The Big Bang Theory is the dominant scientific theory about the origin of the universe. According to the big bang, the universe was created sometime between 10 billion and 20 billion years ago from a cosmic explosion that hurled matter and in all directions.
In 1927, the Belgian priest Georges Lemaître was the first to propose that the universe began with the explosion of a primeval atom. His proposal came after observing the red shift in distant nebulas by astronomers to a model of the universe based on relativity. Years later, Edwin Hubble found experimental evidence to help justify Lemaître's theory. He found that distant galaxies in every direction are going away from us with speeds proportional to their distance.
The big bang was initially suggested because it explains why distant galaxies are traveling away from us at great speeds. The theory also predicts the existence of cosmic background radiation (the glow left over from the explosion itself). The Big Bang Theory received its strongest confirmation when this radiation was discovered in 1964 by Arno Penzias and Robert Wilson, who later won the Nobel Prize for this discovery.
Although the Big Bang Theory is widely accepted, it probably will never be proved; consequentially, leaving a number of tough, unanswered questions.
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.
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 à 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 was able to predict in 1948 the existence of cosmic microwave background radiation (CMB).[2] The CMB was discovered in 1964[3] and corroborated the Big Bang theory, giving it more credence over its chief rival, the steady state theory
The Big Bang theory developed from observations of the structure of the universe and from theoretical considerations. Observers determined that most "spiral nebulae" were receding from Earth, but did not grasp the cosmological implications of this fact, or realize that the supposed nebulae were galaxies outside our Milky Way.[5] Georges Lemaître, a Belgian Roman Catholic priest, independently derived the Friedmann-Lemaître-Robertson-Walker equations from Albert Einstein's equations of general relativity in 1927 and proposed, on the basis of the recession of spiral nebulae, that the universe began as a simple "primeval atom"—now known as the Big Bang.[6]
Edwin Hubble provided an observational basis for Lemaître's theory two years later. He discovered that, seen from Earth, light from other galaxies is redshifted proportionally to their distance from Earth -- a fact 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 implied that the universe was expanding, contradicting the infinite and unchanging static universe scenario developed by Einstein.[9]
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.[10] The other was Lemaître's Big Bang theory, advocated and developed by George Gamow. Hoyle actually coined the name of Lemaître's theory, referring to it sarcastically as "this big bang idea" during a program broadcast on March 28, 1949, by the BBC Third Programme. Hoyle repeated the term in further broadcasts in early 1950, as part of a series of five lectures entitled The Nature of Things. The text of each lecture was published in The Listener a week after the broadcast, the first time that the term "big bang" appeared in print.[11] While Hoyle's "steady state" and Lemaître's "Big Bang" were the two most popular models used to explain Hubble's observations, other ideas were also proposed, including the Milne model,[12] Richard Tolman's oscillatory universe,[13] and Fritz Zwicky's tired light hypothesis.[14]
For a while, support was split between the "steady state" and "Big Bang" theories. Eventually, the observational evidence began to favor the latter. The discovery of the cosmic microwave background radiation in 1964 secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding what happened at the Big Bang, and reconciling observations with the basic theory.
Huge advances in Big Bang cosmology have been made since the late 1990s as a result of major advances in telescope technology as well as copious data from satellites such as COBE, the Hubble Space Telescope, and WMAP. Cosmologists can now calculate many of the parameters of the Big Bang to a new level of precision, leading to the unexpected discovery that the expansion of the universe appears to be accelerating (see dark energy).
Based on measurements of the expansion of the universe using Type 1a supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a calculated age of 13.7 ± 0.2 billion years. The agreement of these three independent measurements strongly supports the so-called ÎCDM model that describes in detail 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. Approximately 10â35 seconds after the Planck epoch a phase transition caused a cosmic inflation, when the universe grew exponentially.
After inflation stopped, the universe consisted of a quark-gluon plasma, perhaps experimentally produced recently as a quark-gluon liquid in which the constituent particles were all moving relativistically -- as well as all other elementary particles.[15] At some point a reaction (as yet unknown) which violated conservation of baryon number led to a very small excess of quarks and leptons over antiquarks and anti-leptons (of the order of 1 part in 1010) - this unknown process is called baryogenesis.
As the universe continued growing in size, the temperature dropped. Symmetry breaking phase transitions put the forces of physics and elementary particles into their present form. Quarks and gluons combined into baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. Below a certain temperature, new proton/antiproton pairs could no longer be created, and a mass annihilation between the remaining protons and antiprotons resulted in the complete disappearance of antiprotons and the almost complete disappearance of protons. A similar process happened for neutrons/antineutrons, and, at a lower temperature, for electrons/positrons.
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 particles gradually slowed down from moving relativistically and its rest mass energy density came to gravitationally dominate that of radiation. After about 380,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is 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 ÎCDM 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 10â33 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.[16] 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.
Theoretical underpinnings
The Big Bang theory depends on three assumptions:
The universality of physical laws
The cosmological principle
The Copernican principle
These ideas were initially taken as postulates, but today there are efforts 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.[17] 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.[18] There are efforts to test the Copernican Principle by looking at the interaction of galaxy groups and clusters with the CMB through the Sunyaev-Zel'dovich effect to a level of 1% accuracy.[19]
These assumptions, combined with Einstein's theory of general relativity, imply that spacetime should be described by a homogeneous and isotropic metric, which must therefore be a FRW metric. These metrics rely on a coordinate chart or grid being laid down over all spacetime, with which we can specify the location of points (e.g., galaxies, stars...) in the universe. The specific chart used is called a comoving coordinate system, since the grid is designed to expand along with the universe, and so objects that are carried along by the expansion of the universe remain at fixed points on the grid. While their coordinate distance (comoving distance) remains constant, the physical distance between two such comoving points expands proportionally with the scale factor of the universe. See also metric expansion of space.
As the universe can be described by such coordinates, the Big Bang is not an explosion of matter moving outward to fill an empty universe; space itself expanded and caused the physical distance between two comoving points to increase. Objects that are bound together (such as atoms, people, stars, the solar system, and galaxies) do not expand with spacetime's expansion because the forces that bind them together are strong compared with the Hubble expansion that is pulling them apart.
One can also define a conformal time η, in which case the full spacetime metric takes the form of a static metric multiplied by an overall scale factor. The conformal time coordinate is quite useful since the comoving distance traveled by a light ray is equal to the conformal time interval of the trip. This enables understanding of the causal structure of spacetime. For example, the Big Bang occurred at a finite interval of conformal time η0 to the past. Objects whose comoving distance is greater than cη0 are too far away for light to have had time to travel to us since the Big Bang: therefore we cannot see all of the past universe and there is a past horizon. If the universe is accelerating, then there is only a finite amount of conformal time ηF to the future (though this finite amount of conformal time corresponds to an infinite amount of clock or proper time). Objects located at comoving distances further than cηF can never be reached by a light ray emitted by us today, therefore we cannot influence all of the future universe and there is a future horizon. See also cosmological horizon.
Observational evidence
It is generally stated that there are three observational pillars that support the Big Bang theory of cosmology. These are the Hubble-type expansion seen in the redshifts of galaxies, the detailed measurements of the cosmic microwave background, and the abundance of light elements (see Big Bang nucleosynthesis). Additionally, the observed correlation function of large-scale structure of the cosmos fits well with standard Big Bang theory.
Hubble's law expansion
Main article: Hubble's law
Hubble's original data from his 1929 paper.[20]Observations of distant galaxies and quasars show that these objects are redshifted -- 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:
where
v is the recessional velocity of the galaxy or other distant object
D is the distance to the object and
H0 is Hubble's constant, measured to be (70 +2.4/-3.2) km/s/Mpc by the WMAP probe.[21]
The Hubble's law observation has two possible explanations, one of which -- that we are at the center of an explosion of galaxies -- is untenable given the Copernican principle. The other explanation is that the universe is uniformly expanding everywhere as a unique property of spacetime. This universal expansion was developed mathematically in the context of general relativity well before Hubble made his analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann-Lemaître-Robertson-Walker.
Cosmic microwave background radiation
Main article: Cosmic microwave background radiation
WMAP image of the cosmic microwave background radiationThe 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 in a process called 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, reducing the high temperature of the blackbody spectrum. The radiation is thought to be observable at every point in the universe as coming from all directions.
In 1964, Arno Penzias and Robert Wilson accidentally discovered the cosmic background radiation while conducting diagnostic observations using a new microwave receiver owned by Bell Laboratories.[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.[22] 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.[23]
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.[24] 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.[25] Indeed there is no obvious reason outside of the Big Bang that, for example, the young universe (i.e., before star formation, as determined by studying matter essentially free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than 3He, and in constant ratios, too.
Galactic evolution and distribution
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.[26]
Features, issues and problems
While currently there are very few researchers who doubt the Big Bang occurred, in the past the community was divided between supporters of the Big Bang and supporters of alternative cosmological models. Throughout the historical development of the subject, problems with the Big Bang theory were posed in the context of a scientific controversy regarding which model could best describe the cosmological observations (see history section above). With the overwhelming consensus in the community today supporting the Big Bang model, many of these problems are remembered as being mainly of historical interest; the solutions to them have been obtained either through modifications to the theory or as the result of better observations. Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as they can be addressed through further refinements of the theory.
The Big Bang model admits very exotic physical phenomena that include dark matter, dark energy, and cosmic inflation which rely on conditions and physics that have not yet been observed in terrestrial laboratory experiments. While explanations for such phenomena remain at the frontiers of inquiry in physics, independent observations of Big Bang nucleosynthesis, the cosmic microwave background, large scale structure and Type Ia supernovae strongly suggest the phenomena are important and real cosmological features of our universe. The gravitational effects of these features are understood observationally and theoretically but they have not yet been successfully incorporated into the Standard Model of particle physics. Though some aspects of the theory remain inadequately explained by fundamental physics, almost all cosmologists accept that the close agreement between Big Bang theory and observation have firmly established all the basic parts of the theory.
The following is a short list of Big Bang "problems" and puzzles:
Horizon problem
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.[24] The observed isotropy of the cosmic microwave background (CMB) is problematic in this regard, because the horizon size at that time corresponds to a size that is about 2 degrees on the sky. If the universe has had the same expansion history since the Planck epoch, there is no mechanism to cause these regions to have the same temperature.
A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at a time 10-35 seconds after the Planck epoch. During inflation, the universe undergoes exponential expansion, and regions in causal contact expand so as to be beyond each other's horizons. Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the universe. After inflation, the universe expands according to Hubble's law, and regions that were out of causal contact come back into the horizon. This explains the observed isotropy of the CMB. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian which has been accurately confirmed by measurements of the CMB.
Flatness problem
The overall geometry of the universe is determined by whether the Omega cosmological parameter is less than, equal to or greater than 1. From top to bottom: geometry in a closed universe, an open universe and a flat universe.Main article: Flatness problem
The flatness problem is an observational problem associated with a Friedmann-Lemaître-Robertson-Walker metric.[24] In general, the universe can have three kinds of geometries -- hyperbolic geometry, Euclidean geometry, or elliptic geometry -- depending on the total energy density of the universe as measured by means of the stress-energy tensor. It is hyperbolic if its density is less than the critical density, elliptic if greater, and Euclidean at the critical density. The universe must have been within one part in 1015 of the critical density in its earliest stages, or it would have caused either a Heat Death or a Big Crunch, and the universe would not exist as it does today.
A possible resolution to this problem is again offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that any residual curvature associated with it would have been smoothed out to a high degree of precision. Thus, it is believed that inflation drove the universe to be very nearly spatially flat.
Magnetic monopoles
The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted point defects in space that would manifest as magnetic monopoles with a density much higher than was consistent with observations, given that searches have never found any monopoles. This problem is also resolvable by cosmic inflation, which removes all point defects from the observable universe in the same way that it drives the geometry to flatness.[24]
Baryon asymmetry
It is not yet understood why the universe has more matter than antimatter.[24] It is generally assumed that when the universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and anti-baryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. An unknown process called baryogenesis created the asymmetry. For baryogenesis to occur, the Sakharov conditions, which were laid out by Andrei Sakharov, must be satisfied. They require that baryon number be not conserved, that C-symmetry and CP-symmetry be violated, and that the universe depart from thermodynamic equilibrium.[27] All these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry.[28] Experiments taking place at CERN near Geneva seek to trap enough anti-hydrogen to compare its spectrum with hydrogen. Any difference would be evidence of a CPT symmetry violation and therefore a Lorentz violation.
Globular cluster age
In the mid-1990s, observations of globular clusters appeared to be inconsistent with the Big Bang. Computer simulations that matched the observations of the stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13.7-billion-year age of the universe. This issue was generally resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters.[29] There still remain some questions as to how accurately the ages of the clusters are measured, but it is clear that these objects are some of the oldest in the universe.
Dark matter
Main article: Dark matter
A pie chart indicating the proportional composition of different energy-density components of the universe, according to the best ÎCDM model fits. Roughly ninety-five percent is in the exotic forms of dark matter and dark energy.During the 1970s and 1980s, various observations (notably of galactic rotation curves) showed that there 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 not normal or baryonic matter but rather dark 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 is far less lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now widely accepted in standard cosmology due to observations of the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and x-ray measurements from galaxy clusters. In August 2006, dark matter was definitively observed through measurements of colliding galaxies in the Bullet Cluster.[30][31]
The detection of dark matter is sensitive only to its gravitational signature, and no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, however, and several projects to detect them directly are underway.
Dark energy
Main article: Dark energy
In the 1990s, detailed measurements of the mass density of the universe revealed a value that was 30% that of the critical density.[9] Since the universe is very nearly spatially flat, as is indicated by measurements of the cosmic microwave background, about 70% of the energy density of the universe was left unaccounted for. This mystery now appears to be connected to another one: Independent measurements of Type Ia supernovae have revealed that the expansion of the universe is undergoing a non-linear acceleration. To explain this acceleration, general relativity requires that much of the universe consist of an energy component with large negative pressure. This dark energy is now thought to make up the missing 70%. Its nature remains one of the great mysteries of the Big Bang. Possible candidates include a scalar cosmological constant and quintessence making up physical vacuum. Observations to help understand this are ongoing. Results from WMAP in 2006 indicate that the universe is 74% dark energy, 22% dark matter, and 4% regular matter.
The future according to the Big Bang theory
Main article: 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 is above the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state that was similar to that in which it started—a Big Crunch. Alternatively, if the density in the universe is equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as the universe grows less dense. The average temperature of the universe would asymptotically approach absolute zero—a Big Freeze. Black holes would evaporate. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death. Moreover, if proton decay exists, then hydrogen, the predominant form of baryonic matter in the universe today, would disappear, leaving only radiation.
Modern observations of accelerated expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ÎCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the universe cools and expands. Other explanations of dark energy — so-called phantom energy theories — suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.
Speculative physics beyond the Big Bang
A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the metric seen on the left. Image from WMAP press release, 2006. (Detail)While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest universe, when inflation is hypothesized to have occurred. There may also be parts of the universe well beyond what can be observed in principle. In the case of inflation this is required: exponential expansion has pushed large regions of space beyond our observable horizon. It may be possible to deduce what happened when physics at very high energy scales is better understood. Speculations about this often involve theories of quantum gravitation.
Some proposals are:
models including the Hartle-Hawking boundary condition in which the whole of space-time is finite;
brane cosmology models, including brane inflation, in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically.
chaotic inflation, in which inflation starts from random initial conditions for the universe.
Some of these scenarios are qualitatively compatible with one another. Each entails untested hypotheses.
Philosophical and religious interpretations
As always been in the history of science, new scientific discoveries were used by philosophers in creating certain "schools of thought", and adapted by some religions to promote certain views.
The Big Bang, a scientific theory, is not based on any religion. Some people have found similarities, however, that they believe have both theological and philosophical implications, since some religious interpretations and world views conflict with the Big Bang origin of the universe.
Some interpretations of the Big Bang theory go beyond science, and some purport to explain the cause of the Big Bang itself (first cause). These views have been criticized by some naturalist philosophers as being modern creation myths. Some people believe that the Big Bang theory is inconsistent with traditional views of creation such as that in Genesis, for example, while others, like astronomer Hugh Ross, believe that the Big Bang theory lends support to the idea of creation ex nihilo.[32]
The following is a list of various religious interpretations of the Big Bang theory:
A number of Christian and traditional Jewish sources have accepted the Big Bang as a possible description of the origin of the universe, interpreting it to allow for a philosophical first cause. Pope Pius XII was an enthusiastic proponent of the Big Bang even before the theory was scientifically well-established and consequently the Roman Catholic Church has been a prominent advocate for the idea that creation ex nihilo can be interpreted as consistent with the Big Bang. This view is shared by many religious Jews in all branches of rabbinic Judaism. Some groups contend the Big Bang is also consistent with the teaching of creation according to Kabbalah. [33]
Some modern Islamic scholars believe that the Qur'an parallels the Big Bang in its account of creation, described as follows: "Do not the unbelievers see that the heavens and the earth were joined together as one unit of creation, before We clove them asunder?" (Ch:21,Ver:30). The claim has also been made that the Qur'an describes an expanding universe: "The heaven, We have built it with power. And verily, We are expanding it." (Ch:51,Ver:47).[34] Parallels with the Big Crunch and an oscillating universe have also been suggested: "On the day when We will roll up the heavens like the rolling up of the scroll for writings, as We originated the first creation, (so) We shall reproduce it; a promise (binding on Us); surely We will bring it about." (Ch:21,Ver:104).
Certain theistic branches of Hinduism, such as in Vaishnavism, conceive of a creation event with similarities to the Big Bang. For example in the third book of the Bhagavata Purana (primarily, chapters 10 and 26), describes a primordial state which bursts forth as the Great Vishnu glances over it, transforming into the active state of the sum-total of matter ("prakriti"). Other forms of Hinduism assert a universe without beginning or end.
Buddhism has a concept of universes that have no initial creation event, but instead go through infinitely repeated cycles of expansion, stability, destruction, and quiescence. The Big Bang may be reconciled with this view, since there are ways to conceive an eternal creation and destruction of universes within the paradigm. A number of popular Zen philosophers were intrigued, in particular, by the concept of the oscillatory universe.
SIZE OF UNIVERSE:-Universe" is a word derived from the Old French univers, which in turn comes from the Latin roots unus ("one") and versus (a form of vertere, "to turn"). Based on observations of the observable universe, Physicists' attempt to describe the whole of space-time, including all matter and energy and events which occur, as a single system corresponding to a mathematical model.
The currently-accepted theory of the universe's formation is the Big Bang model, which describes the expansion of space-time from a gravitational singularity. The universe underwent a rapid period of cosmic inflation that flattened out nearly all initial irregularities. Thereafter the universe expanded and became steadily cooler and less dense. Minor variations in the distribution of mass resulted in fractal segregation into features that are found in the current universe; such as clusters of galaxies.
Theoretical and observational cosmologists vary in their usage of the term Universe to mean either this whole system or just a part of this system.[1]
As used by observational cosmologists, the Universe (upper case "U") most frequently refers to the finite part of space-time which is directly observable by making observations using telescopes and other detectors and using the methods of theoretical and empirical physics for studying the basic components of the Universe and their interactions. Physical cosmologists assume that the observable part of (comoving) space (also called: "our universe") corresponds to a part of a model of the whole of space, and usually not to the whole space. They frequently use the term the Universe to mean either the observable part of space, the observable part of space-time or the entire space-time.[citation needed]
A majority of cosmologists believe that the observable universe is an extremely tiny part of the "whole" (theoretical) Universe and that it is impossible to observe the whole of comoving space. It is presently unknown whether or not this is correct, since according to studies of the shape of the Universe, it is possible that the observable universe is of nearly the same size as the whole of space, but the question remains under debate.[2][3] If a version of the cosmic inflation scenario is correct, then there is no known way to determine whether the (theoretical) universe is finite or infinite, in which case the observable Universe is just a tiny speck of the (theoretical) universe.
[edit] Theory
Theoretical cosmologists study models of the whole of space-time which is connected together, and search for models which are consistent with physical cosmologists' model of space-time on the scale of the observable universe.[citation needed] Their models are speculative but use the methods of theoretical physics. These models are usually referred to using the term universe (lower case "u"). Sometimes theorists use the Universe (upper case "U") to refer to the whole of the specific space-time in which we live.[4]
[edit] Multiverse
Main article: Multiverse (science)
Some theorists extend their model of "all of space-time" beyond a single connected space-time to a set of disconnected space-times, or multiverse. In order to clarify the terminology, George Ellis, U. Kirchner and W.R. Stoeger recommend using the term the Universe for the theoretical model of all of the connected space-time in which we live, universe domain for the observable universe or a similar part of the same space-time, universe for a general space-time (either our own Universe or another one disconnected from our own), "multiverse" for a set of disconnected space-times, and "multi-domain universe" to refer to a model of the whole of a single connected space-time in the sense of chaotic inflation models.[4]
For example, matter that falls into a black hole in our universe could emerge as a Big Bang, starting another universe. However, all such ideas are currently untestable and cannot be regarded as anything more than speculation. The concept of parallel universes is understood only when related to string theory. String theorist Michio Kaku offered several explanations to possible parallel universe phenomena.
Physicist David Deutsch suggests that a multiverse is a consequence of the many-worlds interpretation, which he considers to be the best alternative explanation to the Copenhagen explanations of Quantum theory first presented by Niels Bohr, over half a century ago.
[edit] Evolution
[edit] Formation
Main articles: Age of the universe and Big bang
The most important result of physical cosmology—that the universe is expanding—is derived from redshift observations and quantified by Hubble's Law. That is, astronomers observe that there is a direct relationship between the distance to a remote object (such as a galaxy) and the velocity with which it is receding. Conversely, if this expansion has continued over the entire age of the universe, then in the past these distant, receding objects must once have been closer together.
By extrapolating this expansion back in time, one approaches a gravitational singularity where everything in the universe was compressed into an infinitesimal point; an abstract mathematical concept that may or may not correspond to reality. This idea gave rise to the Big Bang theory, the dominant model in cosmology today.
During the earliest era of the big bang, the universe is believed to have formed a hot, dense plasma. As expansion proceeded, the temperature steadily dropped until a point was reached when atoms could form. At about this time the background energy (in the form of photons) became decoupled from the matter, and was free to travel through space. The left-over energy continued to cool as the universe expanded, and today it forms the cosmic microwave background radiation. This background radiation is remarkably uniform in all directions, which cosmologists have attempted to explain by an early period of inflationary expansion following the Big Bang.
Examination of small variations in the microwave background radiation provides information about the nature of the universe, including the age and composition. The age of the universe from the time of the Big Bang, according to current information provided by NASA's WMAP (Wilkinson Microwave Anisotropy Probe), is estimated to be about 13.7 billion (1.37 à 1010) years, with a margin of error of about 1 % (± 200 million years). Other methods of estimation give different ages ranging from 11 billion to 20 billion.[5] Most of the estimates cluster in the 13–15 billion year range.[6][7]
In the 1977 book The First Three Minutes, Nobel Prize-winner Steven Weinberg laid out the physics of what happened just moments after the Big Bang. Additional discoveries and refinements of theories prompted him to update and reissue that book in 1993.
See also: Timeline of the Big Bang
[edit] Pre-matter soup
Until recently, the first hundredth of a second was a bit of a mystery, leaving Weinberg and others unable to describe exactly what the universe would have been like. New experiments at the Relativistic Heavy Ion Collider in Brookhaven National Laboratory have provided physicists with a glimpse through this curtain of high energy, so they can directly observe the sorts of behavior that might have been taking place in this time frame.[8]
At these energies, the quarks that comprise protons and neutrons were not yet joined together, and a dense, superhot mix of quarks and gluons, with some electrons thrown in, was all that could exist in the microseconds before it cooled enough to form into the sort of matter particles we observe today.[9]
you've ever wondered how big the universe is, you're not alone. Astronomers have long pondered this, too, and they've had a hard time figuring it out. Now an estimate has been made, and its a whopper.
The universe is at least 156 billion light-years wide.
In the new study, researchers examined primordial radiation imprinted on the cosmos. Among their conclusions is that it is less likely that there is some crazy cosmic "hall of mirrors" that would cause one object to be visible in two locations. And they've ruled out the idea that we could peer deep into space and time and see our own planet in its youth.
First, let's see why the size is a number you've never heard of before.
Stretching reality
The universe is about 13.7 billion years old. Light reaching us from the earliest known galaxies has been travelling, therefore, for more than 13 billion years. So one might assume that the radius of the universe is 13.7 billion light-years and that the whole shebang is double that, or 27.4 billion light-years wide.
But the universe has been expanding ever since the beginning of time, when theorists believe it all sprang forth from an infinitely dense point in a Big Bang.
"All the distance covered by the light in the early universe gets increased by the expansion of the universe," explains Neil Cornish, an astrophysicist at Montana State University. "Think of it like compound interest."
Need a visual? Imagine the universe just a million years after it was born, Cornish suggests. A batch of light travels for a year, covering one light-year. "At that time, the universe was about 1,000 times smaller than it is today," he said. "Thus, that one light-year has now stretched to become 1,000 light-years."
All the pieces add up to 78 billion-light-years. The light has not traveled that far, but "the starting point of a photon reaching us today after travelling for 13.7 billion years is now 78 billion light-years away," Cornish said. That would be the radius of the universe, and twice that -- 156 billion light-years -- is the diameter. That's based on a view going 90 percent of the way back in time, so it might be slightly larger.
"It can be thought of as a spherical diameter is the usual sense," Cornish added comfortingly.
(You might have heard the universe is almost surely flat, not spherical. The flatness refers to its geometry being "normal," like what is taught in school; two parallel lines can never cross.)
Hall of mirrors
The scientists studied the cosmic microwave background (CMB), radiation unleashed about 380,000 years after the Big Bang, when the universe had first expanded enough to cool and allow atoms to form. Temperature differences in the CMB left an imprint on the sky that was used last year to reveal the age of the universe and confirm other important cosmological measurements.
The CMB is like a baby picture of the cosmos, before any stars were born.
The focus of the new work, which was published last week in the journal Physical Review Letters, was a search of CMB data for paired circles that would have indicated the universe is like a hall of mirrors, in which multiple images of the same object could show up in different locations in space-time. A hall of mirrors could mean the universe is finite but tricks us into thinking it is infinite.
Think of it as a video game in which an object disappearing on the right side of the screen reappears on the left.
"Several years ago we showed that any finite universe in which light had time to 'wrap around' since the Big Bang would have the same pattern of cosmic microwave background temperature fluctuations around pairs of circles," Cornish explained. They looked for the most likely patterns that would be evident in a CMB map generated by NASA's Wilkinson Microwave Anisotropy Probe (WMAP).
They didn't find those patterns.
Don't look back
"Our results don't rule out a hall-of-mirrors effect, but they make the possibility far less likely," Cornish told SPACE.com, adding that the findings have shown "no sign that the universe is finite, but that doesn't prove that it is infinite."
The results do render impossible a "soccer ball" shape for the universe, proposed late last year by another team. "However, if they were to 'pump up' their soccer ball to make it larger, they could evade our bounds" and still be in the realm of possibility, Cornish said. Other complex shapes haven't been ruled out.
The findings eliminate any chance of seeing our ancient selves, however, unless we can master time travel.
"If the universe was finite, and had a size of about 4 billion to 5 billion light-years, then light would be able to wrap around the universe, and with a big enough telescope we could view the Earth just after it solidified and when the first life formed," Cornish said. "Unfortunately, our results rule out this tantalizing possibility."
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Impossible? Cornish Explains Further
Update, 8:25 a.m. Tuesday, May 25
This article generated quite a few e-mails from readers who were perplexed or flat out could not believe the universe was just 13.7 billion years old yet 158 billion light-years wide. That suggests the speed of light has been exceeded, they argue. So SPACE.com asked Neil Cornish to explain further. Here is his response:
"The problem is that funny things happen in general relativity which appear to violate special relativity (nothing traveling faster than the speed of light and all that).
"Let's go back to Hubble's observation that distant galaxies appear to be moving away from us, and the more distant the galaxy, the faster it appears to move away. The constant of proportionality in that relationship is known as Hubble's constant.
"One seemingly paradoxical consequence of Hubble's observation is that galaxies sufficiently far away will be receding from us at a velocity faster than the speed of light. This distance is called the Hubble radius, and is commonly referred to as the horizon in analogy with a black hole horizon.
"In terms of special relativity, Hubble's law appears to be a paradox. But in general relativity we interpret the apparent recession as being due to space expanding (the old raisins in a rising fruit loaf analogy). The galaxies themselves are not moving through space (at least not very much), but the space itself is growing so they appear to be moving apart. There is nothing in special or general relativity to prevent this apparent velocity from exceeding the speed of light. No faster-than-light signals can be sent via this mechanism, and it does not lead to any paradoxes.
"Indeed, the WMAP data [on cosmic microwave background radiation] contain strong evidence that the very early universe underwent a period of accelerated expansion in which the distance been two points increased so quickly that light could not outrace the expansion so there was a true horizon -- in precise analogy with a black hole horizon. Indeed, the fluctuations we see in the CMB are thought to be generated by a process that is closely analogous to Hawking radiation from black holes.
"Even more amazing is the picture that emerges when you combine the WMAP data with [supernova] observations, which imply that the universe has started inflating again. If this is true, we have started to move away from the distant galaxies at a rate that is increasing, and in the future we will not be able to see as many galaxies as they will appear to be moving away from us faster than the speed of light (due to the expansion of space), so their light will not be able to reach us."
Until recently, the first hundredth of a second was a bit of a mystery, leaving Weinberg and others unable to describe exactly what the universe would have been like. New experiments at the Relativistic Heavy Ion Collider in Brookhaven National Laboratory have provided physicists with a glimpse through this curtain of high energy, so they can directly observe the sorts of behavior that might have been taking place in this time frame.[8]
At these energies, the quarks that comprise protons and neutrons were not yet joined together, and a dense, superhot mix of quarks and gluons, with some electrons thrown in, was all that could exist in the microseconds before it cooled enough to form into the sort of matter particles we observe today.[9]
Fast forwarding to after the existence of matter, more information is coming in on the formation of galaxies. It is believed that the earliest galaxies were tiny "dwarf galaxies" that released so much radiation they stripped gas atoms of their electrons. This gas, in turn, heated up and expanded, and thus was able to obtain the mass needed to form the larger galaxies that we know today.
Current telescopes are just now beginning to have the capacity to observe the galaxies from this distant time. Studying the light from quasars, they observe how it passes through the intervening gas clouds. The ionization of these gas clouds is determined by the number of nearby bright galaxies, and if such galaxies are spread around, the ionization level should be constant. It turns out that in galaxies from the period after cosmic reionization there are large fluctuations in this ionization level. The evidence seems to confirm the pre-ionization galaxies were less common and that the post-ionization galaxies have 100 times the mass of the dwarf galaxies.
The next generation of telescopes should be able to see the dwarf galaxies directly, which will help resolve the problem that many astronomical predictions in galaxy formation theory predict more nearby small galaxies
Depending on the average density of matter and energy in the universe, it will either keep on expanding forever or it will be gravitationally slowed down and will eventually collapse back on itself in a "Big Crunch". Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the universe seems to be accelerating and will accelerate for eternity (see accelerating universe). Other ideas of the fate of our universe include the Big Rip, the Big Freeze, and Heat death of the universe theory. For a more detailed discussion of other theories, see the ultimate fate of the universe.
The currently observable universe appears to have a geometrically flat space-time containing the equivalent mass-energy density of 9.9 Ã 10-30 grams per cubic centimetre. The primary constituents appear to consist of 73% dark energy, 23% cold dark matter and 4% atoms. Thus the density of atoms is on the order of a single hydrogen nucleus (or atom) for every four cubic meters of volume.[10] The exact nature of dark energy and cold dark matter remain a mystery.
During the early phases of the big bang, equal amounts of matter and anti-matter were formed. However, through a CP-violation, physical processes resulted in an asymmetry in the amount of matter as compared to anti-matter. This asymmetry explains the amount of residual matter found in the universe today, as nearly all the matter and anti-matter would otherwise have annihilated each other when they come into contact.[11]
Prior to the formation of the first stars, the chemical composition of the Universe consisted primarily of hydrogen (75% of total mass), with a lesser amount of helium-4 (4He) (24% of total mass) and trace amounts of other elements.[12] A small portion of these elements were in the form of the isotopes deuterium, 3He and lithium (7Li).[13] Subsequently the interstellar medium within galaxies has been steadily enriched by heavier elements. These are introduced as a result of Supernovae explosions, stellar winds and the expulsion of the outer envelope of evolved stars.[14]
The big bang left behind a background flux of photons and neutrinos. The temperature of the background radiation has steadily decreased as the universe expands, and now primarily consists of microwave energy equivalent to a temperature of 2.725 K.[15] The current density of background neutrinos is about 150 per cubic centimetreVery little is known about the size of the universe. It may be trillions of light years across, or even infinite in size. A 2003 paper[17] claims to establish a lower bound of 24 gigaparsecs (78 billion light years) on the size of the universe, but there is no reason to believe that this bound is anywhere near tight. See shape of the Universe for more information.
The observable (or visible) universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The comoving distance to the edge of the visible universe is about 46.5 billion light years in all directions from the earth; thus the visible universe may be thought of as a perfect sphere with the earth at its center and a diameter of about 93 billion light years. Note that many sources have reported a wide variety of incorrect figures for the size of the visible universe, ranging from 13.7 to 180 billion light years. See Observable universe for a list of incorrect figures published in the popular press with explanations of each.
While there is considerable fractalized structure at the local level (arranged in a hierarchy of clustering), on the highest orders of distance the universe is very homogeneous. On these scales the density of the universe is very uniform, and there is no preferred direction or significant asymmetry to the universe. This homogeneity is a requirement of the Friedmann-Lemaître-Robertson-Walker metric employed in modern cosmological models.[18]
Fluctuations in the microwave background radiation. NASA/WMAP image.The question of anisotropy in the early universe was significantly answered by the Wilkinson Microwave Anisotropy Probe, which looked for fluctuations in the microwave background intensity.[19] The measurements of this anisotropy have provided useful information and constraints about the evolution of the universe.
To the limit of the observing power of astronomical instruments, objects radiate and absorb energy according to the same physical laws as they do within our own galaxy.[20] Based on this, it is believed that the same physical laws and constants are universally applicable throughout the observable universe. No confirmed evidence has yet been found to show that physical constants have varied since the big bang, and the possible variation is becoming well constrained
The observable universe is a term used in Big Bang cosmology to describe a ball-shaped region of space surrounding the observer that is close enough that we might observe objects in it, i.e. there has been sufficient time for light emitted by an object to arrive at the observer. Every position has its own observable universe which may or may not overlap with the one centered around the Earth.
The word observable used in this sense has nothing to do with whether modern technology actually permits us to detect radiation from an object in this region. It simply means that it is possible for light or other radiation from the object to reach an observer on earth. In practice, we can only observe objects as far as the surface of last scattering, when the universe became transparent. However, it may be possible to infer information from before this time through the detection of gravitational waves.
The comoving distance from the Earth to the edge of the visible universe is about 46.5 billion light-years in any direction; this is the comoving radius of the visible universe. It is sometimes quoted as a diameter of 92-94 billion light-years. Since the visible universe is a perfect sphere and space is roughly flat, this size corresponds to a comoving volume of about 4/3 Ï R3 = 4.0Ã1032 cubic light-years or 3.4Ã1080 cubic meters.
The figures quoted above are distances now (in cosmological time), not distances at the time the light was emitted. For example, the cosmic microwave background radiation that we see right now was emitted about 13.7 billion years ago by matter that has, in the intervening time, condensed into galaxies. Those galaxies are now about 46 billion light-years from us, but at the time the light was emitted, that matter was only about 40 million light-years away from the matter that would eventually become the Earth
13.7 billion light-years. The age of the universe is about 13.7 billion years, and nothing travels faster than light. Some believe that it follows that the radius of the observable universe must be 13.7 billion light-years. This reasoning makes sense in the flat spacetime of special relativity, but in the real universe, spacetime is highly curved at cosmological scales, and light does not move rectilinearly. Distances obtained as the speed of light times a cosmological time interval have no direct physical significance. [3]
15.8 billion light-years. This is obtained in the same way as the 13.7 billion light-year figure, but starting from an incorrect age of the universe which was reported in the popular press in mid-2006 (e.g. [1] [2] [3]). For an analysis of this claim and the paper that prompted it, see [4].
27 billion light-years. This is a diameter obtained from the (incorrect) radius of 13.7 billion light-years.
78 billion light-years. This figure, as mentioned above, is a lower bound on the size of the whole universe, and has nothing to do with the size of the visible universe.
156 billion light-years. This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius. Since 78 billion light-years is already a diameter (or rather a circumference), the doubled figure is meaningless even in its original context. This figure was very widely reported (e.g. [4] [5] [6]).
180 billion light-years. This estimate accompanied the age estimate of 15.8 billion years in some sources; it was obtained by incorrectly adding 15% to the incorrect figure of 156 billion light-years.
[edit] Matter content
The observable universe contains about 3 to 5 Ã 1022 stars, organized in around 80 billion galaxies, which themselves form clusters and superclusters.
Two back-of-envelope calculations give the number of atoms in the observable universe to be around 1080.
The critical density of the universe is 3 H2 / 8 Ï G, which works out to be 1Ã10â29 grams/cubic centimeter or about 5Ã10â6 atoms of hydrogen/cc. It is believed that only 4 percent of the critical density is in the form of normal atoms, so this leaves 2Ã10â7 hydrogen atoms/cc. Multiplying this by the volume of the visible universe, you get about 7Ã1079 hydrogen atoms.
A typical star has a mass of about 2Ã1033 grams, which is about 1Ã1057 atoms of hydrogen per star. A typical galaxy has about 400 billion stars so that means each galaxy has 1Ã1057 Ã 4Ã1011 = 4Ã1068 hydrogen atoms. There are possibly 80 billion galaxies in the Universe, so that means that there are about 4Ã1068 Ã 8Ã1010 = 3Ã1079 hydrogen atoms in the Universe. But this is definitely a lower limit calculation, and ignores many possible atom sources. [5]
The simple answer is that the observable Universe is about 10 billion light years in radius. That number is obtained by multiplying how old we think the Universe is by the speed of light. The reasoning there is quite straightforward: we can only see out to that distance from which light can have reached us since the Universe began. (But see my note marked * below).
We determine the age of the Universe in a number of ways. One is to estimate the age of the oldest stars we see. Our knowledge of how stars of a given size evolve with time is very good (based on what we know about atomic and nuclear physics) so the major uncertainty here is usually measuring how far away (and so how big) such stars are. The standard method is to look for very small changes in the apparent positions of the stars as the Earth moves around the Sun. (This effect is called parallax). A second way to get an age for the Universe is to try to figure out the time of the big bang itself. Here the method is to use a series of techniques (based on how bright things appear to be - like Cepheid variable stars - that we think we know the true brightness of) to determine first the distance of the nearby galaxies, then increasingly distant galaxies, until we have estimated distances for many galaxies for which relative velocity measurements have been made (using the Doppler red shift of features in their spectra). The relative velocities we observe for distant galaxies have been largely determined by the expansion of the Universe begun with the 'big bang'. So, once we've determined how expansion velocity correlates with distance for some range of distances, it's possible to extrapolate back (with some assumptions) to calculate the instant of the big bang, when all the matter in the Universe was at a single point.
(If any of these terms like 'parallax', 'Cepheid' and 'red shift' are unfamiliar, try entering them in the search window on our home page).
The determination of greater and greater distances is one of the great themes of astronomy. Most introductory books will give you an outline of the story, which you can then fill in to any level of detail with further reading.
The first systematic theory of the size and shape of the universe that attempted to explain observed data was constructed by Ptolemy in the 2d cent. In this theory the solar system was thought to be the entire universe, with the earth at its center and the distant stars located just beyond the farthest planet. This belief was held until the 16th cent., when Copernicus advanced the idea that the sun, rather than the earth, is at the center of the system and that the stars are at very great distances compared to the planets. During the first part of the 20th cent., astronomers discovered that the sun is only one of billions of stars in the Milky Way galaxy and is located far from the galactic center.
Estimates of the size of the universe have been refined as methods of measuring galactic and extragalactic distances have improved. Close stellar distances were at first found by measuring a star's trigonometric parallax. A more powerful contemporary method is to analyze the light reaching the earth from an object by means of a spectroscope; the distance of a very faint object can be estimated by comparing its apparent brightness to those of similar objects at known distances. Another method depends on the fact that the universe as a whole appears to be expanding, as indicated by red shifts (see Doppler effect) in the spectral lines of distant galaxies. Hubble's law makes it possible to estimate their distances from the speed with which they are rushing away from the earth. At present the universe is believed to be at least 10 billion light-years in diameter. One problem with estimating the size of the universe is that space itself (or more properly, space-time) may be curved, as held by the general theory of relativity. This curvature would affect measurements of distance based on the passage of light through space from objects as far away as 5 billion light-years or more.