Editor’s Note: This article is a part of the series Understanding Natural Phenomena.
What emerged at the Big Bang as a quantum fluctuation was an energy field, with no structure. But so much order and structure has emerged and evolved since then: Elementary particles, atoms, molecules, stars, galaxies, life, and so on. All this can be attributed to the following factors: the universe is expanding; the universe is cooling; the gravitational interaction was present right from the beginning; and ultra-minute quantum fluctuations occurred during the so-called ‘inflation‘ period, ~10-35 seconds after the birth of the universe. These fluctuations got amplified over time and were the original source of the cosmic structure we see today, including galaxies and clusters of galaxies.
Gradients of various types get created because of the expansion and the cooling of the universe. And these gradients are a measure of departure from equilibrium. The tendency to move towards equilibrium, so as to achieve stability, creates much of the order and structure. In addition, the inflation mentioned above was a one-off episode which created gradients, and its effects continue to affect the evolution of our universe.
I have already introduced the notion of free energy in Part 6. The second law of thermodynamics says that phenomena occur because their occurrence lowers the overall free energy. In particular, PHASE TRANSITIONS can occur for lowering the free energy. I consider the case of water to illustrate the notion of phase transitions. Above 100oC water exists as steam (at atmospheric pressure). Between 100oC and 0oC it exists as liquid water, and below 0oC it is ice. Thus there are three phases of water, namely steam, liquid water, and ice, each stable in an appropriate temperature (and pressure) regime. There is a change or transition of phase (or phase transition) from steam to liquid water on cooling to 100oC, because liquid water is more stable than steam below this temperature. Another phase transition occurs on cooling to 0oC, when liquid water changes to crystalline ice.
Let us begin at the beginning, and take a look at the figure below. It depicts the main events in the history of our universe. The time scale is not linear. The temperature rises as we go backwards in time towards the Big Bang, and physical processes happen more rapidly.
In the beginning the temperature was so high that no structure or order was possible, and there was only an energy field. As the very hot universe expanded after the cosmic explosion, it also cooled. 10-43 seconds after the Big Bang the temperature was ~1032 K (here K stands for ‘Kelvin’; 0oC = 273 K; no temperature can be equal to or lower than 0 K). The gravitational interaction was present at this stage.
The next important event in our cosmic history occurred 10-35 seconds after the Big Bang, and the technical term for it is INFLATION. During this very brief episode the rate of expansion of the newly born universe was much much higher than what it settled for after a while. In a way, it was this event which provided the real bang in the Big Bang.
As I said above, this inflation was one factor responsible for the later formation of structure in the universe. Why? The rate of expansion during the inflation was so very high that even the tiniest of quantum fluctuations got amplified and was enough for the nucleation and growth of structure. The temperature was ~1027 K, and matter (‘quarks’, ‘leptons’, ‘gauge bosons’, and several other elementary particles) appeared, as also ‘antimatter’. The appearance of matter and antimatter can be attributed to quantum fluctuations in the density of the universe, amplified by the effects of gravity. Even a miniscule increase in local density could attract more matter towards it, with a corresponding decrease in the surrounding density. That is how cosmic inhomogeneity arose and evolved.
At a certain stage of the inflation episode, a cosmic phase transition occurred, which freed enormous amounts of trapped energy (rather like the release of latent heat when steam condenses to water). After this prelude of inflation and cosmic phase transition, the normal (much slower) expansion of the universe set in, and has continued ever since.
During the inflation prelude, the universe grew extremely rapidly from a volume smaller than that of the nucleus of an atom to the size of a tennis ball. Why have cosmologists postulated the occurrence of inflation almost right after the Big Bang? With the postulation of the inflation episode certain cosmological mysteries get resolved. For example, when the universe was just the size of a tennis ball, regions that are very far apart today could have been in communicable contact then, resulting in the observed near-homogenization of the universe.
[Yes, the universe does look remarkably uniform or homogeneous in all directions (though not completely homogeneous). The present age of the universe is ~13.7 billion years. There are regions (e.g. the opposite sides of the horizon) that are so far apart that even light (the fastest moving signal anywhere) cannot travel from one end to the other in 13.7 billion years; so they cannot possibly be causally connected. But even they exhibit the same degree of homogeneity. This is known as the ‘horizon problem‘. The inflation hypothesis solves it.]
As you will see as we progress in this series of posts, an enormous number of facts about the evolution of order and complexity, as also the emergence and evolution of life, can be understood in terms of the following ultimate causes (in conjunction with the laws of quantum mechanics, relativity, and thermodynamics):
- Expansion and cooling of the universe.
- Emergence of the gravitational interaction at the birth of our universe.
- Occurrence of the very rapid and brief cosmic inflation soon after the birth of the universe.
- Occurrence and consolidation of quantum fluctuations during the inflation period.