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.
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