A Space & astronomy forum. SpaceBanter.com

Go Back   Home » SpaceBanter.com forum » Astronomy and Astrophysics » Research
Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

Advantage Inhomogeneity



 
 
Thread Tools Display Modes
  #11  
Old February 6th 16, 04:11 AM posted to sci.astro.research
Steve Willner
external usenet poster
 
Posts: 1,172
Default Advantage Inhomogeneity

In article ,
"Phillip Helbig (undress to reply)" writes:
As for homogeneity, this was more an assumption. Obviously stars
indicate a very inhomogeneous distribution of mass (no dark matter
back then);


If the question is history, inhomogeneity was the dominant assumption
starting with the solar system itself. At the beginning of the
twentieth century, dark patches in the Milky Way were thought to be
regions lacking stars. A good history is at
http://adsabs.harvard.edu/abs/1940PASP...52...80S
(I confess the early date at which dust was recognized surprised me.)

Of course the distribution of the nebulae was obviously
inhomogeneous, even a century ago.

If the question is present understanding:
the idea (then an assumption, now something for which there is much
observational evidence) is that homogeneity nevertheless exists on large
enough scales.


I think that's a little too simplistic. Everyone recognizes
inhomogeneity on small scales, and the microwave background is very
nearly -- but not quite -- homogeneous. Current work is a matter of
_quantifying_ the amount of inhomogeneity on different scales, the
jargon for this being "cosmic variance." Both observations and
simulations are relevant to the problem. There is even a web-based
cosmic variance calculator at
http://casa.colorado.edu/~trenti/CosmicVariance.html
(though its defaults may not be quite up to date, and I wouldn't be
surprised if other such calculators exist).

As I understand it, the definition of a fractal is that the amount of
inhomogeneity is independent of scale. That does not describe the
universe we live in.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #12  
Old February 6th 16, 04:12 AM posted to sci.astro.research
Jos Bergervoet
external usenet poster
 
Posts: 126
Default Advantage Inhomogeneity

On 2/5/2016 6:30 AM, Phillip Helbig (undress to reply) wrote:
In article ,

.. 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,


But don't we have the hierarchical sequence:
galaxies - clusters - superclusters - filaments and voids,
all up to the scale you mention?

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.


So your argument is that above the scale of the filaments,
and up to that of the observed part of the observable universe,
there is a range where no additional structuring has been seen,
so we could have the situation that no new inhomogeneities will
exist at any larger scale. (Also possible, but couldn't there
still be percolating bubbles in some inflation schemes?)

..
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.


So both possibilities exist, but I wonder what is the favored
view by current theories?

A few hundred years
ago, many new islands and even continents were discovered, but that
process stopped. One cannot just extrapolate forever.


I expect that better telescopes will one day discover lots of
new continents! (But we will need quite good telescopes..)

--
Jos
  #13  
Old February 6th 16, 04:13 AM posted to sci.astro.research
Robert L. Oldershaw
external usenet poster
 
Posts: 617
Default Advantage Inhomogeneity

On Friday, February 5, 2016 at 9:59:50 AM UTC-5, Martin Hardcastle wrote:
In article ,


Firstly Helbig cleverly changes my specific year of "1920", into
the phrase "in the 1920s". Those who know their history of the
crucial discovery that nature's hierarchy has an additional galactic
scale level, also know that in 1920 that idea was rigorously doubted
and by 1930 was it accepted by most astronomers. It was most assuredly
NOT accepted in 1920.

Secondly, and quite interestingly, I planned to argue in today's
post that the fact that Obler's Paradox was a dominant enigma in
1920, I repeat 1920, is strong evidence for the case I am making,
because Obler's Paradox was based on the assumption of an infinite
cosmos made up of a statistically homogeneous distribution of stars.

Additionally: JT, New Scientist, and space.com should also acknowledge
that there are many different types of fractal models, and that
ruling out simplistic continuous fractal models does NOT rule out
all fractal models.

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

And FEEL THE BERN!!!!
  #14  
Old February 10th 16, 01:37 AM posted to sci.astro.research
Robert L. Oldershaw
external usenet poster
 
Posts: 617
Default Advantage Inhomogeneity

On Friday, February 5, 2016 at 10:12:17 PM UTC-5, Jos Bergervoet wrote:
On 2/5/2016 6:30 AM, Phillip Helbig (undress to reply) wrote:
In article ,

.. 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,


But don't we have the hierarchical sequence:
galaxies - clusters - superclusters - filaments and voids,
all up to the scale you mention?


Good point. Helbig and those who have a vested interested in
preserving the assumption of "cosmological homogeneity" simply use
coarse-graining to hand-wave away the well-known and well-observed
large-scale structures you specifically mention in your post.

RLO
http://www3.amherst.edu/~rloldershaw
[Question argument from authority and inadequately tested assumptions]
  #15  
Old February 10th 16, 04:16 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
external usenet poster
 
Posts: 273
Default Advantage Inhomogeneity

In article ,
"Robert L. Oldershaw" writes:

.. 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,


But don't we have the hierarchical sequence:
galaxies - clusters - superclusters - filaments and voids,
all up to the scale you mention?


Good point. Helbig and those who have a vested interested in


Why do you think that I have a vested interest?

preserving the assumption


It's not an assumption. It follows from the observed isotropy unless we
are in a special position. In some cases (i.e. inferring the CMB
temperature at high redshift from excitation levels of atoms and so on)
it can even be observed.

of "cosmological homogeneity"


What is the difference with and without the scare quotes?

simply use
coarse-graining


Coarse-graining has a very specific meaning in physics which is
unrelated to what you are discussing here.

to hand-wave


Hand-waving is an argument which has no substance; you merely claim that
other opinions are hand-waving and have not demonstrated it.

away the well-known and well-observed
large-scale structures you specifically mention in your post.


No-one doubts that there are large-scale structures. The point is that
the largest structures are significantly smaller than the size of the
observable universe and that there is no evidence of a fractal
distribution (at least in the way that term is normally understood) even
at smaller scales.
  #16  
Old February 10th 16, 04:17 PM posted to sci.astro.research
Jos Bergervoet
external usenet poster
 
Posts: 126
Default Advantage Inhomogeneity

On 2/10/2016 1:37 AM, Robert L. Oldershaw wrote:
On Friday, February 5, 2016 at 10:12:17 PM UTC-5, Jos Bergervoet wrote:
On 2/5/2016 6:30 AM, Phillip Helbig (undress to reply) wrote:
In article ,

.. 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,


But don't we have the hierarchical sequence:
galaxies - clusters - superclusters - filaments and voids,
all up to the scale you mention?


Good point. Helbig and those who have a vested interested in
preserving the assumption of "cosmological homogeneity" simply use
coarse-graining to hand-wave away the well-known and well-observed
large-scale structures you specifically mention in your post.


That was not my impression. I think we all agree that this
sequence exists, but seems to stop a few hundred Mpc (i.e.
from that point on no larger structures have been seen.)
The only question is whether the sequence will *resume*,
at some scale larger than what we can see today.

My point is that it could go either way. Just like at
the other end of the scale, the elementary particles that
we know *might* be truly elemental, but could also turn
out to be composite. For the particles (or more correctly
for the quantum fields we use to describe them) we have
the fact that the standard model survived now for several
decades without a discovery of further compositeness. But
that is not usually taken as an indication that it doesn't
exist.

--
Jos
  #17  
Old February 10th 16, 04:29 PM posted to sci.astro.research
Nicolaas Vroom
external usenet poster
 
Posts: 216
Default Advantage Inhomogeneity

Op dinsdag 2 februari 2016 16:02:44 UTC+1 schreef David Staup:
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.


IMO it is rather "simple" to simulate local inhomogeneities, but that
does not say "anything" about our entire universe at present.

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]]


IMO a better question is can we humans decide if our universe
at present (now) is either homogeneous or inhomogeneous.
This ofcourse depents about the definition.
I specific make here a difference between our universe versus
the (entire) universe. Our universe was created "after" the Big Bang.
The universe is something larger. If it applies.
What ever the answer does it matter?
It matter in the sense that assuming our universe is inhomogeneous
you can raise the following question: how inhomogeneous is our universe?
A related question is: Was our universe always as inhomogeneous?
When you study the book: "The Big Bang" Joseph Silk at page 72
maskes a distinction between 13 Era.
IMO the most reasonable assumption is that not all these Era or
changes in (average) constitution happened every where throughout
our entire Universe at the same time.
If that assumption is correct than the previous assumption that
our universe is inhomogeneous becomes more reasonable.
IMO the most logical physical situation even is an increase.
To keep our Universe homogeneous (uniform) requires "communication".

What makes for me this whole issue so complicated is, that the accepted
view is that our universe has no center and no rim, while at the
same time the our universe expands (at an accelerated rate?)
and becomes larger and emptier i.e. less dense? This is in conflict
with the idea that our universe resembles the surface of a balloon.
At the same time there exist no observational evidence for this
because all what we can observe of the present is locally.
To get a picture of what we can observe study this:

http://users.telenet.be/nicvroom/friedmann's%20equation.htm

Nicolaas Vroom

[[Mod. note -- A few comments:
1. The phrases "the universe" and "our universe" have fairly
well-established meanings in cosmology
https://en.wikipedia.org/wiki/Universe
which differ somewhat from the meanings you describe.
2. Silk's book "The Big Bang" is a fine book. However, so far as I
can determine from some quick web searching, its newest edition
is the 3rd edition, published in Dec 2000. So... it's 16 years
old, or older still if you're referencing a prior edition.
-- jt]]
  #18  
Old February 10th 16, 09:54 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
external usenet poster
 
Posts: 273
Default Advantage Inhomogeneity

In article , Jos Bergervoet
writes:

.. 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,

But don't we have the hierarchical sequence:
galaxies - clusters - superclusters - filaments and voids,
all up to the scale you mention?


Good point. Helbig and those who have a vested interested in
preserving the assumption of "cosmological homogeneity" simply use
coarse-graining to hand-wave away the well-known and well-observed
large-scale structures you specifically mention in your post.


That was not my impression. I think we all agree that this
sequence exists,


Yes.

but seems to stop a few hundred Mpc (i.e.
from that point on no larger structures have been seen.)


No; I don't think all agree on this. By definition, a fractal
distribution doesn't "stop" at some scale...

The only question is whether the sequence will *resume*,
at some scale larger than what we can see today.


....and then resume again.

We won't ever be able to see farther than we do today.* (Well,
technically, the size of the observable universe is increasing, but
negligibly within our lifetimes.) So, this is speculation about what is
beyond the horizon. That doesn't put it out of the realm of science,
but one would need a good reason to believe that the universe just on
the other side of our horizon is substantially different.

My point is that it could go either way. Just like at
the other end of the scale, the elementary particles that
we know *might* be truly elemental, but could also turn
out to be composite.


There is no evidence for this. But this is really different, since we
could in principle probe smaller scales than we do today. As Feynman
said, there is a lot of room at the bottom.

For the particles (or more correctly
for the quantum fields we use to describe them) we have
the fact that the standard model survived now for several
decades without a discovery of further compositeness. But
that is not usually taken as an indication that it doesn't
exist.


But there are several orders of magnitude here, and further elementary
particles would not necessarily change the observed phenomena. On the
other hand, it would be a coincidence---unless there were an
explanation---why inhomogeneity should pick up again just beyond our
horizon. (It might pick up again on much larger scales, but this would
not be in accord with any fractal model.)

___
*When asked in an interview if he would have made the universe any
differently, Maarten Schmidt, after overcoming his surprise at the
question, said that he would have made it bigger, pointing out that with
the help of pieces of glass which fit in a (big) room, one can see as
far as it is possible to see.
  #19  
Old February 10th 16, 09:54 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
external usenet poster
 
Posts: 273
Default Advantage Inhomogeneity

In article ,
Nicolaas Vroom writes:

IMO the most reasonable assumption is that not all these Era or
changes in (average) constitution happened every where throughout
our entire Universe at the same time.


On the contrary, there is much evidence for this.

If that assumption is correct than the previous assumption that
our universe is inhomogeneous becomes more reasonable.
IMO the most logical physical situation even is an increase.
To keep our Universe homogeneous (uniform) requires "communication".


Yes. This is known as the isotropy (or flatness) problem. Without
inflation, or something similar, there is no solution. This is
independent of the degree of inhomogeneity in large-scale structure and
so on; you still have to explain the isotropy of the CMB.

What makes for me this whole issue so complicated is, that the accepted
view is that our universe has no center and no rim, while at the
same time the our universe expands (at an accelerated rate?)
and becomes larger and emptier i.e. less dense?


Yes.

This is in conflict
with the idea that our universe resembles the surface of a balloon.


Why? The VOLUME of the universe corresponds to the SURFACE of the
balloon.

At the same time there exist no observational evidence for this
because all what we can observe of the present is locally.


No. We can observe what is on our backward light cone (through
astronomical observations) and, much more locally, what is inside it.
We can infer stuff about the region near the light cone from
measurements of the temperature of the CMB at higher redshift and so on.
  #20  
Old February 11th 16, 03:11 AM posted to sci.astro.research
Jos Bergervoet
external usenet poster
 
Posts: 126
Default Advantage Inhomogeneity

On 2/10/2016 9:54 PM, Phillip Helbig (undress to reply) wrote:
In article , Jos Bergervoet

..
.. By definition, a fractal
distribution doesn't "stop" at some scale...

The only question is whether the sequence will *resume*,
at some scale larger than what we can see today.


...and then resume again.


So a fractal distribution is ruled out (but I think
it was ruled out already in the hierarchy of scales
mentioned, unless you are very liberal about the
definition).

But new very large scale structures are not ruled out.

We won't ever be able to see farther than we do today.* (Well,
technically, the size of the observable universe is increasing, but
negligibly within our lifetimes.) So, this is speculation about what is
beyond the horizon. That doesn't put it out of the realm of science,
but one would need a good reason to believe that the universe just on
the other side of our horizon is substantially different.


That would be a coincidence. But instead of looking just
on the other side of the horizon, we could consider the
scales incredibly far beyond the horizon, and then perhaps
the opposite holds. It would be a coincidence to have a
desert of perfect homogeneity all the way upwards.

...
There is no evidence for this.


And I wonder if there ever will be..

.. But this is really different, since we
could in principle probe smaller scales than we do today. As Feynman
said, there is a lot of room at the bottom.


The Planck scale, and *much* further the Landau pole of QED,
those are at least landmarks to look forward to! :-) But
practically speaking they are just as unobservable now as
the Teraparsec scale of the universe..

...
For the particles (or more correctly
for the quantum fields we use to describe them) we have
the fact that the standard model survived now for several
decades without a discovery of further compositeness. But
that is not usually taken as an indication that it doesn't
exist.


But there are several orders of magnitude here, and further elementary
particles would not necessarily change the observed phenomena. On the
other hand, it would be a coincidence---unless there were an
explanation---why inhomogeneity should pick up again just beyond our
horizon.


But *never* to pick up any more is just as coincidental..

(It might pick up again on much larger scales, but this would
not be in accord with any fractal model.)


A fractal model is not needed. It would be a coincidence by
itself! A completely irregular sequence of scales is the
most unprejudiced assumption, I would think.

..
*When asked in an interview if he would have made the universe any
differently, Maarten Schmidt, after overcoming his surprise at the
question, said that he would have made it bigger, pointing out that with
the help of pieces of glass which fit in a (big) room, one can see as
far as it is possible to see.


He doesn't have enough patience.

--
Jos
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Do the Libyan Rebels Have a Big Advantage? Jonathan Policy 5 March 26th 11 02:59 AM
Japs Gain Evolutionary Advantage HVAC[_2_] Misc 1 March 23rd 11 07:09 PM
modest advantage outside glimpse Grover[_2_] Amateur Astronomy 0 August 14th 07 11:36 PM
The Zubrin Advantage Scott Lowther Policy 0 July 5th 04 05:08 AM
SCT Focal length advantage, is there one? Francis Marion Amateur Astronomy 11 May 23rd 04 09:51 PM


All times are GMT +1. The time now is 06:34 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 SpaceBanter.com.
The comments are property of their posters.