A possible solution

I have still not explainedhowstars and galaxies could have formed
in the first place. This could only have happened if there had been
irregularities, or wrinkles, in the fabric of space that would cause
the matter there to be more dense than average. As long as space
does not expand too fast it isnowinevitable that the matter in those
regions will clump together further. This is similar to the example
of the molecules of air in the box that I discussed earlier. In that
case the volume inside the box did not expand, and the regions of
slightly higher density arose by pure chance. In the early Universe,
those regions where matter was clumping together would have
eventually heated up somuchthat nuclear fusion would have been
triggered and stars were born. However, the amount of wrinkling
had to be just right. If too little, matter would never have clumped
together and galaxies and stars (and hence us) would never have
formed. On the other hand if space had been too crumpled then
the high density of matter in those regions would have quickly
resulted in the formation of huge black holes.
Even if we do not understand the origin of these irregularities
we should at least look for experimental evidence that they existed
in the early Universe. It was predicted theoretically that they
should show up as tiny temperature variations in the microwave
background radiation which, as I mentioned in Chapter 3, is the
afterglow of the Big Bang. This effect had to be so small however
that it could not be detected fromEarth. In 1992,NASAannounced
that the COBE satellite (which stands for COsmic Background
Explorer) had detected a difference in the temperature of the
background radiation of just the right magnitude. The discovery
was hailed as the final proof that the Big Bang model was correct.
Some astronomers, however, argue that this statement is too strong
and that the COBE result did nothing more than support our
notions of galaxy formation. Does everything fit together now? Did the entropy of the
Universe start from a low value at the Big Bang? Will it keep
increasing even if the Universe one day collapses to a Big Crunch,
and hence provide us with an arrow of time that does not flip
over? I believe so, assuming of course that the Universe will one
day collapse (not likely, I know).
Just after the Big Bang, the Universe was hot and energetic
and thus in a state of low entropy. As it expanded it cooled. Its
entropy increased rapidly, not due to any heat transfer but rather
because its energy can be thought of as being used up to provide
the work for the expansion.
As the Universe cooled, a tiny amount of its energy became
locked up inside hydrogen atoms. Then, thanks to the wrinkles in
space which provided the seeds for the formation of stars, gravity
was soon able to cause these atoms to clump together to form the
galaxies and the stars within them. It then provided the means for
tapping this energy within the atoms through nuclear fusion.
If galaxies and stars had not formed, the Universe would
have died a heat death long ago. It would now be a cold black
place. The energy locked up within stars is just delaying the
inevitable. In a sense, the heat death of the Universe has already
taken place. The galaxies are really only small isolated pockets
of resistance to the rapidly increasing entropy around them. The
microwave background radiation with its temperature of just three
degrees above absolute zero is proof that the Universe has almost
completely unwound already.
Some authors have claimed that the heat death of the Universe
will never happen even if it continues to expand forever. Since the
space available for the matter in the Universe is always increasing,
they argue, there will always be more room for it to spread into.
This is wrong. Once matter and radiation are uniformly spread
throughout space then any further expansion will just reduce the
density (amount of matter in a given volume). It will not alter the
state of equilibrium.
If the Universe is destined to collapse again under its own
gravity then this would represent a further increase in entropy. It
does not matter if all it contains by then is cold radiation because no gravitational clumping in the usual sense is necessary. The Big
Crunch is not like the formation of galaxies in the early Universe.
During the collapse the whole Universe is closing in on itself. The
best way to describe this is to think of the Universe as a spring. The
expansion is like the stretching spring. If stretched too hard it will
never return to its original coiled state. If the stretching is more
gentle then it will allow itself to be pulled so far before it snaps
back into position. In the same way, the Universe at maximum
expansion still has gravitational potential energy. As it collapses its
entropy increases still further. The maximum entropy is reached
at the Big Crunch which marks the end of time; the tip of the
thermodynamic arrow of time.
The above explanation is an over-simplification. I have
mentioned that there is no real consensus yet on the arrows of
time in cosmology and the reasoning I have offered will be far
from the last word on the subject.
Now that you have seen just how confusing time can be as a
concept on its own, you are finally ready to meet Einstein’s special
theory of relativity in which he managed to lump time together
with space to form four-dimensional spacetime. Don’t be too
alarmed. Compared with this chapter’s often surreal metaphysical
ramblings, special relativity should be a breath of fresh air.
I sense scepticism.