Credit to W.Rex for the subject heading...
Excerpt from God & The New Physics, by Paul Davies [initially published in 1983] --
Although the entropy of a general gravitating system is not known,
work by Jacob Bekenstein and Stephen Hawking, in which the
quantum theory is applied to black holes, has yielded a formula for the
entropy of these objects. As expected, it is enormously greater than the
entropy of, for instance, a star of the same mass. Assuming that the
relationship between entropy and probability extends to the gravitating
case, this result may be expressed in an interesting way. Given a
random distribution of (gravitating) matter, it is overwhelmingly
more probable that it will form a black hole than a star or a cloud of
dispersed gas. These considerations give a new slant, therefore, to the
question of whether the universe was created in an ordered or disordered
state. If the initial state were chosen at random, it seems
exceedingly probable that the big bang would have coughed out black
holes rather than dispersed gases. The present arrangement of matter
and energy, with matter spread thinly at relatively low density, in the
form of stars and gas clouds would, apparently, only result from a very
special choice of initial conditions. Roger Penrose has computed the
odds against the observed universe appearing by accident, given that a
black-hole cosmos is so much more likely on a priori grounds. He
estimates a figure of 10 to the power of 10 to the power of 30 to one.
The absence (or at least lack of predominance) of black holes is not
the only issue. The large scale structure and motion of the universe is
equally remarkable. The accumulated gravity of the universe operates
to restrain the expansion, causing it to decelerate with time. In the
primeval phase the expansion was much faster than it is today. The
universe IS thus the product of a competition between the explosive'
vigour of the big bang, and the force of gravity which tries to pull the
pieces back together again. In recent years, astrophysicists have come
to realize just how delicately this competition has been balanced Had,
the big bang been weaker, the cosmos would have soon fallen back on
itself in a big crunch. On the other hand, had it been stronger, the
cosmic material would have dispersed so rapidly that galaxies would
not have formed. Either way, the observed structure of the universe
seems to depend very sensitively on the precise matching of explosive
vigour to gravitating power.
Just how sensitively is revealed by calculation. At the so-called
Planck time (10 to the power of -43 seconds) (which is the earliest moment at which
the concept of space and time has meaning) the matching was accurate
to a staggering one part in 10 to the power of 60. That is to say, had the explosion
differed in strength at the outset by only one part in 10 to the power of 60,
the universe we now perceive would not exist. To give some meaning to these
numbers, suppose you wanted to fire a bullet at a one-inch target on the
other side of the observable universe, twenty billion light years away
Your aim would have to be accurate to that same part in 10 to the power of 60.
Quite apart from the accuracy of this overall matching, there is the
mystery of why the universe is so extraordinarily uniform, both in the
distribution of matter, and the rate of expansion. Most explosions are
chaotic affairs, and one might expect the big bang to have varied in its
degree of vigour from place to place. This was not so. The expansion
of the universe in our own cosmic neighbourhood is indistinguishable
in rate from that on the far side of the universe.
This coherence of behaviour over the whole cosmos seems all the
more remarkable when account is taken of what are known as light
horizons. When light spreads out across the universe it has to chase the
retreating galaxies which are being swept apart by the expansion. The
rate of recession of a galaxy depends on its distance from the observer.
Distant galaxies recede faster. Imagine a flash of light emitted from a
particular place at the instant of the creation.. The light will have
travelled about twenty billion light years across space by now.
Regions of the universe farther away than this will not yet have
received the light. Observers there would not be able to see the light
source. Conversely, observers near the light source would not be able
to see those regions. It follows that no observer in the universe can see
beyond twenty billion light years at this time. There is a sort of horizon
in space, which conceals everything that lies beyond. And because no
signal or influence can travel faster than light, it follows that no
physical connection at all can exist between regions of the universe
that lie beyond each other's horizon.
When telescopes are turned on the outer limits of the observable
universe, they probe regions that have apparently never been in causal
contact with each other. The reason is that distant regions which lie on
opposite sides of the sky as viewed from Earth are so far apart from
each other that they are beyond each other's horizon. The situation is
closely analogous to ordinary horizons. A lookout on a ship at sea may
just be able to discern two other ships - one ahead, one astern - near
his horizon, but these other ships will be invisible from each other
because of their greater separation. Similarly, the remote galaxies
which lie on opposite sides of the sky are located beyond each other's
light horizon. Because all physical influences or communications are
limited by the speed of light, it is not possible that these galaxies can
have coordinated their behaviour.
The mystery is, why are those regions of the universe that are
causally disconnected so similar in structure and behaviour? Why do
they contain galaxies of the same average size and form, retreating
from each other at the same rate? The mystery becomes all the more
profound when we realize that this behaviour is a remnant of long ago
when the galaxies first formed. But in the past light had travelled less
far since the creation, so the horizons were closer. At one million years
they were a million light years across, at one hundred years a hundred
light years, and so on. If we go back to the Planck time again, the
horizons were a mere 10 to the power of -33 cm in size. Even allowing
for the expansion of the universe, regions as small as this would not, according
to the standard theory, have swelled to a visible size by now. It seems that the
entire observable universe was, at that time, separated into at least 10 to the
power of 80 causally disconnected regions. How is it possible to explain this
cooperation without communication?
A related problem is the extreme degree of cosmic isotropy: uniformity
with orientation. Looking outwards from Earth, the universe presents the
same aspect on the large scale in whichever direction we choose to look.
Careful measurements of the relic cosmic background heat radiation show
that the incoming flux is accurately matched from all sides to better than one
part in a thousand . Had the big bang been a random event, such exceptional
uniformity would be almost impossibly unlikely.
