Advantage Inhomogeneity

http://arxiv.org/abs/1505.07800

also

https://telescoper.wordpress.com/201...oes-it-matter/

RLO
http://www3.amherst.edu/~rloldershaw
Fractal Cosmology

In article ,
"Robert L. Oldershaw" writes:

http://arxiv.org/abs/1505.07800

also

https://telescoper.wordpress.com/201...oes-it-matter/

And?

The blog post essentially points to the same paper, which is ONE SIDE in
a current debate about whether backreaction can be neglected or not
(i.e. how strong the effect is and to what extent, IF ANY, standard
conclusions could be wrong by failing to take it properly into account).
While some well known pundits are authors of the above paper, one of the
main players on the other side is Bob Wald, who is in the same league.

Yes, the universe is not completely homogeneous. This is obvious. The
question is whether it matters.

And, just to be clear, the above paper is not concerned with any sort of
fractal stuff. Yes, fractals are inhomogeneous, but not all
inhomogeneities are fractal.

On Friday, January 22, 2016 at 10:55:32 PM UTC-5, Phillip Helbig (undress to reply) wrote:


Yes, the universe is not completely homogeneous. This is obvious. The
question is whether it matters.

And, just to be clear, the above paper is not concerned with any sort of
fractal stuff. Yes, fractals are inhomogeneous, but not all
inhomogeneities are fractal.

---------------------------------------

Wow! Could we be in agreement?

Almost. To me everything matters.

On 1/22/2016 9:55 PM, Phillip Helbig (undress to reply) wrote:
Yes, the universe is not completely homogeneous. This is obvious. The
question is whether it matters.

Wouldn't a better question be: why is the universe not completely
homogeneous?

[[Mod. note -- A region of the universe which is slightly denser than
the average density tends to contract due to its self-gravitation,
amplifying the inhomogeneity. (This is basically just the Jeans
instability.) Numerical simulations of this process produce
inhomogeneities which are pretty similar to those we see in the
universe today.

Of course, we still have to figure out where the initial (small)
inhomogeneities came from. Here we start getting into the realm
of inflation, quantum fluctuations in the big bang, etc.
-- jt]]

In article , David Staup
writes:

Wouldn't a better question be: why is the universe not completely
homogeneous?

I don't think so. First, if it were completely homogeneous, there could
be no-one around to note that fact. Second, complete or even near
homogeneity implies special initial conditions or some sort of
interaction to smooth it out. In other words, the default expectation
is that it is inhomogeneous. The problem is to try to explain the
observed near homogeneity, in other words why it is not completely
inhomogeneous.

If you imagine the expansion of the universe running in reverse, you
will find that parts of the universe which now are widely separated were
not in causal contact in the early universe, so the question arises why
they are so similar. This is the isotropy problem (sometimes called the
horizon problem). Inflation provides an answer: the very early universe
is not just a backwards extrapolation of the current expansion, but
there was a short phase of exponential inflation, which allows the
entire observable universe to have expanded from a patch so small that
it was in equilibrium. This also provides an explanation for the origin
of the fluctuations which we do see, namely quantum fluctuations.

This sounds like a just-so story, but, even though there are many
varieties of inflation, a fairly generic prediction was made regarding
the perturbation spectrum, i.e. the relative strength of perturbations
on different scales. This was made long before there was any
observational evidence one way or the other, and the prediction has been
confirmed. So, this lends some credence to the inflationary idea.

On Wednesday, February 3, 2016 at 11:25:09 PM UTC-5, Phillip Helbig (undres=
s to reply) wrote:

is that it is inhomogeneous. The problem is to try to explain the=20
observed near homogeneity, in other words why it is not completely=20
inhomogeneous.
=20
-------------------------------

At the risk of repeating myself yet again on a message that never
seems to register in certain places, consider this simple historical
fact.

In 1920 astronomers considered the most likely distribution of
matter in the observable universe, and beyond, to be a statistically
homogeneous distribution stars. That is the way the cosmos looked
to most physicists and astronomers at the time.

Then came the the discovery of galaxies at far greater distances
than had previously been considered to be observable. We could be
in a roughly analogous situation right now today. We look out and
say, "Hmmm, it looks very homogeneous to us", but when our observational
capacities improve enough to increase the observable part of the
local universe, the evidence for strong inhomogeneity on larger
scales may begin to accrue once again.

This has happened in the past several times where one started with
an assumption of statistical homogeneity, only to later learn that
this assumption was quite incorrect and had to be rejected for a
more natural inhomogeneous model.

Sigh. Or should I say - Duh!

RLO
http://www3.amherst.edu/~rloldershaw

[[Mod. note -- I don't think the statement "In 1920 astronomers considered
the most likely distribution of matter in the observable universe, and
beyond, to be a statistically homogeneous distribution stars." is true:
Galileo's observations of the Milky Way resolved it into individual stars,
which were manifestly distributed in a highly inhomogeneous manner (i.e.,
the brightness and number-of-stars-per-square-degree of the Milky Way vary
a lot from one part of the sky to another).
-- jt]]

On Thursday, February 4, 2016 at 11:27:53 AM UTC-5, Robert L. Oldershaw wro=
te:

=20
In 1920 astronomers considered the most likely distribution of
matter in the observable universe, and beyond, to be a statistically
homogeneous distribution stars. That is the way the cosmos looked
to most physicists and astronomers at the time.
=20
=20
RLO
http://www3.amherst.edu/~rloldershaw
=20
[[Mod. note -- I don't think the statement "In 1920 astronomers considere=
d
the most likely distribution of matter in the observable universe, and
beyond, to be a statistically homogeneous distribution stars." is true:
Galileo's observations of the Milky Way resolved it into individual stars=
,
which were manifestly distributed in a highly inhomogeneous manner (i.e.,
the brightness and number-of-stars-per-square-degree of the Milky Way var=
y
a lot from one part of the sky to another).
-- jt]]
--------------------------------------------------

Well, that [a statistically homogeneous distribution of stars]
explicitly was Einstein's opinion in 1920 and early evidence for
galactic scale structure was vigorously resisted by the conservative
wing of the physics/astronomy community of the time - not the least
bit surprisingly.

This is a matter of record.


[[Mod. note -- It's important not to confuse the hypotheses
(1) Stars are uniformly distributed throughout the universe at any
(or some particular) time.
(2) Galaxies are uniformly distributed throughout the universe at any
(or some particular) time.
(3) Averaged over a sufficiently large volume, galaxies are uniformly
distributed throughout the universe at any (or some particular) time.
(4) Averaged over a sufficiently large volume, mass(-energy) is uniformly
distributed throughout the universe at any (or some particular) time.

As I noted above, (1) is manifestly false, and its falsity has been
known for a long time.

(2) is also false, though this fact wasn't well-known until the advent
of large-scale galaxy redshift surveys and the discovery of the
"great wall" in the early 1980s.

(3) and (4) are assumed to be true by almost all cosmological models
(including Einstein's). A fractal cosmology necessarily implies that
(3) and (4) are false. Large-scale galaxy redshift surveys argue that
(3) is true, and observations of the cosmic microwave background argue
that (4) is true.
-- jt]]

In article ,
"Robert L. Oldershaw" writes:

On Wednesday, February 3, 2016 at 11:25:09 PM UTC-5, Phillip Helbig (undress to reply) wrote:

is that it is inhomogeneous. The problem is to try to explain the
observed near homogeneity, in other words why it is not completely
inhomogeneous.

-------------------------------

At the risk of repeating myself yet again on a message that never
seems to register in certain places, consider this simple historical
fact.

In 1920 astronomers considered the most likely distribution of
matter in the observable universe, and beyond, to be a statistically
homogeneous distribution stars. That is the way the cosmos looked
to most physicists and astronomers at the time.

This was never a viable model, and never was a model at all, at least in
the last few hundred years in Europe. In the 1920s, it was determined
that many nebulae are extragalactic systems, i.e. other galaxies. Up
until then, the "island universe" model was common, i.e. a finite
assembly of stars, surrounded by the void.

Then came the the discovery of galaxies at far greater distances
than had previously been considered to be observable. We could be
in a roughly analogous situation right now today.

We could, but we are not.

We look out and
say, "Hmmm, it looks very homogeneous to us", but when our observational
capacities improve enough to increase the observable part of the
local universe, the evidence for strong inhomogeneity on larger
scales may begin to accrue once again.

You are speculating about what happens beyond the horizon. Observations
show that the universe is homogeneous on scales larger than, at most, a
few hundred Mpc, much smaller than the size of the observable universe
(and very much smaller than the observable universe). So, your claim
boils down to the universe becoming inhomogeneous beyond the horizon.
Possible, but there is no evidence for it.

This has happened in the past several times where one started with
an assumption of statistical homogeneity, only to later learn that
this assumption was quite incorrect and had to be rejected for a
more natural inhomogeneous model.

Yes, but that doesn't mean it will always happen. A few hundred years
ago, many new islands and even continents were discovered, but that
process stopped. One cannot just extrapolate forever.

In article ,
"Robert L. Oldershaw" writes:

In 1920 astronomers considered the most likely distribution of
matter in the observable universe, and beyond, to be a statistically
homogeneous distribution stars. That is the way the cosmos looked
to most physicists and astronomers at the time.

Well, that [a statistically homogeneous distribution of stars]
explicitly was Einstein's opinion in 1920

Yes, but Einstein was not an astronomer. Just yesterday, I read
"astronomers, as opposed to cosmologists". Yes, there is some overlap,
but not always.

The early days of cosmology occurred at the same time as, but largely in
isolation from, the debate (including the Curtis-Shapley "Great Debate")
about the size of the Galaxy, whether the nebulae were extragalactic,
etc.

Einstein's main initial assumption was a STATIC universe. As for
homogeneity, this was more an assumption. Obviously stars indicate a
very inhomogeneous distribution of mass (no dark matter back then); the
idea (then an assumption, now something for which there is much
observational evidence) is that homogeneity nevertheless exists on large
enough scales. Einstein's main mistake was assuming that the universe
is static.

In article ,
Phillip Helbig (undress to reply) wrote:
This was never a viable model, and never was a model at all, at least in
the last few hundred years in Europe.

(For those who don't know why, look up 'Olbers' paradox'.)

Martin
--
Martin Hardcastle
School of Physics, Astronomy and Mathematics, University of Hertfordshire, UK
Please replace the xxx.xxx.xxx in the header with herts.ac.uk to mail me