When you notice something out of the ordinary, you apply your mind to it. You consider all the possibilities that made it extraordinary. Then you choose the probability that stands tall against all the others.
Now you have a hypothesis. You test the hypothesis using the available tools – mathematics, instruments such as telescope or microscope, oscillographs, the computer, whatever. Sometimes data that come through fit like the gears in a clock, sometimes you have to work to fit them together. Then you invite peers to test your hypotheses. When a majority verify your research, and do some of their own and finally agree that you are right, subject to further verifications and modifications if necessary, you conclude you have a good Theory.
Big Bang is one such Theory.
How the universe came to existence has been a question that exercised the minds of many philosophers of yore, and scientists of the past few centuries. The probability of a big bang (though he never gave it that name) was first proposed by a polymath Catholic priest, Georges Lemaitre (1894-1966). When he suggested his idea to Einstein and showed his calculations, Einstein (1879-1955) commented wryly : “Your mathematics is perfect. But your physics is awful.” Three years later, when the expansion of the universe was proved by Edwin Hubble (1889-1953), Einstein, like a true scientist, ate his words and congratulated Lemaitre. The expansion solved many problems that vexed great minds such as that of Newton (1643–1727). Newton, in the absence of data that became available to science only in earlyy 1900, believed that God kept making periodic adjustments to stop collapse of the universe. Many years after Einstein and Lemaitre were long gone. the Hubble Space Telescope launched by NASA in 1990 positively confirmed the theory of expansion.
When you notice continuously accelerating expansion, it is easy to rewind the process in your mind as well as on the computer and arrive at a point where it looks like the original size. Mathematicians and physicists have ways of computing ordinary expansion with acceleration, and the expansion at the initial stages of an explosion. Such computations led to the conclusion that an event of sudden inflation occurred at a time between 13.7 (per Planck group) to 13.82 ( per NASA) Billion years ago. A stray but nearly uniform isotropic microwave radiation (Cosmic Microwave Background Radiation universely present – CMBR) independently discovered and measured at 2.72 K by a team of two scientists – Arno Penzias ( b. 1933) and Robert Wilson (b.1936) which fetched them the Nobel prize for Physics (1978) broadly confirmed the timing of the explosive event.
The best of the telescopes cannot see beyond some 5 billion years, but what we can see gives an idea of the development that happened during the 5 billion, you work out the events before the 5 billion partly by guesswork and more precisely by quantum mechanics. The name, Big Bang, was a term derisively used by the eminent but controversial scientist and author of science fiction Fred Hoyle (1915-2001) who did not subscribe to the theory, but the name has stuck. Whether there was a bang or an inflation, or inflation after big bang is a mute question. The current explanation is that at the point of inflation was singularity, where the laws of nature as we know them break down and hence it is difficult to know what happened before. Though this explanation sounds somewhat like the religious contention that God is beyond comprehension, Science will certainly break the ice wall sooner than later.
Big bang was a milestone, not the starting point of the universe. In Einstein’s famous equation, E=MC^2, neither energy E nor mass M is a function of volume. So it is also conceivable that the universe flew in from a near-infinitely small volume and is blowing like a horn to infinitely (to our mind, but infinity has no quantizable limit) large volume retaining virtually the same total amount of mass M and energy E while C remains constant. There is no limit to how small at the beginning, nor how large at the end. Our inability to comprehend the limitlessness of limits does not mean that the least and the most have a limit. If you think of n as the highest possible number, you can’t escape accepting that there could be a n+1.
On the other hand, the universe could be one of no boundaries – just as our earth has a finite circumference, but no boundaries. It recycles, blowing up from a certain low size to a certain high size and then starts crunching (the Big Crunch) back to where it began. Despite the universal expansion that is noticeable, we see that stars do come together to become giant stars and then crush themselves into black holes. It is not unlikely that when the black energy revealed by mathematical computations diminishes and loses steam, expansion reverses, galaxies get closer and finally become the greatest (and thence the smallest) black hole and then again begins with the nth order Big Bang,
Just because we cannot see beyond the hill, we must not conclude that there is nothing beyond it. Science will find what is out there some day. It could be other milestones on the other side of the Big Bang, or the Big Crunch, or even something our imagination has not yet caught on.