Our universe
has been expanding. This means that if we extrapolate backwards in time, there
must have been an instant when there was (almost) a point universe. That was
the Big Bang moment, followed by continual expansion. But for explaining our
observations of the cosmos, something additional had to be postulated, namely a
very brief period of exponentially rapid expansion ('inflation') very soon
after the Big Bang. After this inflation interlude our universe settled to a far
slower expansion rate.
Alan Guth was
the originator of the inflation postulate, which solves, among other things,
the Horizon Problem (cf. Part 8) and the
Flatness Problem (Part 15).
For
postulating inflation, Guth was inspired by the analogy of a 'first-order'phase transition. As an
example, consider the freezing phenomenon in water. As water is cooled, its
free energy changes. There comes a temperature (the freezing point, 0oC)
below which a different phase, namely ice, has a lower free energy than liquid
water. Therefore, as demanded by the second law of thermodynamics, a phase
transition should occur from liquid water to ice. Yes, but that is not the full
story.
Suppose you
take extremely pure water, and also ensure that there are no disturbances like
vibrations etc. Then you find that you can 'supercool' it; i.e., it continues
to be a liquid even below 0oC. This happens because each phase of H2O
is stable in a certain range of temperatures, and, since it is a first-order
phase transition, there is no reason that 0oC be the temperature
where one stability range ends and the other begins. There is a small range of
temperatures around 0oC in which both phases are stable,
meaning that liquid water and ice can coexist. But below 0oC ice does
have a lower free energy than liquid water. Therefore even the slightest
disturbance to supercooled water can make it undergo rapidly the
arrested phase transition to ice. When this happens, the trapped excess free
energy gets released. We call it 'latent heat'.
Something
similar happened to the nascent universe. As it was cooling after the Big Bang,
it got trapped in a supercooled or metastable state. On further cooling, a
sudden, explosive, phase transition occurred. Here the equivalent of the
'latent heat' released was the positive vacuum energy (of the 'false vacuum' of
the metastable state). The volume of the tiny universe increased by a factor of
at least 1078 during the inflation, which occurred during the early
part of the electroweak epoch: It started ~10-36 seconds after time-zero,
and ended ~5 x 10-33 seconds later.
Let us put in
some numbers. Even for the entire inflation period, ∆t ~ 10-33
sec only. The uncertainty principle says that ∆E∆t ≥ h/(4π).
Since h/(4π) ~ 10-27 erg sec, we get ∆E ~106
erg, or ~109 GeV. Just about any value of ∆E larger than this
is also possible as a quantum fluctuation (resulting in the appearance and
disappearance of 'virtual particles') if ∆t is appropriately smaller
than 10-33 sec. Normally this would be of no serious significance
if the virtual particles could disappear within the time ∆t. But space
was doubling in length every 10-37 seconds during inflation, so THE
MOMENTARY INHOMOGENEITIES CREATED BY QUANTUM FLUCTUATIONS GOT YANKED APART
RAPIDLY, AND WERE FROZEN IN SPACE WHEN INFLATION ENDED; there was no going
back.
The 'false
vacuum' energy was of essentially the same nature as that coming from
Einstein's cosmological constant, or dark energy.
Predictions of
this cosmic-inflation model are in conformity with the characteristic density inhomogeneities
recorded in 2006 in the CMB map (for a flat-geometry universe at the end of the
inflation epoch).
The expansion
of space during the inflation interlude was much faster than the speed of
light. This is possible. According to the special theory of relativity (cf.
Part 10), nothing can
travel through space at a speed faster than that of light. But there is no
limit on the speed with which space itself can expand.
Let us note
two more things here:
(i) There was
and is gravitational interaction among the frozen fluctuations, and the
gravitational potential makes a negative contribution to the total energy (cf.
Part 2), so the
total energy content of the universe was and is close to zero.
(ii) 380,000
years after the Big Bang the evolution of the structure frozen at the end of
the inflation episode led to what we see in the CMB map today. In due course,
this structure evolved into the present configuration of galaxies, clusters of
galaxies, life, people.
You can appreciate
why the inflation postulate is such an integral part of modern cosmology. There
is more to it, still. INFLATION ALSO EXPLAINS WHY THERE SHOULD BE A MULTIVERSE
(i.e. many universes)
To understand
that, let us hark back to the phase transitions in water, but this time let us
consider boiling instead of freezing. Above 100oC, vapour or steam
is a more stable phase of H2O than the liquid phase. And it is again
a first-order phase transition, meaning that there is a range of temperatures
in which liquid water and steam can coexist. Because of thermal fluctuations
and other local variations, bubbles of steam start appearing even below 100oC,
and they do so with increasing frequency as the temperature is increased, till
the whole system starts boiling.
Something
similar happened during cosmic inflation, and this phenomenon is called 'chaotic
inflation'. There was a time interval during which inflation occurred, and
at any instant during this interval a different universe could inflate and go
its separate way, like a bubble of steam in the above picture.
So, not just
our universe, but a whole lot of universes, emerged before the end of the
inflation era, each with its own value for the cosmological constant and other
fundamental constants and laws of physics. There is a multiverse, rather
than just one (i.e., our) universe.
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