A brief history of the big bang

After recently reading Lawrence Krauss's 'A universe from nothing' I have understood in greater detail the current state of our understanding of the beginnings of our universe. A fascinating and beautiful picture has emerged over the last 80 years (but more specifically the last 30). A picture which is clearer in some parts and uncertain in others but which is hard to dispute in certain key details. It is also a picture which is infinitely more nuanced and imaginative than any other creation stories that the various religions of the world have come up with. This is not surprising because the people who contributed to the religions didn't know what we know now, even though they may have been quite intelligent themselves.

The story begins around 13.72 billion years with what George Lemaitre called the primeval atom. This primeval atom of infinite density and temperature has been undergoing a constant expansion since then and has resulted in the universe that we currently live in. The theory of the universe emerging from this primeval atom is the theory of big bang. Before this the universe was assumed (by Einstein no less and many others) to be steady and to have existed forever. Three important experimental observations contributed to the advancement of the theory of big bang and no other scientific theory exists which provides better or even nearly as good explanation for these observations:

• It was discovered by Hubble in the first half of the 20th century that far away galaxies (which are not gravitationally bound to our galaxy) are all receding away from us. The further they are the faster they are receding. This would suggest that we are somehow at the center of the expansion which would lend a special place to the humans and the Earth. This runs counter to the Copernican ideas and to the more general cosmological principle which states that the universe, on a large enough scale, is same everywhere and in every direction. If this is assumed to be true, and it is not a difficult assumption, then the explanation for Hubble expansion would be that everything in the universe which is not gravitationally bound to something would be receding away from it. There's uniform expansion everywhere. Extrapolating backwards, the idea suggests that some time in the past all the matter of the universe must have been concentrated at one point, at the moment of the big bang. By measuring how fast the expansion is now, we can determine roughly how long ago did the big bang occur.
• There is a significant abundance of light elements (hydrogen, helium, lithium) in the universe. Once big bang has been proposed it is possible to calculate what the fractions of the light elements should be in the current universe. These theoretical calculations have been found to be in close agreement with experimental measurements.
• When the big bang model was being developed one of its prediction was the existence of its afterglow which could be measured from the Earth now. The measurement was predicted to be in the form of an electromagnetic signature in the microwave regime. This afterglow corresponds to a time around 330,000 years after the big bang. Before this time the universe was, as they say, opaque to such radiation meaning that no observation can possibly be made now about the time before this 'last scattering surface'. This afterglow which goes by the technical name 'Cosmic Microwave Background Radiation' (CMBR) was discovered in 1964 and its properties were found to be in excellent agreement with those predicted by theory. This discovery is also, arguably, the most important observation in all of cosmology.

Big bang, therefore, provides a time frame to our past with a very high degree of confidence in its veracity. Through high energy particle experiments on the Earth the evolution of the Big Bang expansion is reasonably well understood beyond the first microsecond of the universe. What happened in the first microsecond is less well understood and there is no understanding of what happened in the first .

The next question is what is the future of the universe. Which is to say how would the continuing expansion of the universe behave in the future. The answer to this question depends upon the nature of the current universe, specifically the nature of its curvature on large scales. This curvature can be measured from the CMBR profile and our universe has been found to be 'flat' from such measurements. Since the universe is governed by the general theory of relativity which relates the curvature of the universe to the amount of matter in it, it is possible to estimate how much matter there should be in the universe from the fact that its curvature is zero. However, after taking into account all the visible and measurable matter in the universe it was found that we were short of the required amount by a factor of around 50. This apparent discrepancy can be seen in other independent measurements such as gravitational lensing as well. This suggests that there exists another form of matter which only makes itself felt through its gravitational effect and which cannot be seen or measured otherwise. This is named dark matter. It was further discovered that the expansion of the universe is accelerating. This has been accounted for by an additional term in the Einstein's field equations. The term itself has been called the cosmological constant and its physical effect, dark energy. There appears ways to investigate what dark matter is but scientist are at quite a loss about what dark energy is. However, what is unequivocal is that the expansion of the universe is accelerating, so much so that, everything in the universe except the structures which are gravitationally bound to us would disappear from our view in about 2 trillion years (unless dark energy behaves in an unexpected way in the future). This is our fate.

Now the concept of big bang is intricately related to the idea of genesis which sits very uncomfortably with religious thoughts of all kinds which claim such stories as only within their own domains of treatment. Big bang itself doesn't overrule the existence of a God because it doesn't explain the initial conditions (where did the primeval atom come from for instance). But various sub-fields of science have together severely restricted the regions where the concept of God needs to be invoked. They have made the idea of God redundant and unnecessary to a large degree. To me it is only applicable now as an explanation of the first cause but even this applicability is unnatural. There may still be a God who started it all but even if that is the case what is beyond doubt is that it bears no resemblance to the gods of the humans because religious ideas are incredibly local and naive when compared to reality. This is not surprising either. People who came up with such ideas had no clue as to what is really out there.