According to the Big Bang theory, the universe erupted from an enormously energetic, singular event, some 13.82 billion years ago. The theory does not explain the conditions that led to the creation of the universe, nor does it say what came before the Big Bang, or even what lies outside the universe. Contrary to the popular belief, the Big Bang did not occur at a single point in space as an “explosion.” It is better thought of as the simultaneous appearance of space everywhere in the universe. Surprisingly, it began no bigger than a point about 14 billion years ago.
In the early moments of Big Bang the universe went through a period of exponential expansion. Such an inflationary phase provides an explanation of why the universe looks the same in every direction, because, in the early universe, there would be time for light to travel from one region to another.
The universe was a very dark place until around 400 million years after the Big Bang. There were no stars or galaxies and the entire universe was made up of a plasma consisting mainly of protons and electrons (unbound charged particles are called ions and the ionized gas is called plasma), all of which were unbound and bouncing off photons. In the plasma, the very energetic particles and photons were tightly coupled, which means that they were all mixed up and indistinguishable.
Epoch of Recombination
As spacetime expanded the universe cooled down, electrons and protons began to combine to form neutral hydrogen atoms (hydrogen atoms with one electron and one proton). This process of pairing up is called recombination and it occurred about 300,000 years after the Big Bang. The universe was opaque before the recombination but became increasingly transparent as more and more free electrons became bound to the protons and light could travel unimpeded since it was not so much scattered by the free electrons (also by free protons but to a significantly lesser extent), giving rise to the cosmic microwave background (CMB). CMB is the free-streaming photons through the universe freshly released from the great primordial fireball.
Thus, at the epoch of recombination the gas went from being ionized to neutral. Recombination is the earliest phase in the cosmic history that can be studied with the light emanating from it in form of CMB.
While electrons in neutral hydrogen can also absorb photons of certain wavelengths by transitioning into an excited state, a universe full of neutral hydrogen is relatively opaque only at those absorbed wavelengths, but transparent throughout most of the spectrum. The Dark Ages start at that point, because there were no light sources other than the gradually darkening cosmic microwave background (CMB) radiation.
The neutral gas in the universe slowly coalesced into clumps via gravity. These clumps, which had seeds in the fluctuations in the CMB spectrum (shown in Figure 3), were denser than their surroundings by only about one part in 100,000. Those tiny blips in the CMB were enough to tip the scales of gravity to draw more matter together into bigger and bigger clumps. Sometime between 100 and 500 million years after the big bang interesting things started to happen when these little clumps became dense enough to form the first stars.
Epoch of Reionization
Then the first stars and galaxies lighted up the universe. According to astronomers’ best models, the first stars were 30 to 300 times as massive as our sun and millions of times brighter. But they lasted only for a few million years before exploding as massive supernovae. About 600 million years after the Big Bang, the universe went through another phase called reionization when the energetic ultraviolet light from these first stars began splitting the hydrogen atoms back into electrons and protons (or ionizing them).
Reionization offers a wealth of information on the first sources of light in the universe. It can teach us how stars were formed in the first galaxies and how those galaxies were, in turn, formed from the intergalactic medium. The first sources of light act as seeds for the later formation of larger objects.
Epoch of Accelerating Expansion
Since inflation the universe continued to expand since the Big Bang but at a much slower rate. The universe now is full of matter and the attractive force of gravity pulls all matter together. Gravity was expected to slow down the expansion with time and eventually reverse the expansion. However, in 1998 the Hubble Space Telescope (HST) observations of very distant supernovae showed that before the universe was 9 billion years ago, its expansion rate was slower than it is today. Thus, contrary to everyone’s expectations, the expansion of the universe did not slow down due to gravity, rather, it accelerated. Something had to be causing it.
The accelerating expansion phase of the universe happened around 9 billion years after the Big Bang. This is when the force of gravity begins to lose out to the anti-gravitational effect of “dark energy”, a mysterious force which has been accelerating the cosmic expansion ever since.