Where it All Started

THE BIG BANG THEORY

The Bang theory is a cosmological model of the the universe from the earliest known period through its subsequent full-scale evolution. The model describes how the universe expanded from its initial state of very high density and high temperature and offers a comprehensive explanation for a broad range of analysed phenomena, including the abundance of light elements, the Cosmic Microwave Background (CMB) radiation, large-scale structure, and Hubble's law – the farther away galaxies , the faster they are moving away from Earth. If the observed conditions are generalised backwards in time using the known laws of physics, the prediction is that just before a period of very high density there was a singularity. Current knowledge is insufficient to determine if anything existed prior to the singularity.

Georges Lemaître first observed in 1927 that the expanding universe could be traced back in time to an originating single point, calling his theory that of the "Primeval atom". For much of the 20th century scientific community was divided between supporters of the Big Bang and the rival steady-state model, but a wide range of evidence has strongly favored the Big Bang, which is these days universally accepted. Edwin Hubble concluded from analysis of galactic redshifts in 1929 that galaxies are drifting apart; this is an important observable evidence for an expanding universe. In 1964, the CMB was discovered, which was critical evidence in favor of the hot Big Bang model, since than the theory predicted the existence of a background radiation throughout the universe.

The known laws of physics can allow us to calculate the characteristics of the universe in detail back in time to its initial state of extreme density and temperature.  Detailed measurements about the expansion rate of the universe the Big Bang is placed at around 13.8 billion years ago, which is also considered the age of the universe.After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later atoms. Giant clouds of these primordial elements – mostly hydrogen, with some helium and lithium – later came together through gravity, forming early stars and galaxies, whih are the ancestors of stars and universes which are visible today. 

Besides these primordial materials, astronomers also observe the gravitational effects of an unknown dark matter surrounding galaxies. Most of the gravitational potential in the universe seems to be in this form, and the Big Bang theory and various observations indicate that it is not the conventional baryonic matter that forms atoms. Measurements of the redshifts of Supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy's existence.