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Big Bang

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   According to the Big Bang, the universe emerged from an extremely dense
   and hot state (bottom). Since then, space itself has expanded with the
   passage of time, carrying the galaxies with it.
   Enlarge
   According to the Big Bang, the universe emerged from an extremely dense
   and hot state (bottom). Since then, space itself has expanded with the
   passage of time, carrying the galaxies with it.

   In physical cosmology, the Big Bang is the scientific theory that the
   universe emerged from a tremendously dense and hot state about 13.7
   billion years ago. The theory is based on the observations indicating
   the expansion of space (in accord with the Robertson-Walker model of
   general relativity) as indicated by the Hubble redshift of distant
   galaxies taken together with the cosmological principle.

   Extrapolated into the past, these observations show that the universe
   has expanded from a state in which all the matter and energy in the
   universe was at an immense temperature and density. Physicists do not
   widely agree on what happened before this, although general relativity
   predicts a gravitational singularity (for reporting on some of the more
   notable speculation on this issue, see cosmogony).

   The term Big Bang is used both in a narrow sense to refer to a point in
   time when the observed expansion of the universe (Hubble's law) began —
   calculated to be 13.7 billion ( 1.37 × 10^10) years ago (±2%) — and in
   a more general sense to refer to the prevailing cosmological paradigm
   explaining the origin and expansion of the universe, as well as the
   composition of primordial matter through nucleosynthesis as predicted
   by the Alpher-Bethe-Gamow theory.

   From this model, George Gamow in 1948 was able to predict, at least
   qualitatively, the existence of cosmic microwave background radiation
   (CMB). The CMB was discovered in 1964 and further corroborated the Big
   Bang theory, giving it an additional advantage over its chief rival,
   the steady state theory.
          Physical cosmology

     * Age of the universe
     * Big Bang
     * Blueshift
     * Comoving distance
     * Cosmic microwave background
     * Dark energy
     * Dark matter
     * FLRW metric
     * Friedmann equations
     * Galaxy formation
     * Hubble's law
     * Inflation
     * Large-scale structure
     * Lambda-CDM model
     * Metric expansion of space
     * Nucleosynthesis
     * Observable universe
     * Redshift
     * Shape of the universe
     * Structure formation
     * Timeline of the Big Bang
     * Timeline of cosmology
     * Ultimate fate of the universe
     * Universe

            Related topics
     * Astrophysics
     * General relativity
     * Particle physics
     * Quantum gravity


History

   The Big Bang theory developed from observations of the structure of the
   universe and from theoretical considerations. Observers determined that
   most "spiral nebulae" were receding from Earth; but the observers
   themselves were unaware of the cosmological implications of this fact,
   or that the supposed nebulae were actually galaxies outside our own
   Milky Way. Georges Lemaître, a Belgian Roman Catholic priest,
   independently derived the Friedmann-Lemaître-Robertson-Walker equations
   from Albert Einstein's equations of general relativity in 1927 and
   proposed, on the basis of the recession of spiral nebulae, that the
   universe began as a simple "primeval atom"—what was later called the
   Big Bang.

   Soon after, in 1929, Edwin Hubble provided an observational basis for
   Lemaître's theory. He discovered that, seen from Earth, light from
   other galaxies is redshifted proportionally to their distance from
   Earth. This fact is now known as Hubble's law. Given the cosmological
   principle whereby the universe, when viewed on sufficiently large
   distance scales, has no preferred directions or preferred places,
   Hubble's law implied that the universe was expanding, contradicting the
   infinite and unchanging static universe scenario developed by Einstein.
   Artist's depiction of the WMAP satellite gathering data to help
   scientists understand the Big Bang.
   Enlarge
   Artist's depiction of the WMAP satellite gathering data to help
   scientists understand the Big Bang.

   This idea allowed for two distinct possibilities. One possibility 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. It was actually Hoyle
   who coined the name of Lemaître's theory, referring to it sarcastically
   as "this big bang idea" during a program broadcast on March 28, 1949,
   by the BBC Third Programme. Hoyle repeated the term in further
   broadcasts in early 1950, as part of a series of five lectures entitled
   The Nature of Things. The text of each lecture was published in The
   Listener a week after the broadcast, the first time that the term "big
   bang" appeared in print. While Hoyle's "steady state" and Lemaître's
   "Big Bang" were the two most popular models used to explain Hubble's
   observations, other ideas were also proposed. Some of these
   alternatives included the Milne model, Richard Tolman's oscillatory
   universe, and Fritz Zwicky's tired light hypothesis.

   For a while, support was split between the "steady state" and "Big
   Bang" theories. However, the observational evidence eventually began to
   favour the latter. The discovery of the cosmic microwave background
   radiation in 1964 secured its place 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 what happened at the Big Bang and reconciling
   observations with the basic theory.

   Huge advances in Big Bang cosmology were made in the late 1990s and the
   early 21st century as a result of major advances in telescope
   technology in combination with large amounts of satellite data such as
   that from COBE, the Hubble Space Telescope and WMAP. Such data have
   allowed cosmologists to calculate many of the parameters of the Big
   Bang to a new level of precision and led to the unexpected discovery
   that the expansion of the universe appears to be accelerating. (See
   dark energy.)

   See also: Timeline of cosmology

Overview

   Based on measurements of the expansion of the universe using Type 1a
   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 13.7 ± 0.2 billion
   years. The agreement of these three independent measurements is
   considered strong evidence for the so-called ΛCDM model that describes
   the detailed nature of the contents of the universe.

   The early universe was filled homogeneously and isotropically with an
   incredibly high energy density and concomitantly huge temperatures and
   pressures. It expanded and cooled, going through phase transitions
   pertinent to elementary particles.

   Approximately 10^−35 seconds after the Planck epoch a phase transition
   caused the universe to experience exponential growth during a period
   called cosmic inflation. After inflation stopped, the material
   components of the universe were in the form of a quark-gluon plasma
   (also including all other particles—and perhaps experimentally produced
   recently as a quark-gluon liquid ) in which the constituent particles
   were all moving relativistically. As the universe continued growing in
   size, the temperature dropped. At a certain temperature, by an
   as-yet-unknown transition called baryogenesis, the quarks and gluons
   combined into baryons such as protons and neutrons, somehow producing
   the observed asymmetry between matter and antimatter. Still lower
   temperatures led to further symmetry breaking phase transitions that
   put the forces of physics and elementary particles into their present
   form. Later, some protons and neutrons combined to form the universe's
   deuterium and helium nuclei in a process called Big Bang
   nucleosynthesis. As the universe cooled, matter gradually stopped
   moving relativistically and its rest mass energy density came to
   gravitationally dominate that of radiation. After about 300,000 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 the cosmic microwave background.

   Over 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 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 20% of the matter in the
   universe.

   The universe today appears to be dominated by a mysterious form of
   energy known as dark energy. Approximately 70% of the total energy
   density of today's universe is in this form. This dark energy causes
   the expansion of the universe to deviate from a linear
   velocity-distance relationship, observed as a faster than expected
   expansion at very large distances. Dark energy in its simplest
   formulation takes the form of a cosmological constant term in
   Einstein's field equations of general relativity, but its composition
   is 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 these observations are encapsulated in the ΛCDM model of cosmology,
   which is a mathematical model of the Big Bang with six free parameters.
   Mysteries appear as one looks closer to the beginning, when particle
   energies were higher than can yet be studied by experiment. There is no
   compelling physical model for the first 10^−33 seconds of the universe,
   before the phase transition that grand unification theory predicts. At
   the "first instant", Einstein's theory of gravitation predicts a
   gravitational singularity where densities become infinite. To resolve
   this paradox, a theory of quantum gravitation is needed. Understanding
   this period of the history of the universe is one of the greatest
   unsolved problems in physics.

   See also: Timeline of the Big Bang

Theoretical underpinnings

   As it stands today, the Big Bang is dependent on three assumptions:
    1. The universality of physical laws
    2. The cosmological principle
    3. The Copernican principle

   When first developed, these ideas were simply taken as postulates, but
   today there are efforts underway to test each of them. Tests of the
   universality of physical laws have found that the largest possible
   deviation of the fine structure constant over the age of the universe
   is of order 10^-5. The isotropy of the universe that defines the
   Cosmological Principle has been tested to a level of 10^-5 and the
   universe has been measured to be homogeneous on the largest scales to
   the 10% level. There are efforts underway to test the Copernican
   Principle by means of looking at the interaction of galaxy groups and
   clusters with the CMB through the Sunyaev-Zel'dovich effect to a level
   of 1% accuracy.

   Using these assumptions, combined with Einstein's theory of general
   relativity, one finds that spacetime should be described by a
   homogeneous and isotropic metric, which must therefore be a FRW metric.
   These metrics rely on a coordinate chart or grid being laid down over
   all spacetime, with which we can specify the location of points (e.g.,
   galaxies, stars...) in the universe. The specific chart used is called
   a comoving coordinate system, since the grid is designed to expand
   along with the universe, and so objects that are carried along by 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. See also metric
   expansion of space.

   As the universe can be described by such coordinates, the Big Bang is
   not an explosion of matter moving outward to fill an empty universe;
   what is expanding is space itself. It is this expansion that causes the
   physical distance between two comoving points to increase. Objects that
   are bound together (such as atoms, people, stars, the solar system, or
   galaxies) do not expand with spacetime's expansion because the forces
   that bind them together are strong compared with the Hubble expansion
   that is pulling them apart.

   One can also define a conformal time η, in which case the full
   spacetime metric takes the form of a static metric multiplied by an
   overall scale factor. The conformal time coordinate is quite useful
   since the comoving distance traveled by a light ray is equal to the
   conformal time interval of the trip. This enables one to understand the
   causal structure of spacetime. For example, the Big Bang occurred at a
   finite interval of conformal time η[0] to the past. Objects whose
   comoving distance is greater than cη[0] are too far away for light to
   have had time to travel to us since the Big Bang: therefore we cannot
   see all of the past universe and there is a past horizon. If the
   universe is accelerating, then there is only a finite amount of
   conformal time η[F] to the future (though this finite amount of
   conformal time corresponds to an infinite amount of clock or proper
   time). Objects located at comoving distances further than cη[F] can
   never be reached by a light ray emitted by us today, therefore we
   cannot influence all of the future universe and there is a future
   horizon. See also cosmological horizon.

Observational evidence

   It is generally stated that there are three observational pillars that
   support the Big Bang theory of cosmology. These are the Hubble-type
   expansion seen in the redshifts of galaxies, the detailed measurements
   of the cosmic microwave background, and the abundance of light
   elements. (See Big Bang nucleosynthesis.) Additionally, the observed
   correlation function of large-scale structure of the cosmos fits well
   with standard Big Bang theory.

Hubble's law expansion

   Observations of distant galaxies and quasars show that these objects
   are redshifted, meaning that the light emitted from them has been
   shifted to longer wavelengths. This is seen by taking a frequency
   spectrum of the objects and then matching the spectroscopic pattern of
   emission lines or absorption lines corresponding to atoms of the
   chemical elements interacting with the light. From this analysis, a
   redshift corresponding to a Doppler shift for the radiation can be
   measured which is explained by a recessional velocity. When the
   recessional velocities are plotted against the distances to the
   objects, a linear relationship, known as Hubble's law, is observed:

                v = H_0 D \,

   where

          v is the recessional velocity of the galaxy or other distant
          object
          D is the distance to the object and
          H[0] is Hubble's constant, measured to be (70 +2.4/-3.2) km/ s/
          Mpc by the WMAP probe.

   The Hubble's law observation has two possible explanations. One is that
   we are at the centre of an explosion of galaxies, a position which is
   untenable given the Copernican principle. The second explanation is
   that the universe is uniformly expanding everywhere as a unique
   property of spacetime. This type of universal expansion was developed
   mathematically in the context of general relativity well before Hubble
   made his analysis and observations, and it remains the cornerstone of
   the Big Bang theory as developed by
   Friedmann-Lemaître-Robertson-Walker.

Cosmic microwave background radiation

   WMAP image of the cosmic microwave background radiation
   Enlarge
   WMAP image of the cosmic microwave background radiation

   The Big Bang theory predicted the existence of the cosmic microwave
   background radiation or CMB which is composed of photons first emitted
   during baryogenesis. Because the early universe was in thermal
   equilibrium, the temperature of the radiation and the plasma were equal
   until the plasma recombined. Before atoms formed, radiation was
   constantly absorbed and re-emitted in a process called Compton
   scattering: the early universe was opaque to light. However, cooling
   due to the expansion of the universe allowed the temperature to
   eventually fall below 3,000  K at which point electrons and nuclei
   combined to form atoms and the primordial plasma turned into a neutral
   gas. This is known as photon decoupling. A universe with only neutral
   atoms allows radiation to travel largely unimpeded.

   Because the early universe was in thermal equilibrium, the radiation
   from this time had a blackbody spectrum and freely streamed through
   space until today, becoming redshifted because of the Hubble expansion.
   This reduces the high temperature of the blackbody spectrum. The
   radiation should be observable at every point in the universe to come
   from all directions of space.

   In 1964, Arno Penzias and Robert Wilson, while conducting a series of
   diagnostic observations using a new microwave receiver owned by Bell
   Laboratories, discovered the cosmic background radiation. 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 favour of the Big Bang hypothesis. Penzias and Wilson were awarded
   the 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 determined that the CMB was isotropic to
   about one part in 10^5. During the 1990s, CMB anisotropies were further
   investigated by a large number of ground-based experiments and the
   universe was shown to be almost geometrically flat by measuring the
   typical angular size (the size on the sky) of the anisotropies. (See
   shape of the universe.)

   In early 2003, the 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. (See cosmic
   microwave background radiation experiments.) This satellite also
   disproved several specific cosmic inflation models, but the results
   were consistent with the inflation theory in general.

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. 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.

   The measured abundances all agree with those predicted from a single
   value of the baryon-to-photon ratio. The agreement is relatively poor
   for ^7Li and ^4He, the two elements for which the systematic
   uncertainties are least understood. This is considered strong evidence
   for the Big Bang, as the theory is the only known explanation for the
   relative abundances of light elements. 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
   essentially 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

   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.

Features, issues and problems

   While currently there are very few researchers who doubt the Big Bang
   occurred, in the past the community was divided between supporters of
   the Big Bang and supporters 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 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. Other issues, such as the cuspy
   halo problem and the dwarf galaxy problem of cold dark matter, are not
   considered to be fatal as they can be addressed through further
   refinements of the theory.

   The Big Bang model admits very exotic physical phenomena that include
   dark matter, dark energy, and cosmic inflation which rely on conditions
   and physics that have not yet been observed in terrestrial laboratory
   experiments. While explanations for such phenomena remain at the
   frontiers of inquiry in physics, independent observations of Big Bang
   nucleosynthesis, the cosmic microwave background, large scale structure
   and Type Ia supernovae strongly suggest the phenomena are important and
   real cosmological features of our universe. The gravitational effects
   of these features are understood observationally and theoretically but
   they have not yet been successfully incorporated into the Standard
   Model of particle physics. Though some aspects of the theory remain
   inadequately explained by fundamental physics, almost all cosmologists
   accept that the close agreement between Big Bang theory and observation
   have firmly established all the basic parts of the theory.

   The following is a short list of Big Bang "problems" and puzzles:

Horizon problem

   The horizon problem results from the premise that information cannot
   travel faster than light, and hence two regions of space which are
   separated by a greater distance than the speed of light multiplied by
   the age of the universe cannot be in causal contact. The observed
   isotropy of the cosmic microwave background (CMB) is problematic in
   this regard, because the horizon size at that time corresponds to a
   size that is about 2 degrees on the sky. If the universe has had the
   same expansion history since the Planck epoch, there is no mechanism to
   cause these 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 a time 10^-35 seconds after the Planck epoch.
   During inflation, the universe undergoes exponential expansion, and
   regions in causal contact expand so as to be beyond each other's
   horizons. 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. After inflation, the
   universe expands according to Hubble's law, and regions that were out
   of causal contact come back into the horizon. This explains the
   observed isotropy of the CMB. Inflation predicts that the primordial
   fluctuations are nearly scale invariant and Gaussian which has been
   accurately confirmed by measurements of the CMB.

Flatness problem

   The overall geometry of the universe is determined by whether the Omega
   cosmological parameter is less than, equal to or greater than 1. From
   top to bottom: geometry in a closed universe, an open universe and a
   flat universe.
   Enlarge
   The overall geometry of the universe is determined by whether the Omega
   cosmological parameter is less than, equal to or greater than 1. From
   top to bottom: geometry in a closed universe, an open universe and a
   flat universe.

   The flatness problem is an observational problem that results from
   considerations of the geometry associated with a
   Friedmann-Lemaître-Robertson-Walker metric. In general, the universe
   can have three different kinds of geometries: hyperbolic geometry,
   Euclidean geometry, or elliptic geometry. The geometry is determined by
   the total energy density of the universe (as measured by means of the
   stress-energy tensor): hyperbolic results from a density less than the
   critical density, elliptic from a density greater than the critical
   density, and Euclidean from exactly the critical density. The universe
   is required to be within one part in 10^15 of the critical density in
   its earliest stages. Any greater deviation would have caused either a
   Heat Death or a Big Crunch, and the universe would not exist as it does
   today.

   A possible resolution to this problem is again offered by inflationary
   theory. During the inflationary period, spacetime expanded to such an
   extent that any residual curvature associated with it would have been
   smoothed out to a high degree of precision. Thus, it is believed that
   inflation drove the universe to be very nearly spatially flat.

Magnetic monopoles

   The magnetic monopole objection was raised in the late 1970s. Grand
   unification theories predicted point defects in space that would
   manifest as magnetic monopoles with a density much higher than was
   consistent with observations, given that searches have never found any
   monopoles. This problem is also resolvable by cosmic inflation, which
   removes all point defects from the observable universe in the same way
   that it drives the geometry to flatness.

Baryon asymmetry

   It is not yet understood why the universe has more matter than
   antimatter. It is generally assumed that when the universe was young
   and very hot, it was in statistical equilibrium and contained equal
   numbers of baryons and anti-baryons. 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, which
   were laid out by Andrei Sakharov, must be satisfied. They require that
   baryon number be not conserved, that C-symmetry and CP-symmetry be
   violated, and that the universe depart from thermodynamic equilibrium.
   All these conditions occur in the Standard Model, but the effect is not
   strong enough to explain the present baryon asymmetry. Experiments
   taking place at CERN near Geneva seek to trap enough anti-hydrogen to
   compare its spectrum with hydrogen. Any difference would be evidence of
   a CPT symmetry violation and therefore a Lorentz violation.

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

   A pie chart indicating the proportional composition of different
   energy-density components of the universe, according to the best ΛCDM
   model fits. Roughly ninety-five percent is in the exotic forms of dark
   matter and dark energy.
   Enlarge
   A pie chart indicating the proportional composition of different
   energy-density components of the universe, according to the best ΛCDM
   model fits. Roughly ninety-five percent is in the exotic forms of dark
   matter and dark energy.

   During the 1970s and 1980s, various observations (notably of galactic
   rotation curves) showed that there was 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 not normal or baryonic matter but
   rather dark matter. In addition, assuming that the universe was mostly
   normal matter led to predictions that were strongly inconsistent with
   observations. In particular, the universe is far less lumpy and
   contains far less deuterium than can be accounted for without dark
   matter. While dark matter was initially controversial, it is now a
   widely accepted part of standard cosmology due to observations of the
   anisotropies in the CMB, galaxy cluster velocity dispersions,
   large-scale structure distributions, gravitational lensing studies, and
   x-ray measurements from galaxy clusters. In August 2006, dark matter
   was definitively observed through measurements of colliding galaxies in
   the Bullet Cluster. This and other detections of dark matter are only
   sensitive to its gravitational signature; no dark matter particles have
   yet been observed in laboratories. However, there are many particle
   physics candidates for dark matter, and several projects to detect them
   directly are underway.

Dark energy

   In the 1990s, detailed measurements of the mass density of the universe
   revealed a value that was 30% that of the critical density. Since the
   universe is very nearly spatially flat, as is indicated by measurements
   of the cosmic microwave background, about 70% of the energy density of
   the universe was left unaccounted for. This mystery now appears to be
   connected to another one: Independent measurements of Type Ia
   supernovae have revealed that the expansion of the universe is
   undergoing a non-linear acceleration. To explain this acceleration,
   general relativity requires that much of the universe consist of an
   energy component with large negative pressure. This dark energy is now
   thought to make up the missing 70%. Its nature remains one of the great
   mysteries of the Big Bang. Possible candidates include a scalar
   cosmological constant and quintessence. Observations to help understand
   this are ongoing. Results from WMAP in 2006 indicate that the universe
   is 74% dark energy, 22% dark matter, and 4% regular matter (see
   external link).

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 is above 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. Alternatively, if the density in the universe is
   equal to or below the critical density, the expansion would slow down,
   but never stop. Star formation would cease as the universe grows less
   dense. The average temperature of the universe would asymptotically
   approach absolute zero—a Big Freeze. Black holes would evaporate. 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. Moreover, if proton decay exists, then hydrogen, the predominant
   form of baryonic matter in the universe today, would disappear, leaving
   only radiation.

   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 cools and expands.
   Other explanations of dark energy — so-called phantom energy theories —
   suggest that ultimately galaxy clusters and eventually galaxies
   themselves will be torn apart by the ever-increasing expansion in a
   so-called Big Rip.

Speculative physics beyond the Big Bang

   A graphical representation of the expansion of the universe with the
   inflationary epoch represented as the dramatic expansion of the metric
   seen on the left. Image from WMAP press release, 2006. (Detail)
   Enlarge
   A graphical representation of the expansion of the universe with the
   inflationary epoch represented as the dramatic expansion of the metric
   seen on the left. Image from WMAP press release, 2006. (Detail)

   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
   universe, when inflation is hypothesized to have occurred. There may
   also be parts of the universe well beyond what can be observed in
   principle. In the case of inflation this is required: exponential
   expansion has pushed large regions of space beyond our observable
   horizon. It may be possible to deduce what happened when we better
   understand physics at very high energy scales. Speculations about this
   often involve theories of quantum gravitation.

   Some proposals are:
     * models including the Hartle-Hawking boundary condition in which the
       whole of space-time is finite;
     * brane cosmology models, including brane inflation, 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 starts from random initial
       conditions for the universe.

   Some of these scenarios are qualitatively compatible with one another.
   Each entails untested hypotheses.

Philosophical and religious interpretations

   While the Big Bang is a scientific theory that is not based on any
   religion, some similarities have not gone unnoticed. There are both
   theological and philosophical implications, since some religious
   interpretations and world views conflict with the Big Bang origin of
   the universe.

   There are a number of interpretations of the Big Bang theory that go
   beyond science, some of them purporting to explain the cause of the Big
   Bang itself ( first cause). These views have been criticized by some
   naturalist philosophers as being modern creation myths. Some people
   believe that the Big Bang theory is inconsistent with traditional views
   of creation such as that in Genesis, for example, while others, like
   astronomer Hugh Ross, believe that the Big Bang theory lends support to
   the idea of creation ex nihilo.

   Initially, many scientists rejected the Big Bang theory because they
   thought it was religious in nature. The prevailing view at the time was
   that the universe was eternal, having always existed. Some felt the
   idea that the universe had a beginning would imply a creator (see Kalam
   cosmological argument), which would be unscientific. These connotations
   troubled astronomer Fred Hoyle and others, who developed the now
   discredited steady state theory as an alternative to the Big Bang which
   would allow for an eternal universe. Astrophysicist Arthur Eddington
   had no such qualms, arguing that evidence of a Big Bang and start to
   the universe made "religion possible for a reasonable man of science."

   The following is a list of various religious interpretations of the Big
   Bang theory:
     * A number of Christian and traditional Jewish sources have accepted
       the Big Bang as a possible description of the origin of the
       universe, interpreting it to allow for a philosophical first cause.
       Pope Pius XII was an enthusiastic proponent of the Big Bang even
       before the theory was scientifically well-established and
       consequently the Roman Catholic Church has been a prominent
       advocate for the idea that creation ex nihilo can be interpreted as
       consistent with the Big Bang. This view is shared by many religious
       Jews in all branches of rabbinic Judaism. Some groups, such as the
       Kabbalah Centre, contend the Big Bang is also consistent with the
       teaching of creation according to Isaac Luria and Kabbalah.
     * Some modern Islamic scholars believe that the Qur'an parallels the
       Big Bang in its account of creation, described as follows: "Do not
       the unbelievers see that the heavens and the earth were joined
       together as one unit of creation, before We clove them asunder?"
       (Ch:21,Ver:30). The claim has also been made that the Qur'an
       describes an expanding universe: "The heaven, We have built it with
       power. And verily, We are expanding it." (Ch:51,Ver:47). Parallels
       with the Big Crunch and an oscillating universe have also been
       suggested: "On the day when We will roll up the heavens like the
       rolling up of the scroll for writings, as We originated the first
       creation, (so) We shall reproduce it; a promise (binding on Us);
       surely We will bring it about." (Ch:21,Ver:104).
     * Certain theistic branches of Hinduism, such as in Vaishnavism,
       conceive of a creation event with similarities to the Big Bang. For
       example in the third book of the Bhagavata Purana (primarily,
       chapters 10 and 26), describes a primordial state which bursts
       forth as the Great Vishnu glances over it, transforming into the
       active state of the sum-total of matter (" prakriti"). Other forms
       of Hinduism assert a universe without beginning or end.
     * Buddhism has a concept of universes that have no initial creation
       event, but instead go through infinitely repeated cycles of
       expansion, stability, destruction, and quiescence. The Big Bang,
       however, is not seen to be in conflict with this since there are
       ways to conceive an eternal creation and destruction of universes
       within the paradigm. A number of popular Zen philosophers were
       intrigued, in particular, by the concept of the oscillatory
       universe.

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