Big Bang

In physical cosmology, the Big Bang is the scientific theory of the origin of the universe as an explosion of space and matter, starting from an enormously dense and hot state at some finite time in the past. The central idea is that the observed redshift of the galaxies (Hubble's law) shows that the galaxies are receding from each other, which implies that they were much closer together in the past. Extrapolated to the extreme, this line of reasoning leads to the conclusion that all the constituents of the universe began at very high density and temperature (perhaps even a gravitational singularity). Since then, space itself has expanded with the passage of time, carrying the galaxies with it.

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—measured to be 13.7 billion (13.7 × 10⁹) years ago—and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and evolution of the universe.

One consequence of the Big Bang is that the conditions of today's universe are different from the conditions in the past or in the future. From this model, George Gamow in 1948 was able to predict the cosmic microwave background radiation (CMB). The CMB was discovered in the 1960s and served as a confirmation of the Big Bang theory over its chief rival, the steady state theory. Template:Cosmology

History

The Big Bang theory developed from observations and theoretical considerations. Observationally, in the 1910s, Vesto Slipher and later Carl Wilhelm Wirtz determined that most spiral nebulae were receding from Earth, but they weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way.

Also in the 1910s, Albert Einstein's theory of general relativity was found to admit no static cosmological solutions given the basic assumptions of cosmology described in this section (http://en.wikipedia.org/wiki/Big_Bang#Theoretical_underpinnings). The universe was described by a metric tensor that was either expanding or shrinking, a result that Einstein himself considered wrong and he tried to fix by adding a cosmological constant. The first person to seriously apply general relativity to cosmology without the stabilizing cosmological constant was Alexander Friedmann, whose equations describe the Friedmann-Lemaître-Robertson-Walker universe.

In 1927, the Belgian Jesuit priest Georges Lemaître independently derived the Friedmann-Lemaître-Robertson-Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"—what was later called the Big Bang.

In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. Hubble proved that the spiral nebulae were galaxies and measured their distances by observing Cepheid variable stars. He discovered that the galaxies are receding in every direction at speeds (relative to the Earth) directly proportional to their distance. This fact is now known as Hubble's law (see Edwin Hubble: Mariner of the Nebulae by Edward Christianson).

Given the cosmological principle, Hubble's law suggested that the universe was expanding. This idea allowed for two opposing possibilities. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other possibility was Fred Hoyle's steady state model in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time. It was actually Hoyle who coined the name of Lemaître's theory, referring to it sarcastically as "this 'big bang' idea" during a 1949 BBC radio program, The Nature of Things, the text of which was published in 1950.

For a number of years the support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. Since the discovery of the cosmic microwave background radiation in 1965 it has been regarded as the best theory of the origin and evolution of the cosmos.

Before the late 1960s, many cosmologists thought the infinitely dense and physically paradoxical singularity at the starting time of Friedmann's cosmological model could be avoided by allowing for a universe which was contracting before entering the hot dense state and starting to expand again. This was formalized as Richard Tolman's oscillating universe. In the sixties, Stephen Hawking and others demonstrated that this idea was unworkable, and the singularity is an essential feature of the physics described by Einstein's gravity. This led the majority of cosmologists to accept the notion that the universe as currently described by the physics of general relativity has a finite age. However, due to a lack of a theory of quantum gravity, there is no way to say whether the singularity is an actual origin point for the universe or whether the physical processes that govern the regime cause the universe to be effectively eternal in character.

Virtually all theoretical work in cosmology now involves extensions and refinements to the basic Big Bang theory. 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. These 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 Ia supernovae, measurements of the lumpiness of the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a measured age of 13.7 ± 0.2 billion years. The agreement of these three independent measurements is considered strong evidence for the so-called Lambda-CDM model that describes the detailed nature of the contents of the universe.

The early universe was filled homogeneously and isotropically with a incredibly high energy density and concomitantly huge temperatures and pressures. It expanded and cooled, going through phase transitions analogous to the condensation of steam or freezing of water as it cools, but related 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^{[1] (http://www.aip.org/pnu/2005/split/728-1.html)}) 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 component of the universe's composition is revealed by its property of causing the expansion of the universe to deviate from a linear velocity-distance relationship by causing spacetime to expand faster than expected at very large distances. Dark energy in its simplest formation 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 Lambda-CDM 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 called for by grand unification theory. At the "first instant", Einstein's theory of gravity predicts a gravitational singularity where densities become infinite. To resolve this paradox, a theory of quantum gravity 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 homogenous 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 clusters with the CMB through the Sunyaev-Zeldovich effect to a level of 1% accuracy.

The Big Bang theory uses Weyl's postulate to unambiguously measure time at any point as the "time since the Planck epoch". Measurements in this system rely on conformal coordinates in which so-called comoving distances and conformal times remove the expansion of the universe from consideration of spacetime measurements. The comoving distances and conformal times are defined so that objects moving with the cosmological flow are always the same comoving distance apart and the particle horizon or observational limit of the local universe is set by the conformal time.

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 spacetime itself. It is this expansion that causes the physical distance between any two fixed points in our universe to increase. Objects that are bound together (for example, by gravity) do not expand with spacetime's expansion because the physical laws that govern them are assumed to be uniform and independent of the metric expansion. Moreover, the expansion of the universe on today's local scales is so small that any dependence of physical laws on the expansion is unmeasurable by current techniques.

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 law expansion

Observations of distant galaxies and quasars show that these objects are redshifted, meaning that the light emitted from them has been proportionately 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 elements interacting with the radiation. From this analysis, a measured redshift can be determined which is explained by a recessional velocity corresponding to a Doppler shift for the radiation. When the recessional velocities are plotted against the distances to the objects, a linear relationship, known as the Hubble Law, is observed:

v = H₀ D

where v is the recessional velocity, D is the distance to the object and H₀ is the Hubble constant measured to be 71 ± 4 km/s/Mpc by the WMAP probe.

Cosmic microwave background radiation

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 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 adsorbed and reemitted 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 3000 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 favor 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 10⁵. During the 1990s, CMB anisotropies were further investigated by a large number of ground-based experiments and the universe was shown to be 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 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. 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 universe should have more helium than deuterium or more deuterium than ³He.

Galactic evolution and distribution

Detail 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

Historically, a number of problems have arisen within the Big Bang theory. Some of them are today mainly of historical interest, and have been avoided 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 refinements of the theory.

There are a small number of proponents of non-standard cosmologies who believe that there was no Big Bang at all. They claim that solutions to standard problems in the Big Bang involve ad hoc modifications and addenda to the theory. Most often attacked are the parts of standard cosmology that include dark matter, dark energy, and cosmic inflation. However, while explanations for these features remain at the frontiers of inquiry in physics, each one is strongly suggested to exist in some form by observations of the cosmic microwave background, large scale structure and type IA supernovae. The gravitational effects of these features are understood observationally and theoretically even as models for them have not yet been fully incorporated into the standard model of particle physics in an accepted way. Though such aspects of standard cosmology remain inadequately explained, the vast majority of astronomers and physicists accept that the close agreement between Big Bang theory and observation have firmly established all the basic parts of the theory.

What follows is a short list of standard Big Bang "problems" and puzzles:

The 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.

This apparent inconsistency is resolved 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 a Hubble 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

The flatness problem is an observational problem that results from considerations of the geometry associated with 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): the 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 measured to be required to be within one part in 10¹⁵ 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.

The 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, inflation drove the universe to be 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 not be conserved, that C-symmetry and CP-symmetry be violated, and that the universe depart from thermodynamic equilibrium. All these conditions occur in the big bang, but the effect is not strong enough to explain the present baryon asymmetry. New developments in high energy particle physics are necessary to explain the 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

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. Dark matter has only been detected through its gravitational signature; no particles that might make it up have yet been observed in laboratories. However, there are many particle physics candidates for dark matter, and several projects to detect them 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 flat, as is indicated by measurements of the cosmic microwave background, fully 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 rather than following a strict Hubble Law. 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.

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. 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 Lambda-CDM 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.

See also Ultimate fate of the universe.

Speculative physics beyond the Big Bang

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 gravity.

Some proposals are:

• chaotic inflation
• brane cosmology models, including the ekpyrotic model in which the Big Bang is the result of a collision between branes
• an oscillatory universe in which the early universe's hot, dense state resulted from the Big Crunch of a universe similar to ours. The universe could have gone through an infinite number of big bangs and big crunches. The cyclic extension of the ekpyrotic model is a modern version of such a scenario.
• models including the Hartle-Hawking boundary condition in which the whole of space-time is finite.

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

Philosophical and religious interpretations

There are a number of interpretations of the Big Bang theory that are entirely speculative or extra-scientific. Some of these ideas purport to explain the cause of the Big Bang itself (first cause), and have been criticized by some naturalist philosophers as being modern creation myths. Some people believe that the Big Bang theory lends support to traditional views of creation, for example as given in Genesis, while others believe that all Big Bang theories are inconsistent with such views.

The Big Bang as a scientific theory is not associated with any religion. While certain fundamentalist interpretations of religions conflict with the Big Bang history of the universe, there are more liberal interpretations that do not.

The following is a list of various religious interpretations of the Big Bang theory:

• A number of Christian apologists, the Roman Catholic Church in particular, have accepted the Big Bang as a 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.
• Some students of Kabbalah, deism and other non-anthropomorphic faiths concord with the Big Bang theory, for example connecting it with the theory of "divine retraction" (tzimtzum) as explained by the Jewish scholar Moses Maimonides.
• 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? We have made every living thing out of the water." (Ch:21,Ver:30). The Qur'an also appears to describe an expanding universe: "The heaven, We have built it with power. And verily, We are expanding it." (Ch:51,Ver:47).
• Certain theistic branches of Hinduism, such as the Vaishnava-traditions, conceive of a theory of creation with similarities to the theory of the Big Bang. The Hindu mythos, narrated 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").
• Buddhism has a concept of a universe that has no creation event. The Big Bang, however, is not seen to be in conflict with this since there are ways to get an eternal universe within the paradigm. A number of popular Zen philosophers were intrigued, in particular, by the concept of the oscillating universe.

External links and references

Big Bang overviews

• Open Directory Project: Cosmology (http://www.dmoz.org/Science/Astronomy/Cosmology/)
• PBS.org, "From the Big Bang to the End of the Universe. The Mysteries of Deep Space Timeline" (http://www.pbs.org/deepspace/timeline/)
• "Welcome to the History of the Universe" (http://www.historyoftheuniverse.com/). Penny Press Ltd.
• Cambridge University Cosmology, "The Hot Big Bang Model (http://www.damtp.cam.ac.uk/user/gr/public/bb_home.html)". Includes a discussion of the problems with the big bang.
• Smithsonian Institution, "UNIVERSE! - The Big Bang and what came before (http://cfa-www.harvard.edu/seuforum/bigbanglanding.htm)".
• D'Agnese, Joseph, "The last Big Bang man left standing, physicist Ralph Alpher devised Big Bang Theory of universe (http://www.findarticles.com/p/articles/mi_m1511/is_7_20/ai_55030837)". Discover, July 1999.
• Felder, Gary, "The Expanding Universe (http://www.ncsu.edu/felder-public/kenny/papers/cosmo.html)".
• LaRocco, Chris and Blair Rothstein, "THE BIG BANG: It sure was BIG!!" (http://www.umich.edu/~gs265/bigbang.htm).
• Mather, John C., and John Boslough 1996, The very first light: the true inside story of the scientific journey back to the dawn of the universe. ISBN 0-465-01575-1 p.300
• Shestople, Paul, ""Big Bang Primer" (http://cosmology.berkeley.edu/Education/IUP/Big_Bang_Primer.html).
• Singh, Simon, Big Bang: the origin of the universe, Fourth Estate (2005). A historical review of the big bang. Sample text and reviews can be found at [2] (http://www.321books.co.uk/reviews/big-bang-simon-singh.htm).
• Wright, Edward L., "Brief History of the Universe" (http://www.astro.ucla.edu/~wright/BBhistory.html).

Technical introductions

• S. Dodelson, Modern Cosmology, Academic Press (2003). Released slightly before the WMAP results, this is the most modern introductory textbook.
• E. W. Kolb and M. S. Turner, The Early Universe, Addison-Wesley (1990). This is the classic reference for cosmologists.
• P. J. E. Peebles, Principles of Physical Cosmology, Princeton University Press (1993). Peebles' book has a strong historical focus.

Some primary sources

• G. Lemaître, "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques" (A homogeneous Universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae), Annals of the Scientific Society of Brussels 47A (1927):41—General Relativity implies the universe has to be expanding. Einstein brushed him off in the same year. Lemaître's note was translated in Monthly Notices of the Royal Astronomical Society 91 (1931): 483–490.
• G. Lemaître, Nature 128 (1931) suppl.: 704, with a reference to the primeval atom.
• R. A. Alpher, H. A. Bethe, G. Gamow, "The Origin of Chemical Elements,"Physical Review 73 (1948), 803. The so-called αβγ paper, in which Alpher and Gamow suggested that the light elements were created by protons capturing neutrons in the hot, dense early universe. Bethe's name was added for symmetry.
• G. Gamow, "The Origin of Elements and the Separation of Galaxies," Physical Review 74 (1948), 505. These two 1948 papers of Gamow laid the foundation for our present understanding of big-bang nucleosynthesis.
• G. Gamow, Nature 162 (1948), 680.
• R. A. Alpher, "A Neutron-Capture Theory of the Formation and Relative Abundance of the Elements," Physical Review 74 (1948), 1737.
• R. A. Alpher and R. Herman, "On the Relative Abundance of the Elements," Physical Review 74 (1948), 1577. This paper contains the first estimate of the present temperature of the universe.
• R. A. Alpher, R. Herman, and G. Gamow Nature 162 (1948), 774.
• A. A. Penzias and R. W. Wilson, "A Measurement of Excess Antenna Temperature at 4080 Mc/s," Astrophysical Journal 142 (1965), 419. The paper describing the discovery of the cosmic microwave background.
• R. H. Dicke, P. J. E. Peebles, P. G. Roll and D. T. Wilkinson, "Cosmic Black-Body Radiation," Astrophysical Journal 142 (1965), 414. The theoretical interpretation of Penzias and Wilson's discovery.
• A. D. Sakharov, "Violation of CP invariance, C asymmetry and baryon asymmetry of the universe," Pisma Zh. Eksp. Teor. Fiz. 5, 32 (1967), translated in JETP Lett. 5, 24 (1967).
• R. A. Alpher and R. Herman, "Reflections on early work on 'big bang' cosmology" Physics Today Aug 1988 24–34. A review article.

Religion and philosophy

• Leeming, David Adams, and Margaret Adams Leeming, A Dictionary of Creation Myths. Oxford University Press (1995), ISBN 0195102754.

• Pius XII (1952), "Modern Science and the Existence of God," The Catholic Mind 49:182–192.

Research articles

Most scientific papers about cosmology are initially released as preprints on arxiv.org. They are generally technical, but sometimes have introductions in plain English. The most relevant archives, which cover experiment and theory, are the astrophysics (http://arxiv.org/archive/astro-ph) archive, where papers closely grounded in observations are released, and the general relativity and quantum cosmology (http://arxiv.org/archive/gr-qc) archive, which covers more speculative ground. Papers of interest to cosmologists also frequently appear on the high energy phenomenology (http://arxiv.org/archive/hep-th) and high energy theory (http://arxiv.org/archive/hep-th) archives.bg:Теория на Големия взрив ca:Big Bang da:Big Bang de:Urknall et:Suur Pauk es:Teoría del Big Bang eo:Praeksplodo fr:Big-bang id:Big Bang is:Miklihvellur it:Big Bang he:המפץ הגדול hu:Ősrobbanás nl:Oerknal ja:ビッグバン no:Big Bang pl:Wielki Wybuch pt:Big Bang ru:Большой Взрыв simple:Big Bang sk:Veľký tresk sl:prapok sr:Велики Прасак fi:Alkuräjähdys sv:Big Bang th:บิกแบง vi:Big Bang zh:大爆炸理论