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big bang (Friedmann-Lemaitre universe)
subject			fact

big bang	is a kind of cosmology theory
	has definition According to standard cosmology, the explosion that started the universe expanding 10 to 15 billion years ago.
	has definition The initial point of creation.
	has definition The state of extremely high (classically, infinite) density and temperature from which the universe began expanding.
	has definition A model of the Universe which started with an initial singularity. The Friedmann model of a homogeneous, isotropic universe (composed of adiabatically expanding matter and radiation, as a result of a primeval explosion) is the standard example.
	has definition An evolutionary model of cosmology in which the universe began about 10 billion years ago, in a state of extremely high density and temperature. According to this model, the universe has been expanding, thinning out, and cooling since its beginning. It is an observational fact that distant galaxies are all moving away from our own galaxy, as predicted by the big bang model. (See closed universe; flat universe; Friedmann models; open universe.)
	has definition Model of cosmic history in which the universe begins in a state of high density and temperature, both of which decrease as the universe expands. Less a theory than a school of theories that attempt to trace how the universe evolved.
	has definition Theory originally proposed by Georges Lemaitre but elaborated by George Gamow and the α-β-hypothesis-γ that the Universe began with the Big Bang, the superexplosion of all the matter now dispersing in the Universe. Since the nuclear physics involved has been explained, and various supporting evidence - notably helium abundance and the sources of radio emission - has been discovered, the theory is almost universally accepted (although at one time the steady state theory rivaled it in popularity).
	has definition The most widely accepted theory of the origin of the Universe. It asserts that the Universe began some 1010 years ago from a space-time point of infinite energy density (a singularity). The expansion of the Universe since that time is akin to the expansion of the surface of an inflating balloon: every point on the balloon's surface is moving away from every other point. So, microbes living on the surface see their two-dimensional world expanding, yet there is no center to the expansion which is everywhere uniform.
	has definition A homogeneous, isotropic model of the Universe involving nonstatic (i.e., expanding or contracting) solutions to Einstein's field equations (with zero cosmological constant) calculated by the Russian mathematician A. Friedmann in 1922.
	has definition A general class of cosmological models that assume the universe is homogeneous and isotropic on large scales and that allow the universe to evolve in time. Most calculation in the standard big bang model assume a Friedmann cosmology. (See Friedmann equation; homogeneity; isotropy.) A cosmological model that has the same properties as a Friedmann model under some conditions is said to have a Friedmann limit.
	has definition One of three standard Big Bang models that were formulated by Friedmann and Lemaitre of an isotropic and homogeneous universe composed of expanding matter and radiation. In these models space is unbounded.
	has synonym Friedmann-Lemaitre universe
	has antonym big crunch
cosmology theory	has domain cosmology
theory	has validity correct or incorrect with caveats
	has author or reasearch group
	has date or a range of dates for which the theory was active

Kinds of big bang :

• closed universe (1 kind, 10 facts) - Big Bang model that was formulated by Friedmann and Lemaitre which has a positive curvature, like the surface of a sphere, in which case the universe is finite, closed, and will eventually recollapse. This space is unbounded., Any model of the Universe in which the gravity of the matter content can reverse the expansion and cause a collapse., A standard universe with a spherical three-dimensional spatial geometry. Such a universe is finite in both space and time, and recollapses., A homogeneous, isotropic universe is said to be temporally closed if gravity is strong enough to eventually reverse the expansion, causing the universe to recollapse. It is said to be spatially closed if gravity is strong enough to curve the space back on itself, forming a finite volume with no boundary. Triangles would contain more than 180°, the circumference of a circle would be less than π times the diameter, and a traveler intending to travel in a straight line would eventually find herself back at her starting point. If Einstein's cosmological constant is zero, as is frequently assumed, then a universe which is temporally closed is also spatially closed, and vice versa., Cosmological model in which the universe eventually stops expanding and begins to collapse, presumably to end in a fireball like that of the big bang. Compare open universe., A universe that has a finite size. Closed universes expand for a finite time, reach a maximum size, and then collapse. In closed universes, the inward pull of gravity dominates and eventually reverses the outward flying apart of matter; that is, gravitational energy dominates the kinetic energy of expansion. The value of omega is greater than 1 for a closed universe. If a universe begins closed, it remains closed; if it begins open, it remains open; if it begins flat, it remains flat. In the big bang model of the universe, the question of whether the universe is closed, open, or flat is determined by the initial conditions, just as the fate of a rocket launched from earth is determined by its initial upward velocity relative to the strength of earth's gravitational pull. If the initial rate of expansion of the universe was lower than a critical value, determined by the mass density, the universe will expand only for a certain period of time and then collapse, just as a rocket launched with a velocity below a critical value, dependent on the strength of earth's gravity, will reach a maximum height and then fall back to earth. This is the behavior of a closed universe. If the initial rate of expansion of the universe was larger than a critical value, the universe is open and will keep expanding forever. If the initial rate of expansion was precisely the critical value, the universe is flat and will expand forever, but with a rate of expansion that approaches zero. (See flat universe; omega; open universe.)
• flat universe (14 facts) (Einstein-de Sitter universe) - Big Bang model that was formulated by Friedmann and Lemaitre which has a zero curvature, or flat (Euclidian). This space is unbounded., A Universe in which there is no curvature to the spacetime continuum. This means that the kinetic energy of the expansion is exactly balanced by the potential gravitational energy of the matter. Thus, after an infinite amount of time the Universe will stop expanding., A homogeneous, isotropic universe is called flat if it is just on the borderline between being spatially closed and spatially open, so the geometry is precisely Euclidean. If Einstein's cosmological constant is zero, then a flat universe will go on expanding forever, but the velocity of recession between any two objects would approach zero at large times., A universe that is at the boundary between an open and closed universe. In a flat universe, the average mass density always has precisely the critical value. A flat universe has zero total energy and an infinite size. Flat universes have the geometry of an infinite, flat surface, that is, Euclidean geometry. The value of omega is 1 for a flat universe. (See closed universe; critical mass density; Euclidean geometry; omega; open universe.), A Friedmann model of the Universe in which the spacetime continuum is not curved., A homogeneous world model (the simplest relativistic model) of finite density, zero curvature, and nonzero cosmological constant, subject to the field equations of general relativity in an expanding Euclidean space. The radius increases rapidly from zero, and, although it always increases, the rate of increase becomes less as time goes on., The flat (k = 0), pressureless standard model of the universe., A particular solution to Einstein's cosmological equations in which the universe is flat. (See flat universe.), The Friedmann-Lemaitre model in which space is Euclidean was advocated by Einstein and de Sitter in 1932., A cosmological model in which a static (neither expanding nor collapsing) universe is maintained by introducing a cosmological repulsion force (in the form of the cosmological constant) to counterbalance the gravitational force., A world model of a static universe with a positive cosmological constant, whose radius of curvature is constant and independent of time.
• open universe (6 facts) - Big Bang model that was formulated by Friedmann and Lemaitre which has a negative curvature, like a saddle-shaped surface, in which case the universe is infinite, open, and will expand forever. This space is unbounded., Any model of the Universe which does not contain enough matter to halt its expansion., A homogeneous, isotropic Universe is said to be temporally open if gravity is not strong enough to eventually reverse the expansion, so the universe goes on expanding forever. It is said to be spatially open if it curves the opposite way from a closed universe, so that triangles would contain less than 180°, the circumference of a circle would be more than π times the diameter, and the volume would be infinite. If Einstein's Cosmological Constant is zero, as is frequently assumed, then a universe which is temporally open is also spatially open, and vice versa., Cosmological model in which the universe continues to expand forever; its space-time geometry is hyperbolic, or "open".

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