Davoud wrote:
I can think of no reason why the Universe can't have a central region
in three-dimensional space if the Big Bang theory is correct. If the
Universe exhibits perfect radial symmetry, then the center is a simple
point in three-dimensional space; if, due to quantum fluctuations, the
Universe has minor bumps at its edge, like the surface of a rocky
planet, then the center in three-dimensional space would need to be
spread out; a central /region/ rather than a point.

I think you have a notion of the universe's expansion which is at
variance with the actual Big Bang theory. In the simplest case, the
universe is a 3-sphere--what you might think of geometrically as the
usual four-dimensional hypersphere. (The '3' refers to the dimension of
the hypersphere's "surface," not its interior.) By referring to the
"center" of the universe, we assume the existence of a "metric"--a
consistent ruler by which distances *not* within the actual universe
may be measured. Even so, that center is not on the hypersurface of the
hypersphere--it is in the interior. Its location cannot therefore be
specified by the three coordinates that span the universe; a fourth is
needed.

There are more complex cases, but they all involve the center not lying
on the universe itself, but somewhere else, and invariably requiring a
fourth coordinate (if in fact it exists and it is possible to specify
its location in such a way).

The problem is the edge. I can at least grasp the /concept/ of a
center. In my version of the balloon analogy the balloon is perfectly
spherical (or very nearly so, considering the above-mentioned quantum
fluctuations it doesn't need a stem because it is self-inflating.
Every point is moving away from a common center, which is where those
points started out (wrapped in a single dimension?) when the balloon
was infinitely small. But the /edge/ of the Universe? From inside the
balloon I can move to the edge and encounter a material substance that
I cannot penetrate. My balloon is transparent, and I can see what is
outside that object. But I can't get my mind around an immaterial edge
beyond which is nothing whatsoever, not even empty space. One
cosmologist, half joking, said "It could be a brick wall, for all we
know."

In the case of the balloon, the edge *is* the two-dimensional universe.
Beings in that universe cannot see "through" their universe the way you
or I can; that is a benefit conferred by our three-dimensional nature.
They are confined to looking at objects on the surface.

In the same way, we don't see a boundary to our universe, because the
things we can see are confined to the three-dimensional surface of the
four-dimensional hypersphere (in the simplest case). Our eyes don't
work perpendicular to all three familiar spatial directions, so we can't
see through our universe the way that four-dimensional beings could.

Physicists used to say that, ultimately, the Universe could and would
be explained by a few simple laws that everyone could understand at
least in a rudimentary way. I think that all hope for such an
explanation has been abandoned.

I don't think any *physicist* has seriously believed this in the last
century or so. Quantum theory pretty much defies intuitive
understanding; the best that most can hope for is to be able to perform
the mathematical manipulations well enough to get the right answer at
the end. So I suspect physicists abandoned this idea a long time ago;
probably by the time of Poincare.

String theory, Inflation, the notion
that gravity is so weak because it is Not of This World, but is just
leaking in from another dimension; the Universe may be a hologram; the
Universe may not exist at all except in our minds; the physicists
themselves don't agree on or understand this stuff.

They don't agree on the speculative stuff. The basic tenets of the
Big Bang are agreed upon by nearly all practicing cosmologists. There
is rather strong consensus on it. There are interesting phenomena that
people are trying to explain in a number of different ways, but this
divergence shouldn't be taken for fundamental disagreement on the basic
physics underlying the evolution of the universe.

Here is my one prediction, however: There will not be a Theory of
Everything that unifies gravity with the other forces. Gravity, it will
be decided, is distinct, and must be understood on its own terms.

What is the physical basis for your prediction? The reason I ask is
that it is not as though there is *no* unification between gravity and
the other three forces. There are incomplete unifications that make
some successful non-trivial predictions about high-energy particle
behavior. (These theories can be found through an assiduous Web
search.) What is lacking is a *comprehensive* unification. But the
fact that incomplete theories exist is encouraging, in my opinion.