Re: Cosmology and Relativistic vector addition/subtraction of velocities

In article <1174634323.484208.290270@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Chalky <chalkyspam@xxxxxxxxxxxxxxxx> writes:

On Mar 6, 1:48 pm, "John (Liberty) Bell"
<john.b...@xxxxxxxxxxxxxxxxxx> wrote:
On Feb 28, 12:54 pm, hel...@xxxxxxxxxxxxxxxxxxxxxxxx (Phillip Helbig---
remove CLOTHES to reply) wrote:

In article <1172559521.623516.214...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"John (Liberty) Bell" <john.b...@xxxxxxxxxxxxxxxxxx> writes:
especially if one is considering the limiting case (Milne) general
relativistic solution, where the cosmological constant is zero, and
the mass is zero. This point is not totally academic as this is the
reference model against which astronomers plot the observable dynamics
of the accelerating expansion of the universe

Some astronomers, not all.

Point accepted

This model has no special physical
significance.

Well, it _should_ have special physical significance. With no mass,
and no Lambda, test points _should_ then obey the simple laws of
inertial physics (and SR). If not, there would seem to be something
seriously wrong with EFE since I understood that GR is supposed to
reduce to SR, in the absence of gravity and acceleration.

Correction: in fact, in order to reproduce inertial SR conditions
within the context of EFE, it looks like you need to set matter
density to zero, but dark energy density to 1. This is because you
need Omega (total) =1 to reproduce flat space which is, of course, a
defining property of SR.

I have not come across this argument before myself, but it would seem
to be a good theoretical reason why a cosmological constant is a
necessary feature of Einstein's GR solution (irrespective of the
supportive astronomical evidence).

Nevertheless, this apparent requirement of 'dark energy' to preserve
inertial conditions in the absence of matter (in the context of GR),
does still strike me as a bit suspect.

Note that Einstein himself preferred a model with Omega > 1 and
lambda = 0. He started out with no lambda and arbitrary Omega, then
introduced lambda to obtain a static universe. It is well known that he
then abandoned lambda when the universe was shown to be non-static. He
proposed the Omega = 1, lambda = 0 model with de Sitter mainly for
practical reasons, i.e. it wasn't then ruled out by observations and
things were simple to calculate in it. His own preference was for
Omega > 0 (and lambda = 0) since this provided a closed model; otherwise
he would have had to worry about boundary conditions at infinity.

.

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