The Big Bang Theory
Immediately after the Big Bang, the universe was in an extremely hot and optically dense state and began expanding rapidly but cooled sufficiently after the initial expansion to allow energy to be converted into various subatomic particles that included protons, neutrons, and electrons. The simplest atomic nuclei were formed in the first three minutes after the Big Bang, but the first electrically neutral atoms took thousands of years to appear. The majority of atoms that were produced by the Big Bang were hydrogen (H) and helium (He), along with traces of lithium (Li). Colossal 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 explosions.
It is important to note that Big Bang is not the only cosmological theory that gained widespread acceptance. In fact, the scientific community was once divided between supporters of two different models of the universe: the Big Bang and the Steady State theory.
The Steady State theory was originally proposed in 1920 by James Jeans and revised in 1948 by Hermann Bondi, Thomas Gold, and Fred Hoyle 4. A steady-state universe has no beginning or end in time. The universe always expands but maintains a constant average density at all times by allowing matter to be continuously created to form new stars and galaxies. The rate of creation of new stars and galaxies is the same as the rate the old ones become unobservable because of their increasing distance and velocity of recession. The average density of the universe is the same and, on the grand scale, galaxies of all possible ages are uniformly interspersed in space.
The steady-state theory gained many supporters in the 1950s and 1960s but eventually faded away mainly because the theory had no reasonable explanation for CMB. On the other hand, most cosmologists started to believe that the Big Bang model explains how the universe evolved after 14 billion years of its birth because of the mounting observational evidence. This article explores this evidence starting with a very brief overview of Albert Einstein’s general theory of relativity which is basic framework of the Big Bang model.
Einstein’s general theory of relativity is an essential tool in modern astrophysics and it provides the framework of the standard Big Bang model of cosmology. It is a theory of gravitation whose defining feature is its use of the Einstein field equations the solutions of which define the topology of the space-time and how objects move under the influence of gravitational fields. The laws governing cosmic evolution appears to be written in the language of mathematics and Einstein’s geometrical interpretation of space and time is firmly grounded in mathematical equations. While it is not necessary to be well versed in advance mathematics, it is important to understand some of the predictions made by the general theory of relativity to make sense of our current understanding of the evolution of the universe.
As high resolution telescopes and satellite technology were developed, a wealth of observational data started to become available. In the field of cosmology, where the observables are sometimes located millions of light years away from us, the only way we can study them is via the light that they emit or reflect. Furthermore, the light which is detected today has often originated millions of years ago. To make sense of the observational data, one needed a solid theoretical understanding of the nature of space-time. Einstein’s theory gave that foundation to the cosmologists so they knew what to look for.
4 Fred Hoyle famously coined the phrase “Big Bang” although he meant it to derisively oppose his opponents’ views, but the phrase stuck.