The Big Bang Theory

Big Bang Theory is about the origin of Universe. It suggests that about 1370 crore (13.7 billion) years ago, all matter and energy in the universe was concentrated into an area smaller than an atom. At this instant, matter, energy, space and time were not existent.Then suddenly with a bang, the Universe began to expand at an incredible rate and matter, energy, space and time came into being. As the Universe expanded, matter began to coalesce into gas clouds and the stars and planets. Some scientists believe that this expansion is finite and will done day cease. After this point in time, the Universe will begin to collapse until a Big Crunch occurs.

Just before the Big Bang

No one knows what the universe was like at this time. The best current theory, the “inflationary universe” model assumes that all of space is filled with an extremely concentrated, unstable form of energy that will be transformed into particles of matter at the instant of the Big Bang. But no one knows how space and time came into existence in the first place.

The first few minutes to next thousand years

After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, including photons, electrons, protons and neutrons.Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed. The majority of atoms that were produced by the Big Bang are hydrogen, along with helium and traces of lithium.

PS :  If the universe had remained this hot and dense for much longer, the hydrogen would all have been cooked into other chemical elements. Without hydrogen, there would be no water, and therefore no life as we know it!

Big Bang Theory Time Line

Earlier Opaque Universe vs Later Transparent Universe

Photons (light) being elementary particles would have been formed soon after the Big Bang. But these photons would have been scattered by the early electrons. As the Universe continued to cool, it would have eventually reached the temperature where electrons combined with nuclei to form neutral atoms (recombination). Before this “recombination” occurred, the Universe would have been opaque because the free electrons would have caused light (photons) to scatter the way sunlight scatters from the water droplets in clouds. But when the free electrons were absorbed to form neutral atoms, the Universe suddenly became transparent. Those same photons – the afterglow of the Big Bang known as Cosmic Background Radiation – can be observed today.

PS: Opacity of the early Universe before recombination is, in effect, a curtain drawn over those interesting very early events. Fortunately, there is a way to observe the Universe that does not involve photons at all. Gravitational waves, the only known form of information that can reach us undistorted from the instant of the Big Bang, can carry information that we can get no other way. Two missions that are being considered by NASA, LISA and the Big Bang Observer, will look for the gravitational waves from the epoch of inflation.