helping to complete details of the theory.
Other lines of evidence
After some controversy, the age of Universe as estimated from the Hubble expansion and the CMB is now in good agreement with (i.e., slightly larger than) the ages of the oldest stars, both as measured by applying the theory of
stellar evolution to
globular clusters and through
radiometric dating of individual
Population II stars.
The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of temperature-sensitive emission lines in gas clouds at high redshift. This prediction also implies that the amplitude of the
Sunyaev–Zel'dovich effect in
clusters of galaxies does not depend directly on redshift; this seems to be roughly true, but unfortunately the amplitude does depend on cluster properties which do change substantially over cosmic time, so a precise test is impossible.
Features, issues and problems
While scientists now prefer the Big Bang model over other cosmological models, the scientific community was once divided between supporters of the Big Bang and those of
alternative cosmological models. Throughout the historical development of the subject, problems with the Big Bang theory were posed in the context of a scientific controversy regarding which model could best describe the
cosmological observations (see the
history section above). With the overwhelming
consensus in the community today supporting the Big Bang model, many of these problems are remembered as being mainly of historical interest; the solutions to them have been obtained either through modifications to the theory or as the result of better observations.
The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, the formation of galaxies—are derived from many observations that are independent from any cosmological model; these include the
abundance of light elements, the
cosmic microwave background,
large scale structure, and the
Hubble diagram for
Type Ia supernovae.
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 to the most active laboratory investigations. Remaining issues, such as the
cuspy halo problem and the
dwarf galaxy problem of
cold dark matter, are not fatal to the dark matter explanation as solutions to such problems exist which involve only further refinements of the theory.
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.
On the other hand,
inflation and
baryogenesis remain somewhat more speculative features of current Big Bang models: they explain important features of the early universe, but could be replaced by alternative ideas without affecting the rest of the theory.
[If inflation is true, baryogenesis must have occurred, but not vice versa.] Discovering the correct explanations for such phenomena are some of the remaining
unsolved problems in physics.
Horizon problem
The horizon problem results from the premise that information cannot travel
faster than light. In a Universe of finite age, this sets a limit—the
particle horizon—on the separation of any two regions of space that are in
causal contact.
[Kolb and Turner (1988), chapter 8] The observed isotropy of the CMB is problematic in this regard: if the Universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature.
A resolution to this apparent inconsistency is offered by
inflationary theory in which a homogeneous and isotropic scalar energy field dominates the Universe at some very early period (before baryogenesis). During inflation, the Universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable Universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.
Heisenberg's uncertainty principle predicts that during the inflationary phase there would be
quantum thermal fluctuations, which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the Universe. Inflation predicts that the
primordial fluctuations are nearly
scale invariant and
Gaussian, which has been accurately confirmed by measurements of the CMB.
If inflation occurred, exponential expansion would push large regions of space well beyond our observable horizon.
Flatness/oldness problem
thumb|275px|The overall [[Shape of the Universe|geometry of the Universe is determined by whether the
Omega cosmological parameter is less than, equal to or greater than 1. Shown from top to bottom are a
closed Universe with positive curvature, a
hyperbolic Universe with negative curvature and a
flat Universe with zero curvature.]]
The flatness problem (also known as the oldness problem) is an observational problem associated with a
Friedmann–Lemaître–Robertson–Walker metric.
The Universe may have positive, negative or zero spatial
curvature depending on its total energy density. Curvature is negative if its density is less than the
critical density, positive if greater, and zero at the critical density, in which case space is said to be
flat. The problem is that any small departure from the critical density grows with time, and yet the Universe today remains very close to flat.
[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.] Given that a natural timescale for departure from flatness might be the
Planck time, 10
−43 seconds, the fact that the Universe has reached neither a
Heat Death nor a
Big Crunch after billions of years requires some 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 10
14 of its critical value, or it would not exist as it does today.
A resolution to this problem is offered by
inflationary theory. During the inflationary period, spacetime expanded to such an extent that its
curvature would have been smoothed out. Thus, it is theorized that inflation drove the Universe to a very nearly spatially flat state, with almost exactly the critical density.
Magnetic monopoles
The magnetic monopole objection was raised in the late 1970s.
Grand unification theories predicted
topological defects in space that would manifest as
magnetic monopoles. These objects would be produced efficiently in the hot early Universe, resulting in a density much higher than is consistent with observations, given that searches have never found any monopoles. This problem is also resolved by
cosmic inflation, which removes all point defects from the observable Universe in the same way that it drives the geometry to flatness.
[Kolb and Turner, chapter 8]A resolution to the horizon, flatness, and magnetic monopole problems alternative to cosmic inflation is offered by the
Weyl curvature hypothesis.
Baryon asymmetry
It is not yet understood why the Universe has more
matter than
antimatter.
[Kolb and Turner, chapter 6] 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. An unknown process called "
baryogenesis" created 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.
(Translated in Journal of Experimental and Theoretical Physics Letters 5, 24 (1967).)
All these conditions occur in the
Standard Model, but the effect is not strong enough to explain the present baryon asymmetry.
Globular cluster age
In the mid-1990s, observations of
globular clusters appeared to be inconsistent with the Big Bang. Computer simulations that matched the observations of the
stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13.7 billion year age of the Universe. This issue was generally resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to
stellar winds, indicated a much younger age for globular clusters. There still remain some questions as to how accurately the ages of the clusters are measured, but it is clear that these objects are some of the oldest in the Universe.
Dark matter
thumb|right|375px|A [[pie chart indicating the proportional composition of different energy-density components of the Universe, according to the best
ΛCDM model fits roughly 95% is in the exotic forms of dark matter and
dark energy]]
During the 1970s and 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 was initially controversial, it is now indicated by numerous observations: the anisotropies in the CMB,
galaxy cluster velocity dispersions, large-scale structure distributions,
gravitational lensing studies, and
X-ray measurements of galaxy clusters.
The evidence for dark matter comes from its gravitational influence on other matter, and 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.
[ .]Dark energy
Measurements of the
redshift–
magnitude relation for
type Ia supernovae have revealed that the expansion of the Universe has been
accelerating since the Universe was about half its present age. To explain this acceleration,
general relativity requires that much of the energy in the Universe consists of a component with large
negative pressure, dubbed "
dark energy". Dark energy is indicated by several other lines of evidence. Measurements of the
cosmic microwave background indicate that the Universe is very nearly spatially flat, and therefore according to general relativity the Universe must have almost exactly the
critical density of mass/energy. But the
mass density of the Universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density.
Since dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy is also required by two geometrical measures of the overall curvature of the Universe, one using the frequency of
gravitational lenses, and the other using the characteristic pattern of the
large-scale structure as a cosmic ruler.
Negative pressure is a property of
vacuum energy, but the exact nature of dark energy remains one of the great mysteries of the Big Bang. Possible candidates include a
cosmological constant and
quintessence. Results from the WMAP team in 2008, which combined data from the CMB and other sources, indicate that the Universe today is 72% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos.
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.
In the
ΛCDM, the best current model of the Big Bang, dark energy is explained by the presence of a
cosmological constant in the
general theory of relativity. However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about
quantum gravity. Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research.
The future according to the Big Bang theory
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 that was similar to that in which it started—a
Big Crunch.
[Kolb and Turner, 1988, chapter 3] 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 as all the interstellar gas in each galaxy is consumed; stars would burn out leaving
white dwarfs,
neutron stars, and
black holes. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the Universe would asymptotically approach
absolute zero—a
Big Freeze. Moreover, if the proton were
unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting
Hawking radiation. The
entropy of the Universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as
heat death.
Modern observations of
accelerated 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, would remain together, and they too would be subject to
heat death, as the Universe expands and cools. Other explanations of dark energy—so-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.
Speculative physics beyond Big Bang theory
right|300px|thumb|This is an artist's concept of the Universe expansion, where space (including hypothetical non-observable portions of the Universe) is represented at each time by the circular sections. Note on the left the dramatic expansion (not to scale) occurring in the inflationary epoch, and at the center the expansion acceleration. The scheme is decorated with WMAP images on the left and with the representation of stars at the appropriate level of development.
Image from WMAP press release, 2006
While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the Universe's history. The
Penrose-Hawking singularity theorems require the existence of a
singularity at the beginning of cosmic time. However, these theorems assume that
general relativity is correct, but general relativity must break down before the Universe reaches the
Planck temperature, and a correct treatment of
quantum gravity may avoid the singularity.
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 the need for a singularity.
- 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.
- chaotic inflation, in which inflation events start here and there in a random quantum gravity foam, each leading to a bubble universe expanding from its own big bang.
Proposals in the last two categories see the Big Bang as an event in a much larger and older Universe, or
multiverse, and not the literal beginning.
Religious interpretations
The Big Bang is a scientific theory, and as such is dependent on its agreement with observations. But as a theory which addresses the origins of reality, it has always carried theological and philosophical implications. In the 1920s and 1930s almost every major cosmologist preferred an eternal
steady state Universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the
steady state theory. This perception was enhanced by the fact that the originator of the Big Bang theory, Monsignor
Georges Lemaître, was a
Roman Catholic priest.
Since the acceptance of the Big Bang as the dominant physical cosmological paradigm, there have been a variety of reactions by religious groups as to its
implications for their respective
religious cosmologies. Some accept the scientific evidence at face value, while others seek to reconcile the Big Bang with their religious tenets, and others completely reject or ignore the evidence for the Big Bang.
