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developed from an extremely dense and hot state. Space itself has been expanding ever since, carrying Galaxies (and all other matter) with it.]] The Big Bang is the Cosmological model of the Universe whose primary assertion is that the universe has Expanded into its current state from a Primordial Condition of enormous Density and Temperature . The term is also used in a narrower sense to describe the fundamental "fireball" that erupted at or close to an initial timepoint in the history of our observed Spacetime ."Even though the Universe has been expanding and cooling ever since, the sound waves have left their imprint as temperature variations on the afterglow of the '''big bang fireball'''..." 1 Theoretical support for the Big Bang comes from mathematical models, called Friedmann Models . These models show that a Big Bang is consistent with General Relativity and with the Cosmological Principle , which states that the properties of the universe should be independent of position or orientation. Observational Evidence for the Big Bang includes the analysis of the Spectrum of Light from Galaxies , which reveal a Shift Towards Longer Wavelengths Proportional to each galaxy's Distance in a relationship described by Hubble's Law . Combined with the assumption that observers located anywhere in the universe would make similar observations (the Copernican Principle ), this suggests that Space itself is Expanding . The next most important observational evidence was the Discovery Of Cosmic Microwave Background Radiation in 1964. This had been predicted as a relic from when hot ionized plasma of the early universe first cooled sufficiently to form neutral hydrogen and allow space to become transparent to light, and its discovery led to general acceptance among physicists that the Big Bang is the best model for the origin and evolution of the universe. A third important line of evidence is the relative proportion of light elements in the universe, which is a close match to predictions for the formation of light elements in the first minutes of the universe, according to Big Bang Nucleosynthesis . HISTORY See Also: History of the Big Bang theory See Also: Timeline of cosmology History of astronomy The Big Bang theory 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 .2 3 Ten years later, ", echoing previous speculations about the Cosmic Egg origin of the universe.8 Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the Cosmic Distance Ladder , using the 100 inch Hooker telescope at Mount Wilson Observatory . This allowed him to estimate distances to galaxies whose Redshift s had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recession velocity—now known as Hubble's Law .910 Lemaître had already shown that this was expected, given the Cosmological Principle .11}} satellite gathering data to help scientists understand the Big Bang.]] During the 1930s other ideas were proposed as 's Tired Light hypothesis.14 . 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.15 The other was Lemaître's Big Bang theory, advocated and developed by George Gamow , who introduced big bang nucleosynthesis16 and whose associates, Ralph Alpher and Robert Herman , predicted the cosmic microwave background (CMB).17 It is an irony that it was Hoyle who coined the name that would come to be applied to Lemaître's theory, referring to it sarcastically as "this ''big bang'' idea" during a radio broadcast.18 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 of the Cosmic Microwave Background Radiation in 196419 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 ,20 the Hubble Space Telescope and WMAP .21 Cosmologists now have fairly precise measurement 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 (see Dark Energy ). OVERVIEW See Also: Timeline of the Big Bang Extrapolation of the expansion of the universe backwards in time using e, 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.23 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 Isotropic ally with an incredibly high Energy Density , huge Temperature s and Pressure s, and was very rapidly expanding and cooling. Approximately 10−35 seconds into the expansion, a Phase Transition caused a Cosmic Inflation , during which the universe grew Exponentially .24 After inflation stopped, the universe consisted of a Quark-gluon Plasma , as well as all other Elementary Particle s.25 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 Quark s and Lepton s over antiquarks and anti-leptons—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 (and hence the typical energy of each particle was decreasing). Symmetry Breaking phase transitions put the Fundamental Force s 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 Baryon s such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton-antiproton pairs (similarly for neutrons-antineutrons), so a mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons, and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by Photon s (with a minor contribution from Neutrino s). A few minutes into the expansion, when the temperature was about a billion Kelvin 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 .Kolb and Turner (1988), chapter 4 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 380,000 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 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, Star s, 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 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 accelerate, observed as a slower 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 Equation s 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 can be explained by the ΛCDM Model of cosmology, which is a Mathematical Model of the Big Bang with six free parameters. As noted above, there is no compelling physical model for the first 10−11 seconds of the universe. To resolve the Paradox of the initial Singularity , 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 . THEORETICAL UNDERPINNINGS The Big Bang theory depends on two major assumptions: # The universality of Physical Law s # The Cosmological Principle —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 10-5.26 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.27 FLRW metric See Also: Friedmann-Lemaître-Robertson-Walker metric Metric expansion of space General relativity describes spacetime by a Metric , which determines the distances that separate nearby points. The points themselves (galaxies, stars, etc.) 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.28 Chapter 23 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. Sizes of objects which are defined by laws and constants of physics (say, size of an atom, or of a chunk of solid like meter stick, or of a Solar system) do not change (because those laws and constants do not change). Horizons See Also: Cosmological horizon An important feature of the Big Bang spacetime is the presence of Horizons . Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit—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 FRW model that describes our universe. Our understanding of the universe back to very early times Suggests that there was a past horizon, though in practice our view is limited by the opacity of the universe at early times. 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 (sometimes called the three pillars of the Big Bang theory) are the Hubble-type Expansion seen in the Redshift s of galaxies, the detailed measurements of the cosmic microwave background, and the abundance of light elements (see Big Bang Nucleosynthesis ). Many other lines of evidence now support the picture, notably various properties of the Large-scale Structure Of The Cosmos which are predicted to occur due to gravitational growth of structure in the standard Big Bang theory. Hubble's law expansion See Also: Hubble's law See Also: distance measures (cosmology) Observations of distant galaxies and Quasar s show that these objects are Redshift ed—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 Line s or Absorption Line s corresponding to Atom s of the Chemical Element s interacting with the light. From this analysis, a Redshift can be measured. If this is interpreted as a Doppler Shift , the recessional Velocity of the object can be calculated. For some galaxies, it is then 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: :: where : is the recessional Velocity of the Galaxy or other distant object : is the distance to the object and : is Hubble's constant, measured to be (70 +2.4/-3.2) ( Km / S )/ Mpc by the WMAP probe. 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 considered 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 And Walker . The theory requires the relation to hold at all times, where is the Proper Distance , , and , , and all 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 . 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 Cosmic microwave background radiation See Also: Cosmic microwave background radiation 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'', 380,000 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 105. John C. Mather 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 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 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. This satellite is still gathering data. Another satellite will be launched within the next few years, the Planck Surveyor , which 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 . Abundance of primordial elements See Also: Big Bang nucleosynthesis 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.Kolb and Turner (1988), chapter 4 All the abundances depend on a single parameter, the ratio of Photon s to Baryon s, 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.29 }} 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 See Also: Large-scale structure of the cosmos Structure formation Galaxy formation and evolution |
For an annotated list of textbooks and monographs, see Physical Cosmology .
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