<P> This primordial singularity is itself sometimes called "the Big Bang", but the term can also refer to a more generic early hot, dense phase of the universe . In either case, "the Big Bang" as an event is also colloquially referred to as the "birth" of our universe since it represents the point in history where the universe can be verified to have entered into a regime where the laws of physics as we understand them (specifically general relativity and the standard model of particle physics) work . Based on measurements of the expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event--otherwise known as the "age of the universe"--is 13.799 ± 0.021 billion years . The agreement of independent measurements of this age supports the ΛCDM model that describes in detail the characteristics of the universe . </P> <P> Despite being extremely dense at this time--far denser than is usually required to form a black hole--the universe did not re-collapse into a black hole . This may be explained by considering that commonly - used calculations and limits for gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang . </P> <P> 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 a very high energy density and huge temperatures and pressures and was very rapidly expanding and cooling . Approximately 10 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially during which time density fluctuations that occurred because of the uncertainty principle were amplified into the seeds that would later form the large - scale structure of the universe . After inflation stopped, reheating occurred until the universe obtained the temperatures required for the production of a quark--gluon plasma as well as all other elementary particles . 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 . </P> <P> 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 . After about 10 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators . At about 10 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 10 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). </P>

Which term refers to an assumed rapid expansion of the universe immediately after the big bang