[[Mod. note -- I'm sorry for the delay in processing this article,
which arrived in the s.a.r moderation system on 2023-02-06, but was
mis-classified as spam by an over-eager spam filter.
-- jt]]

On Sunday, December 25, 2022 at 1:55:39 AM UTC-7, Phillip Helbig (undress t=
o reply) wrote:
>
> In article <Y6fv4VgT...@gold.bkis-orchard.net>, Gary Harnagel=20
> <hit...@yahoo.com> writes
> >
> > > [[Mod. note --=20
> > > It is very likely that 50 years from now our understanding of cosmolo=
gy=20
> > > will be different than it is today. It is also very likely that 50 ye=
ars=20
> > > from now our understanding of cosmology will in the main *refine* (as=
=20
> > > opposed to overthrow) our understanding today.=20
> >
> > Although "refinement" has been THE process for the past half-century,=
=20
> > "overthrow happened in 1905-1916 with the advent of relativity. It=20
> > happened again with quantum physics in the 1930's and the 1960's with=
=20
> > QFT. The discovery of "island universes" seems to have been more than a=
=20
> > "refinement" and the application of GR to cosmology was, too.
>
> While I agree on the quantum stuff, I'm not so sure about the others.=20
> There had been a debate on whether nebulae, as they were then known,=20
> were within the Milky Way or extragalactic systems at least since the=20
> time of William Herschel. The question was decided about a hundred=20
> years ago, but since it was a decision between two alternatives which=20
> had both been around for a while, I don't see it as a revolution in the=
=20
> same sense as the quantum developments you mention.

It seems to have made a big impact on our psyche. Evem those who believed
the "nebulae" were outside the galaxy, no one considered just how BIG the
universe is.

> As to relativity,=20
> with regard to special relativity I think that Rovelli makes a=20
> convincing case that Einstein's formulation of it grew out of his=20
> conservative attitude to physics (which, famously, led him to part=20
> company with the quantum theory he helped to create). One could argue=20
> that the results of SR were revolutionary, but the theory itself, so=20
> Rovelli, really wasn't. He discusses this in his book about Anaximander=
=20
> which, partly because we don't know much about Anaximander, is also=20
> about many other things. With GR, I think that all agree that it was=20
> revolutionary.=20
>=20
> In general, though, the fact that there were some revolutions in the=20
> past doesn't necessarily mean that there will be any in the future.

Perhaps not, but I wouldn't on that :-)

> > The problem with GR applied to cosmology is that theories such as the=
=20
> > FLRW metric aren't the only possibilities.
>
> A Friedmann-Lema=C3=AEtre-Robertson-Walker (FLRW) model is a model a) bas=
ed=20
> on GR and b) homogeneous and isotropic. It can contain an arbitrary=20
> number of components with various equations of state. Historically, and=
=20
> probably physically as well, the most important have been=20
> non-relativistic matter (known as "dust" in cosmology, where a dust=20
> particle is a supercluster of galaxies), radiation, and the cosmological=
=20
> constant. Due to the different equations of state, the respective=20
> energy densities evolve differently with time, hence radiation is most=20
> important at the beginning, then matter, then the cosmological constant.=
=20
> But as you mention there are many other possibilities for cosmological=20
> models based on GR.=20
>=20
> 50--100 years ago, homogeneity and isotropy were essentially simplifying=
=20
> assumptions. Today, they are observational facts. (To be more exact,=20
> we observe a high degree of isotropy which implies homogeneity unless we=
=20
> are in a special location for which there is no evidence.)
>
> > If a "big bang" could happen=20
> > in our brane (taking a concept from M-theory), it may not have been=20
> > unique. Suppose a "big bang" happened before our present one and it's=
=20
> > way, way out there and expanding faster than we are. What would that=20
> > look like? It happened long, long ago so star formation has stopped.=20
> > All it would consist of would be red dwarfs, whose spectra might look=
=20
> > something like the CMBR.
>
> Regardless of the other points, the idea that the CMB is reprocessed=20
> starlight is an old one, going back at least to Fred Hoyle who wanted to=
=20
> have an explanation for the CMB in the steady-state model. The presence=
=20
> of the CMB doesn't directly contradict the steady-state model (though in=
=20
> that model its temperature would be constant while in conventional=20
> cosmology it decreases with time, something which one could at least in=
=20
> principle measure), but it is not something which arises naturally.=20
> However, today so many details about the CMB are known that reprocessed=
=20
> starlight is not a valid explanation. (By coincidence, the energy in=20
> the CMB is about the same as that in starlight.)

I did a rough calculation taking the average spectra and occurrence of OBAF=
GKM.
It approximated a black-body distribution, so I don't see how it can be sai=
d that
it does't match the CMB if finer detail were considered. Do you have a ref=
erence
explaining why it doesn't match?

> > THAT would overthrow the FLRW model since all=20
> > of our big bang is not all that there is. There's other stuff out there=
=20
> > that has a gravitational effect on us, as well as spacetime itself.
>
> Yes and no. Conceptually, yes. However, it is known that an FLRW model=20
> is a good description of our Universe, and that would still be the case=
=20
> even if the big picture were different.

It assumes that there was ONE, and only one, big bang. Consequently, space=
time
has limited extent (e.g., one of many independent expanding bubbles in a su=
per-
structure). If the BB weren't unique to our universe, spacetime would not =
be
limited: we're tramping over "old" spacetime . That would have an impact o=
n the
assumptions one uses to construct a metric.

> > It might also do away with FLRW's need for dark energy. GR predicts=20
> > that, since we are closer to this expanding side of the previous "big=
=20
> > bang," we would be dragged (accelerated) along its line of motion by th=
e=20
> > Lense-Thirring effect, thus explaining dark energy.
>
> Does that explanation work quantitatively?=20

Don't know :-)

> I always wonder why people think that they have to explain dark energy.=
=20
> GR has two physical constants, G and Lambda. Hardly anyone wants to=20
> explain G in the same manner, i.e. explain why it is non-zero, calculate=
=20
> its value from something else, and so on.

It has always seemed ad hoc to me. It represents an unknown factor
that has some physical cause. When you solce a differential equation
and have arbitrary constant(s) of integration, they are determined by the
boundary conditions.

> That does not rule out that=20
> dark energy might be something which behaves like the cosmological=20
> constant (note that there is no evidence that dark energy---a more=20
> general term---is anything more complicated that the cosmological=20
> constant), but in that case one would have to explain why Lambda, the=20
> "bare" cosmological constant, is zero. Usually if Nature has a degree=20
> of freedom it is used and the absence of something like that, i.e. a=20
> parameter being zero, implies a new symmetry, quantum number,=20
> conservation law, etc., and the burden of proof is on those who make=20
> that claim.

Why couldn't it just be superfluous? Or, as you say, it is explained by
whatever dark energy turns out to be?

> > The ekpyrotic theory of Steinhardt, Khoury, Turok and Ovrut, suitably=
=20
> > modified and verified, could do a GREAT deal of damage to our present=
=20
> > cosmological model.
>
> Suffice it to say that it has not (yet?) convinced the community.=20
> Again, a modified version would have to make concrete, testable=20
> predictions for it to be verified (in the sense that none of those=20
> predictions rule it out).
>
> > Whether this flight of fancy has any semblance of=20
> > reality, I think FLRW is in trouble.
>
> Why do you think that FLRW is in trouble?

The JWST seems to be finding galaxies earlier than when they should
have formed, for one thing. Of course, "super-stars" are being hypothesize=
d which evolve at extreme rates, so maybe things happened a lot faster then.
We'll see ....