With the advent of Cosmic inflation, the Universe expanded at a speed faster than the speed of light resulting in an instantaneous increase in the volume of space by 1078.  Due to this, the Universe continued to decrease in temperature and density, and therefore the average energy per particle was decreasing. Due to a decrease in the temperature, the symmetry between the fundamental forces broke down and put the fundamental forces and framework of standard model particle physics as we observe them today. 

The electromagnetic force and weak nuclear force were separated at about 10-12 seconds. After a period of about 10-11 seconds, the events become quite predictable, as particles have dropped down to levels now achievable at particle accelerators. Initially after a period of  10-6 seconds, quarks and gluons begin to fuse together  to form baryons such as protons and neutrons. 

Due to Baryogenesis, there is one part in 30 million more baryons than antibaryons which creates a small excess of baryons over anti-baryons. As temperatures dropped more, they no longer supported proton-antiproton formation which released in an instantaneous mass annihilation, leaving 1 in 1010 of the original protons and neutrons, and none of their antiparticles. The same process was experienced by electrons and positrons. This led to a drastic fall in the energy of the particles and remaining electrons, protons and neutrons were no longer at relativistic speeds. The energy density of the Universe was dominated now by photons.

After a few minutes of the expansion, when temperatures had fallen to about a billion kelvin and universal density of matter in the Universe was comparable to that of the Earth’s atmosphere, neutrons fused with protons to form universe’s deuterium and helium nuclei in a process known as Big Bang nucleosynthesis.

              Some reactions at the time of Big Bang nucleosynthesis 

With the advent of time, the Universe became cooler, the rest energy density of matter gravitationally dominated that of photon radiation. After a period of about 379,000 years, the electrons and nuclei combine to form atoms. This ancient relic radiation flowing through space largely unimpeded, is also known as Cosmic Background Radiation. 

                                           Cosmic Microwave Background Radiation

Written By- Sukhjit Singh

He is pursuing B.Sc in Mathematics Honours from SGTB Khalsa College, Delhi University. He often takes up multiple exciting projects and aims to develop multiple skills.