On 1/17/2020 3:07 PM, John Clark wrote:
On Fri, Jan 17, 2020 at 5:03 PM Alan Grayson <agrayson2...@gmail.com
<mailto:agrayson2...@gmail.com>> wrote:
>>Yes, you can use that to represent a curved path in 4D (one of
time 3 of space) Minkowski Space where Special Relativity
lives, but as you say that doesn't really get to the
fundamental issue because Minkowski Space is flat and Special
Relativity says nothing about gravity, for that you need
General Relativity and GR doesn't live in Minkowski Space.
In General Relativity curved Spacetime is what gravity is, and
in GR if there is any curvature in the Spacetime of the
universe, and we know there is because we know that gravity
exists, then, unless vacuum energy also exists and is fine
tuned to one very precise value, the universe can not be
stable, it must be either expanding or contracting. There are
thermodynamic reasons to think it can't be contracting so it
must be expanding.
And that is why no physicist would say that Carroll's
statement "/the manifestation of spacetime curvature is
simply the fact that space is expanding/" was controversial .
> /The question is, what does he mean? Is space expanding BECAUSE
of curvature? If so it's expanding because of gravity, since you
wrote that gravity and curvature are equivalent. But since gravity
is attractive (as far as we know), how could it be responsible for
expansion (as distinguished from contraction)? AG /
If the universe consisted of a cloud of particles that were not moving
with respect to each other the gravitational attraction between the
particles would indeed cause the universe to contract, but the
particles ARE moving with respect to each other, so what will happen?
It depends on how they are moving, but General Relativity can tell you
one thing, unless you invoke a very fine tuned vacuum energy (aka the
Cosmological Constant) that cloud of particles will NOT remain the
same size, it will either expand or contract.
And even if you fine tuned to be exactly in equilibrium, it would be an
unstable equilibrium and quickly shift into collapse or expansion...as
was pointed out to Einstein when he used the CC to model a static universe.
Brent
We learn from observation that it's expanding which is consistent with
thermodynamic reasoning.
John K Clark
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