This is part-2 of a series of blog articles, which will explore some of the biggest questions of cosmology and physics through a fun and short conversational format. In this series, I document conversations and Q&A sessions with Swagat Saurav Mishra, a senior research fellow at IUCAA, who works on early universe physics, inflationary cosmology and primordial black holes.

Till about a century ago, the boundaries between philosophy, metaphysics and real ‘science’, were very distinct and rigid, and the scientific revolution was still in its nascency. If you were born in 1820 and you were interested in the fate of the universe or the nature of time, you would be better off studying philosophy rather than science. Today, however, we are far more confident in the ability of science to solve the bigger mysteries of nature, and it is easy to find prominent scientists from around the world talking about the Multiverse theory or how the universe might end someday. From Aristotle’s geocentric model to the Steady State theory developed in 1948, the attempts to understand and solve the puzzle of creation has been a long, arduous journey, only thriving in the last 60 years or so. Gradually, the field of Cosmology found its footing in a dramatic story involving near misses, brilliant scientists, observational astronomers with tremendous patience, accidental discoveries and pigeon poop (we’ll cover this soon in our series).

As you might already know, Cosmology, being the audacious field it is, deals with some of the most challenging questions of science, including the birth of the universe and its eventual fate. As novices to the field, most of us have had that ‘one’ question in specific that we’ve tried asking our teachers or parents, but we eventually gave up because we never got any satisfactory response. The question, of course, is about the creation of matter, space and time. From atoms to ice cream, there is so much matter in the universe! Surely it must have come from somewhere, right? How can all of this pop out of nothing?

The challenge of trying to study the beginning is that you can never truly tell what happened before it. You are hence in an awkward position to make any judgement of how and why the beginning happened. To illustrate this a bit more, assume you're reading a fictional novel that begins with the sentence:

“Imagine a world in which every man and woman could predict the future, and all the children were taught calculus at the age of 6.”

Now, that does sound like a bizarre world, but as you continue to read the novel, you become involved in this new reality and soon everything makes sense to you! Do you know how this world came to be or how the people there could predict the future? No. But you do know that those ‘properties’ must have come from somewhere, or must have been consequences of some initial conditions. You are also aware that if you had to develop a model to explain how this hypothetical world works, you need to work out a theory that precisely and accurately explains all the attributes of this world. You need to be able to do this with the least number of random assumptions while accounting for the fact that coincidence and the proverbial dice of God could be significant factors, while also being aware that this whole world probably only exists in fiction! Most importantly, you need to set up tests or experiments, for example on the abilities of people living there or the fundamental physical laws,  that will help you in verifying your hypothesis about how this world came to be and how it functions. How did the beginning begin?
This analogy, although too naive,  helps in understanding how challenging it could be to study the birth of our universe.

Let's discuss the science in more detail now! We have our guide walking us through an explanation of how inflation helps us solve the problem of 'how' and 'why' our universe came to be. As you might be aware, the more traditional Big Bang Theory doesn't say much about what banged or why it banged and is often thought of as a badly chosen name in scientific circles. Swagat Saurav, a senior research fellow at IUCAA, introduced us to his domain of research in the previous article. Here, he addresses our big question.

Swagat: "The total entropy of the universe is measured by the total number of degrees of freedom which is roughly the total number of particles in the universe (an order of magnitude estimate) which is given by the humongous number 10^88 within our observable universe. So the question of the origin of these particles (baryons, photons, neutrinos, dark matter etc), known as the initial Entropy problem, is a fundamental one.

Our understanding of the modern Cosmology indicates that all most all the degrees of freedom (particles) present today were created at the end of inflation by the decay of the inflaton field energy. This process is known as 'Cosmic Reheating', with the 're' being a bad nomenclature like the 'Big Bang'. I will elaborate on it in the next paragraph. It turns out that the inflaton field had a lot of energy stored in it which was making the universe accelerate at early times. Slowly the value of this field started to change and finally, it started to oscillate. Such an oscillating field, in quantum physics, sprouts out particles and decays, somewhat like a radioactive substance. Thus inflation ended and the universe was reheated to the hot big bang phase. We do not know much about our universe before the epoch of inflation, so no comments on 'where from the inflaton field came?' question.

Regarding reheating, it progresses through the following steps. Initially, the inflaton field was changing very slowly and not oscillating. Thus it was accelerating the universe (this is a natural consequence of Einstein's theory of gravity). Later, it started to speed up and ended the accelerating phase and the universe started to decelerate, still filled with inflaton energy. Quickly after that, the inflaton field started to oscillate rapidly. During these oscillations, the inflaton behaved like a parametric oscillator (a simple harmonic oscillator whose frequency is time-dependent and periodically varying) and produced other particles in a quick and explosive way. This first stage of reheating has been termed as 'preheating'. These produced particles were not among any of our standard model particles. They were unstable and hence started to decay to standard model particles and dark matter and other particles which might have existed in the early universe. Let us call this stage as the 'Elementary Reheating' or more technically 'Perturbative Reheating' stage. Finally, these particles interacted very very efficiently with each other in the high-energy and high-density conditions available in the early universe and came to thermal equilibrium by a time when the universe was much younger than 1 min. This is the beginning of the Hot Big Bang phase.

So this was an accurate and simplified description of our best knowledge of how all the 10^88 particles that we see around today where created at the end of inflation when there were no particles at all, in a process called Reheating. Please keep in mind that Reheating is a highly quantum mechanical phenomenon and can not be understood more easily what I have explained here, with our classical physics analogies."

In the upcoming article, we will explore inflationary cosmology in more detail, and we will compare it with other rivalling theories, such as the cyclic universe or the Big Bounce hypothesis.