According to the Big Bang model, the universe expanded from an extremely dense and hot state and continues to expand today. The Big Bang theory is the prevailing cosmological model for the birth of the universe .[1] The model postulates that at some moment all of space was contained in a single point from which the universe has been expanding ever since. Modern measurements place this moment at approximately 13.8 billion years ago, which is thus considered the age of the universe .[2] After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles , and later simple atoms. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies. The Big Bang theory does not provide any explanation for the initial conditions of the universe; rather, it describes and explains the general evolution of the universe going forward from that point on. Since Georges Lemaître first noted, in 1927, that an expanding universe might be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion. While the scientific community was once divided between supporters of two different expanding universe theories—the Big Bang and the Steady State theory , accumulated empirical evidence provides strong support for the former.[3] In 1929, Edwin Hubble discovered indications that all galaxies are drifting apart at high speeds. In 1964, the cosmic microwave background radiation was discovered, which was crucial evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the universe before it was discovered. The known physical laws of nature can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature .[4][5][6] Overview History of the Universe - gravitational waves are hypothesized to arise from cosmic inflation , an expansion just after the Big Bang. [7][8][9][10] A graphical timeline is available at Graphical timeline of the Big Bang Hubble observed that the distances to faraway galaxies were strongly correlated with their redshifts . This was interpreted to mean that all distant galaxies and clusters are receding away from our vantage point with an apparent velocity proportional to their distance: that is, the farther they are, the faster they move away from us, regardless of direction. [11] Assuming that we are not at the center of a giant explosion (that the Earth is not the center of the universe), the only remaining interpretation is that all observable regions of the universe are receding from all others. Since we know that the distance between galaxies increases today, it must mean that in the past galaxies were closer together. The continuous expansion of the universe implies that the universe was denser and hotter in the past. The fist subatomic particles included protons, neutrons , and electrons . Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed. The majority of atoms produced by the Big Bang were hydrogen, along with helium and traces of lithium . Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies, and the heavier elements were synthesized either within stars or during supernovae . Large particle accelerators can replicate the conditions that prevailed after the earliest moments of the universe, resulting in confirmation and refinement of the details of the Big Bang model. However, these accelerators can only probe so far into high energy regimes . Consequently, the state of the universe in the very earliest instants of the Big Bang expansion is still poorly understood and an area of open investigation. The Big Bang theory offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background, large scale structure , and Hubbles Law. [12] The framework for the Big Bang model relies on Albert Einsteins theory of general relativity and on simplifying assumptions such as homogeneity and isotropy of space. The governing equations were formulated by Alexander Friedmann, and similar solutions were worked on by Willem de Sitter . Since then, astrophysicists have incorporated observational and theoretical additions into the Big Bang model, and its parametrization as the Lambda-CDM model serves as the framework for current investigations of theoretical cosmology. The Lambda-CDM model is the standard model of Big Bang cosmology, the simplest model that provides a reasonably good account of various observations about the universe. Timeline of the Big Bang Main article: Timeline of the Big Bang Singularity See also: Gravitational singularity 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. [13] This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly no closer than the end of the Planck epoch. This singularity is sometimes called the Big Bang, [14] but the term can also refer to the early hot, dense phase itself, [15][notes 1] which can be 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 an estimated age of 13.798 ± 0.037 billion years. [17] The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents of the universe. Inflation and baryogenesis Main articles: Cosmic inflation and baryogenesis 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 and huge temperatures and pressures and was very rapidly expanding and cooling. Approximately 10−37 seconds into the expansion, a phase transition caused a cosmic inflation , during which the universe grew exponentially.[18] After inflation stopped, the universe consisted of a quark–gluon plasma , as well as all other elementary particles .[19] 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. [20] Cooling Main articles: Big Bang nucleosynthesis and cosmic microwave background radiation Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. Galaxies are color-coded by redshift . The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form. [21] 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-) kelvin and the density was about that of air, neutrons combined with protons to form the universes deuterium and helium nuclei in a process called Big Bang nucleosynthesis .[22] 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 . [23] The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the Universe was only 10–17 million years old.[24] [25][26]
Posted on: Sun, 28 Dec 2014 15:02:11 +0000
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