Thinking About Large-Scale Structure

On Sunday, March 13, 2016 at 8:34:32 AM UTC-4, Jos Bergervoet wrote:

If this Atlas of The Universe is accurate:
http://www.atlasoftheuniverse.com/universe.html

then there's a nice resemblance with 3D graphene foam:
http://acsmaterial.com/product.asp?cid=99&id=126

And on the "Atlas" you can nicely zoom in, After two
zoom levels you just see the Virgo supercluster and the
homogeneity is clearly gone.


That's an *extremely* BIG IF, my friend.

The "turnover to homogeneity" scale has jumped from 30 Mpc, to 60 Mpc,
to 80 Mpc, to 120 Mpc, etc., and now it stands at about 260 Mpc.

Is 260 Mpc the final guess? I certainly do not think so, and if
history is any predictor of the future, we know what to expect.

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

On Monday, March 14, 2016 at 2:29:53 AM UTC-6, Robert L. Oldershaw wrote:

On Sunday, March 13, 2016 at 5:16:44 AM UTC-4, Gary Harnagel wrote:

For one thing, the He model really isn't homogeneous if you look at a
small enough scale. There are individual knots of matter called atoms
and then there are clouds of electrons and then there are really, really
tiny knots of matter called nuclei and then there are protons and
neutrons and then there are quarks and gluons. So one might ask the same
question of a sphere with helium inside, yes?

Thanks, Gary, but in the intended analogy the He atoms are analogous
to the galaxies, and galaxies have nuclei and central supermassive
black holes. Also galaxies are surrounded by large halos which we
could say are analogous to electron "clouds.

Point taken. So the analogy fails because gravity is inconsequential for
He atoms but is not for galaxies. Furthermore, some "force" (or phenomenon,
or whatever) must be responsible for the soap-bubble-like structure, or
Craig's metal foam model.

Since we don't understand that "force" I don't see how we can justify
any speculation beyond it. There may be some other "force" beyond the
"foam" level that causes further agglomeration, or maybe there isn't.
The speculated "superfoam" may be fractal or it may not. If this genesis
of the universe truly is 14 billion years old, one can't go much farther
than the "ordinary foam."

Gary

In article , "Robert L.
Oldershaw" writes:

I would direct you to the newest paper,
http://arxiv.org/abs/1603.03260 , on this issue that I have cited in
my 3/13/16 post to the thread entitled something like "Largest
Structure...".

Are we really confident that we can claim that "there is very little
detectable clumpiness"..."beyond a few hundred Mpc"? Is this due to
the fact that the lumpiness is not there, or is it due to the fact
that we have trouble seeing it? Can we *confidently* decide which is
the case with existing data, or are we engaging in wishful thinking?

From time to time people claim such very large-scale structures, but the
stastical significance is usually quite low. Compare this with the
paper I mentioned recently which looked at the distribution of tens of
thousands of quasars.

On 3/14/2016 9:40 AM, Robert L. Oldershaw wrote:
On Sunday, March 13, 2016 at 8:34:32 AM UTC-4, Jos Bergervoet wrote:

If this Atlas of The Universe is accurate:
http://www.atlasoftheuniverse.com/universe.html

then there's a nice resemblance with 3D graphene foam:
http://acsmaterial.com/product.asp?cid=99&id=126

And on the "Atlas" you can nicely zoom in, After two
zoom levels you just see the Virgo supercluster and the
homogeneity is clearly gone.

That's an *extremely* BIG IF, my friend.

Actually it is rather small. There may be some further
double-checking to be done before the map of the visible
universe is known beyond any doubt, but that is basically
Google-maps work. The truly Big IF is what lies at the next
higher zoom level, say a factor of 20 beyond the visible
universe (or further, of course).

The "turnover to homogeneity" scale has jumped from 30 Mpc, to 60 Mpc,
to 80 Mpc, to 120 Mpc, etc., and now it stands at about 260 Mpc.

Is 260 Mpc the final guess? I certainly do not think so, and if
history is any predictor of the future, we know what to expect.

That is another if, again! But indeed, with history as our
guide we could perhaps find a way to "observe" the universe
beyond the horizon, and have the means to find out whether
there are new inhomogeneities at larger scales.

The "history" involved here would probably have to be some
inflation era in the early universe. As soon as we find out
what field exactly drove the inflation, we might very well
be able to conclude how the fluctuations on every possible
scale were seeded at that time, even for scales beyound or
horizon.

--
Jos

On Monday, March 14, 2016 at 5:53:12 PM UTC-4, Phillip Helbig (undress to reply) wrote:

Are we really confident that we can claim that "there is very little
detectable clumpiness"..."beyond a few hundred Mpc"? Is this due to
the fact that the lumpiness is not there, or is it due to the fact
that we have trouble seeing it? Can we *confidently* decide which is
the case with existing data, or are we engaging in wishful thinking?

From time to time people claim such very large-scale structures, but the
stastical significance is usually quite low. Compare this with the
paper I mentioned recently which looked at the distribution of tens of
thousands of quasars.

But my final question above still stands as a valid unanswered
question that we will need a lot more data to answer scientifically.
Currently we do not, and cannot, know if the putative turnover at 260
Mpc will hold up, or fail as so many previous putative "turnovers"
have.

Also, what is the statistical significance of the bottom line results
of the paper you refer to regarding the "tens of thousands of
quasars"?

On Monday, March 14, 2016 at 4:38:40 AM UTC-4, Robert L. Oldershaw wrote:
On Sunday, March 13, 2016 at 5:26:47 AM UTC-4, Craig Markwardt wrote:
.....
Are we really confident that we can claim that "there is very little
detectable clumpiness"..."beyond a few hundred Mpc"?

Look up any paper on galaxy correlations, which is a statistical
measure of clumpiness. There are so many researchers that have worked
on this, and none have found detectable clumping beyond 100-200 Mpc.
The result is (small) upper limits on large spatial scales. That's not
to say that there are rare extreme cases. Homogeneity is a statistical
statement, which implies there will be extremes in the tail of the
statistical distribution.

The item you pointed to, is "significant" at the 20% level. Which
means that if you repeat the experiment 5 times you are likely to
detect one such occurrence. This is hardly significant at the level
usually demanded by researchers I am acquainted with.

CM

[Mod. note: reformatted -- mjh]

In article , "Robert L.
Oldershaw" writes:

Also, what is the statistical significance of the bottom line results
of the paper you refer to regarding the "tens of thousands of
quasars"?

There is no null hypothesis here. They did what you suggest: empirical
research. If the null hypothesis is homogeneity, and they found a lack
of it, then one could quote a probability that this is just due to
chance. If you have a null hypothesis involving a lack of homogeneity,
then you could calculate the probability of their result of homogeneity
being due to chance. However, there are an infinite number of such
hypotheses. This is related to the fact that one can never rule
something IN, only rule something OUT (with some likelihood).

On Thursday, March 17, 2016 at 12:30:13 PM UTC-4, Phillip Helbig (undress to reply) wrote:


There is no null hypothesis here. They did what you suggest: empirical
research. If the null hypothesis is homogeneity, and they found a lack
of it, then one could quote a probability that this is just due to
chance. If you have a null hypothesis involving a lack of homogeneity,
then you could calculate the probability of their result of homogeneity
being due to chance. However, there are an infinite number of such
hypotheses. This is related to the fact that one can never rule
something IN, only rule something OUT (with some likelihood).

So are you saying that in the specific case of cosmological matter distributions we are dealing with open questions and nothing reasonable is ruled out?

On Thursday, March 17, 2016 at 12:23:52 PM UTC-4, Craig Markwardt wrote:

on this, and none have found detectable clumping beyond 100-200 Mpc.
The result is (small) upper limits on large spatial scales. That's not
to say that there are rare extreme cases. Homogeneity is a statistical
statement, which implies there will be extremes in the tail of the
statistical distribution.

Well Craig, the putative "turnover" has already been shifted outside
your 100-200 Mpc limit you cited above.

How about the entire Cosmic Web? Is this not clumping on a VERY large
scale?


The item you pointed to, is "significant" at the 20% level. Which
means that if you repeat the experiment 5 times you are likely to
detect one such occurrence. This is hardly significant at the level
usually demanded by researchers I am acquainted with.


If structures with sizes in the 500-1000 Mpc range were found to be
common, would you begin to doubt the whole concept of cosmological
"homogeneity", or would you recommend moving the goalposts again?


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

In article , "Robert L.
Oldershaw" writes:

There is no null hypothesis here. They did what you suggest: empirical
research. If the null hypothesis is homogeneity, and they found a lack
of it, then one could quote a probability that this is just due to
chance. If you have a null hypothesis involving a lack of homogeneity,
then you could calculate the probability of their result of homogeneity
being due to chance. However, there are an infinite number of such
hypotheses. This is related to the fact that one can never rule
something IN, only rule something OUT (with some likelihood).

So are you saying that in the specific case of cosmological matter
distributions we are dealing with open questions and nothing reasonable
is ruled out?

No. I'm saying that this paper did not rule out specific alternative
scenarios, not because it can't, but because that wasn't the goal.