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Big Bang

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For other articles with similar names, see Big Bang (disambiguation).

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

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

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

The term Big Bang is used both in a narrow sense to refer to a point in time when the observed expansion of the universe (Hubble's law) began — calculated to be 13.7 billion (1.37 ? 10¹⁰) years ago (±2%) — and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and expansion of the universe, as well as the composition of primordial matter through nucleosynthesis as predicted by the Alpher-Bethe-Gamow theory.^[1]

From this model, George Gamow in 1948 was able to predict, at least qualitatively, the existence of cosmic microwave background radiation (CMB).^[2] The CMB was discovered in 1964^[3] and further corroborated the Big Bang theory, giving it an additional advantage over its chief rival, the steady state theory.^[4]

[edit] History

Main article: History of the Big Bang

The Big Bang theory developed from observations of the molecular structure of the universe and theoretical considerations. Observationally, it was determined that most spiral nebulae were receding from Earth, but those who made the observation weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way.^[5] In 1927, Georges Lema?tre, a Belgian Roman Catholic priest, independently derived the Friedmann-Lema?tre-Robertson-Walker equations from Albert Einstein's equations of general relativity and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"—what was later called the Big Bang.^[6]

In 1929, Edwin Hubble provided an observational basis for Lema?tre's theory. He discovered that, seen from Earth, light from other galaxies is redshifted in direct proportion to their distance from the Earth. This fact is now known as Hubble's law.^[7]^[8] Given the cosmological principle whereby the universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places, Hubble's law suggested that the universe was expanding, contradicting the infinite and unchanging static universe scenario developed by Einstein.^[9]

Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big Bang.

This idea allowed for two main opposing possibilities. One was Lema?tre's Big Bang theory, advocated and developed by George Gamow. The other possibility was Fred Hoyle's steady state model in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time.^[10] It was actually Hoyle who coined the name of Lema?tre's theory, referring to it sarcastically as "this big bang idea" during a program broadcast on March 28, 1949, by the BBC Third Programme. Hoyle repeated the term in further broadcasts in early 1950, as part of a series of five lectures entitled The Nature of Things. The text of each lecture was published in The Listener a week after the broadcast, the first time that the term "big bang" appeared in print.^[11] While Hoyle's "steady state" and Lema?tre's "Big Bang" were the two most popular models used to explain Hubble's observations, other ideas were also proposed as well. Some of these alternatives included the Milne model,^[12] Richard Tolman's oscillatory universe,^[13] and Fritz Zwicky's tired light hypothesis.^[14]

For a number of years, the support for the "steady state" and "Big Bang" theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. Since the discovery of the cosmic microwave background radiation in 1964, it has been regarded as the best theory of the origin and evolution of the cosmos. Virtually all theoretical work in cosmology now involves extensions and refinements to the basic Big Bang theory. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding what happened at the Big Bang and reconciling observations with the basic theory.

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

[edit] Overview

A graphical timeline is available here:

Graphical timeline of the Big Bang

Based on measurements of the expansion of the universe using Type 1a supernovae, measurements of the lumpiness of the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a calculated age of 13.7 ± 0.2 billion years. The agreement of these three independent measurements is considered strong evidence for the so-called ?CDM model that describes the detailed nature of the contents of the universe.

The early universe was filled homogeneously and isotropically with an incredibly high energy density and concomitantly huge temperatures and pressures. It expanded and cooled, going through phase transitions analogous to the condensation of steam or freezing of water as it cools, but related to elementary particles.

Approximately 10^?35 seconds after the Planck epoch a phase transition caused the universe to experience exponential growth during a period called cosmic inflation. After inflation stopped, the material components of the universe were in the form of a quark-gluon plasma (also including all other particles—and perhaps experimentally produced recently as a quark-gluon liquid [3]) in which the constituent particles were all moving relativistically. As the universe continued growing in size, the temperature dropped. At a certain temperature, by an as-yet-unknown transition called baryogenesis, the quarks and gluons combined into baryons such as protons and neutrons, somehow producing the observed asymmetry between matter and antimatter. Still lower temperatures led to further symmetry breaking phase transitions that put the forces of physics and elementary particles into their present form. Later, some protons and neutrons combined to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. As the universe cooled, matter gradually stopped moving relativistically and its rest mass energy density came to gravitationally dominate that of radiation. After about 300,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is the cosmic microwave background.

Over time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The three possible types are known as cold dark matter, hot dark matter, and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the universe is cold dark matter. The other two types of matter make up less than 20% of the matter in the universe.

The universe today appears to be dominated by a mysterious form of energy known as dark energy. Approximately 70% of the total energy density of today's universe is in this form. This dark energy causes the expansion of the universe to deviate from a linear velocity-distance relationship, observed as a faster than expected expansion at very large distances. Dark energy in its simplest formulation takes the form of a cosmological constant term in Einstein's field equations of general relativity, but its composition is unknown and, more generally, the details of its equation of state and relationship with the standard model of particle physics continue to be investigated both observationally and theoretically.

All these observations are encapsulated in the ?CDM model of cosmology, which is a mathematical model of the Big Bang with six free parameters. Mysteries appear as one looks closer to the beginning, when particle energies were higher than can yet be studied by experiment. There is no compelling physical model for the first 10^?33 seconds of the universe, before the phase transition that grand unification theory predicts. At the "first instant", Einstein's theory of gravitation predicts a gravitational singularity where densities become infinite.^[15] To resolve this paradox, a theory of quantum gravitation is needed. Understanding this period of the history of the universe is one of the greatest unsolved problems in physics.

[edit] Theoretical underpinnings

As it stands today, the Big Bang is dependent on three assumptions:

When first developed, these ideas were simply taken as postulates, but today there are efforts underway to test each of them. Tests of the universality of physical laws have found that the largest possible deviation of the fine structure constant over the age of the universe is of order 10^-5.^[16] The isotropy of the universe that defines the Cosmological Principle has been tested to a level of 10^-5 and the universe has been measured to be homogeneous on the largest scales to the 10% level.^[17] There are efforts underway to test the Copernican Principle by means of looking at the interaction of galaxy groups and clusters with the CMB through the Sunyaev-Zel'dovich effect to a level of 1% accuracy.^[18]

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

As the universe can be described by such coordinates, the Big Bang is not an explosion of matter moving outward to fill an empty universe; what is expanding is space itself. It is this expansion that causes the physical distance between any two fixed points in our universe to increase. Objects that are bound together (for example, by gravity) do not expand with spacetime's expansion because the physical laws that govern them are assumed to be uniform and independent of the metric expansion. Moreover, the expansion of the universe on today's local scales is so small that any dependence of physical laws on the expansion is unmeasurable by current techniques.

[edit] Observational evidence

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

[edit] Hubble's law expansion

Main article: Hubble's law

Hubble's original data from his 1929 paper.^[19]

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

$v = H_0 D \,$

where

v

is the recessional velocity of the galaxy or other distant object

D

is the distance to the object and

H 0

is Hubble's constant, measured to be (70 +2.4/-3.2) km/s/Mpc by the WMAP probe.^[20]

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

[edit] Cosmic microwave background radiation

Main article: Cosmic microwave background radiation

WMAP image of the cosmic microwave background radiation

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

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

In 1964, Arno Penzias and Robert Wilson, while conducting a series of diagnostic observations using a new microwave receiver owned by Bell Laboratories, discovered the cosmic background radiation.^[3] Their discovery provided substantial confirmation of the general CMB predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K—and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded the Nobel Prize for their discovery.

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the Big Bang's predictions regarding the CMB. COBE found a residual temperature of 2.726 K and determined that the CMB was isotropic to about one part in 10⁵.^[21] During the 1990s, CMB anisotropies were further investigated by a large number of ground-based experiments and the universe was shown to be almost geometrically flat by measuring the typical angular size (the size on the sky) of the anisotropies. (See shape of the universe.)

In early 2003, the results of the Wilkinson Microwave Anisotropy satellite (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. (See cosmic microwave background radiation experiments.) This satellite also disproved several specific cosmic inflation models, but the results were consistent with the inflation theory in general.^[22]

[edit] Abundance of primordial elements

Main article: Big Bang nucleosynthesis

Using the Big Bang model it is possible to calculate the concentration of helium-4, helium-3, deuterium and lithium-7 in the universe as ratios to the amount of ordinary hydrogen, H.^[23] All the abundances depend on a single parameter, the ratio of photons to baryons. The ratios predicted (by mass, not by number) are about 0.25 for ⁴He/H, about 10^-3 for ²H/H, about 10^-4 for ³He/H and about 10^-9 for ⁷Li/H.

The measured abundances all agree with those predicted from a single value of the baryon-to-photon ratio. The agreement is relatively poor for ⁷Li and ⁴He, the two elements for which the systematic uncertainties are least understood. This is considered strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements.^[24] Indeed there is no obvious reason outside of the Big Bang that, for example, the young universe (i.e., before star formation, as determined by studying matter essentially free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than ³He, and in constant ratios, too.

[edit] Galactic evolution and distribution

Main articles: Large-scale structure of the cosmos, Structure formation, and Galaxy formation and evolution

Detailed observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as galaxy clusters and superclusters. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions, and larger structures agree well with Big Bang simulations of the formation of structure in the universe and are helping to complete details of the theory.^[25]

[edit] Features, issues and problems

While currently there are very few researchers who doubt the Big Bang occurred, in the past the community was divided between supporters of the Big Bang and supporters of alternative cosmological models. Throughout the historical development of the subject, problems with the Big Bang theory were posed in the context of a scientific controversy regarding which model could best describe the cosmological observations (see history section above). With the overwhelming consensus in the community today supporting the Big Bang model, many of these problems are remembered as being mainly of historical interest; the solutions to them have been obtained either through modifications to the theory or as the result of better observations. Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as they can be addressed through further refinements of the theory.

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

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

[edit] Horizon problem

Main article: Horizon problem

The horizon problem results from the premise that information cannot travel faster than light, and hence two regions of space which are separated by a greater distance than the speed of light multiplied by the age of the universe cannot be in causal contact.^[23] The observed isotropy of the cosmic microwave background (CMB) is problematic in this regard, because the horizon size at that time corresponds to a size that is about 2 degrees on the sky. If the universe has had the same expansion history since the Planck epoch, there is no mechanism to cause these regions to have the same temperature.

A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at a time 10^-35 seconds after the Planck epoch. During inflation, the universe undergoes exponential expansion, and regions in causal contact expand so as to be beyond each other's horizons. Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the universe. After inflation, the universe expands according to Hubble's law, and regions that were out of causal contact come back into the horizon. This explains the observed isotropy of the CMB. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian which has been accurately confirmed by measurements of the CMB.

[edit] Flatness problem

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

Main article: Flatness problem

The flatness problem is an observational problem that results from considerations of the geometry associated with a Friedmann-Lema?tre-Robertson-Walker metric.^[23] In general, the universe can have three different kinds of geometries: hyperbolic geometry, Euclidean geometry, or elliptic geometry. The geometry is determined by the total energy density of the universe (as measured by means of the stress-energy tensor): hyperbolic results from a density less than the critical density, elliptic from a density greater than the critical density, and Euclidean from exactly the critical density. The universe is required to be within one part in 10¹⁵ of the critical density in its earliest stages. Any greater deviation would have caused either a Heat Death or a Big Crunch, and the universe would not exist as it does today.

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

[edit] Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted point defects in space that would manifest as magnetic monopoles with a density much higher than was consistent with observations, given that searches have never found any monopoles. This problem is also resolvable by cosmic inflation, which removes all point defects from the observable universe in the same way that it drives the geometry to flatness.^[23]

[edit] Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter.^[23] It is generally assumed that when the universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and anti-baryons. However, observations suggest that the universe, including i