Big Bang

In physical cosmology, the Big Bang is the scientific theory that the universe emerged from an enormously dense and hot state nearly 14 billion years ago. The Big Bang is a consequence of the observed Hubble's law velocities of distant galaxies that when taken together with the cosmological principle implies that space is expanding according to the Friedmann model of general relativity. Extrapolated into the past, these observations show that the universe has expanded from a primeval state, in which all the matter and energy in the universe was at an immense temperature and density. Physicists do not widely agree on what happened before this, although general relativity predicts a gravitational singularity.

Overview

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

Related Topics:
Type Ia supernova - Cosmic microwave background - Correlation function - Age - 13.7 ± 0.2 billion years - Lambda-CDM model

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

Related Topics:
Energy - Temperature - Pressure - Phase transition

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Approximately 10-35 seconds after the Planck epoch, a phase transition caused the universe to experience exponential growth during a period called cosmic inflation. After inflation stopped, the material components of the universe were in the form of a quark-gluon plasma (also including all other particles—and perhaps experimentally produced recently as a quark-gluon liquid http://www.aip.org/pnu/2005/split/728-1.html) in which the constituent particles were all moving relativistically. As the universe continued growing in size, the temperature dropped. At a certain temperature, by an as-yet-unknown transition called baryogenesis, the quarks and gluons combined into baryons such as protons and neutrons, somehow producing the observed asymmetry between matter and antimatter. Still lower temperatures led to further symmetry breaking phase transitions that put the forces of physics and elementary particles into their present form. Later, some protons and neutrons combined to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. As the universe cooled, matter gradually stopped moving relativistically and its rest mass energy density came to gravitationally dominate that of radiation. After about 300,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is the cosmic microwave background.

Related Topics:
Planck epoch - Exponential growth - Cosmic inflation - Quark-gluon plasma - Relativistically - Baryogenesis - Baryon - Asymmetry - Matter - Antimatter - Symmetry breaking - Forces of physics - Elementary particles - Deuterium - Helium - Nuclei - Big Bang nucleosynthesis - Rest mass - Gravitationally - Radiation - Hydrogen - Decoupled

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

Related Topics:
Star - Cold dark matter - Hot dark matter - Baryonic matter - WMAP

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

Related Topics:
Dark energy - Expansion of the universe - Spacetime - Cosmological constant - Einstein's field equation - Equation of state - Standard model

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

Related Topics:
Lambda-CDM model - Mathematical model - Grand unification theory - Gravitational singularity - Paradox - Quantum gravity - Unsolved problems in physics

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See also: Timeline of the Big Bang

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