Wednesday, August 4, 2010

The Big Bang

The Big Bang

This article is about the cosmological model of the Universe. For other uses, see Big Bang (disambiguation) and Big Bang Theory (disambiguation).

According to the Big Bang model, the Universe expanded from an extremely dense and hot state and continues to expand today. A common analogy explains that space itself is expanding, carrying galaxies with it, like raisins in a rising loaf of bread. The graphic scheme above is an artist's concept illustrating the expansion of a portion of a flat Universe.The Big Bang is the prevailing cosmological theory of the early development of the universe. Cosmologists use the term Big Bang to refer to the idea that the universe was originally extremely hot and dense at some finite time in the past and has since cooled by expanding to the present diluted state and continues to expand today. The theory is supported by the most comprehensive and accurate explanations from current scientific evidence and observation.[1][2] According to the best available measurements as of 2010[update], the initial conditions occurred around 13.3 to 13.9 billion years ago.[3][4]

Georges Lemaître proposed what became known as the Big Bang theory of the origin of the Universe, although he called it his "hypothesis of the primeval atom". The framework for the model relies on Albert Einstein's general relativity and on simplifying assumptions (such as homogeneity and isotropy of space). The governing equations had been formulated by Alexander Friedmann. After Edwin Hubble discovered in 1929 that the distances to far away galaxies were generally proportional to their redshifts, as suggested by Lemaître in 1927, this observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.[5] If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures,[6][7][8] and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of the theory, but these accelerators have limited capabilities to probe into such high energy regimes. Without any evidence associated with the earliest instant of the expansion, the Big Bang theory cannot and does not provide any explanation for such an initial condition; rather, it describes and explains the general evolution of the Universe since that instant. The observed abundances of the light elements throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the Universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.

Fred Hoyle is credited with coining the term Big Bang during a 1949 radio broadcast. It is popularly reported that Hoyle, who favored an alternative "steady state" cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.[9][10][11] Hoyle later helped considerably in the effort to understand stellar nucleosynthesis, the nuclear pathway for building certain heavier elements from lighter ones. After the discovery of the cosmic microwave background radiation in 1964, and especially when its spectrum (i.e., the amount of radiation measured at each wavelength) sketched out a blackbody curve, most scientists were fairly convinced by the evidence that some Big Bang scenario must have occurred.

Motivation and development

Main article: History of the Big Bang theory

The Big Bang theory developed from observations of the structure of the Universe and from theoretical considerations. In 1912 Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way.[12][13] Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einstein's equations of general relativity, showing that the Universe might be expanding in contrast to the static Universe model advocated by Einstein at that time.[14] In 1924, Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, proposed that the inferred recession of the nebulae was due to the expansion of the Universe.[15]

In 1931 Lemaître went further and suggested that the evident expansion in forward time required that the Universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the Universe into a single point, a "primeval atom" where and when the fabric of time and space comes into existence.[16]

Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2,500 mm) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recession velocity—now known as Hubble's law.[5][17] Lemaître had already shown that this was expected, given the Cosmological Principle.[18]

Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big BangDuring the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model,[19] the oscillatory Universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard Tolman)[20] and Fritz Zwicky's tired light hypothesis.[21]

After World War II, two distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the Universe seemed to expand. In this model, the Universe is roughly the same at any point in time.[22] The other was Lemaître's Big Bang theory,[notes 1] advocated and developed by George Gamow, who introduced big bang nucleosynthesis (BBN)[23] and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background radiation (CMB).[24] Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast in March 1949.[25][notes 2] For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor the latter. The discovery and confirmation of the cosmic microwave background radiation in 1964[26] secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the Universe at earlier and earlier times, and reconciling observations with the basic theory.

Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances in telescope technology as well as the analysis of copious data from satellites such as COBE,[27] the Hubble Space Telescope and WMAP.[28] Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the Universe appears to be accelerating.

[edit] Overview

[edit] Timeline of the Big Bang

Main article: Timeline of the Big Bang

A graphical timeline is available at

Graphical timeline of the Big Bang

Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.[29] This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch. The early hot, dense phase is itself referred to as "the Big Bang",[notes 3] and is considered the "birth" of our Universe. Based on measurements of the expansion using Type Ia 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 13.73 ± 0.12 billion years.[30] The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents of the Universe.

The earliest phases of the Big Bang are subject to much speculation. In the most common models, the Universe was filled homogeneously and isotropically with an incredibly high energy density, huge temperatures and pressures, and was very rapidly expanding and cooling. Approximately 10−37 seconds into the expansion, a phase transition caused a cosmic inflation, during which the Universe grew exponentially.[31] After inflation stopped, the Universe consisted of a quark–gluon plasma, as well as all other elementary particles.[32] Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present Universe.[33]

The Universe continued to grow in size and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form.[34] After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was now no longer high enough to create new proton–antiproton pairs (similarly for neutrons–antineutrons), so a mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons, and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the Universe was dominated by photons (with a minor contribution from neutrinos).

A few minutes into the expansion, when the temperature was about a billion (one thousand million; 109; SI prefix giga-) kelvins and the density was about that of air, neutrons combined with protons to form the Universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis.[35] Most protons remained uncombined as hydrogen nuclei. As the Universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation.[36]

The Hubble Ultra Deep Field showcases galaxies from an ancient era when the Universe was younger, denser, and warmer according to the Big Bang theory.Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, 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 of matter are known as cold dark matter, hot dark matter and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the Universe is cold dark matter. The other two types of matter make up less than 18% of the matter in the Universe.[30]

Independent lines of evidence from Type Ia supernovae and the CMB imply the Universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 72% of the total energy density of today's Universe is in this form. When the Universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity had the upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the Universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.[18]

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity. As noted above, there is no well-supported model describing the action prior to 10−15 seconds or so. Apparently a new unified theory of quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the Universe is currently one of the greatest unsolved problems in physics.

[edit] Underlying assumptions

The Big Bang theory depends on two major assumptions: the universality of physical laws, and the Cosmological Principle. The cosmological principle states that on large scales the Universe is homogeneous and isotropic.

These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the Universe is of order 10−5.[37] Also, General Relativity has passed stringent tests on the scale of the solar system and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.[notes 4]

If the large-scale Universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler Copernican Principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10−5 via observations of the CMB.[notes 5] The Universe has been measured to be homogeneous on the largest scales at the 10% level.[38]

[edit] FLRW metric

Main articles: Friedmann–Lemaître–Robertson–Walker metric and Metric expansion of space

General relativity describes spacetime by a metric, which determines the distances that separate nearby points. The points, which can be galaxies, stars, or other objects, themselves are specified using a coordinate chart or "grid" that is laid down over all spacetime. The cosmological principle implies that the metric should be homogeneous and isotropic on large scales, which uniquely singles out the Friedmann–Lemaître–Robertson–Walker metric (FLRW metric). This metric contains a scale factor, which describes how the size of the Universe changes with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates. In this coordinate system, the grid expands along with the Universe, and objects that are moving only due to the expansion of the Universe remain at fixed points on the grid. While their coordinate distance (comoving distance) remains constant, the physical distance between two such comoving points expands proportionally with the scale factor of the Universe.[39]

The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself expands with time everywhere and increases the physical distance between two comoving points. Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our Universe only on large scales—local concentrations of matter such as our galaxy are gravitationally bound and as such do not experience the large-scale expansion of space.

[edit] Horizons

Main article: Cosmological horizon

An important feature of the Big Bang spacetime is the presence of horizons. Since the Universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our Universe. Our understanding of the Universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the Universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the Universe continues to accelerate, there is a future horizon as well.[40]

[edit] Observational evidence

The earliest and most direct kinds of observational evidence are the Hubble-type expansion seen in the redshifts of galaxies, the detailed measurements of the cosmic microwave background, the abundance of light elements (see Big Bang nucleosynthesis), and today also the large scale distribution and apparent evolution of galaxies[41] which are predicted to occur due to gravitational growth of structure in the standard theory. These are sometimes called "the four pillars of the Big Bang theory".[42]

[edit] Hubble's law and the expansion of space

Main articles: Hubble's law and metric expansion of space

See also: distance measures (cosmology) and scale factor (universe)

Observations of distant galaxies and quasars show that these objects are redshifted—the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission lines or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed:[5]


v is the recessional velocity of the galaxy or other distant object

D is the comoving distance to the object and

H0 is Hubble's constant, measured to be 70.1 ± 1.3 km/s/Mpc by the WMAP probe.[30]

Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable given the Copernican Principle—or the Universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Alexander Friedman in 1922[14] and Georges Lemaître in 1927,[15] well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson and Walker.

The theory requires the relation v = HD to hold at all times, where D is the comoving distance, v is the recessional velocity, and v, H, and D varying as the Universe expands (hence we write H0 to denote the present-day Hubble "constant"). For distances much smaller than the size of the observable Universe, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity v. However, the redshift is not a true Doppler shift, but rather the result of the expansion of the Universe between the time the light was emitted and the time that it was detected.[43]

That space is undergoing metric expansion is shown by direct observational evidence of the Cosmological Principle and the Copernican Principle, which together with Hubble's law have no other explanation. Astronomical redshifts are extremely isotropic and homogenous,[5] supporting the Cosmological Principle that the Universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.

Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican Principle, that the Earth is not in a central position, on a cosmological scale.[notes 6] Radiation from the Big Bang was demonstrably warmer at earlier times throughout the Universe. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the Universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

[edit] Cosmic microwave background radiation

Main article: Cosmic microwave background radiation

WMAP image of the cosmic microwave background radiationDuring the first few days of the Universe, the Universe was in full thermal equilibrium, with photons being continually emitted and absorbed, giving the radiation a blackbody spectrum. As the Universe expanded, it cooled to a temperature at which photons could no longer be created or destroyed. The temperature was still high enough for electrons and nuclei to remain unbound, however, and photons were constantly "reflected" from these free electrons through a process called Thomson scattering. Because of this repeated scattering, the early Universe was opaque to light.

When the temperature fell to a few thousand Kelvin, electrons and nuclei began to combine to form atoms, a process known as recombination. Since photons scatter infrequently from neutral atoms, radiation decoupled from matter when nearly all the electrons had recombined, at the epoch of last scattering, 379,000 years after the Big Bang. These photons make up the CMB that is observed today, and the observed pattern of fluctuations in the CMB is a direct picture of the Universe at this early epoch. The energy of photons was subsequently redshifted by the expansion of the Universe, which preserved the blackbody spectrum but caused its temperature to fall, meaning that the photons now fall into the microwave region of the electromagnetic spectrum. The radiation is thought to be observable at every point in the Universe, and comes from all directions with (almost) the same intensity.

In 1964, Arno Penzias and Robert Wilson accidentally discovered the cosmic background radiation while conducting diagnostic observations using a new microwave receiver owned by Bell Laboratories.[26] Their discovery provided substantial confirmation of the general CMB predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K—and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded a Nobel Prize for their discovery.

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the Big Bang's predictions regarding the CMB. COBE found a residual temperature of 2.726 K and in 1992 detected for the first time the fluctuations (anisotropies) in the CMB, at a level of about one part in 105.[27] John C. Mather and George Smoot were awarded Nobels for their leadership in this work. During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably BOOMERanG, found the Universe to be almost spatially flat by measuring the typical angular size (the size on the sky) of the anisotropies. (See shape of the Universe.)

In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. This spacecraft also disproved several specific cosmic inflation models, but the results were consistent with the inflation theory in general,[28] it confirms too that a sea of cosmic neutrinos permeates the Universe, a clear evidence that the first stars took more than a half-billion years to create a cosmic fog. A new space probe named Planck, with goals similar WMAP, was launched in May 2009. It is anticipated to soon provide even more accurate measurements of the CMB anisotropies. Many other ground- and balloon-based experiments are also currently running; see Cosmic microwave background experiments.

The background radiation is exceptionally smooth, which presented a problem in that conventional expansion would mean that photons coming from opposite directions in the sky were coming from regions that had never been in contact with each other. The leading explanation for this far reaching equilibrium is that the Universe had a brief period of rapid exponential expansion, called inflation. This would have the effect of driving apart regions that had been in equilibrium, so that all the observable Universe was from the same equilibrated region.

[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.[35] All the abundances depend on a single parameter, the ratio of photons to baryons, which itself can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for 4He/H, about 10−3 for 2H/H, about 10−4 for 3He/H and about 10−9 for 7Li/H.[35]

The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for 4He, and a factor of two off for 7Li; in the latter two cases there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium.[44] Indeed there is no obvious reason outside of the Big Bang that, for example, the young Universe (i.e., before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than 3He, and in constant ratios, too.

[edit] Galactic evolution and distribution

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

This panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The galaxies are color coded by redshift.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.[45][46]

[edit] Other lines of evidence

After some controversy, the age of Universe as estimated from the Hubble expansion and the CMB is now in good agreement with (i.e., slightly larger than) the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars.

The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of temperature-sensitive emission lines in gas clouds at high redshift. This prediction also implies that the amplitude of the Sunyaev–Zel'dovich effect in clusters of galaxies does not depend directly on redshift; this seems to be roughly true, but unfortunately the amplitude does depend on cluster properties which do change substantially over cosmic time, so a precise test is impossible.

[edit] Features, issues and problems

While scientists now prefer the Big Bang model over other cosmological models, the scientific community was once divided between supporters of the Big Bang and those 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 the 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.

The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, the formation of galaxies—are derived from many observations that are independent from any cosmological model; these include the abundance of light elements, the cosmic microwave background, large scale structure, and the Hubble diagram for Type Ia supernovae.

Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject to the most active laboratory investigations.[47] Remaining issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not fatal to the dark matter explanation as solutions to such problems exist which involve only further refinements of the theory. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible.[48]

On the other hand, inflation and baryogenesis remain somewhat more speculative features of current Big Bang models: they explain important features of the early universe, but could be replaced by alternative ideas without affecting the rest of the theory.[notes 7] Discovering the correct explanations for such phenomena are some of the remaining unsolved problems in physics.

[edit] Horizon problem

Main article: Horizon problem

The horizon problem results from the premise that information cannot travel faster than light. In a Universe of finite age, this sets a limit—the particle horizon—on the separation of any two regions of space that are in causal contact.[49] The observed isotropy of the CMB is problematic in this regard: if the Universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature.

A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the Universe at some very early period (before baryogenesis). During inflation, the Universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable Universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.

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. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian, which has been accurately confirmed by measurements of the CMB.

If inflation occurred, exponential expansion would push large regions of space well beyond our observable horizon.

[edit] Flatness/oldness problem

Main article: 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. Shown from top to bottom are a closed Universe with positive curvature, a hyperbolic Universe with negative curvature and a flat Universe with zero curvature.The flatness problem (also known as the oldness problem) is an observational problem associated with a Friedmann–Lemaître–Robertson–Walker metric.[49] The Universe may have positive, negative or zero spatial curvature depending on its total energy density. Curvature is negative if its density is less than the critical density, positive if greater, and zero at the critical density, in which case space is said to be flat. The problem is that any small departure from the critical density grows with time, and yet the Universe today remains very close to flat.[notes 8] Given that a natural timescale for departure from flatness might be the Planck time, 10−43 seconds, the fact that the Universe has reached neither a Heat Death nor a Big Crunch after billions of years requires some explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the Universe density must have been within one part in 1014 of its critical value, or it would not exist as it does today.[50]

A resolution to this problem is offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that its curvature would have been smoothed out. Thus, it is theorized that inflation drove the Universe to a very nearly spatially flat state, with almost exactly the critical density.

[edit] Magnetic monopoles

Main article: Magnetic monopole

The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted topological defects in space that would manifest as magnetic monopoles. These objects would be produced efficiently in the hot early Universe, resulting in a density much higher than is consistent with observations, given that searches have never found any monopoles. This problem is also resolved by cosmic inflation, which removes all point defects from the observable Universe in the same way that it drives the geometry to flatness.[49]

A resolution to the horizon, flatness, and magnetic monopole problems alternative to cosmic inflation is offered by the Weyl curvature hypothesis.[51][52]

[edit] Baryon asymmetry

Main article: Baryon asymmetry

It is not yet understood why the Universe has more matter than antimatter.[33] It is generally assumed that when the Universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and antibaryons. However, observations suggest that the Universe, including its most distant parts, is made almost entirely of matter. An unknown process called "baryogenesis" created the asymmetry. For baryogenesis to occur, the Sakharov conditions must be satisfied. These require that baryon number is not conserved, that C-symmetry and CP-symmetry are violated and that the Universe depart from thermodynamic equilibrium.[53] All these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry.

[edit] 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.[54] 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.

[edit] 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 ΛCDM model fits – roughly 95% is in the exotic forms of dark matter and dark energyDuring the 1970s and 1980s, various observations showed that there is not sufficient visible matter in the Universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the Universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the Universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the Universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now indicated by numerous observations: the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy clusters.[55]

The evidence for dark matter comes from its gravitational influence on other matter, and no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.[56]

[edit] Dark energy

Main article: Dark energy

Measurements of the redshift–magnitude relation for type Ia supernovae indicate that the expansion of the Universe has been accelerating since the Universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the Universe consists of a component with large negative pressure, dubbed "dark energy". Dark energy is indicated by several other lines of evidence. Measurements of the cosmic microwave background indicate that the Universe is very nearly spatially flat, and therefore according to general relativity the Universe must have almost exactly the critical density of mass/energy. But the mass density of the Universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density.[18] Since dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy is also required by two geometrical measures of the overall curvature of the Universe, one using the frequency of gravitational lenses, and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.

Negative pressure is a property of vacuum energy, but the exact nature of dark energy remains one of the great mysteries of the Big Bang. Possible candidates include a cosmological constant and quintessence. Results from the WMAP team in 2008, which combined data from the CMB and other sources, indicate that the Universe today is 72% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos.[30] The energy density in matter decreases with the expansion of the Universe, but the dark energy density remains constant (or nearly so) as the Universe expands. Therefore matter made up a larger fraction of the total energy of the Universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.

In the ΛCDM, the best current model of the Big Bang, dark energy is explained by the presence of a cosmological constant in the general theory of relativity. However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about quantum gravity. Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research.

[edit] 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 were greater than the critical density, then the Universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state that was similar to that in which it started—a Big Crunch.[40] Alternatively, if the density in the Universe were equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as all the interstellar gas in each galaxy is consumed; stars would burn out leaving white dwarfs, neutron stars, and black holes. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the Universe would asymptotically approach absolute zero—a Big Freeze. Moreover, if the proton were unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the Universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.

Modern observations of 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 ΛCDM model of the Universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the Universe expands and cools. 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.[57]

[edit] Speculative physics beyond Big Bang theory

This is an artist's concept of the Universe expansion, where space (including hypothetical non-observable portions of the Universe) is represented at each time by the circular sections. Note on the left the dramatic expansion (not to scale) occurring in the inflationary epoch, and at the center the expansion acceleration. The scheme is decorated with WMAP images on the left and with the representation of stars at the appropriate level of development.

Image from WMAP press release, 2006While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the Universe's history. The Penrose-Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time. However, these theorems assume that general relativity is correct, but general relativity must break down before the Universe reaches the Planck temperature, and a correct treatment of quantum gravity may avoid the singularity.[58]

Some proposals, each of which entails untested hypotheses, are:

models including the Hartle–Hawking no-boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.[59]

brane cosmology models[60] in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically. In the latter model, the Big Bang was preceded by a Big Crunch and the Universe endlessly cycles from one process to the other.[61][62][63]

chaotic inflation, in which universal inflation ends locally here and there in a random fashion, each end-point leading to a bubble universe expanding from its own big bang.[64][65][66]

Proposals in the last two categories see the Big Bang as an event in a much larger and older Universe, or multiverse, and not the literal beginning.

[edit] Religious interpretations

Main article: Religious interpretations of the Big Bang theory

The Big Bang is a scientific theory, and as such is dependent on its agreement with observations. But as a theory which addresses the origins of reality, it has always carried theological and philosophical implications. In the 1920s and 1930s almost every major cosmologist preferred an eternal steady state Universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the steady state theory.[67] This perception was enhanced by the fact that the originator of the Big Bang theory, Monsignor Georges Lemaître, was a Roman Catholic priest.[68] Pope Pius XII, declared at the November 22, 1951 opening meeting of the Pontifical Academy of Sciences that the Big Bang theory accorded with the Catholic concept of creation.[69]

Since the acceptance of the Big Bang as the dominant physical cosmological paradigm, there have been a variety of reactions by religious groups as to its implications for their respective religious cosmologies. Some accept the scientific evidence at face value, while others seek to reconcile the Big Bang with their religious tenets, and others completely reject or ignore the evidence for the Big Bang theory.[70]



India, officially the Republic of India (Hindi: भारत गणराज्य Bhārat Gaṇarājya; see also Official names of India), is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with 1.18 billion people, and the most populous democracy in the world.[16][17] Mainland India is bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal on the east; and it is bordered by Pakistan to the west;[note] China, Nepal, and Bhutan to the north; and Bangladesh and Burma to the east. India is in the vicinity of Sri Lanka, and the Maldives in the Indian Ocean, its Andaman and Nicobar Islands are also in the vicinity of the Indonesian island of Sumatra in the Andaman Sea, and in the Andaman Sea India also shares a maritime border with Thailand.[18] India has a coastline of 7,517 kilometres (4,700 mi).[19]

Home to the ancient Indus Valley Civilisation and a region of historic trade routes and vast empires, the Indian subcontinent was identified with its commercial and cultural wealth for much of its long history.[20] Four major religions, Hinduism, Buddhism, Jainism and Sikhism originated here, while Zoroastrianism, Judaism, Christianity and Islam arrived in the first millennium CE and shaped the region's diverse culture. Gradually annexed by the British East India Company from the early eighteenth century and colonised by the United Kingdom from the mid-nineteenth century, India became an independent nation in 1947 after a struggle for independence that was marked by widespread non-violent resistance.[21]

India is a federal constitutional republic with a parliamentary democracy consisting of 28 states and seven union territories. A pluralistic, multilingual and multiethnic society, India is also home to a diversity of wildlife in a variety of protected habitats. The Indian economy is the world's eleventh largest economy by nominal GDP and the fourth largest by purchasing power parity. Since the introduction of market-based economic reforms in 1991, India has become one of the fastest growing major economies in the world;[22] however, it still suffers from poverty,[23] illiteracy,[24] corruption,[25] disease,[26] and malnutrition.[27] India is classified as a newly industrialized country[28][29] and is one of the four BRIC nations. It is a nuclear weapons state and has the third-largest standing armed force in the world.[30] while its military expenditure ranks tenth in the world. It is a founding member of the United Nations, the East Asia Summit, the South Asian Association for Regional Cooperation and the Non-Aligned Movement and a member of the Commonwealth of Nations and the G-20 major economies.


Main article: Names of India

The name India (pronounced /ˈɪndiə/) is derived from Indus, which is derived from the Old Persian word Hindu, from Sanskrit सिन्धु Sindhu, the historic local appellation for the Indus River.[31] The ancient Greeks referred to the Indians as Indoi (Ινδοί), the people of the Indus.[32] The Constitution of India and common usage in various Indian languages also recognise Bharat (pronounced [ˈbʱɑːrʌt̪] ( listen)) as an official name of equal status.[33] The name Bharat is derived from the name of the legendary king Bharata in Hindu scriptures. Hindustan ([hɪnd̪ʊˈstɑːn] ( listen)), originally a Persian word for “Land of the Hindus” referring to northern India, is also occasionally used as a synonym for all of India.[34]


Main articles: History of India and History of the Republic of India

Stone Age rock shelters with paintings at the Bhimbetka rock shelters in Madhya Pradesh are the earliest known traces of human life in India. The first known permanent settlements appeared over 9,000 years ago and gradually developed into the Indus Valley Civilisation,[35] dating back to 3400 BCE in western India. It was followed by the Vedic period, which laid the foundations of Hinduism and other cultural aspects of early Indian society, and ended in the 500s BCE. From around 550 BCE, many independent kingdoms and republics known as the Mahajanapadas were established across the country.[36]

Paintings at the Ajanta Caves in Aurangabad, Maharashtra, sixth centuryIn the third century BCE, most of South Asia was united into the Maurya Empire by Chandragupta Maurya and flourished under Ashoka the Great.[37] From the third century CE, the Gupta dynasty oversaw the period referred to as ancient "India's Golden Age".[38][39] Empires in Southern India included those of the Chalukyas, the Cholas and the Vijayanagara Empire. Science, technology, engineering, art, logic, language, literature, mathematics, astronomy, religion and philosophy flourished under the patronage of these kings.

Following invasions from Central Asia between the 10th and 12th centuries, much of North India came under the rule of the Delhi Sultanate and later the Mughal Empire. Under the rule of Akbar the Great, India enjoyed much cultural and economic progress as well as religious harmony.[40][41] Mughal emperors gradually expanded their empires to cover large parts of the subcontinent. However, in North-Eastern India, the dominant power was the Ahom kingdom of Assam, among the few kingdoms to have resisted Mughal subjugation. The first major threat to Mughal imperial power came from a Hindu Rajput king Maha Rana Pratap of Mewar in the 16th century and later from a Hindu state known as the Maratha confederacy, that ruled much of India in the mid-18th century.[42]

From the 16th century, European powers such as Portugal, the Netherlands, France, and Great Britain established trading posts and later took advantage of internal conflicts to establish colonies in the country. By 1856, most of India was under the control of the British East India Company.[43] A year later, a nationwide insurrection of rebelling military units and kingdoms, known as India's First War of Independence or the Sepoy Mutiny, seriously challenged the Company's control but eventually failed. As a result of the instability, India was brought under the direct rule of the British Crown.

Mahatma Gandhi (right) with Jawaharlal Nehru, 1937. Nehru would go on to become India's first prime minister in 1947.In the 20th century, a nationwide struggle for independence was launched by the Indian National Congress and other political organisations.[44] Indian leader Mahatma Gandhi led millions of people in several national campaigns of non-violent civil disobedience.[21]

On 15 August 1947, India gained independence from British rule, but at the same time the Muslim-majority areas were partitioned to form a separate state of Pakistan.[45] On 26 January 1950, India became a republic and a new constitution came into effect.[46]

Since independence, India has faced challenges from religious violence, casteism, naxalism, terrorism and regional separatist insurgencies, especially in Jammu and Kashmir and Northeast India. Since the 1990s terrorist attacks have affected many Indian cities. India has unresolved territorial disputes with the People's Republic of China, which, in 1962, escalated into the Sino-Indian War, and with Pakistan, which resulted in wars in 1947, 1965, 1971 and 1999. India is a founding member of the United Nations (as British India) and the Non-Aligned Movement.

In 1974, India conducted an underground nuclear test[47] and five more tests in 1998, making India a nuclear state.[47] Beginning in 1991, significant economic reforms[48] have transformed India into one of the fastest-growing economies in the world, increasing its global clout.[22]


Main article: Government of India

National Symbols of India[49][50]

Flag Tricolour

Emblem Sarnath Lion Capital

Anthem Jana Gana Mana

Song Vande Mataram

Animal Royal Bengal Tiger

Bird Indian Peacock

Aquatic animal Dolphin

Flower Lotus

Tree Banyan

Fruit Mango

Sport Field hockey

Calendar Saka

River Ganges

India is federation with a parliamentary form of government, governed under the Constitution of India.[51] It is a constitutional republic and representative democracy, "in which majority rule is tempered by minority rights protected by law." Federalism in India defines the power distribution between the center and the states. The government is regulated by a checks and balances defined by Indian Constitution, which serves as the country's supreme legal document.


The Constitution of India, the longest and the most exhaustive among constitutions of independent nations in the world, came into force on 26 January 1950.[52] The preamble of the constitution defines India as a sovereign, socialist, secular, democratic republic.[53] India has a bicameral parliament operating under a Westminster-style parliamentary system. Its form of government was traditionally described as being 'quasi-federal' with a strong centre and weaker states,[54] but it has grown increasingly federal since the late 1990s as a result of political, economic and social changes.[55]

President and Prime Minister

The President of India is the head of state[56] elected indirectly by an electoral college[57] for a five-year term.[58][59] The Prime Minister is the head of government and exercises most executive power.[56] Appointed by the President,[60] the Prime Minister is by convention supported by the party or political alliance holding the majority of seats in the lower house of Parliament.[56] The executive branch consists of the President, Vice-President, and the Council of Ministers (the Cabinet being its executive committee) headed by the Prime Minister. Any minister holding a portfolio must be a member of either house of parliament. In the Indian parliamentary system, the executive is subordinate to the legislature, with the Prime Minister and his Council being directly responsible to the lower house of the Parliament.[61]


The Legislature of India is the bicameral Parliament, which consists of the upper house called the Rajya Sabha (Council of States) and the lower house called the Lok Sabha (House of People).[62] The Rajya Sabha, a permanent body, has 245 members serving staggered six year terms.[63] Most are elected indirectly by the state and territorial legislatures in proportion to the state's population.[63] 543 of the Lok Sabha's 545 members are directly elected by popular vote to represent individual constituencies for five year terms.[63] The other two members are nominated by the President from the Anglo-Indian community if the President is of the opinion that the community is not adequately represented.[63]


India has a unitary three-tier judiciary, consisting of the Supreme Court, headed by the Chief Justice of India, 21 High Courts, and a large number of trial courts.[64] The Supreme Court has original jurisdiction over cases involving fundamental rights and over disputes between states and the Centre, and appellate jurisdiction over the High Courts.[65] It is judicially independent,[64] and has the power to declare the law and to strike down Union or State laws which contravene the Constitution.[66] The role as the ultimate interpreter of the Constitution is one of the most important functions of the Supreme Court.[67]

Administrative divisions

Main article: Administrative divisions of India

India consists of 28 states and seven Union Territories.[68] All states, and the two union territories of Puducherry and the National Capital Territory of Delhi, have elected legislatures and governments patterned on the Westminster model. The other five union territories are directly ruled by the Centre through appointed administrators. In 1956, under the States Reorganisation Act, states were formed on a linguistic basis.[69] Since then, this structure has remained largely unchanged. Each state or union territory is further divided into administrative districts.[70] The districts in turn are further divided into tehsils and eventually into villages.

The 28 states and 7 union territories of IndiaStates:

1.Andhra Pradesh

2.Arunachal Pradesh







9.Himachal Pradesh

10.Jammu and Kashmir




14.Madhya Pradesh










24.Tamil Nadu


26.Uttar Pradesh


28.West Bengal

Union Territories:

A.Andaman and Nicobar Islands


C.Dadra and Nagar Haveli

D.Daman and Diu


F.National Capital Territory of Delhi



Main article: Politics of India

The Secretariat Building, in New Delhi, houses key government offices.India is the most populous democracy in the world.[16][17] It has operated under a multi-party system for most of its history. For most of the years since independence, the federal government has been led by the Indian National Congress (INC).[68] Politics in the states have been dominated by national parties like the INC, the Bharatiya Janata Party (BJP) and various regional parties. From 1950 to 1990, barring two brief periods, the INC enjoyed a parliamentary majority.

Within Indian political culture, the Indian National Congress is considered center-left or "liberal" and the Bharatiya Janata Party is considered center-right or "conservative". The INC was out of power between 1977 and 1980, when the Janata Party won the election owing to public discontent with the state of emergency declared by the then Prime Minister Indira Gandhi. In 1989, a Janata Dal-led National Front coalition in alliance with the Left Front coalition won the elections but managed to stay in power for only two years.[71] As the 1991 elections gave no political party a majority, the INC formed a minority government under Prime Minister P.V. Narasimha Rao and was able to complete its five-year term.[72]

The years 1996–1998 were a period of turmoil in the federal government with several short-lived alliances holding sway. The BJP formed a government briefly in 1996, followed by the United Front coalition that excluded both the BJP and the INC. In 1998, the BJP formed the National Democratic Alliance (NDA) with several other parties and became the first non-Congress government to complete a full five-year term.[73]

In the 2004 Indian elections, the INC won the largest number of Lok Sabha seats and formed a government with a coalition called the United Progressive Alliance (UPA), supported by various Left-leaning parties and members opposed to the BJP. The UPA again came into power in the 2009 general election; however, the representation of the Left leaning parties within the coalition has significantly reduced.[74] Manmohan Singh became the first prime minister since Jawaharlal Nehru in 1962 to be re-elected after completing a full five-year term.[75]

Foreign relations and military

Main articles: Foreign relations of India and Indian Armed Forces

Jointly developed by Sukhoi and Hindustan Aeronautics, the Su-30 MKI "Flanker-H" is the Indian Air Force's prime air superiority fighter.[76]Since its independence in 1947, India has maintained cordial relationships with most nations. It took a leading role in the 1950s by advocating the independence of European colonies in Africa and Asia.[77] India is a member of the Commonwealth of Nations and a founding member of the Non-Aligned Movement.[78] India was involved in two brief military interventions in neighbouring countries – Indian Peace Keeping Force in Sri Lanka and Operation Cactus in Maldives. After the Sino-Indian War and the Indo-Pakistani War of 1965, India's relationship with the Soviet Union warmed and continued to remain so until the end of the Cold War. India has fought two wars with Pakistan over the Kashmir dispute. A third war between India and Pakistan in 1971 resulted in the creation of Bangladesh (then East Pakistan).[79] Additional skirmishes have taken place between the two nations over the Siachen Glacier. In 1999, India and Pakistan fought an undeclared war over Kargil.

In recent years, India has played an influential role in the SAARC and the WTO.[80] India has provided as many as 55,000 Indian military and police personnel to serve in thirty-five UN peacekeeping operations across four continents.[14] India is also an active participant in various mutlilateral forums, particularly the East Asia Summit[81] and the G8+5.[82] Recent overtures by the Indian government have strengthened relations with the United States and China. In the economic sphere, India has close relationships with other developing nations in South America, Asia and Africa.

India and Russia share an extensive economic, defence and technological relationship.[83] Shown here is PM Manmohan Singh with President Dmitry Medvedev at the 34th G8 Summit.India maintains the third-largest military force in the world, which consists of the Indian Army, Navy, Air Force[46] and auxiliary forces such as the Paramilitary Forces, the Coast Guard, and the Strategic Forces Command. The official Indian defence budget for 2010 stood at US$31.9 billion (or 2.12% of GDP).[84] According to a 2008 SIPRI report, India's annual military expenditure in terms of PPP stood at US$72.7 billion.[85] The President of India is the supreme commander of the Indian Armed Forces. India maintains close defence cooperation with Russia, Israel and France, who are the chief suppliers of arms. Defence contractors, such as the Defence Research and Development Organisation (DRDO) and Hindustan Aeronautics (HAL), oversee indigenous development of sophisticated arms and military equipment, including ballistic missiles, fighter aircraft and main battle tanks, to reduce India's dependence on foreign imports.

India became a nuclear power in 1974 after conducting an initial nuclear test, known as the Operation Smiling Buddha, and carried out further underground testing in 1998. Despite criticism and military sanctions, India has consistently refused to sign the CTBT and the NPT. India maintains a "no first use" nuclear policy[86] and is developing nuclear triad capability as a part of its "minimum credible deterrence" doctrine.[86] On 10 October 2008, a civilian nuclear agreement between India and the United States was signed, prior to which India received waivers from the IAEA and the NSG which ended restrictions on nuclear technology commerce and recognized India as the world's de facto sixth nuclear weapons state.[87] On 12 March 2010, Russia signed with India a nuclear reactor deal which will build 16 nuclear reactors in India as part of defence and energy deals .[88] On 28 June 2010, Canada signs with India a nuclear co-operation deal to promote and develop co-operation in civilian nuclear energy .[89]


Main article: Geography of India

See also: Geological history of India and Climate of India

Topographic map of India.India, the major portion of the Indian subcontinent, sits atop the Indian tectonic plate, a minor plate within the Indo-Australian Plate.[90]

India's defining geological processes commenced seventy-five million years ago, when the Indian subcontinent, then part of the southern supercontinent Gondwana, began a northeastwards drift—lasting fifty million years—across the then unformed Indian Ocean.[90] The subcontinent's subsequent collision with the Eurasian Plate and subduction under it, gave rise to the Himalayas, the planet's highest mountains, which now abut India in the north and the north-east.[90] In the former seabed immediately south of the emerging Himalayas, plate movement created a vast trough, which, having gradually been filled with river-borne sediment,[91] now forms the Indo-Gangetic Plain.[92] To the west of this plain, and cut off from it by the Aravalli Range, lies the Thar Desert.[93]

The original Indian plate now survives as peninsular India, the oldest and most geologically stable part of India, and extends as far north as the Satpura and Vindhya ranges in central India. These parallel ranges run from the Arabian Sea coast in Gujarat in the west to the coal-rich Chota Nagpur Plateau in Jharkhand in the east.[94] To their south, the remaining peninsular landmass, the Deccan Plateau, is flanked on the left and right by the coastal ranges, Western Ghats and Eastern Ghats respectively;[95] the plateau contains the oldest rock formations in India, some over one billion years old. Constituted in such fashion, India lies to the north of the equator between 6°44' and 35°30' north latitude[96] and 68°7' and 97°25' east longitude.[97]

India's coast is 7,517 kilometres (4,700 mi) long; of this distance, 5,423 kilometres (3,400 mi) belong to peninsular India, and 2,094 kilometres (1,300 mi) to the Andaman, Nicobar, and Lakshadweep Islands.[19] According to the Indian naval hydrographic charts, the mainland coast consists of the following: 43% sandy beaches, 11% rocky coast including cliffs, and 46% mudflats or marshy coast.[19]

The Himalayas form the mountainous landscape of Northern India. Seen here is Ladakh in Jammu & Kashmir.Major Himalayan-origin rivers that substantially flow through India include the Ganges (Ganga) and the Brahmaputra, both of which drain into the Bay of Bengal.[98] Important tributaries of the Ganges include the Yamuna and the Kosi, whose extremely low gradient causes disastrous floods every year. Major peninsular rivers whose steeper gradients prevent their waters from flooding include the Godavari, the Mahanadi, the Kaveri, and the Krishna, which also drain into the Bay of Bengal;[99] and the Narmada and the Tapti, which drain into the Arabian Sea.[100] Among notable coastal features of India are the marshy Rann of Kutch in western India, and the alluvial Sundarbans delta, which India shares with Bangladesh.[101] India has two archipelagos: the Lakshadweep, coral atolls off India's south-western coast; and the Andaman and Nicobar Islands, a volcanic chain in the Andaman Sea.[102]

India's climate is strongly influenced by the Himalayas and the Thar Desert, both of which drive the monsoons.[103] The Himalayas prevent cold Central Asian Katabatic wind from blowing in, keeping the bulk of the Indian subcontinent warmer than most locations at similar latitudes.[104][105] The Thar Desert plays a crucial role in attracting the moisture-laden southwest summer monsoon winds that, between June and October, provide the majority of India's rainfall.[103] Four major climatic groupings predominate in India: tropical wet, tropical dry, subtropical humid, and montane.[106]

Flora and fauna

Main articles: Flora of India and Fauna of India

See also: List of ecoregions in India

The Indian peacock is India's national bird and is found primarily in semi-desert grasslands, scrubs and deciduous forests of India.[107]India, which lies within the Indomalaya ecozone, displays significant biodiversity. One of eighteen megadiverse countries, it is home to 7.6% of all mammalian, 12.6% of all avian, 6.2% of all reptilian, 4.4% of all amphibian, 11.7% of all fish, and 6.0% of all flowering plant species.[108] Many ecoregions, such as the shola forests, exhibit extremely high rates of endemism; overall, 33% of Indian plant species are endemic.[109][110]

India's forest cover ranges from the tropical rainforest of the Andaman Islands, Western Ghats, and North-East India to the coniferous forest of the Himalaya. Between these extremes lie the sal-dominated moist deciduous forest of eastern India; the teak-dominated dry deciduous forest of central and southern India; and the babul-dominated thorn forest of the central Deccan and western Gangetic plain.[111] Important Indian trees include the medicinal neem, widely used in rural Indian herbal remedies. The pipal fig tree, shown on the seals of Mohenjo-daro, shaded Gautama Buddha as he sought enlightenment. According to latest report, less than 12% of India's landmass is covered by dense forests.[112]

Many Indian species are descendants of taxa originating in Gondwana, from which the Indian plate separated. Peninsular India's subsequent movement towards, and collision with, the Laurasian landmass set off a mass exchange of species. However, volcanism and climatic changes 20 million years ago caused the extinction of many endemic Indian forms.[113] Soon thereafter, mammals entered India from Asia through two zoogeographical passes on either side of the emerging Himalaya.[111] Consequently, among Indian species, only 12.6% of mammals and 4.5% of birds are endemic, contrasting with 45.8% of reptiles and 55.8% of amphibians.[108] Notable endemics are the Nilgiri leaf monkey and the brown and carmine Beddome's toad of the Western Ghats. India contains 172, or 2.9%, of IUCN-designated threatened species.[114] These include the Asiatic Lion, the Bengal Tiger, and the Indian white-rumped vulture, which suffered a near-extinction from ingesting the carrion of diclofenac-treated cattle.

In recent decades, human encroachment has posed a threat to India's wildlife; in response, the system of national parks and protected areas, first established in 1935, was substantially expanded. In 1972, India enacted the Wildlife Protection Act[115] and Project Tiger to safeguard crucial habitat; in addition, the Forest Conservation Act[116] was enacted in 1980. Along with more than five hundred wildlife sanctuaries, India hosts thirteen biosphere reserves,[117] four of which are part of the World Network of Biosphere Reserves; twenty-five wetlands are registered under the Ramsar Convention.[118]


Main article: Economy of India

See also: Economic history of India, Economic development in India, and Transport in India

The Bombay Stock Exchange, in Mumbai, is Asia's oldest and India's largest stock exchange by market capitalisation.In 2009, India's nominal GDP stood at US$1.243 trillion, which makes it the eleventh-largest economy in the world.[119] If PPP is taken into account, India's economy is the fourth largest in the world at US$3.561 trillion,[120] corresponding to a per capita income of US$3,100.[121] The country ranks 139th in nominal GDP per capita and 128th in GDP per capita at PPP.[119] With an average annual GDP growth rate of 5.8% for the past two decades, India is one of the fastest growing economies in the world.[122]

India has the world's second largest labour force, with 516.3 million people. In terms of output, the agricultural sector accounts for 28% of GDP; the service and industrial sectors make up 54% and 18% respectively. Major agricultural products include rice, wheat, oilseed, cotton, jute, tea, sugarcane, potatoes; cattle, water buffalo, sheep, goats, poultry; fish.[68] Major industries include textiles, telecommunications, chemicals, food processing, steel, transport equipment, cement, mining, petroleum, machinery, software.[68] India's trade has reached a relatively moderate share of 24% of GDP in 2006, up from 6% in 1985.[123] In 2008, India's share of world trade was about 1.68%.[124] Major exports include petroleum products, textile goods, gems and jewelry, software, engineering goods, chemicals, and leather manufactures.[68] Major imports include crude oil, machinery, gems, fertilizer, chemicals.[68]

From the 1950s to the 1980s, India followed socialist-inspired policies. The economy was shackled by extensive regulation, protectionism, and public ownership, leading to pervasive corruption and slow economic growth.[125] In 1991, the nation liberalised its economy and has since moved towards a free-market economy.[123][126] The policy change in 1991 came after an acute balance of payments crisis, and the emphasis since then has been to use foreign trade and foreign investment as integral parts of India's economy.[127]

The Tata Nano, the world's cheapest car.[128] India's annual car exports have surged fivefold in the past five years.[129]In the late 2000s, India's economic growth averaged 7.5% a year.[123] Over the past decade, hourly wage rates in India have more than doubled.[130] In 2009, the Global Competitiveness Report ranked India 16th in financial market sophistication, 24th in banking sector, 27th in business sophistication and 30th in innovation; ahead of several advanced economies.[131] Seven of the world's top 15 technology outsourcing companies are based in India and the country is viewed as the second most favourable outsourcing destination after the United States.[132]

Despite India's impressive economic growth over recent decades, it still contains the largest concentration of poor people in the world.[133] The percentage of people living below the World Bank's international poverty line of $1.25 a day (PPP, in nominal terms Rs. 21.6 a day in urban areas and Rs. 14.3 in rural areas in 2005) decreased from 60% in 1981 to 42% in 2005.[134] Since 1991, inter-state economic inequality in India has consistently grown; the per capita net state domestic product of India's richest states is about 3.2 times that of the poorest states.[135] Even though India has avoided famines in recent decades, half of children are underweight[136] and about 46% of Indian children under the age of three suffer from malnutrition.[133][137]

A 2007 Goldman Sachs report projected that "from 2007 to 2020, India’s GDP per capita will quadruple," and that the Indian GDP will surpass that of the United States before 2050, but India "will remain a low-income country for several decades, with per capita incomes well below its other BRIC peers."[138] Although the Indian economy has grown steadily over the last two decades; its growth has been uneven when comparing different social groups, economic groups, geographic regions, and rural and urban areas.[133] The World Bank suggests that India must continue to focus on public sector reform, infrastructure, agricultural and rural development, removal of labor regulations, improvement in transport, energy security, and health and nutrition.[139]


Main article: Demographics of India

See also: Religion in India, Languages of India, Ethnic groups of South Asia, and List of most populous metropolitan areas in India

Population density map of India.With an estimated population of 1.2 billion,[10] India is the world's second most populous country. The last 50 years have seen a rapid increase in population due to medical advances and massive increase in agricultural productivity due to the "green revolution".[140][141] India's urban population increased 11-fold during the twentieth century and is increasingly concentrated in large cities. By 2001 there were 35 million-plus cities in India, with the largest cities, with a population of over 10 million each, being Mumbai, Delhi and Kolkata. However, as of 2001, more than 70% of India's population continues to reside in rural areas.[142][143]

India is the world's most culturally, linguistically and genetically diverse geographical entity after the African continent.[68] India is home to two major linguistic families: Indo-Aryan (spoken by about 74% of the population) and Dravidian (spoken by about 24%). Other languages spoken in India come from the Austro-Asiatic and Tibeto-Burman linguistic families. Neither the Constitution of India, nor any Indian law defines any national language.[8] Hindi, with the largest number of speakers,[144] is the official language of the union.[145] English is used extensively in business and administration and has the status of a 'subsidiary official language;'[146] it is also important in education, especially as a medium of higher education. In addition, every state and union territory has its own official languages, and the constitution also recognises in particular 21 "scheduled languages".

As per the 2001 census, over 800 million Indians (80.5%) were Hindu. Other religious groups include Muslims (13.4%), Christians (2.3%), Sikhs (1.9%), Buddhists (0.8%), Jains (0.4%), Jews, Zoroastrians and Bahá'ís.[147] Tribals constitute 8.1% of the population.[148] India has the third-highest Muslim population in the world and has the highest population of Muslims for a non-Muslim majority country.

India's literacy rate is 64.8% (53.7% for females and 75.3% for males).[46] The state of Kerala has the highest literacy rate at 91% while Bihar has the lowest at 47%.[149][150] The national human sex ratio is 944 females per 1,000 males. India's median age is 24.9, and the population growth rate of 1.38% per annum; there are 22.01 births per 1,000 people per year.[46] Though India has one of the world's most diverse and modern healthcare systems, the country continues to face several public health-related challenges.[151] According to the World Health Organization, 900,000 Indians die each year from drinking contaminated water and breathing in polluted air.[152] There are about 60 physicians per 100,000 people in India.[153]


Main article: Culture of India

The Taj Mahal in Agra was built by Shah Jahan as memorial to wife Mumtaz Mahal. It is one of the New Seven Wonders of the World and a UNESCO World Heritage Site considered to be of "outstanding universal value".[154]India's culture is marked by a high degree of syncretism[155] and cultural pluralism.[156] India's cultural tradition dates back to 8,000 BCE[157] and has a continuously recorded history for over 2,500 years.[158] With its roots based in the Indus Valley Tradition, the Indian culture took a distinctive shape during the 11th century BCE Vedic age which laid the foundation of Hindu philosophy, mythology, literary tradition and beliefs and practices, such as dhárma, kárma, yóga and mokṣa.[159] It has managed to preserve established traditions while absorbing new customs, traditions, and ideas from invaders and immigrants and spreading its cultural influence to other parts of Asia, mainly South East and East Asia.

Indian religions form one of the most defining aspects of Indian culture.[160] Major dhármic religions which were founded in India include Hinduism, Buddhism and Jainism. Considered to be a successor to the ancient Vedic religion,[161] Hinduism has been shaped by several schools of thoughts such as the Advaita Vedanta,[162] the Yoga Sutras and the Bhakti movement.[160] Buddhism originated in India in 5th century BCE and prominent early Buddhist schools, such as Theravāda and Mahāyāna, gained dominance during the Maurya Empire.[160] Though Buddhism entered a period of gradual decline in India 5th century CE onwards,[163] it played an influential role in shaping Indian philosophy and thought.[160]

Traditional Indian society is defined by relatively strict social hierarchy. The Indian caste system describes the social stratification and social restrictions in the Indian subcontinent, in which social classes are defined by thousands of endogamous hereditary groups, often termed as jātis or castes.[164] Several influential social reform movements, such as the Bramho Shômaj, the Arya Samāja and the Ramakrishna Mission, have played a pivotal role in the emancipation of Dalits (or "untouchables") and other lower-caste communities in India.[165] However, the majority of Dalits continue to live in segregation and are often persecuted and discriminated against.[166]

Traditional Indian family values are highly respected, and multi-generational patriarchal joint families have been the norm, although nuclear family are becoming common in urban areas.[125] An overwhelming majority of Indians have their marriages arranged by their parents and other respected family members, with the consent of the bride and groom.[167] Marriage is thought to be for life,[167] and the divorce rate is extremely low.[168] Child marriage is still a common practice, with half of women in India marrying before the legal age of 18.[169][170]

Indian cuisine is characterised by a wide variety of regional styles and sophisticated use of herbs and spices. The staple foods in the region are rice (especially in the south and the east) and wheat (predominantly in the north).[171] Spices, such as black pepper which are now consumed world wide, are originally native to the Indian subcontinent. Chili pepper, which was introduced by the Portuguese, is also widely used in Indian cuisine.[172]

One of the fourteen gopurams of the Meenakshi Temple complex in Madurai, Tamil Nadu. Dedicated to Hindu God Shiva and his consort Meenakshi, the temple is considered to be the foremost religious and cultural center of Tamil people[173] and is one of the holiest Hindu sites in India.[174]Traditional Indian dress varies across the regions in its colours and styles and depends on various factors, including climate. Popular styles of dress include draped garments such as sari for women and dhoti or lungi for men; in addition, stitched clothes such as salwar kameez for women and kurta-pyjama and European-style trousers and shirts for men, are also popular.

Many Indian festivals are religious in origin, although several are celebrated irrespective of caste and creed. Some popular festivals are Diwali, Ganesh Chaturthi, Ugadi, Thai Pongal, Holi, Onam, Vijayadashami, Durga Puja, Eid ul-Fitr, Bakr-Id, Christmas, Buddha Jayanti, Moharram and Vaisakhi.[175][175] India has three national holidays which are observed in all states and union territories — Republic Day, Independence Day and Gandhi Jayanthi. Other sets of holidays, varying between nine and twelve, are officially observed in individual states. Religious practices are an integral part of everyday life and are a very public affair.

Indian architecture is one area that represents the diversity of Indian culture. Much of it, including notable monuments such as the Taj Mahal and other examples of Mughal architecture and South Indian architecture, comprises a blend of ancient and varied local traditions from several parts of the country and abroad. Vernacular architecture also displays notable regional variation.

Indian music covers a wide range of traditions and regional styles. Classical music largely encompasses the two genres – North Indian Hindustani, South Indian Carnatic traditions and their various offshoots in the form of regional folk music. Regionalised forms of popular music include filmi and folk music; the syncretic tradition of the bauls is a well-known form of the latter.

Indian dance too has diverse folk and classical forms. Among the well-known folk dances are the bhangra of the Punjab, the bihu of Assam, the chhau of West Bengal, Jharkhand , sambalpuri of Orissa , the ghoomar of Rajasthan and the Lavani of Maharashtra. Eight dance forms, many with narrative forms and mythological elements, have been accorded classical dance status by India's National Academy of Music, Dance, and Drama. These are: bharatanatyam of the state of Tamil Nadu, kathak of Uttar Pradesh, kathakali and mohiniyattam of Kerala, kuchipudi of Andhra Pradesh, manipuri of Manipur, odissi of Orissa and the sattriya of Assam.[176]

Theatre in India often incorporates music, dance, and improvised or written dialogue.[177] Often based on Hindu mythology, but also borrowing from medieval romances, and news of social and political events, Indian theatre includes the bhavai of state of Gujarat, the jatra of West Bengal, the nautanki and ramlila of North India, the tamasha of Maharashtra, the burrakatha of Andhra Pradesh, the terukkuttu of Tamil Nadu, and the yakshagana of Karnataka.[178]

The Indian film industry is the largest in the world.[179] Bollywood, based in Mumbai, makes commercial Hindi films and is the most prolific film industry in the world.[180] Established traditions also exist in Assamese, Bengali, Kannada, Malayalam, Marathi, Tamil, and Telugu language cinemas.[181]

The earliest works of Indian literature were transmitted orally and only later written down.[182] These included works of Sanskrit literature – such as the early Vedas, the epics Mahābhārata and Ramayana, the drama Abhijñānaśākuntalam (The Recognition of Śakuntalā), and poetry such as the Mahākāvya[183] – and the Tamil language Sangam literature.[184] Among Indian writers of the modern era active in Indian languages or English, Rabindranath Tagore won the Nobel Prize in 1913.


Main article: Sport in India

A 2008 Indian Premier League Twenty20 cricket match being played between the Chennai Super Kings and Kolkata Knight RidersIndia's official national sport is field hockey, administered by Hockey India. The Indian field hockey team won the 1975 Hockey World Cup and 8 gold, 1 silver and 2 bronze medals at the Olympic games,the highest from any national team. However, cricket is the most popular sport; the India national cricket team won the 1983 Cricket World Cup and the 2007 ICC World Twenty20, and shared the 2002 ICC Champions Trophy with Sri Lanka. India has also won the Asia Cup a record five times.Cricket in India is administered by the Board of Control for Cricket in India (BCCI); and domestic competitions include the Ranji Trophy, the Duleep Trophy, the Deodhar Trophy, the Irani Trophy and the NKP Salve Challenger Trophy. In addition, BCCI conducts the Indian Premier League, a Twenty20 competition.

Tennis has become increasingly popular, owing to the victories of the India Davis Cup team. Association football is also a popular sport in northeast India, West Bengal, Goa,Tamil Nadu and Kerala.[185] The Indian national football team has won the South Asian Football Federation Cup several times. Chess, commonly held to have originated in India, is also gaining popularity with the rise in the number of Indian Grandmasters.[186] Vishwanathan Anand,an Indian Grandmaster,has won the World Chess Championship four times.

Traditional sports include kabaddi, kho kho, and gilli-danda, which are played nationwide. India is also home to the ancient martial arts, Kalarippayattu and Varma Kalai. The Rajiv Gandhi Khel Ratna and the Arjuna Award are India's highest awards for achievements in sports, while the Dronacharya Award is awarded for excellence in coaching.

The Jaypee Group Circuit in Greater Noida,will be the upcoming hosts of the Indian Grand Prix in 2011.India has hosted several high-profile international sporting events, including the 2003 Afro-Asian Games and the 2007 Military World Games. India has also hosted or co-hosted the 1951 and the 1982 Asian Games, the 1987 and 1996 Cricket World Cup. It has also successfully hosted the 2010 Hockey World Cup and is scheduled to host the 2010 Commonwealth Games and later the 2011 Cricket World Cup.