The Big Bang theory is the most accepted scientific theory which explains the origin of the universe. It states that the universe was formed by expansion of an extremely hot and dense singularity about 13.8 billion years ago. It is believed that universe is expanding even today.
Right at the Big Bang the universe had zero volume and was infinitely hot. The curvature of space-time was infinite and all the matter was compressed into a single point of infinite density.
The universe expanded at a dramatically fast rate at the Big Bang. Within a fraction of seconds, space spread out at a mind boggling rate and temperatures dropped rapidly.
TIMELINES OF THE BIG BANG
The process of Big Bang unfolded into a sequence of events that can be studied under various timelines.
THE PLANCK EPOCH ( t = 0 to 10-43 seconds)
All matter and energy of universe were compressed into an infinitely small, dense and heated point of singularity. All known laws of physics break down at this point.
COSMIC INFLATION ( 10-36 to 10-32 seconds)
The space underwent exponential expansion at an extremely high rate. The sudden ballooning smoothened the irregularities and laid foundation for the uniform structure of the present universe that we observe.
QUARK AND HADRON ERA ( FIRST SECOND)
As the universe expanded, the temperature of the radiation started to decrease. One second after the Big Bang it decreased to about 10,000 million degrees. This is about a thousand times the temperature at the center of the sun.
Subatomic particles like quarks and leptons were generated. These combined to form protons and neutrons. At this time, the universe mainly consisted of photons, electrons and neutrinos along with their antiparticles, some protons and neutrons.
NUCLEOSYNTHESIS (3 to 20 minutes)
Temperatures fell to about 1,000 million degrees. At this temperature, protons and neutrons were unable to overcome the attraction of the strong nuclear force. They began to combine together to form nuclei of deuterium atom or heavy hydrogen, which has one proton and one neutron.
The nuclei of deuterium combined with more protons and neutrons to form nuclei of helium, which contained two protons and two neutrons. There were also small amounts of few heavier elements such as lithium and beryllium.
* Just a few hours after the big bang, the production of helium and other elements stopped. The universe just continued to expand and cool without any significant change for the next million years or so.
RECOMBINATION AND FIRST LIGHT (380,000 years)
Eventually, temperature dropped to a few thousand degrees. As the temperature lowered to this level, the electrons and nuclei were no longer able to overcome the force of electromagnetic attraction between them. They started to bind together by electromagnetic force to form atoms.
Earlier than this, the universe was an opaque, hot fog of plasma. After the creation of atoms, light could travel freely across space. The thermal radiation that travelled since then is observed today as the cosmic microwave background radiation. This provides a strong proof of the big bang model.
STAR AND GALAXY FORMATION (150 million to 1 billion years)
In regions that were slightly denser than average, the expansion was slowed down by gravitational attraction. The particles in these regions started to clump together under the influence of gravity. This led to the formation of galaxies and planets.
The gravitational pull of matter outside the collapsing regions caused the collapsing regions to rotate slightly. The smaller the collapsing region became, the faster it began to rotate to balance the gravitational attraction from outside. In this manner, disc like rotating galaxies were born.
As time passed, the gas from galaxies disintegrated into smaller clouds. These clouds collapsed under their own gravity and temperature inside these increased. Once the clouds became heated enough, nuclear reactions started occurring inside them to convert hydrogen into helium. The heat generated during nuclear reaction raised the pressure, restricting further contraction of clouds. In this way, stars including our sun were formed.
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