Big Bang — The Big Bang theory is the dominant theory in cosmology about the early development and current shape of the universe. According to this theory, the universe expanded rapidly starting 13.7 0.2 billion years ago.
Extrapolating the history of the universe backwards using current physical models leads to a gravitational singularity, at which all distances become zero and temperatures and pressures become infinite. What this means is unclear and most physicists believe that this result is due to a lack of understanding of what the laws of physics are, necessary to describe this situation, and in particular, the lack of a theory of quantum gravity.
The universe was initially almost uniformly filled with energy and extremely hot. As the distances in the universe rapidly grew, the temperature dropped, leading to the creation of the known forces of physics, elementary particles, and eventually hydrogen and helium atoms in a process called Big bang nucleosynthesis.
Over time, the slightly denser regions of the almost, but not quite, uniformly distributed matter were pulled together by gravity into clumps, forming gas clouds, stars, galaxies, and the other astronomical structures seen today. The details of how the process of galaxy formation occurred depends on the type of matter in the universe, and the three competing pictures of how this occurred are known as cold dark matter, hot dark matter, and baryonic matter.
It is at present unknown whether the singularity of spacetime described above is a physical reality or just a mathematical extrapolation of general relativity beyond its limits of applicability. The resolution of this question has to wait until a confirmed theory of quantum gravity is available.
In general relativity, one usually talks about spacetime and cannot cleanly separate space from time. In the Big Bang theory, this difficulty does not arise; Weyl’s postulate is assumed and time can be unambiguously measured at any point as the “time since the Big Bang”.
The Big Bang was not an explosion of matter moving outward to fill an empty universe. Instead, it involved the rapid growth of the universe itself. Because of this, the distance (in the sense of comoving distance) between far removed galaxies increases faster than the speed of light. This does not violate the laws of special relativity, a theory which is physically valid only as a local theory. It states, among other things, that matter and information cannot travel through space faster than the speed of light, and it is empirically invalid for global space-time concepts (because it ignores gravity).
In 1927, the Belgian priest Georges Lematre was the first to propose that the universe began with the explosion of a “primeval atom”. Earlier, in 1918, the Strasbourg astronomer Wirtz had measured a systematic redshift of certain “nebulae”, and called this the K-correction, but he wasn’t aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way.
Years later, Edwin Hubble found experimental evidence to help justify Lematre’s theory. Again using redshift measurements, Hubble determined that distant galaxies are receding in every direction at speeds (relative to the Earth) directly proportional to their distance, a fact now known as Hubble’s law.
Since galaxies were receding, this suggested two possibilities. One, proposed by George Gamow, was that the universe began a finite time in the past and has been expanding ever since. The other was Fred Hoyle’s steady state model in which new matter would be created as the galaxies moved away from each other and that the universe at one point in time would look roughly like any other point in time. For a number of years the support for these two opposing theories was evenly divided.
In the intervening period however, all observational evidence gathered has provided overwhelming support for the Big Bang theory, and since the mid-1960s it has been regarded as the best available theory of the origin and evolution of the cosmos, and virtually all theoretical work in cosmology involves extensions and refinements to the basic big bang theory. Much of the current work in cosmology includes understanding how galaxies form within the context of the big bang, understanding what happened at the big bang, and reconciling observations with the basic theory.
Over the decades a number of weaknesses have been identified in the big bang theory, but these have thus far all been addressed by extensions and refinements such as cosmic inflation. As of 2003, there are no weaknesses in the big bang theory which are regarded as fatal by most or even a large minority of cosmologists. However, there remain small numbers of who still support non-standard cosmologies in which the big bang is considered incorrect.