Stephen Hawking was undoubtedly one of the brilliant minds we have seen in our era. He was a great physicist, cosmologist and one of best authors of popular science books. His book A Brief History of Time sold more than 10 million copies so far and became one of the most popular science books. In his writings, Hawking widely discusses about his research work on black holes and gravitational singularity, which have helped us to reconfigure our view towards the universe and its origins. This book, The Theory of Everything: The Origin and Fate of the Universe is similar to ‘A Brief History of Time’ and is based on a series of lectures Hawking delivered at Cambridge University.
Hawking begins the book with the history of ideas about the universe right from the time of Aristotle (340 B.C) where Greek people were the first to discover that the Earth was spherical and Aristotle proposed simple cosmological model with Earth as the centre of the universe. However, after many centuries, these models were firmly replaced in mid 1600s by Heliocentric models proposed by Copernicus and Galileo. During this time, the idea of gravity first came into the picture with the laws proposed by Sir Isaac Newton and the universe was thought to be static in existence. But, in 1929, Edwin Hubble’s observations that the distant stars are moving away from us, finally brought the question of beginnings of the universe into the realm of science.
The second lecture of the book discusses this landmark observation by Hubble that the universe is expanding (i.e distant stars are moving away from us). At present, the average density of the universe is much less than the critical value in order to halt the expansion by gravitational attraction. This suggest that the universe will probably expand forever. Friedmann assumptions and models further describe that the galaxies are all moving directly away from each other. So, at some point in the past, the distance between galaxies must have been zero and the universe was infinitely dense with infinite curvature of space-time, which we call The Big Bang, the beginning of the universe.
Hawking then moves on to discuss his studies on Black holes in the third lecture. A Black hole is a place in space where the matter has been squeezed into a tiny space and the corresponding gravitational field is so much that even light cannot escape from it. This can happen when a star is dying. This infinite space and infinite space-time curvature (known as the point of singularity) is similar to the conditions of Big bang. At this point of singularity, the classical laws of science and our ability to predict future would breakdown. So, we technically cannot comment what happened before Big Bang.
Hawking continues the discussion on his research work in the next lecture. He argues that the black holes could have been formed if the early universe had not been perfectly uniform, because then a small region was denser than average could be compressed as a black hole. He mathematically proved that the black holes can even emit radiation (Hawking radiation) as X rays and gamma rays in order to prevent violations of the second law of thermodynamics. However, the search for the practical evidence of primordial blackholes and Hawking radiation is still going on.
In the fifth lecture, he talks about the origin of the universe. According to hot big bang model, universe was initially infinitely hot and dense, but as it expanded, temperature began to drop rapidly. At this stage it contained only subatomic particles, and later the protons and neutrons started to combine to produce hydrogen and helium. At further drop in temperature, electrons and nuclei started combining to form atoms. As the expansion still continued, the regions of higher dense space experienced more gravitational pull from the matter outside, thus rotating them slightly, creating disk like galaxies. Later the gas in the galaxies broke up into small clouds, which started collapsing under their own gravity, initiating nuclear reactions and rising temperature in this process, finally becoming as burning balls (stars). This picture of the universe leaves us with most fundamental questions. Why was the early universe so hot? Why did it start with critical rate of expansion to avoid collapse? What was the origin of density fluctuations i.e non uniformity ?
Hawking tries to explore the answers with quantum theory since the classical laws and Einstein’s equations would breakdown at the point of singularity. He argues that the curved space-time must be taken to be Euclidean space, i.e time as an imaginary coordinate and indistinguishable from directions in space. He proposed that if one is using the Euclidean space-time, it is possible for space-time to be finite with no singularities that formed a boundary or edge. “Imagine this no boundary space-time as the surface of the Earth, with two more dimensions”, Hawking says. Using this no boundary condition, universe might have started with minimum possible non uniformity according to uncertainty principle, and then expanded in real time like inflationary model.
In the last lecture, Hawking quests for the unified theories of physics, with a special mention of string theory. In this theory, the fundamental particles are not particles that just occupy a single point in space. Rather, they are like infinitely thin loop of strings. For instance, an electron or a proton could be regarded as waves on a string, with differed oscillations. These strings seem to be consistent in higher dimensions of space-time (either ten or twenty six dimensions). But we cannot notice these extra dimensions as they are curved upto into space of very small size (as small as 10^-35 meters). Right now, string theory is too young to be called as the unified theory as we still don’t know how to relate the waves on a string to the particles we observe.
So, can there be a unified theory of physics that will include quantum mechanics and gravity? If there is such a theory, can it explain why the universe bother of existing in the very first place? Where might the conclusion lead to? a self evolving universe or a need for a creator?
Think about it.