Jesse Mazer wrote:

<snip>

In theory, one can imagine a universe where all the matter is

collected

in a sphere of finite radius, but space goes on forever beyond the

edge

of the sphere. So while the distribution of matter would have an

edge,

the laws of physics would continue to work the same way beyond this

edge.

You could never know.

In a universe in which the space available to matter has an 'edge,' one
cannot "do physics" (observe the interactions of matter and energy)
beyond the edge to determine empirically whether or not the laws of
physics would continue to work the same way 'out there.'

On Wed, 12 Jan 2005 14:35:41 GMT, Jesse Mazer <vze2ztqw@mail.verizon.net> wrote:

Rob Dekker wrote:

<ilya_shambat2004@yahoo.com> wrote in message news:1105115949.854438.306130@z14g2000cwz.googlegroups.com...

I've been reading Richard Feynman's Six Not-So-Easy Pieces.

In his first chapter he makes the point that, because the laws of the
universe will be perceived the same from wherever the person observes
it, there is no way to find the center of the universe.

That is a fallacy.

In theory, one can imagine a universe where all the matter is collected
in a sphere of finite radius, but space goes on forever beyond the edge
of the sphere. So while the distribution of matter would have an edge,
the laws of physics would continue to work the same way beyond this edge.

Jesse

It appears to me that you both are trifling with a basic premise of
Einstein and relativity.

Could the big bang be a black or white hole all the same?

In the previous answer I was careful to only argue that the standard
FRW big bang model is distinct from a black or white hole. The real
universe may be different from the FRW universe so can we rule out the
possibility that it is a black or white hole? I am not going to enter
into such issues as to whether there was actually a singularity and I
will assume that general relativity is effectively correct as for as
we are concerned here.

The previous argument against the big bang being a black hole still
applies. The black hole singularity always lies in the future light
cone whereas astronomical observation clearly indicate a hot big bang
in the past. The possibility that the big bang is actually a white
hole remains.

The major assumption of the FRW cosmologies is that the universe is
homogeneous and isotropic on large scales. That is to say that it
looks the same everywhere and in every direction at any given
cosmological time. There is good astronomical evidence that the
distribution of galaxies is fairly homogeneous and isotropic on scales
larger than a few hundred million light years. The high level of
isotropy of the cosmic background radiation (CBR) is strong supporting
evidence for homogeneity. However, the size of the observable
universe is limited by the speed of light, and the age of the
universe. We see only as far as about ten to twenty billion light
years which is about 100 times larger than the scales on which
structure is seen in galaxy distributions.

Homogeniety has always been a debated topic. The universe itself may
well be many orders of magnitude larger than what we can observe, or
it may be infinite. Astronomer Martin Rees compares our view like
looking out to sea from a ship in the middle of the ocean. As we look
out beyond the local disturbances of the waves we see an apparently
endless and featureless seascape. From a ship the horizon will be only
a few miles away and the ocean may stretch for hundreds of miles
before there is land. When we look out into space with our largest
telescopes our view is also limited to a finite distance. No matter
how smooth it seems, we cannot assume that it continues like that
beyond where we can see. So homogeneity is not certain on scales much
larger than the observable universe. We might argue in favour of it
on philosophical grounds but we cannot prove it.

In that case we must ask if there is a white hole model for the
universe which would be as consistent with observations as the FRW
models. Some people initially think that the answer must be no
because white holes (like black holes) have tidal forces which stretch
and compress in different directions. Hence they are quite different
from what we observe. This is not conclusive because it applies only
to the space-time of a black hole in the absence of matter. Inside a
star the tidal forces can be absent.

A white hole model which fitted cosmological observation would have to
be the time reversal of a star collapsing to form a black hole. To a
good approximation we could ignore pressure and treat it like a
spherical cloud of dust with no internal forces other than gravity.
Stellar collapse has been intensively studied since the seminal work
of Snyder and Oppenheimer in 1939 and this simple case is well
understood. It is possible to construct an exact model of stellar
collapse in the absence of pressure by gluing together any FRW
solution inside the spherical star and a Schwarzschild solution
outside. Space-time within the star remains homogeneous and isotropic
during the collapse.

It follows that the time reversal of this model for a collapsing
sphere of dust is indistinguishable from the FRW models if the dust
sphere is larger than the observable universe. In other words, we
cannot rule out the possibility that the universe is a very large
white hole. Only by waiting many billions of years until the edge of
the sphere comes into view could we know.

It has to be admitted that if we drop the assumptions of homogeneity
and isotropy then there are many other possible cosmological models
including many with non-trivial topologies. This makes it difficult
to derive anything concrete from such theories. But this has not
stopped some brave and imaginative cosmologists thinking about them.
One of the most exciting possibilities was considered by C. Hellaby,
in 1987 who envisaged the Universe being created as a string of beads
of isolated while holes that explode independently and coalesce into
one Universe at a certain moment. This is all described by a single
exact solution of general relativity.