Back to the drawing board

Phillip Helbig (undress to reply)

2022-12-25 08:55:38 UTC

Post by Gary Harnagel
[[Mod. note --
It is very likely that 50 years from now our understanding of cosmology
will be different than it is today. It is also very likely that 50 years
from now our understanding of cosmology will in the main *refine* (as
opposed to overthrow) our understanding today.

Although "refinement" has been THE process for the past half-century,
"overthrow happened in 1905-1916 with the advent of relativity. It
happened again with quantum physics in the 1930's and the 1960's with
QFT. The discovery of "island universes" seems to have been more than a
"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.
There had been a debate on whether nebulae, as they were then known,
were within the Milky Way or extragalactic systems at least since the
time of William Herschel. The question was decided about a hundred
years ago, but since it was a decision between two alternatives which
had both been around for a while, I don't see it as a revolution in the
same sense as the quantum developments you mention. As to relativity,
with regard to special relativity I think that Rovelli makes a
convincing case that Einstein's formulation of it grew out of his
conservative attitude to physics (which, famously, led him to part
company with the quantum theory he helped to create). One could argue
that the results of SR were revolutionary, but the theory itself, so
Rovelli, really wasn't. He discusses this in his book about Anaximander
which, partly because we don't know much about Anaximander, is also
about many other things. With GR, I think that all agree that it was
revolutionary.

In general, though, the fact that there were some revolutions in the
past doesn't necessarily mean that there will be any in the future.

Post by Gary Harnagel
The problem with GR applied to cosmology is that theories such as the
FLRW metric aren't the only possibilities.

A Friedmann-Lemaître-Robertson-Walker (FLRW) model is a model a) based
on GR and b) homogeneous and isotropic. It can contain an arbitrary
number of components with various equations of state. Historically, and
probably physically as well, the most important have been
non-relativistic matter (known as "dust" in cosmology, where a dust
particle is a supercluster of galaxies), radiation, and the cosmological
constant. Due to the different equations of state, the respective
energy densities evolve differently with time, hence radiation is most
important at the beginning, then matter, then the cosmological constant.
But as you mention there are many other possibilities for cosmological
models based on GR.

50--100 years ago, homogeneity and isotropy were essentially simplifying
assumptions. Today, they are observational facts. (To be more exact,
we observe a high degree of isotropy which implies homogeneity unless we
are in a special location for which there is no evidence.)

Post by Gary Harnagel
If a "big bang" could happen
in our brane (taking a concept from M-theory), it may not have been
unique. Suppose a "big bang" happened before our present one and it's
way, way out there and expanding faster than we are. What would that
look like? It happened long, long ago so star formation has stopped.
All it would consist of would be red dwarfs, whose spectra might look
something like the CMBR.

Regardless of the other points, the idea that the CMB is reprocessed
starlight is an old one, going back at least to Fred Hoyle who wanted to
have an explanation for the CMB in the steady-state model. The presence
of the CMB doesn't directly contradict the steady-state model (though in
that model its temperature would be constant while in conventional
cosmology it decreases with time, something which one could at least in
principle measure), but it is not something which arises naturally.
However, today so many details about the CMB are known that reprocessed
starlight is not a valid explanation. (By coincidence, the energy in
the CMB is about the same as that in starlight.)

Post by Gary Harnagel
THAT would overthrow the FLRW model since all
of our big bang is not all that there is. There's other stuff out there
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
is a good description of our Universe, and that would still be the case
even if the big picture were different.

Post by Gary Harnagel
It might also do away with FLRW's need for dark energy. GR predicts
that, since we are closer to this expanding side of the previous "big
bang," we would be dragged (accelerated) along its line of motion by the
Lense-Thirring effect, thus explaining dark energy.

Does that explanation work quantitatively?

I always wonder why people think that they have to explain dark energy.
GR has two physical constants, G and Lambda. Hardly anyone wants to
explain G in the same manner, i.e. explain why it is non-zero, calculate
its value from something else, and so on. That does not rule out that
dark energy might be something which behaves like the cosmological
constant (note that there is no evidence that dark energy---a more
general term---is anything more complicated that the cosmological
constant), but in that case one would have to explain why Lambda, the
"bare" cosmological constant, is zero. Usually if Nature has a degree
of freedom it is used and the absence of something like that, i.e. a
parameter being zero, implies a new symmetry, quantum number,
conservation law, etc., and the burden of proof is on those who make
that claim.

Post by Gary Harnagel
The ekpyrotic theory of Steinhardt, Khoury, Turok and Ovrut, suitably
modified and verified, could do a GREAT deal of damage to our present
cosmological model.

Suffice it to say that it has not (yet?) convinced the community.
Again, a modified version would have to make concrete, testable
predictions for it to be verified (in the sense that none of those
predictions rule it out).

Post by Gary Harnagel
Whether this flight of fancy has any semblance of
reality, I think FLRW is in trouble.

Why do you think that FLRW is in trouble?