The Big Bang theory is a cosmological model of the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from a very high-density and high-temperature state, and offers a comprehensive explanation for a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, large-scale structure and Hubble's law (the farther away galaxies are, the faster they are moving away from Earth). If the observed conditions are extrapolated backwards in time using the known laws of physics, the prediction is that just before a period of very high density there was a singularity which is typically associated with the Big Bang. Current knowledge is insufficient to determine if the singularity was primordial.
Georges Lemaître first noted in 1927 that an expanding universe could be traced back in time to an originating single point, calling his theory that of the "primeval atom". For much of the rest of the 20th century scientific community was divided between supporters of the Big Bang and the rival steady state model, but a wide range of empirical evidence has strongly favored the Big Bang which is now universally accepted. Edwin Hubble concluded that galaxies are drifting apart in 1929 from analysis of galactic redshifts; this is important observational evidence for an expanding universe. In 1964, the CMB was discovered, which was crucial evidence in favor of the hot Big Bang model, since that theory predicted the existence of background radiation throughout the universe.
The known laws of physics can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature. Detailed measurements of the expansion rate of the universe place the Big Bang at around 13.8 billion years ago, which is thus considered the age of the universe. After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later atoms. Giant clouds of these primordial elements (mostly hydrogen, with some helium and lithium) later coalesced through gravity, eventually forming early stars and galaxies, the descendants of which are visible today. Astronomers also observe the gravitational effects of dark matter surrounding galaxies. Most of the matter in the universe seems to be in the form of dark matter, and the Big Bang theory and various observations indicate that it is not conventional baryonic matter (atoms). It is still not known exactly what dark matter is. More recently, measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy's existence.
- 1 Features of the model
- 2 Timeline
- 3 History
- 4 Observational evidence
- 5 Problems and related issues in physics
- 6 Ultimate fate of the universe
- 7 Misconceptions
- 8 Pre-Big Bang cosmology
- 9 Religious and philosophical interpretations
- 10 See also
- 11 Notes
- 12 References
- 13 Further reading
- 14 External links
Features of the model
The Big Bang theory offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the CMB, large-scale structure, and Hubble's law. The theory depends on two major assumptions: the universality of physical laws and the cosmological principle. The universality of physical laws is the underlying principle of the theory of relativity. The cosmological principle states that on large scales the universe is homogeneous and isotropic.
These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine-structure constant over much of the age of the universe is of order 10−5. Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars.[notes 1]
The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, the cosmological principle can be derived from the simpler Copernican principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10−5 via observations of the temperature of the CMB. The universe has been measured to be homogeneous on the largest scales at the 10% level.[clarification needed]
Expansion of space
The expansion of the Universe was inferred from the early astronomical observations and is an essential ingredient of the Big Bang theory. Mathematically, general relativity describes spacetime by a metric, which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, are themselves 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 (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 because of 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 co-moving points expands proportionally with the scale factor of the universe.
The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distances between comoving points. In other words, the Big Bang is not an explosion in space, but rather an expansion of space. 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.
An important feature of the Big Bang spacetime is the presence of particle horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not yet had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe.
Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well.
|A graphical timeline is available at|
Graphical timeline of the Big Bang
According to the Big Bang theory, the universe at the beginning was very hot and very small, and since then it is expanding and cooling down.
Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This irregular behavior, known as the gravitational singularity, indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone can not extrapolate toward the singularity — beyond the end of the so called Planck epoch.
This primordial singularity is itself sometimes called "the Big Bang", but the term can also refer to a more generic early hot, dense phase[notes 2] of the universe. In either case, "the Big Bang" as an event is also colloquially referred to as the "birth" of our universe since it represents the point in history where the universe can be verified to have entered into a regime where the laws of physics as we understand them (specifically general relativity and the Standard Model of particle physics) work. Based on measurements of the expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event — known as the "age of the universe" — is 13.799 ± 0.021 billion years. The agreement of independent measurements of this age supports the Lambda-CDM (ΛCDM) model that describes in detail the characteristics of the universe.
Despite being extremely dense at this time—far denser than is usually required to form a black hole—the universe did not re-collapse into a black hole. This may be explained by considering that commonly-used calculations and limits for gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang.
Inflation and baryogenesis
The earliest phases of the Big Bang are subject to much speculation, since astronomical data about them are not available. In the most common models the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures and was very rapidly expanding and cooling. Approximately 10−37 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially and during which time density fluctuations that occurred because of the Heisenberg's uncertainty principle were amplified into the seeds that would later form the large-scale structure of the universe. After inflation stopped, reheating occurred until the universe obtained the temperatures required for the production of a quark–gluon plasma as well as all other elementary particles. Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point, an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.
The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form. After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators. At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was 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 photons (with a minor contribution from neutrinos).
A few minutes into the expansion, when the temperature was about a billion kelvin and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis (BBN). Most protons remained uncombined as hydrogen nuclei.
As the universe cooled, the rest energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years, the electrons and nuclei combined into atoms (mostly hydrogen), which were able to emit redaiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background. The chemistry of life may have begun during a habitable epoch when the universe was only 10–17 million years old.
Over a long period of time, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter, warm dark matter, hot dark matter, and baryonic matter. The best measurements available, from the Wilkinson Microwave Anisotropy Probe (WMAP), show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be cold (warm dark matter is ruled out by early reionization), and is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. In an "extended model" which includes hot dark matter in the form of neutrinos, then if the "physical baryon density" is estimated at about 0.023 (this is different from the 'baryon density' expressed as a fraction of the total matter/energy density, which is about 0.046), and the corresponding cold dark matter density is about 0.11, the corresponding neutrino density is estimated to be less than 0.0062.
Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 73% of the total energy density of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity predominated, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the universe to slowly begin to accelerate.
Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theoretically.
All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and general relativity. There are no well-supported models or direct experimental observations describing the action prior to approximately 10−15 seconds. Apparently a new unified theory of quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the universe is currently one of the greatest unsolved problems in physics.
English astronomer Fred Hoyle is credited with coining the term "Big Bang" during a talk for a March 1949 BBC Radio broadcast, saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past."
It is popularly reported that Hoyle, who favored an alternative "steady state" cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.
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. Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Einstein field equations, showing that the universe might be expanding in contrast to the static universe model advocated by Albert Einstein at that time. In 1924, American astronomer Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist, proposed that the inferred recession of the nebulae was due to the expansion of the universe.
In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence.
Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2.5 m) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recessional velocity—now known as Hubble's law. Lemaître had already shown that this was expected, given the cosmological principle.
In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the steady state theory. This perception was enhanced by the fact that the originator of the Big Bang theory, Lemaître, was a Roman Catholic priest. Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz., that matter is eternal. A beginning in time was "repugnant" to him. Lemaître, however, disagreed:
"If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time."
During the 1930s, other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model, the oscillatory universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard C. Tolman) and Fritz Zwicky's tired light hypothesis.
After World War II, two distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time. The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced BBN and whose associates, Ralph Alpher and Robert Herman, predicted the CMB. Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast in March 1949.[notes 3] For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe. 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.
In 1968 and 1970, Roger Penrose, Stephen Hawking, and George F. R. Ellis published papers where they showed that mathematical singularities were an inevitable initial condition of relativistic models of the Big Bang. Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, Alan Guth made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang theory with the introduction of an epoch of rapid expansion in the early universe he called "inflation". Meanwhile, during these decades, two questions in observational cosmology that generated much discussion and disagreement were over the precise values of the Hubble Constant and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual fate of the universe).
In the mid-1990s, observations of certain globular clusters appeared to indicate that they were about 15 billion years old, which conflicted with most then-current estimates of the age of the universe (and indeed with the age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. While there still remain some questions as to how accurately the ages of the clusters are measured, globular clusters are of interest to cosmology as some of the oldest objects in the universe.
Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as the Cosmic Background Explorer (COBE), the Hubble Space Telescope and WMAP. Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.
The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by BBN. More recent evidence includes observations of galaxy formation and evolution, and the distribution of large-scale cosmic structures, These are sometimes called the "four pillars" of the Big Bang theory.
Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently subjected to the most active laboratory investigations. Remaining issues include the cuspy halo problem and the dwarf galaxy problem of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible. Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved problems in physics.
Hubble's law and the expansion of space
Observations of distant galaxies and quasars show that these objects are redshifted: the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed: where
- is the recessional velocity of the galaxy or other distant object,
- is the comoving distance to the object, and
- is Hubble's constant, measured to be 70.4+1.3
−1.4 km/s/Mpc by the WMAP.
Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable under the assumption of the Copernican principle—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Friedmann in 1922 and Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson, and Walker.
The theory requires the relation to hold at all times, where is the comoving distance, v is the recessional velocity, and , , and vary as the universe expands (hence we write to denote the present-day Hubble "constant"). For distances much smaller than the size of the observable universe, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity . 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.
That space is undergoing metric expansion is shown by direct observational evidence of the cosmological principle and the Copernican principle, which together with Hubble's law have no other explanation. Astronomical redshifts are extremely isotropic and homogeneous, supporting the cosmological principle that the universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.
Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican principle, that, on a cosmological scale, the Earth is not in a central position. Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the CMB over billions of years is explainable only if the universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.
Cosmic microwave background radiation
In 1964, Arno Penzias and Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the microwave band. Their discovery provided substantial confirmation of the big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a blackbody spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded the 1978 Nobel Prize in Physics.
The surface of last scattering corresponding to emission of the CMB occurs shortly after recombination, the epoch when neutral hydrogen becomes stable. Prior to this, the universe comprised a hot dense photon-baryon plasma sea where photons were quickly scattered from free charged particles. Peaking at around 372±14 kyr, the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent.
In 1989, NASA launched COBE, which made two major advances: in 1990, high-precision spectrum measurements showed that the CMB frequency spectrum is an almost perfect blackbody with no deviations at a level of 1 part in 104, and measured a residual temperature of 2.726 K (more recent measurements have revised this figure down slightly to 2.7255 K); then in 1992, further COBE measurements discovered tiny fluctuations (anisotropies) in the CMB temperature across the sky, at a level of about one part in 105. John C. Mather and George Smoot were awarded the 2006 Nobel Prize in Physics for their leadership in these results.
During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably BOOMERanG, found the shape of the universe to be spatially almost flat by measuring the typical angular size (the size on the sky) of the anisotropies.
In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe were released, yielding what were at the time the most accurate values for some of the cosmological parameters. The results disproved several specific cosmic inflation models, but are consistent with the inflation theory in general. The Planck space probe was launched in May 2009. Other ground and balloon based cosmic microwave background experiments are ongoing.
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. The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for , about 10−3 for , about 10−4 for and about 10−9 for .
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 , and off by a factor of two for (this anomaly is known as the cosmological lithium problem); in the latter two cases, there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium. 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 , and in constant ratios, too.:182–185
Galactic evolution and distribution
Detailed observations of the morphology and distribution of galaxies and quasars are in agreement with the current state of the Big Bang theory. 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.
Primordial gas clouds
In 2011, astronomers found what they believe to be pristine clouds of primordial gas by analyzing absorption lines in the spectra of distant quasars. Before this discovery, all other astronomical objects have been observed to contain heavy elements that are formed in stars. These two clouds of gas contain no elements heavier than hydrogen and deuterium. Since the clouds of gas have no heavy elements, they likely formed in the first few minutes after the Big Bang, during BBN.
Other lines of evidence
The age of the universe as estimated from the Hubble expansion and the CMB is now in good agreement with other estimates using the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars.
The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of very low temperature absorption lines in gas clouds at high redshift. This prediction also implies that the amplitude of the Sunyaev–Zel'dovich effect in clusters of galaxies does not depend directly on redshift. Observations have found this to be roughly true, but this effect depends on cluster properties that do change with cosmic time, making precise measurements difficult.
As with any theory, a number of mysteries and problems have arisen as a result of the development of the Big Bang theory. Some of these mysteries and problems have been resolved while others are still outstanding. Proposed solutions to some of the problems in the Big Bang model have revealed new mysteries of their own. For example, the horizon problem, the magnetic monopole problem, and the flatness problem are most commonly resolved with inflationary theory, but the details of the inflationary universe are still left unresolved and many, including some founders of the theory, say it has been disproven. What follows are a list of the mysterious aspects of the Big Bang theory still under intense investigation by cosmologists and astrophysicists.
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 antibaryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the universe depart from thermodynamic equilibrium. All these conditions occur in the Standard Model, but the effects are not strong enough to explain the present baryon asymmetry.
Measurements of the redshift–magnitude relation for type Ia supernovae indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy".
Dark energy, though speculative, solves numerous problems. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy also helps to explain two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses, and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.
Negative pressure is believed to be a property of vacuum energy, but the exact nature and existence of dark energy remains one of the great mysteries of the Big Bang. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos. According to theory, the energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore, matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.
The dark energy component of the universe has been explained by theorists using a variety of competing theories including Einstein's cosmological constant but also extending to more exotic forms of quintessence or other modified gravity schemes. A cosmological constant problem, sometimes called the "most embarrassing problem in physics", results from the apparent discrepancy between the measured energy density of dark energy, and the one naively predicted from Planck units.
During the 1970s and the 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter has always been controversial, it is inferred by various observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters.
Indirect evidence for dark matter comes from its gravitational influence on other matter, as no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.
Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the dwarf galaxy problem and the cuspy halo problem. Alternative theories have been proposed that do not require a large amount of undetected matter, but instead modify the laws of gravity established by Newton and Einstein; yet no alternative theory has been as successful as the cold dark matter proposal in explaining all extant observations.
The horizon problem results from the premise that information cannot travel faster than light. In a universe of finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in causal contact. The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature.:191–202
A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.:180–186
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.:207 Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB.:sec 6
If inflation occurred, exponential expansion would push large regions of space well beyond our observable horizon.:180–186
A related issue to the classic horizon problem arises because in most standard cosmological inflation models, inflation ceases well before electroweak symmetry breaking occurs, so inflation should not be able to prevent large-scale discontinuities in the electroweak vacuum since distant parts of the observable universe were causally separate when the electroweak epoch ended.
The magnetic monopole objection was raised in the late 1970s. Grand Unified theories (GUTs) predicted topological defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that no monopoles have been found. This problem is also resolved by cosmic inflation, which removes all point defects from the observable universe, in the same way that it drives the geometry to flatness.
The flatness problem (also known as the oldness problem) is an observational problem associated with a FLRW. The universe may have positive, negative, or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density; positive if greater; and zero at the critical density, in which case space is said to be flat.
The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat.[notes 4] Given that a natural timescale for departure from flatness might be the Planck time, 10−43 seconds, the fact that the universe has reached neither a heat death nor a Big Crunch after billions of years requires an explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the density of the universe must have been within one part in 1014 of its critical value, or it would not exist as it does today.
Ultimate fate of the universe
Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started—a Big Crunch.
Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out, leaving white dwarfs, neutron stars, and black holes. Collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would very gradually asymptotically approach absolute zero—a Big Freeze. Moreover, if protons are unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.
Modern observations of accelerating expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death as the universe expands and cools. Other explanations of dark energy, called phantom energy theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.
One of the common misconceptions about the Big Bang model is that it fully explains the origin of the universe. However, the Big Bang model does not describe how energy, time, and space was caused, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state. It is misleading to visualize the Big Bang by comparing its size to everyday objects. When the size of the universe at Big Bang is described, it refers to the size of the observable universe, and not the entire universe.
Hubble's law predicts that galaxies that are beyond Hubble distance recede faster than the speed of light. However, special relativity does not apply beyond motion through space. Hubble's law describes velocity that results from expansion of space, rather than through space.
Astronomers often refer to the cosmological redshift as a Doppler shift which can lead to a misconception. Although similar, the cosmological redshift is not identical to the classically derived Doppler redshift because most elementary derivations of the Doppler redshift do not accommodate the expansion of space. Accurate derivation of the cosmological redshift requires the use of general relativity, and while a treatment using simpler Doppler effect arguments gives nearly identical results for nearby galaxies, interpreting the redshift of more distant galaxies as due to the simplest Doppler redshift treatments can cause confusion.
Pre-Big Bang cosmology
The Big Bang explains the evolution of the universe from a density and temperature that is well beyond humanity's capability to replicate, so extrapolations to most extreme conditions and earliest times are necessarily more speculative. Lemaître called this initial state the "primeval atom" while Gamow called the material "ylem". How the initial state of the universe originated is still an open question, but the Big Bang model does constrain some of its characteristics. For example, specific laws of nature most likely came to existence by random way, but as inflation models show, some combinations of them are far more probable. In any case, observations indicate the universe is consistent with being flat which implies a balance between gravitational potential energy and other forms requiring no additional energy to be created, while quantum fluctuations in the early universe can provide the circumstances for dense regions of matter (such as superclusters) to form. Ultimately, the Big Bang theory, built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time, and such an infinite energy density may be a physical impossibility. In any case, the physical theories of general relativity and quantum mechanics as currently realized are not applicable before the Planck epoch, and correcting this will require the development of a correct treatment of quantum gravity. Certain quantum gravity treatments, such as the Wheeler–DeWitt equation, imply that time itself could be an emergent property. As such, physics may conclude that time did not exist before the Big Bang.
While it is not known what could have preceded the hot dense state of the early universe or how and why it originated, or even whether such questions are sensible, speculation abounds as the subject of "cosmogony".
Some speculative proposals in this regard, each of which entails untested hypotheses, are:
- Models including the Hartle–Hawking no-boundary condition, in which the whole of spacetime is finite; the Big Bang does represent the limit of time but without any singularity.  In such case, the universe is self-sufficient and caused its own existence by itself.
- Brane cosmology models, in which inflation is due to the movement of branes in string theory; the pre-Big Bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically. In the latter model the Big Bang was preceded by a Big Crunch and the universe cycles from one process to the other.
- Eternal inflation, in which universal inflation ends locally here and there in a random fashion, each end-point leading to a bubble universe, expanding from its own big bang.
Jim Peebles, awarded the 2019 Nobel Prize in Physics for his "theoretical discoveries in physical cosmology", noted in his award presentation that he does not support the Big Bang theory, due to the lack of concrete supporting evidence, and stated, "It's very unfortunate that one thinks of the beginning whereas in fact, we have no good theory of such a thing as the beginning."
Religious and philosophical interpretations
As a description of the origin of the universe, the Big Bang has significant bearing on religion and philosophy. As a result, it has become one of the liveliest areas in the discourse between science and religion. Some believe the Big Bang implies a creator, and some see its mention in their holy books, while others argue that Big Bang cosmology makes the notion of a creator superfluous.
- Big Bounce – A hypothetical cosmological model for the origin of the known universe
- Big Crunch – Theoretical scenario for the ultimate fate of the universe
- Cold Big Bang – A designation of an absolute zero temperature at the beginning of the Universe
- Cosmic Calendar
- Eureka: A Prose Poem – A lengthy non-fiction work by American author Edgar Allan Poe, a Big Bang speculation
- Shape of the universe – The local and global geometry of the universe
- Detailed information of and references for tests of general relativity are given in the article tests of general relativity.
- There is no consensus about how long the Big Bang phase lasted. For some writers, this denotes only the initial singularity, for others the whole history of the universe. Usually, at least the first few minutes (during which helium is synthesized) are said to occur "during the Big Bang".
- It is commonly reported that Hoyle intended this to be pejorative. However, Hoyle later denied that, saying that it was just a striking image meant to emphasize the difference between the two theories for radio listeners.
- Strictly, dark energy in the form of a cosmological constant drives the universe towards a flat state; however, our universe remained close to flat for several billion years before the dark energy density became significant.
- Silk 2009, p. 208.
- Singh 2004, p. 560. Book limited to 532 pages. Correct source page requested.
- NASA/WMAP Science Team (6 June 2011). "Cosmology: The Study of the Universe". Universe 101: Big Bang Theory. Washington, D.C.: NASA. Archived from the original on 29 June 2011. Retrieved 18 December 2019.
The second section discusses the classic tests of the Big Bang theory that make it so compelling as the most likely valid and accurate description of our universe.
- Bridge, Mark (Director) (30 July 2014). First Second of the Big Bang. How The Universe Works. Silver Spring, MD. Science Channel.
- Kragh 1996, p. 319: "At the same time that observations tipped the balance definitely in favor of relativistic big-bang theory, ..."
- Partridge 1995, p. xvii
- Chow 2008, p. 211
- "Planck reveals an almost perfect universe". Planck. Paris: ESA. 21 March 2013. Archived from the original on 6 June 2019. Retrieved 15 April 2017.
- Peebles, P. J. E.; Ratra, Bharat (22 April 2003). "The cosmological constant and dark energy". Reviews of Modern Physics. College Park, MD: American Physical Society. 75 (2): 559–606. arXiv:astro-ph/0207347. Bibcode:2003RvMP...75..559P. doi:10.1103/RevModPhys.75.559. ISSN 0034-6861.
- Wright, Edward L. (24 May 2013). "Frequently Asked Questions in Cosmology: What is the evidence for the Big Bang?". Ned Wright's Cosmology Tutorial. Los Angeles: Division of Astronomy & Astrophysics, University of California, Los Angeles. Archived from the original on 20 June 2013. Retrieved 25 November 2019.
- Ivanchik, Alexandre V.; Potekhin, Alexander Y.; Varshalovich, Dmitry A. (March 1999). "The fine-structure constant: a new observational limit on its cosmological variation and some theoretical consequences". Astronomy & Astrophysics. Les Ulis: EDP Sciences for European Southern Observatory. 343 (2): 439–445. arXiv:astro-ph/9810166. Bibcode:1999A&A...343..439I. ISSN 0004-6361.
- Goodman, Jeremy (15 August 1995). "Geocentrism reexamined" (PDF). Physical Review D. College Park, MD: American Physical Society. 52 (4): 1821–1827. arXiv:astro-ph/9506068. Bibcode:1995PhRvD..52.1821G. doi:10.1103/PhysRevD.52.1821. ISSN 1550-7998. PMID 10019408. Archived (PDF) from the original on 2 May 2019. Retrieved 2 December 2019.
- d'Inverno 1992, chpt. 23
- Davis, Tamara M.; Lineweaver, Charles H. (31 March 2004). "Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe". Publications of the Astronomical Society of Australia. Cambridge, UK: Cambridge University Press on behalf of the Astronomical Society of Australia. 21 (1): 97–109. arXiv:astro-ph/0310808. Bibcode:2004PASA...21...97D. doi:10.1071/as03040. ISSN 1323-3580.
- Kolb & Turner 1988, chpt. 3
- Hawking & Ellis 1973
- Roos 2012, p. 216: "This singularity is termed the Big Bang."
- Drees 1990, pp. 223–224
- Planck Collaboration (October 2016). "Planck 2015 results. XIII. Cosmological parameters" (PDF). Astronomy & Astrophysics. Les Ulis: EDP Sciences for European Southern Observatory. 594: Article A13. arXiv:1502.01589. Bibcode:2016A&A...594A..13P. doi:10.1051/0004-6361/201525830. ISSN 0004-6361. Archived (PDF) from the original on 9 October 2019. Retrieved 29 November 2019. (See PDF, page 32, Table 4, Age/Gyr, last column.)
- Guth 1998
- Schewe, Phillip F.; Stein, Ben P. (20 April 2005). "An Ocean of Quarks". Physics News Update. Vol. 728 no. 1. College Park, MD: American Institute of Physics. OCLC 40221781. Archived from the original on 23 April 2005. Retrieved 30 November 2019.
- Kolb & Turner 1988, chpt. 6
- Kolb & Turner 1988, chpt. 7
- Kolb & Turner 1988, chpt. 4
- Peacock 1999, chpt. 9
- Loeb, Abraham (October 2014). "The habitable epoch of the early Universe" (PDF). International Journal of Astrobiology. Cambridge, UK: Cambridge University Press. 13 (4): 337–339. arXiv:1312.0613. Bibcode:2014IJAsB..13..337L. CiteSeerX 10.1.1.748.4820. doi:10.1017/S1473550414000196. ISSN 1473-5504. Archived (PDF) from the original on 29 April 2019. Retrieved 30 November 2019.
- Dreifus, Claudia (1 December 2014). "Much-Discussed Views That Go Way Back - Avi Loeb Ponders the Early Universe, Nature and Life". Science. The New York Times. New York: The New York Times Company. ISSN 0362-4331. Archived from the original on 27 March 2015. Retrieved 3 December 2014. "A version of this article appears in print on Dec. 2, 2014, Section D, Page 2 of the New York edition with the headline: Much-Discussed Views That Go Way Back."
- Clavin, Whitney; Jenkins, Ann; Villard, Ray (7 January 2014). "NASA's Hubble and Spitzer Team up to Probe Faraway Galaxies". Jet Propulsion Laboratory. Washington, D.C.: NASA. Archived from the original on 3 September 2019. Retrieved 8 January 2014.
- Spergel, David N.; Verde, Licia; Peiris, Hiranya V.; et al. (September 2003). "First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters". The Astrophysical Journal Supplement Series. Chicago: University of Chicago Press for the American Astronomical Society. 148 (1): 175–194. arXiv:astro-ph/0302209. Bibcode:2003ApJS..148..175S. doi:10.1086/377226. ISSN 1538-4365.
- Jarosik, Norman; Bennett, Charles L.; Dunkley, Jo; et al. (February 2011). "Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results" (PDF). The Astrophysical Journal Supplement Series. Bristol: IOP Publishing for the American Astronomical Society. 192 (2): Article 14. arXiv:1001.4744. Bibcode:2011ApJS..192...14J. doi:10.1088/0067-0049/192/2/14. ISSN 1538-4365. Archived (PDF) from the original on 14 September 2019. Retrieved 2 December 2019. (See PDF, page 39, Table 8.)
- Manly 2011, chpt. 7: "The Ultimate Free Lunch"[page needed].
- "Hoyle on the Radio: Creating the 'Big Bang'". Fred Hoyle: An Online Exhibition. Cambridge, UK: St John's College. Archived from the original on 26 May 2014. Retrieved 2 December 2019.
- Kragh, Helge (April 2013). "Big Bang: the etymology of a name". Astronomy & Geophysics. Oxford, UK: Oxford University Press. 54 (2): 2.28–2.30. doi:10.1093/astrogeo/att035. ISSN 1366-8781.
- Mattson, Barbara (Project Leader) (8 December 2017). "Hoyle Scoffs at 'Big Bang' Universe Theory". Cosmic Times (hosted by Imagine the Universe!). Greenbelt, MD: NASA: High Energy Astrophysics Science Archive Research Center. OCLC 227004453. Archived from the original on 10 March 2018. Retrieved 2 December 2019.
- "'Big bang' astronomer dies". Sci/Tech. BBC News. London: BBC. 22 August 2001. Archived from the original on 3 September 2019. Retrieved 2 December 2019.
- Croswell 1995, chpt. 9
- Mitton 2011, p. 129: "To create a picture in the mind of the listener, Hoyle had likened the explosive theory of the universe's origin to a 'big bang'."
- Moskowitz, Clara (25 September 2012). "Hubble Telescope Reveals Farthest View Into Universe Ever". Space.com. New York: Future plc. Archived from the original on 12 October 2019. Retrieved 3 December 2019.
- Slipher, Vesto M. (1913). "The Radial Velocity of the Andromeda Nebula". Lowell Observatory Bulletin. Flagstaff, AZ: Lowell Observatory. 1: 56–57. Bibcode:1913LowOB...2...56S. ISSN 0024-7057.
- Slipher, Vesto M. (January 1915). "Spectrographic Observations of Nebulae". Popular Astronomy. Northfield, MN: Goodsell Observatory. 23: 21–24. Bibcode:1915PA.....23...21S. OCLC 499322421.
- Friedman, Alexander (December 1922). "Über die Krümmung des Raumes". Zeitschrift für Physik (in German). Springer Berlin Heidelberg. 10 (1): 377–386. Bibcode:1922ZPhy...10..377F. doi:10.1007/BF01332580. ISSN 0044-3328.
- Lemaître, Georges (April 1927). "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques". Annales de la Société scientifique de Bruxelles (in French). Namur: Société scientifique de Bruxelles. 47: 49–59. Bibcode:1927ASSB...47...49L. ISSN 0037-959X. OCLC 1765868.
- Translated in: Lemaître, Georges (March 1931). "A Homogeneous Universe of Constant Mass and Increasing Radius accounting for the Radial Velocity of Extra-galactic Nebulæ". Monthly Notices of the Royal Astronomical Society. London: Royal Astronomical Society. 91 (5): 483–490. Bibcode:1931MNRAS..91..483L. doi:10.1093/mnras/91.5.483. ISSN 0035-8711.
- Lemaître, Abbé Georges (24 October 1931). "Contributions to a British Association Discussion on the Evolution of the Universe". Nature. London: Nature Publishing Group. 128 (3234): 704–706. Bibcode:1931Natur.128..704L. doi:10.1038/128704a0. ISSN 0028-0836. OCLC 4650144498.
- Hubble, Edwin (15 March 1929). "A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae". Proceedings of the National Academy of Sciences. Washington, D.C.: National Academy of Sciences. 15 (3): 168–173. Bibcode:1929PNAS...15..168H. doi:10.1073/pnas.15.3.168. ISSN 0027-8424. PMC 522427. PMID 16577160. Archived from the original on 1 October 2006. Retrieved 28 November 2019.
- Christianson 1995
- Kragh 1996
- "Big bang theory is introduced – 1927". A Science Odyssey. Boston, MA: WGBH Boston. 1998. Archived from the original on 23 April 1999. Retrieved 31 July 2014.
- Eddington, Arthur S. (21 March 1931). "The End of the World: from the Standpoint of Mathematical Physics". Nature. London: Nature Publishing Group. 127 (3203): 447–453. Bibcode:1931Natur.127..447E. doi:10.1038/127447a0. ISSN 0028-0836.
- Appolloni, Simon (17 June 2011). "'Repugnant', 'Not Repugnant at All': How the Respective Epistemic Attitudes of Georges Lemaitre and Sir Arthur Eddington Influenced How Each Approached the Idea of a Beginning of the Universe". IBSU Scientific Journal. Tbilisi, Georgia: International Black Sea University. 5 (1): 19–44. ISSN 2233-3002.
- Lemaître, Georges (9 May 1931). "The Beginning of the World from the Point of View of Quantum Theory". Nature. London: Nature Publishing Group. 127 (3210): 706. Bibcode:1931Natur.127..706L. doi:10.1038/127706b0. ISSN 0028-0836.
- Milne 1935
- Tolman 1934
- Zwicky, Fritz (15 October 1929). "On the Red Shift of Spectral Lines through Interstellar Space". Proceedings of the National Academy of Sciences. Washington, D.C.: National Academy of Sciences. 15 (10): 773–779. Bibcode:1929PNAS...15..773Z. doi:10.1073/pnas.15.10.773. ISSN 0027-8424. PMC 522555. PMID 16577237.
- Hoyle, Fred (October 1948). "A New Model for the Expanding Universe". Monthly Notices of the Royal Astronomical Society. London: Royal Astronomical Society. 108 (5): 372–382. Bibcode:1948MNRAS.108..372H. doi:10.1093/mnras/108.5.372. ISSN 0035-8711.
- Alpher, Ralph A.; Bethe, Hans; Gamow, George (1 April 1948). "The Origin of Chemical Elements". Physical Review. College Park, MD: American Physical Society. 73 (7): 803–804. Bibcode:1948PhRv...73..803A. doi:10.1103/PhysRev.73.803. ISSN 0031-899X.
- Alpher, Ralph A.; Herman, Robert (13 November 1948). "Evolution of the Universe". Nature. London: Nature Publishing Group. 162 (4124): 774–775. Bibcode:1948Natur.162..774A. doi:10.1038/162774b0. ISSN 0028-0836.
- Penzias, Arno A.; Wilson, R. W. (July 1965). "A Measurement of Excess Antenna Temperature at 4080 Mc/s". The Astrophysical Journal. Chicago: University of Chicago Press for the American Astronomical Society. 142: 419–421. Bibcode:1965ApJ...142..419P. doi:10.1086/148307. ISSN 0004-637X. Archived from the original on 14 October 2019. Retrieved 5 December 2019.
- Hawking, Stephen W.; Ellis, George F. R. (April 1968). "The Cosmic Black-Body Radiation and the Existence of Singularities in our Universe". The Astrophysical Journal. Chicago: University of Chicago Press for the American Astronomical Society. 152: 25. Bibcode:1968ApJ...152...25H. doi:10.1086/149520. ISSN 0004-637X.
- Hawking, Stephen W.; Penrose, Roger (27 January 1970). "The Singularities of Gravitational Collapse and Cosmology". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. London: Royal Society. 314 (1519): 529–548. Bibcode:1970RSPSA.314..529H. doi:10.1098/rspa.1970.0021. ISSN 1364-5021. Archived from the original on 2 May 2019. Retrieved 5 December 2019.
- Guth, Alan (15 January 1981). "Inflationary universe: A possible solution to the horizon and flatness problems". Physical Review D. College Park, MD: American Physical Society. 23 (2): 347–356. Bibcode:1981PhRvD..23..347G. doi:10.1103/PhysRevD.23.347. ISSN 1550-7998.
- Huchra, John P. (2008). "The Hubble Constant". John Huchra's Website. Cambridge, MA: Harvard-Smithsonian Center for Astrophysics. Archived from the original on 30 September 2019. Retrieved 5 December 2019.
- Livio 2000, p. 160
- Navabi, Ali Akbar; Riazi, Nematollah (March 2003). "Is the Age Problem Resolved?". Journal of Astrophysics and Astronomy. New Delhi: Springer India, Indian Academy of Sciences, and Astronomical Society of India. 24 (1–2): 3–10. Bibcode:2003JApA...24....3N. doi:10.1007/BF03012187. ISSN 0250-6335.
- Boggess, Nancy W.; Mather, John C.; Weiss, Rainer; et al. (1 October 1992). "The COBE Mission: Its Design and Performance Two Years after the launch". The Astrophysical Journal. Chicago: University of Chicago Press for the American Astronomical Society. 397: 420–429. Bibcode:1992ApJ...397..420B. doi:10.1086/171797. ISSN 0004-637X. Retrieved 6 December 2019.
- Spergel, David N.; Bean, Rachel; Doré, Olivier; et al. (June 2007). "Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Implications for Cosmology". The Astrophysical Journal Supplement Series. Chicago: University of Chicago Press for the American Astronomical Society. 170 (2): 377–408. arXiv:astro-ph/0603449. Bibcode:2007ApJS..170..377S. doi:10.1086/513700. ISSN 1538-4365.
- Krauss 2012, p. 118
- Gladders, Michael D.; Yee, H. K. C.; Majumdar, Subhabrata; et al. (20 January 2007). "Cosmological Constraints from the Red-Sequence Cluster Survey". The Astrophysical Journal. Chicago: University of Chicago Press for the American Astronomical Society. 655 (1): 128–134. arXiv:astro-ph/0603588. Bibcode:2007ApJ...655..128G. doi:10.1086/509909. ISSN 0004-637X.
- Shellard, Paul; et al., eds. (2012). "The Four Pillars of the Standard Cosmology". Outreach. Cambridge, UK: Centre for Theoretical Cosmology; University of Cambridge. Archived from the original on 2 November 2013. Retrieved 6 December 2019.
- Sadoulet, Bernard; et al. "Direct Searches for Dark Matter" (PDF). Astro2010: The Astronomy and Astrophysics Decadal Survey (white paper). Washington, D.C.: National Academies Press on behalf of the National Research Council of the National Academy of Sciences. OCLC 850950122. Archived from the original on 13 April 2009. Retrieved 8 December 2019.
- Diemand, Jürg; Zemp, Marcel; Moore, Ben; Stadel, Joachim; Carollo, C. Marcella (December 2005). "Cusps in cold dark matter haloes". Monthly Notices of the Royal Astronomical Society. London: Royal Astronomical Society. 364 (2): 665–673. arXiv:astro-ph/0504215. Bibcode:2005MNRAS.364..665D. doi:10.1111/j.1365-2966.2005.09601.x. ISSN 0035-8711.
- Martínez-Delgado 2013, pp. 95–122, chpt. 3 "Notes on the Missing Satellites Problem" by James S. Bullock. Bibcode: 2010arXiv1009.4505B
- Cahn, Robert N.; et al. (2009). "Whitepaper: For a Comprehensive Space-Based Dark Energy Mission" (PDF). Astro2010: The Astronomy and Astrophysics Decadal Survey, Science White Papers, no. 35 (white paper). Washington, D.C.: National Academies Press on behalf of the National Research Council of the National Academy of Sciences. Bibcode:2009astro2010S..35B. OCLC 850950122. Archived from the original on 7 August 2011. Retrieved 8 December 2019.
- Peacock 1999, chpt. 3
- Srianand, Raghunathan; Petitjean, Patrick; Ledoux, Cédric (21 December 2000). "The cosmic microwave background radiation temperature at a redshift of 2.34". Nature. London: Nature Publishing Group. 408 (6815): 931–935. arXiv:astro-ph/0012222. Bibcode:2000Natur.408..931S. doi:10.1038/35050020. ISSN 0028-0836. PMID 11140672. Lay summary – European Southern Observatory (20 December 2000).
- White, Martin (1999). "Anisotropies in the CMB" (PDF). In Arisaka, Katsushi; Bern, Zvi (eds.). DPF 99: Proceedings of the Los Angeles Meeting. Division of Particles and Fields Conference 1999 (DPF '99). Los Angeles: University of California, Los Angeles on behalf of the American Physical Society. arXiv:astro-ph/9903232. Bibcode:1999dpf..conf.....W. OCLC 43669022. Talk #9-10: The Cosmic Microwave Background. Archived (PDF) from the original on 4 February 2017. Retrieved 9 December 2019.
- Bennett, Charles L.; Larson, Davin; Weiland, Janet L.; et al. (October 2013). "Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Final Maps and Results". The Astrophysical Journal Supplement Series. Bristol: IOP Publishing for the American Astronomical Society. 208 (2): Article 20. arXiv:1212.5225. Bibcode:2013ApJS..208...20B. doi:10.1088/0067-0049/208/2/20. ISSN 1538-4365.
- Gannon, Megan (21 December 2012). "New 'Baby Picture' of Universe Unveiled". Space.com. New York: Future plc. Archived from the original on 29 October 2019. Retrieved 9 December 2019.
- Wright 2004, p. 291
- Melchiorri, Alessandro; Ade, Peter A.R.; de Bernardis, Paolo; et al. (20 June 2000). "A Measurement of Ω from the North American Test Flight of Boomerang". The Astrophysical Journal Letters. Chicago: University of Chicago Press for the American Astronomical Society. 536 (2): L63–L66. arXiv:astro-ph/9911445. Bibcode:2000ApJ...536L..63M. doi:10.1086/312744. ISSN 2041-8213. PMID 10859119.
- de Bernardis, Paolo; Ade, Peter A.R.; Bock, James J.; et al. (27 April 2000). "A Flat Universe from High-Resolution Maps of the Cosmic Microwave Background Radiation" (PDF). Nature. London: Nature Publishing Group. 404 (6781): 955–959. arXiv:astro-ph/0004404. Bibcode:2000Natur.404..955D. doi:10.1038/35010035. hdl:10044/1/60851. ISSN 0028-0836. PMID 10801117. Archived (PDF) from the original on 2 May 2019. Retrieved 10 December 2019.
- Miller, Andre D.; Caldwell, Robert H.; Devlin, Mark Joseph; et al. (10 October 1999). "A Measurement of the Angular Power Spectrum of the Cosmic Microwave Background from l = 100 to 400". The Astrophysical Journal Letters. Chicago: University of Chicago Press for the American Astronomical Society. 524 (1): L1–L4. arXiv:astro-ph/9906421. Bibcode:1999ApJ...524L...1M. doi:10.1086/312293. ISSN 2041-8213.
- Steigman, Gary (February 2006). "Primordial Nucleosynthesis: Successes And Challenges". International Journal of Modern Physics E. Singapore: World Scientific. 15 (1): 1–36. arXiv:astro-ph/0511534. Bibcode:2006IJMPE..15....1S. CiteSeerX 10.1.1.337.542. doi:10.1142/S0218301306004028. ISSN 0218-3013.
- Ryden 2003
- Bertschinger, Edmund (2000). "Cosmological Perturbation Theory and Structure Formation". arXiv:astro-ph/0101009.
- Bertschinger, Edmund (September 1998). "Simulations of Structure Formation in the Universe". Annual Review of Astronomy and Astrophysics. Palo Alto, CA: Annual Reviews. 36 (1): 599–654. Bibcode:1998ARA&A..36..599B. doi:10.1146/annurev.astro.36.1.599. ISSN 0066-4146.
- "BICEP2 March 2014 Results and Data Products". The BICEP and Keck Array CMB Experiments. Cambridge, MA: FAS Research Computing, Harvard University. 16 December 2014 [Results originally released on 17 March 2014]. Archived from the original on 18 March 2014. Retrieved 10 December 2019.
- Clavin, Whitney (17 March 2014). "NASA Technology Views Birth of the Universe". Jet Propulsion Laboratory. Washington, D.C.: NASA. Archived from the original on 10 October 2019. Retrieved 10 December 2019.
- Overbye, Dennis (17 March 2014). "Space Ripples Reveal Big Bang's Smoking Gun". Space & Cosmos. The New York Times. New York: The New York Times Company. ISSN 0362-4331. Archived from the original on 17 March 2014. Retrieved 11 December 2019. "A version of this article appears in print on March 18, 2014, Section A, Page 1 of the New York edition with the headline: Space Ripples Reveal Big Bang's Smoking Gun." The online version of this article was originally titled "Detection of Waves in Space Buttresses Landmark Theory of Big Bang".
- Overbye, Dennis (24 March 2014). "Ripples From the Big Bang". Out There. The New York Times. New York: The New York Times Company. ISSN 0362-4331. Archived from the original on 25 March 2014. Retrieved 24 March 2014. "A version of this article appears in print on March 25, 2014, Section D, Page 1 of the New York edition with the headline: Ripples From the Big Bang."
- Fumagalli, Michele; O'Meara, John M.; Prochaska, J. Xavier (2 December 2011). "Detection of Pristine Gas Two Billion Years After the Big Bang". Science. Washington, D.C.: American Association for the Advancement of Science. 334 (6060): 1245–1249. arXiv:1111.2334. Bibcode:2011Sci...334.1245F. doi:10.1126/science.1213581. ISSN 0036-8075. PMID 22075722.
- Stephens, Tim (10 November 2011). "Astronomers find clouds of primordial gas from the early universe". University News & Events. Santa Cruz, CA: University of California, Santa Cruz. Archived from the original on 14 November 2011. Retrieved 11 December 2019.
- Perley, Daniel (21 February 2005). "Determination of the Universe's Age, to". Berkeley, CA: Department of Astronomy, University of California, Berkeley. Archived from the original on 11 September 2006. Retrieved 11 December 2019.
- Srianand, Raghunathan; Noterdaeme, Pasquier; Ledoux, Cédric; et al. (May 2008). "First detection of CO in a high-redshift damped Lyman-α system". Astronomy & Astrophysics. Les Ulis: EDP Sciences for European Southern Observatory. 482 (3): L39–L42. Bibcode:2008A&A...482L..39S. doi:10.1051/0004-6361:200809727. ISSN 0004-6361.
- Avgoustidis, Anastasios; Luzzi, Gemma; Martins, Carlos J.A.P.; et al. (14 February 2012). "Constraints on the CMB temperature-redshift dependence from SZ and distance measurements". Journal of Cosmology and Astroparticle Physics. 2012 (2): Article 013. arXiv:1112.1862. Bibcode:2012JCAP...02..013A. CiteSeerX 10.1.1.758.6956. doi:10.1088/1475-7516/2012/02/013. ISSN 1475-7516.
- Belušević 2008, p. 16
- Ghosh, Pallab (11 February 2016). "Einstein's gravitational waves 'seen' from black holes". Science & Environment. BBC News. London: BBC. Archived from the original on 11 February 2016. Retrieved 13 April 2017.
- Billings, Lee (12 February 2016). "The Future of Gravitational Wave Astronomy". Scientific American. Stuttgart: Springer Nature. ISSN 0036-8733. Archived from the original on 13 February 2016. Retrieved 13 April 2017.
- Earman, John; Mosterín, Jesús (March 1999). "A Critical Look at Inflationary Cosmology". Philosophy of Science. Chicago: University of Chicago Press on behalf of the Philosophy of Science Association. 66 (1): 1–49. doi:10.1086/392675. ISSN 0031-8248. JSTOR 188736.
- Hawking & Israel 2010, pp. 581–638, chpt. 12: "Singularities and time-asymmetry" by Roger Penrose.
- Penrose 1989
- Steinhardt, Paul J. (April 2011). "The Inflation Debate: Is the theory at the heart of modern cosmology deeply flawed?" (PDF). Scientific American. Vol. 304 no. 4. Stuttgart: Springer Nature. pp. 36–43. doi:10.1038/scientificamerican0411-36. ISSN 0036-8733. Archived (PDF) from the original on 1 November 2019. Retrieved 23 December 2019.
- Sakharov, Andrei D. (10 January 1967). "Нарушение СР-инвариантности, С-асимметрия и барионная асимметрия Вселенной" [Violation of CP-invariance, C-asymmetry and baryon asymmetry of the Universe] (PDF). Pis'ma v ZhETF (in Russian). Moscow: Nauka. 5 (1): 32–35. doi:10.1070/PU1991v034n05ABEH002497. Archived (PDF) from the original on 28 July 2018.
- Translated in: Sakharov, Andrei D. (10 January 1967). "Violation of CP Invariance, С Asymmetry, and Baryon Asymmetry of the Universe" (PDF). JETP Letters. Moscow: Nauka. 5 (1): 24–27. Archived (PDF) from the original on 9 November 2019. Retrieved 13 December 2019.
- Reprinted in: Kolb & Turner 1988, pp. 371-373
- Translated in: Sakharov, Andrei D. (10 January 1967). "Violation of CP Invariance, С Asymmetry, and Baryon Asymmetry of the Universe" (PDF). JETP Letters. Moscow: Nauka. 5 (1): 24–27. Archived (PDF) from the original on 9 November 2019. Retrieved 13 December 2019.
- Tanabashi, M. 2018, pp. 406–413, chpt. 27: "Dark Energy" (Revised September 2017) by David H. Weinberg and Martin White.
- Rugh, Svend E.; Zinkernagel, Henrik (December 2002). "The quantum vacuum and the cosmological constant problem". Studies in History and Philosophy of Science Part B. Amsterdam: Elsevier. 33 (4): 663–705. arXiv:hep-th/0012253. Bibcode:2002SHPMP..33..663R. doi:10.1016/S1355-2198(02)00033-3. ISSN 1355-2198.
- Keel, William C. (October 2009) [Last changes: February 2015]. "Dark Matter". Bill Keel's Lecture Notes - Galaxies and the Universe. Archived from the original on 3 May 2019. Retrieved 15 December 2019.
- Tanabashi, M. 2018, pp. 396–405, chpt. 26: "Dark Matter" (Revised September 2017) by Manuel Drees and Gilles Gerbier.
- Dodelson, Scott (31 December 2011). "The Real Problem with MOND". International Journal of Modern Physics D. Singapore: World Scientific. 20 (14): 2749–2753. arXiv:1112.1320. Bibcode:2011IJMPD..20.2749D. doi:10.1142/S0218271811020561. ISSN 0218-2718.
- Kolb & Turner 1988, chpt. 8
- Penrose 2007
- Hawking & Israel 2010, pp. 504–517, chpt. 9: "The big bang cosmology — enigmas and nostrums" by Robert H. Dicke and Phillip J.E. Peebles.
- NASA/WMAP Science Team (29 June 2015). "What is the Ultimate Fate of the Universe?". Universe 101: Big Bang Theory. Washington, D.C: NASA. Archived from the original on 15 October 2019. Retrieved 18 December 2019.
- Adams, Fred C.; Laughlin, Gregory (April 1997). "A dying universe: the long-term fate and evolution of astrophysical objects". Reviews of Modern Physics. College Park, MD: American Physical Society. 69 (2): 337–372. arXiv:astro-ph/9701131. Bibcode:1997RvMP...69..337A. doi:10.1103/RevModPhys.69.337. ISSN 0034-6861..
- Caldwell, Robert R.; Kamionkowski, Marc; Weinberg, Nevin N. (15 August 2003). "Phantom Energy: Dark Energy with w<−1 Causes a Cosmic Doomsday". Physical Review Letters. College Park, MD: American Physical Society. 91 (7): 071301. arXiv:astro-ph/0302506. Bibcode:2003PhRvL..91g1301C. doi:10.1103/PhysRevLett.91.071301. ISSN 0031-9007. PMID 12935004.
- "Brief Answers to Cosmic Questions". Universe Forum. Cambridge, MA: Harvard–Smithsonian Center for Astrophysics. Archived from the original on 13 April 2016. Retrieved 18 December 2019. Archival site: "The Universe Forum's role as part of NASA's Education Support Network concluded in September, 2009."
- Hawking 1988, p. 69.
- Filippenko, Alexei V.; Pasachoff, Jay M. (March–April 2002). "A Universe from Nothing". Mercury. Vol. 31 no. 2. San Francisco: Astronomical Society of the Pacific. p. 15. Bibcode:2002Mercu..31b..15F. ISSN 0047-6773. Archived from the original on 22 October 2013. Retrieved 10 March 2010.
- Lawrence M. Krauss (Speaker); R. Elisabeth Cornwell (Producer) (21 October 2009). 'A Universe From Nothing' by Lawrence Krauss, AAI 2009 (Video). Washington, D.C.: Richard Dawkins Foundation for Reason and Science. Retrieved 17 October 2011.
- Carroll n.d.
- Beckers, Mike (16 February 2015). "Quantentrick schafft Urknall-Singularität ab" [Quantum Trick Eliminates the Big Bang Singularity]. Cosmology. Spektrum der Wissenschaft (in German). Stuttgart: Springer Nature. ISSN 0170-2971. Archived from the original on 21 July 2017. Retrieved 19 December 2019. Google translation
- Ali, Ahmed Farag; Das, Saurya (4 February 2015). "Cosmology from quantum potential". Physics Letters B. Amsterdam: Elsevier. 741: 276–279. arXiv:1404.3093v3. doi:10.1016/j.physletb.2014.12.057. ISSN 0370-2693.
- Das, Saurya; Rajat K., Bhaduri (21 May 2015). "Dark matter and dark energy from a Bose–Einstein condensate". Classical and Quantum Gravity. Bristol: IOP Publishing. 32 (10): 105003. arXiv:1411.0753. Bibcode:2015CQGra..32j5003D. doi:10.1088/0264-9381/32/10/105003. ISSN 0264-9381.
- Hawking, Stephen W. (1996). "The Beginning of Time". Stephen Hawking (Lecture). London: The Stephen Hawking Foundation. Archived from the original on 6 November 2019. Retrieved 26 April 2017.
- Hartle, James H.; Hawking, Stephen W. (15 December 1983). "Wave function of the Universe". Physical Review D. College Park, MD: American Physical Society. 28 (12): 2960–2975. Bibcode:1983PhRvD..28.2960H. doi:10.1103/PhysRevD.28.2960. ISSN 1550-7998.
- Hawking 1988, p. 71.
- Langlois, David (June 2002). "Brane Cosmology". Progress of Theoretical Physics Supplement. Oxford, UK: Oxford University Press on behalf of the Physical Society of Japan. 148: 181–212. arXiv:hep-th/0209261. Bibcode:2002PThPS.148..181L. doi:10.1143/PTPS.148.181. ISSN 0375-9687.
- Gibbons, Shellard & Rankin 2003, pp. 801–838, chpt. 43: "Inflationary theory versus the ekpyrotic/cyclic scenario" by Andrei Linde. Bibcode: 2003ftpc.book..801L
- Than, Ker (8 May 2006). "Recycled Universe: Theory Could Solve Cosmic Mystery". Space.com. New York: Future plc. Archived from the original on 6 September 2019. Retrieved 19 December 2019.
- Kennedy, Barbara K. (1 July 2007). "What Happened Before the Big Bang?". News and Events. University Park, PA: Eberly College of Science, Pennsylvania State University. Archived from the original on 15 December 2019. Retrieved 19 December 2019.
- Linde, Andrei D. (May 1986). "Eternal Chaotic Inflation". Modern Physics Letters A. Singapore: World Scientific. 1 (2): 81–85. Bibcode:1986MPLA....1...81L. doi:10.1142/S0217732386000129. ISSN 0217-7323. Archived from the original on 17 April 2019.
- Linde, Andrei D. (14 August 1986). "Eternally Existing Self-Reproducing Chaotic Inflationary Universe". Physics Letters B. Amsterdam: Elsevier. 175 (4): 395–400. Bibcode:1986PhLB..175..395L. doi:10.1016/0370-2693(86)90611-8. ISSN 0370-2693.
- Hooper, Dan (12 October 2019). "A Well-Deserved Physics Nobel - Jim Peebles's award honors modern cosmological theory at last". Observations. Scientific American (blog). Stuttgart: Springer Nature. ISSN 0036-8733. Archived from the original on 22 October 2019. Retrieved 13 October 2019.
- Couronne, Ivan; Ahmed, Issam (14 November 2019). "Top cosmologist's lonely battle against 'Big Bang' theory". Paris. Agence France-Presse. Archived from the original on 14 November 2019. Retrieved 20 December 2019.
- Harris 2002, p. 128
- Frame 2009, pp. 137–141
- Harrison 2010, p. 9
- Harris 2002, p. 129
- Craig, William Lane (December 1999). "The Ultimate Question of Origins: God and the Beginning of the Universe". Astrophysics and Space Science (Lecture). Dordrecht: Kluwer Academic Publishers. 269–270 (1–4): 721–738. doi:10.1023/A:1017083700096. ISSN 0004-640X.
- Block et al. 2000, pp. 723–740 doi:10.1007/978-94-011-4114-7_85
- Craig, William Lane. "The Ultimate Question of Origins: God and the Beginning of the Universe". Scholarly Writings: The Existence of God. Reasonable Faith. Dallas, TX. Archived from the original on 27 June 2019. Retrieved 21 December 2019.
- Asad 1980
- Hawking 1988, Introduction: "... a universe with no edge in space, no beginning or end in time, and nothing for a Creator to do." — Carl Sagan.
- Asad, Muhammad (1980). The Message of the Qur'an. Gibraltar, British Overseas Territory: Dar al-Andalus Limited. ISBN 978-0-614-21062-0. OCLC 754875650.
- Belušević, Radoje (2008). Relativity, Astrophysics and Cosmology. 1. Weinheim: Wiley-VCH. ISBN 978-3-527-40764-4. OCLC 876678499.
- Block, David L.; Puerari, Ivânio; Stockton, Alan; et al., eds. (2000). Toward a New Millennium in Galaxy Morphology: Proceedings of an International Conference 'Toward a New Millennium in Galaxy Morphology: from z=0 to the Lyman Break, held at the Eskom Conference Centre, Midrand, South Africa, September 13–18, 1999. Dordrecht: Kluwer Academic Publishers. doi:10.1007/978-94-011-4114-7. ISBN 978-94-010-5801-8. LCCN 00042415. OCLC 851369444. "Reprinted from Astrophysics and Space Science Volumes 269–270, Nos. 1–4, 1999".
- Block, David L. (2012). "Georges Lemaître and Stigler's Law of Eponymy". In Holder, Rodney D.; Mitton, Simon (eds.). Georges Lemaître: Life, Science and Legacy. Astrophysics and Space Science Library. 395. Heidelberg; New York: Springer. arXiv:1106.3928v2. Bibcode:2012ASSL..395...89B. doi:10.1007/978-3-642-32254-9_8. ISBN 978-3-642-32253-2. LCCN 2012956159. OCLC 839779611.
- Carroll, Sean M. (n.d.). "Why Is There Something, Rather Than Nothing?". In Knox, Eleanor; Wilson, Alastair (eds.). Routledge Companion to the Philosophy of Physics. London: Routledge. arXiv:1802.02231v2. Bibcode:2018arXiv180202231C.
- Chow, Tai L. (2008). Gravity, Black Holes, and the Very Early Universe: An Introduction to General Relativity and Cosmology. New York: Springer. ISBN 978-0-387-73629-7. LCCN 2007936678. OCLC 798281050.
- Christianson, Gale E. (1995). Edwin Hubble: Mariner of the Nebulae. New York: Farrar, Straus and Giroux. ISBN 978-0-374-14660-3. LCCN 94045995. OCLC 461940674.
- Croswell, Ken (1995). Alchemy of the Heavens: Searching for Meaning in the Milky Way. Illustrations by Philippe Van (1st Anchor Books ed.). New York: Anchor Books. ISBN 978-0-385-47213-5. LCCN 94030452. OCLC 1100389944.
- d'Inverno, Ray (1992). Introducing Einstein's Relativity. Oxford, UK; New York: Clarendon Press; Oxford University Press. ISBN 978-0-19-859686-8. LCCN 91024894. OCLC 554124256.
- Drees, William B. (1990). Beyond the Big Bang: Quantum Cosmologies and God. La Salle, IL: Open Court Publishing Company. ISBN 978-0-8126-9118-4. LCCN 90038498. OCLC 1088758264.
- Farrell, John (2005). The Day Without Yesterday: Lemaître, Einstein, and the Birth of Modern Cosmology. New York: Thunder's Mouth Press. ISBN 978-1-56025-660-1. LCCN 2006272995. OCLC 61672162.
- Frame, Tom (2009). Losing My Religion: Unbelief in Australia. Sydney: UNSW Press. ISBN 978-1-921410-19-2. OCLC 782015652.
- Gibbons, Gary W.; Shellard, E.P.S.; Rankin, Stuart John, eds. (2003). The Future of Theoretical Physics and Cosmology: Celebrating Stephen Hawking's 60th Birthday. Cambridge, UK; New York: Cambridge University Press. ISBN 978-0-521-82081-3. LCCN 2002041704. OCLC 1088190774.
- Guth, Alan H. (1998) [Originally published 1997]. The Inflationary Universe: Quest for a New Theory of Cosmic Origins. Foreword by Alan Lightman. London: Vintage Books. ISBN 978-0-09-995950-2. LCCN 96046117. OCLC 919672203.
- Harris, James F. (2002). Analytic Philosophy of Religion. Handbook of Contemporary Philosophy of Religion. 3. Dordrecht: Kluwer Academic Publishers. ISBN 978-1-4020-0530-5. LCCN 2002071095. OCLC 237734029.
- Harrison, Peter, ed. (2010). The Cambridge Companion to Science and Religion. Cambridge Companions to Religion. Cambridge, UK; New York: Cambridge University Press. ISBN 978-0-521-71251-4. LCCN 2010016793. OCLC 972341489.
- Hawking, Stephen W.; Ellis, George F. R. (1973). The Large-Scale Structure of Space-Time. Cambridge, UK: Cambridge University Press. ISBN 978-0-521-20016-5. LCCN 72093671. OCLC 1120809270.
- Hawking, Stephen W. (1988). A Brief History of Time: From the Big Bang to Black Holes. Introduction by Carl Sagan; illustrations by Ron Miller. New York: Bantam Dell Publishing Group. ISBN 978-0-553-10953-5. LCCN 87033333. OCLC 39256652.
- Hawking, Stephen W.; Israel, Werner, eds. (2010) [Originally published 1979]. General Relativity: An Einstein Centenary Survey. Cambridge, UK: Cambridge University Press. ISBN 978-0-521-13798-0. LCCN 78062112. OCLC 759923541.
- Kolb, Edward; Turner, Michael, eds. (1988). The Early Universe. Frontiers in Physics. 70. Redwood City, CA: Addison-Wesley. ISBN 978-0-201-11604-5. LCCN 87037440. OCLC 488800074.
- Kragh, Helge (1996). Cosmology and Controversy: The Historical Development of Two Theories of the Universe. Princeton, NJ: Princeton University Press. ISBN 978-0-691-02623-7. LCCN 96005612. OCLC 906709898.
- Krauss, Lawrence M. (2012). A Universe From Nothing: Why there is Something Rather than Nothing. Afterword by Richard Dawkins (1st Free Press hardcover ed.). New York: Free Press. ISBN 978-1-4516-2445-8. LCCN 2011032519. OCLC 709673181.
- Livio, Mario (2000). The Accelerating Universe: Infinite Expansion, the Cosmological Constant, and the Beauty of the Cosmos (Audio book performance by Tom Parks, Brilliance Audio). Foreword by Allan Sandage. New York: John Wiley & Sons. ISBN 978-0-471-32969-5. LCCN 99022278. OCLC 226086793.
- Manly, Steven L. (2011). Brandon, Jodi (ed.). Visions of the Multiverse. Pompton Plains, NJ: New Page Books. ISBN 978-1-60163-720-8. LCCN 2010052741. OCLC 609531953.
- Martínez-Delgado, David, ed. (2013). Local Group Cosmology. Cambridge, UK: Cambridge University Press. ISBN 978-1-107-02380-2. LCCN 2013012345. OCLC 875920635. "Lectures presented at the XX Canary Islands Winter School of Astrophysics, held in Tenerife, Spain, November 17-18, 2008."
- Milne, Edward Arthur (1935). Relativity, Gravitation and World-Structure. The International Series of Monographs on Physics. Oxford, UK; London: Clarendon Press; Oxford University Press. LCCN 35019093. OCLC 1319934.
- Mitton, Simon (2011). Fred Hoyle: A Life in Science. Cambridge, UK; New York: Cambridge University Press. ISBN 978-0-521-18947-7. LCCN 2011293530. OCLC 774201415.
- Olive, K.A.; et al. (Particle Data Group) (2014). "Review of Particle Physics" (PDF). Chinese Physics C. Bristol: IOP Publishing on behalf of the Chinese Physical Society. 38 (9): 090001. doi:10.1088/1674-1137/38/9/090001. ISSN 1674-1137. OCLC 5631153824. Archived (PDF) from the original on 30 January 2017. Retrieved 13 December 2019.
- Partridge, R. Bruce (1995). 3K: The Cosmic Microwave Background Radiation. Cambridge Astrophysics Series. 25 (Illustrated ed.). Cambridge, UK: Cambridge University Press. ISBN 978-0-521-35808-8. LCCN 94014980. OCLC 1123849709.
- Peacock, John A. (1999). Cosmological Physics. Cambridge Astrophysics Series. Cambridge, UK; New York: Cambridge University Press. ISBN 978-0-521-42270-3. LCCN 98029460. OCLC 60157380.
- Penrose, Roger (1989). "Difficulties with Inflationary Cosmology". In Fenyves, Ervin J. (ed.). Fourteenth Texas Symposium on Relativistic Astrophysics. Annals of the New York Academy of Sciences. 571. New York: New York Academy of Sciences. pp. 249–264. Bibcode:1989NYASA.571..249P. doi:10.1111/j.1749-6632.1989.tb50513.x. ISBN 978-0-89766-526-1. ISSN 0077-8923. LCCN 89014030. OCLC 318253659. "Symposium held in Dallas, Tex., Dec. 11-16, 1988."
- Penrose, Roger (2007) [Originally published: London: Jonathan Cape, 2004]. The Road to Reality (1st Vintage Books ed.). New York: Vintage Books. ISBN 978-0-679-77631-4. LCCN 2008274126. OCLC 920157277. The 2004 edition of the book is available from the Internet Archive. Retrieved 20 December 2019.
- Roos, Matts (2012) [Chapter originally published 2008]. "Expansion of the Universe – Standard Big Bang Model". In Engvold, Oddbjørn; Stabell, Rolf; Czerny, Bozena; Lattanzio, John (eds.). Astronomy and Astrophysics. Encyclopedia of Life Support Systems. II. Ramsey, Isle of Man: UNESCO in partnership with Eolss Publishers Co. Ltd. arXiv:0802.2005. Bibcode:2008arXiv0802.2005R. ISBN 978-1-84826-823-4. OCLC 691095693.
- Ryden, Barbara Sue (2003). Introduction to Cosmology. San Francisco: Addison-Wesley. ISBN 978-0-8053-8912-8. LCCN 2002013176. OCLC 1087978842.
- Silk, Joseph (2009). Horizons of Cosmology: Exploring Worlds Seen and Unseen. Templeton Science and Religion Series. Conshohocken, PA: Templeton Press. ISBN 978-1-59947-341-3. LCCN 2009010014. OCLC 818734366.
- Singh, Simon (2004). Big Bang: The Origin of the Universe (1st U.S. ed.). New York: Fourth Estate. Bibcode:2004biba.book.....S. ISBN 978-0-00-716220-8. LCCN 2004056306. OCLC 475508230.
- Tanabashi, M.; et al. (Particle Data Group) (2018). "Review of Particle Physics". Physical Review D. College Park, MD: American Physical Society. 98 (3): 030001. doi:10.1103/PhysRevD.98.030001. ISSN 1550-7998. OCLC 7814919666. Archived from the original on 14 October 2019. Retrieved 28 December 2019.
- Tolman, Richard C. (1934). Relativity, Thermodynamics and Cosmology. The International Series of Monographs on Physics. Oxford, UK; London: Clarendon Press; Oxford University Press. ISBN 978-0-486-65383-9. LCCN 34032023. OCLC 919976.
- Woolfson, Michael (2013). Time, Space, Stars & Man: The Story of Big Bang (2nd ed.). London: Imperial College Press. ISBN 978-1-84816-933-3. LCCN 2013371163. OCLC 835115510.
- Wright, Edward L. (2004). "Theoretical Overview of Cosmic Microwave Background Anisotropy". In Freedman, Wendy L. (ed.). Measuring and Modeling the Universe. Carnegie Observatories Astrophysics Series. 2. Cambridge, UK: Cambridge University Press. arXiv:astro-ph/0305591. Bibcode:2004mmu..symp..291W. ISBN 978-0-521-75576-4. LCCN 2005277053. OCLC 937330165.
- Yao, W.-M.; et al. (Particle Data Group) (2006). "Review of Particle Physics" (PDF). Journal of Physics G: Nuclear and Particle Physics. Bristol: IOP Publishing. 33 (1): 1–1232. Bibcode:2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001. ISSN 0954-3899. OCLC 938374545. Archived (PDF) from the original on 12 February 2017. Retrieved 16 December 2019.
- Alpher, Ralph A.; Herman, Robert (August 1988). "Reflections on Early Work on 'Big Bang' Cosmology". Physics Today. College Park, MD: American Institute of Physics. 41 (8): 24–34. Bibcode:1988PhT....41h..24A. doi:10.1063/1.881126. ISSN 0031-9228.
- Barrow, John D. (1994). The Origin of the Universe. Science Masters. London: Weidenfeld & Nicolson. ISBN 978-0-297-81497-9. LCCN 94006343. OCLC 490957073.
- Davies, Paul (1992). The Mind of God: The Scientific Basis for a Rational World. New York: Simon & Schuster. ISBN 978-0-671-71069-9. LCCN 91028606. OCLC 59940452.
- Lineweaver, Charles H.; Davis, Tamara M. (March 2005). "Misconceptions about the Big Bang" (PDF). Scientific American. Vol. 292 no. 3. Stuttgart: Springer Nature. pp. 36–45. ISSN 0036-8733. Archived (PDF) from the original on 9 October 2019. Retrieved 23 December 2019.
- Mather, John C.; Boslough, John (1996). The Very First Light: The True Inside Story of the Scientific Journey Back to the Dawn of the Universe (1st ed.). New York: Basic Books. ISBN 978-0-465-01575-7. LCCN 96010781. OCLC 34357391.
- Riordan, Michael; Zajc, William A. (May 2006). "The First Few Microseconds" (PDF). Scientific American. Vol. 294 no. 5. Springer Nature. pp. 34–41. Bibcode:2006SciAm.294e..34R. doi:10.1038/scientificamerican0506-34a. ISSN 0036-8733. Archived (PDF) from the original on 30 November 2014.
- Weinberg, Steven (1993) [Originally published 1977]. The First Three Minutes: A Modern View of the Origin of the Universe (Updated ed.). New York: Basic Books. ISBN 978-0-465-02437-7. LCCN 93232406. OCLC 488469247. 1st edition is available from the Internet Archive. Retrieved 23 December 2019.
- Big-bang model at the Encyclopædia Britannica
- Once Upon a Universe – STFC funded project explaining the history of the universe in easy-to-understand language
- "Big Bang Cosmology" – NASA/WMAP Science Team
- "The Big Bang" – NASA Science
- "Big Bang, Big Bewilderment" – Big bang model with animated graphics by Johannes Koelman
- Cosmology at Curlie