Answer to Question #203736 in Astronomy | Astrophysics for Narendranath

Question

What is the so called cosmic reionization process?

Answer 1

According to faculty.washington.edu, "Hydrogen reionization is the epoch when ultraviolet photons produced by the first galaxies ionized almost all of the hydrogen in the Universe." This process took place several hundred million years after the Big Bang.

According to www.nsf.gov, "The epoch of reionization is a period in the history of the universe that likely arose as a result of the arrival of the first stars and galaxies. Prior to this, the universe was dark, suffused with a dense, obscuring fog of primordial gas. As the first stars switched on, their ultraviolet energy began to reionize the cosmos, punching ever-larger holes in their murky surroundings. Eventually, the effect of these young, massive stars and their infant galaxies enabled light to shine freely through space", "Once the majority of the universe was reionized, approximately one billion years after the Big Bang, light across much of the electromagnetic spectrum could travel unimpeded through the cosmos, eventually revealing the universe as we see it today."

So, at the first stage of evolution our Universe was full of dense and ionized gas. Then due to expansion and cooling of the Universe, the electrons and protons could form neutral atoms. After the formation of stars and galaxies their emission reionized the gas.