In the widely accepted standard Hot Big-Bang cosmology, our Universe at its earliest time was extremely hot and dense and has been expanding ever since. Very close to the beginning, somewhere between 10-36 to 10-34 seconds, the cosmological expansion was exponential - this period is known as `inflation'. During this epoch, the Universe doubled in size around 90 times. The expansion continued after the end of this period, but at a slower pace. Scientists believe that quantum fluctuations in the very early universe were amplified by the exponential expansion in the inflation phase to form the density inhomogenieties that eventually grew due to gravity to form structures like galaxies and clusters of galaxies.
And the Universe grows transparent
At the end of the first three minutes, the universe was a hot dense ionized plasma of primordial elements - Hydrogen, Helium, Lithium, Deuterium nuclei, plus electrons, in thermal equilibrium with radiation; all at a common temperature. The high density and temperature kept all elements completely ionized. However, as the Universe continued to expand the density dropped and the temperature dropped and the nuclei began to acquire electrons. First at about when the cosmic time was 20,000 years the helium nuclei acquired a single electron. Then at about when the clock showed 90,000 years the helium nuclei acquired a second electron and became neutral. Then when the cosmic time reached about 400,000 years after the Big Bang the Universe was sufficiently cool and tenuous so that remaining charged particles - protons and electrons - combined to form hydrogen atoms. And the universe was now composed of just neutral particles - hydrogen, helium and a little deutrium and lithium - and radiation and was transparent. The time when the Universe transitioned from being completely ionized to being almost neutral is known as the Epoch of Recombination.
The process of electron capture by Hydrogen and Helium nuclei, and the trickle down of the electrons down quantum states, resulted in the release of radiation or photons. These photons add to the relic radiation of the Big Bang - the Cosmic Microwave Background radiation (CMB). Since the photon released by capture of an electron by an atom can very well knock off the electron from another atom effectively nullifying the process, recombination proceeds over a long period gradually progressing with the expansion and cooling of the Universe. Once the photons decoupled from matter as the universe became almost wholly composed of neutral atoms, the photons traveled freely through the Universe without interacting with matter. CMB is not just a blackbody radiation of Planck form - it must inevitably contain the recombination lines emitted during cosmological recombination - and is hence a storehouse of information about the early times of the Universe, this thermal history, and is undoubtedly a powerful probe in Cosmology.
Shown below is a not-to-scale schematic depicting our view of the universe as we look outwards in space and backwards in time into the Early Universe - we are in the Milky Way galaxy at the center of the picture. We see history as we look out into space!
The transition of the Universe from ionized to neutral state during the Epoch of Recombination at about 400,000 years cosmic time was followed by what we call the `Dark Ages'. We cannot hope to see the universe during this age since there was no little interaction between matter and radiation, and, however, matter and radiation were in thermal equilibrium. But at about 10 million years the density became sufficiently low so that the matter cooled away from the radiation and from that time the gas is potentially observable.
Around 100 million years after the Big Bang, the initial density perturbations in the Universe are expected to have grown sufficiently to collapse and form structures: The first stars? The first ultradwarf galaxies? These might have lit up the whole Universe - the `Cosmic Dawn'. All this changed the temperature of matter, including hydrogen, and the populations of its electrons amongst its internal energy states, and the visibility of the gas evolves over cosmic time. The formation of first sources of radiation also resulted in the neutral Universe being reionized and we usher in the Epoch of Reionization. By 550 million years after the Big Bang, the Universe was completely reionized and it still remains so!
However, the rather long history is not without questions. There are plenty unresolved issues at these epochs of the Early Universe - in particular during the times when the history depends on complex astrophysics of star and galaxy formation and radiative escape from these first objects. We specifically focus on two of them, namely the Epochs of Recombination and Reionization. Can we actually study the thermal history of the Universe through observations? Were there any non standard processes during the Epoch of Recombination? How were the first stars formed? How different were they from present ones? How did they heat and ionize the Universe again? We aim to get answers to these by detecting signals of cosmological origins through our dedicated experiments: APSERa and SARAS