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Fractal Hubble Bubbles?

Posted April 12, 2011 4:00 AM by Jorrie
Pathfinder Tags: cosmology Hubble-bubble

In the standard ΛCDM model of the cosmos, the broad structures we see today (galaxies clusters) are thought to have formed from primordial quantum fluctuations in the early (observable) universe. These fluctuations were random variations in density so that over-dense locations formed galaxies and under-dense areas formed what is called 'voids'. The over-dense regions became increasingly over-dense and voids became increasingly under-dense as the cosmos expanded. On top of the normal cosmic expansion, matter inside a void would tend to move outward due to the gravitational pull of the surrounding denser regions.

Even if the cosmos at large was not expanding, an observer near the center of a void would still have noticed something like Hubble's law, i.e. more distant galaxies receding faster than nearer ones. Hence, this concept is called the "Hubble Bubble".1 It has been shown that under certain conditions (e.g. our region with all its superclusters, is near the center of a 'super-sized void'), it could even give an apparent accelerated expansion in the larger universe - hence mimicking dark energy.

The main problems with this idea are: (i) it seemingly requires our galaxy to be near the center of a void, which is apparently not the case; (ii) the rate at which galaxies would recede from us would be too the low for what we observe today, unless the void was unrealistically large (some billion years across) (iii) We are much more likely to live inside a "void wall", the dense structures that surround voids. Surprisingly, this location would give the same type of apparently accelerated expansion.

For the relatively nearby "Hubble galaxies",2 it would give redshifts according to Hubble's law, but when we look at the range of the supernovae where the accelerated expansion was observed, we observe on the far side of a void. Due to the void characteristics, those galaxies would be farther away than they should be for a matter-only cosmos - hence apparent acceleration of expansion. There were still quite a few problems with this idea, amongst others, it would still require some dark energy to fit observations.

Since late 2007, a New Zealand cosmologist, David Wiltshire, has been advocating an improvement to this scheme, apparently solving most (plus a few extra) of the Hubble Bubble problems. It has not been convincingly refuted up to now. He calls it the 'Fractal Bubble' (FB) 3.1 or 'Timescape'3.2 cosmic model, which includes a few very interesting, but entirely reasonable ideas. The main three are: (i) the voids and walls of the cosmos forms a fractal structure (i.e. scale-independent); (ii) the cosmic time of the voids and walls are different, because they comply strictly to Einstein's gravitational energy and time dilation; (iii) the cosmos is about one billion years older (on average) than what the standard ΛCDM model predicts, which also solves some additional problems.

The standard ΛCDM model works only on the average density of the cosmos (i.e. homogeneous matter), giving a surprisingly good fit to observations; but - many of the interpretations and values derived from the data using the homogeneous ΛCDM model as departure point. Hence, the results are model dependent. For one thing, it uses a 'cosmic time' that ticks at the same rate everywhere in the model. As can be seen in NASA's 'cosmic map of local structure',4 the roughly 500 million light years radius around us is not homogeneous at all. At larger distances, the voids are apparently even larger (not shown on this diagram).

Wiltshire has shown that when he uses a non-homogeneous density model and assumes that the large voids comprise more than 50% of the volume of the universe, there may be up to a 43% difference between 'void wall clocks' and 'void center clocks'. This effect is due to Einstein's standard gravitational time dilation and the combined inhomogeneous density and time progression [Wiltshire3.2 Fig 1(b), right] cause effects that appear to be an accelerating cosmic expansion.

If he factors this into his FB model (with voids expanding faster than walls), the predictions are an equal or even better fit to observations than the standard model. Wiltshire's Table 2 shows a comparison of the ΛCDM and FB models.

If his sums are right, this table is very impressive. What is even more interesting, it requires no dark energy! He labels the FB a 'conservative' model, because it requires no new physics. Further, it needs only half as much dark matter as the ΛCDM model, meaning that normal matter makes up at least one third of the total energy of the cosmos (against the mere 4% in the standard model). On average, it means we may live in a standard flat Einstein-de Sitter universe that will not expand forever, but neither will it ever collapse. A sort of relief…

Personally, I do not understand his model very well (I doubt if very many cosmologists fully understand it, or have even seriously looked at it), but I did not see any devastating critiques against it. Some of the critiques seem to confuse it with 'Hubble Bubble models', where the observer is near the center of a void. In the FB model, bubbles play a very important role, but they do not have to be excessively large. The only requirement is that they make up more half of the cosmos by volume, which is apparently the case (counting in the mini-voids, the FB model requires a void-to-wall volume ratio of about 70:30 presently).

Jorrie

Notes

1. The pretty bubble picture top left is from David Wiltshire's home page: http://www2.phys.canterbury.ac.nz/~dlw24/. 'Fractal Bubbles" are not quite the same as "Hubble Bubbles". I combined them in the title for effect... :)

2. 'Hubble galaxies' refer to those on which Edwin Hubble originally based his linear redshift-distance law. They were all well within 100 million light years from us. He was a bit out on the value of the constant Ho, but that was due to distance measurement errors, which were corrected much later.

3. David L. Wiltshire, (3.1) Dark energy without dark energy, Dec, 2007 and (3.2) Gravitational energy as dark energy: Cosmic structure and apparent acceleration, Feb, 2011.

4. A better resolution diagram can be viewed at http://heasarc.nasa.gov/docs/cosmic/gifs/atlas_sc.jpg.

5. I want to acknowledge the stimuli given by jonmtkisco and Cedar in my Blog Alternative cosmologies. Jon originally drew my attention to it, but I thought it an improbable theory at that time. Cedar's ideas made me look at it again and I now no longer think it is that unlikely...

-J

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#1

Re: Fractal Hubble Bubbles?

04/12/2011 7:17 AM

The model makes sense and does not require any new leaps of faith in the realm of physics to explain what we see.

It will be interesting to see if support for this model gains acceptance.

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#7
In reply to #1

Re: Fractal Hubble Bubbles?

04/13/2011 7:43 AM

The key thing you said in your comment is "what we see". Almost our entire understanding of neighboring dust and gases is based on gathering light and a few other type of waves. I have no proof (other than dark matter theories) but I just have to think there's so much more surrounding us we just can't "see" it.

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#8
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Re: Fractal Hubble Bubbles?

04/13/2011 8:18 AM

I think the first order of business in situations like these is to find consistency with our theories for the limits of our observations.

If a theory works for what we can observe and explains those observations satisfactorily, we have come a long way in our path of understanding.

As far as we can tell, the laws of one part of the universe work just as consistently as they do with any other part of the universe. It is reasonable to believe that the physics behind any portion of the universe that we can not directly observe will follow suit. There is no evidence nor any compelling theory that would suggest otherwise.

The Standard Model has been a good approximation of what we observe. Just like Newtonian physics, it describes and predicts observations pretty closely to the expected result. Yes, there are some inconsistencies with Newton's laws and Einstein's work brought us much closer to the mark.

I suspect that the Fractal Hubble Bubble theory may do to the Standard Model what Einstein did to Newton's work; that is, better refine our understanding of our universe if the theory (Fractal Hubble Bubbles) holds up to peer review.

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#9
In reply to #8

Re: Fractal Hubble Bubbles?

04/13/2011 1:08 PM

Hi AH, you wrote: "... that is, better refine our understanding of our universe if the theory (Fractal Hubble Bubbles) holds up to peer review"

Just note that 'Fractal Hubble Bubbles' is my own creation for a title of this Blog entry, not Wiltshire's. He originally called it the 'Fractal Bubble' (FB) model, but lately he only uses 'Timescape' (TS) model, as I mentioned in the Blog, read with my end note (3).

As far as peer review is concerned, I've just read a Nov. 2010 paper by Wiltshire and his post-doc assistant, Peter Smale: (arXiv:1009.5855v2), titled 'Supernova tests of the timescape cosmology'. They answered one review that claimed that the TS theory shows a relatively poor fit with a data set known as the 'Union and Constitution super-novae compilations'. Here is his abstract:

"The timescape cosmology has been proposed as a viable alternative to homogeneous cosmologies with dark energy. It realises cosmic acceleration as an apparent effect that arises in calibrating average cosmological parameters in the presence of spatial curvature and gravitational energy gradients that grow large with the growth of inhomogeneities at late epochs. Recently Kwan, Francis and Lewis [arXiv:0902.4249] have claimed that the timescape model provides a relatively poor fit to the Union and Constitution supernovae compilations, as compared to the standard Lambda CDM model. We show this conclusion is a result of systematic issues in supernova light curve fitting, and of failing to exclude data below the scale of statistical homogeneity, z < 0.033. Using all currently available supernova datasets (Gold07, Union, Constitution, MLCS17, MLCS31, SDSS-II, CSP, Union2), and making cuts at the statistical homogeneity scale, we show that data reduced by the SALT/SALT-II fitters provides Bayesian evidence that favours the spatially flat Lambda CDM model over the timescape model, whereas data reduced with MLCS2k2 fitters gives Bayesian evidence which favours the timescape model over the Lambda CDM model. We discuss the questions of extinction and reddening by dust, and of intrinsic colour variations in supernovae which do not correlate with the decay time, and the likely impact these systematics would have in a scenario consistent with the timescape model."

Quite interesting how model-dependent interpretations of the data can be.

-J

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#10
In reply to #9

Re: Fractal Hubble Bubbles?

04/13/2011 1:35 PM

Yes, I should have been more careful with the nomenclature. ;-)

Not that I have the time and the expertise to mathematically confirm this, but the apparent time-shift required for the effects between voids and regions of higher densities would seem pretty small, yet I surmise that the distances are vast, so that would account for the disparities that drive the apparent issues with the Standard Model?

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#12
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Re: Fractal Hubble Bubbles?

04/13/2011 2:58 PM

Hi AH, you wrote: "...but the apparent time-shift required for the effects between voids and regions of higher densities would seem pretty small, yet I surmise that the distances are vast, so that would account for the disparities that drive the apparent issues with the Standard Model?"

It is more the vast time over which the differences in clock rates are accumulated. Even for adjacent walls/voids, the present age difference could apparently be some 40%, but that translates to only about 3 billionths of one percent per year (which if I'm not mistaken is some one millisecond per year).

It reminds me of a saying of Lewis Carrol Epstein in his book Relativity Visualized: "Age is impossible". Not only differences in gravitational potential, but also following different paths through spacetime make various parts of a system age at different rates.

-J

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#13
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Re: Fractal Hubble Bubbles?

04/13/2011 3:37 PM

Understood. Distance implied time as far as what I meant, but your version of it is clearer.

Thanks.

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#14
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Re: Fractal Hubble Bubbles?

04/13/2011 5:17 PM

"Quite interesting how model-dependent interpretations of the data can be."

If you are looking for a banana, there is a good chance you will miss the elephant...The most important information is in that portion of the universe of data that does not adhere to a particular model- what most people will discard as "noise".

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#2

Re: Fractal Hubble Bubbles?

04/12/2011 10:39 AM

Great post. I'll be reading the papers.

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#3

Re: Fractal Hubble Bubbles?

04/12/2011 11:50 AM

"Hubble bubble" eh? What would Shakespeare the Engineer say?

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Re: Fractal Hubble Bubbles?

04/12/2011 5:16 PM

"...toil and trouble"?

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#5

Re: Fractal Hubble Bubbles?

04/12/2011 5:19 PM

Excellent post, Jorrie. 5 Stars.

Clever thinking by Wiltshire.

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#6

Re: Fractal Hubble Bubbles?

04/13/2011 12:06 AM

Excellent- I will be reading some of your references. There seems to be some attention to some of the issues I have with the Standard Model...

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#11

Re: Fractal Hubble Bubbles?

04/13/2011 2:53 PM

Well, duh!

(More precisely: why didn't I think of that? Corollary: I hope I'm understanding it as well as I think I am.)

If meteorologists used a homogeneous density model of the atmosphere, they'd be able to give us planetary average temperature. Reeeall useful!

OF COURSE the universe is not uniformly dense throughout... and Whiltshire's model indicates that the density variations are on the same scale at which we identify breakdowns in the old homogeneous models, and at which gravity becomes very noticeable.

Did this guy just make Stephen Hawking, Kip Thorne and every other astrophysicist look dumb?

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#15

Re: Fractal Hubble Bubbles?

04/13/2011 5:24 PM

When an "expert" has invested all of his time and energies for more than 20 years establishing credentials within the establishment, it is very likely that he or she will welcome a new concept that chips away at the very foundation of one's accumulated knowledge. No one is going to like being told that the work of a lifetime is no longer valid...

I have noted on many occasions that it seems most "great" scientists make their major contributions early in life (Einstein, Dirac, Watson and Crick, etc.). Why is this? Could it be because they have yet to invest in the status quo?

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#18
In reply to #15

Re: Fractal Hubble Bubbles?

04/14/2011 1:12 AM

Hi CW, you wrote: "... Why is this? Could it be because they have yet to invest in the status quo?"

Good point.

It may also be due (partly) to them "standing on the shoulders of Giants" and being young, they may have the better agility to 'climb up there' rather rapidly. I do not think blind alleys and dead-ends on the past journeys have been wasted - it may be like when on a sightseeing tour, those getting-lost-detours are sometimes very worthwhile.

-J

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#16

Re: Fractal Hubble Bubbles?

04/13/2011 8:56 PM

Hi Jorrie,

Great post. There was a similar theory several year ago, but without the evidence to back it up, so I just forgot about it. Your sentence staring with "Wiltshire has shown that..." seems to be missing words. Please take a look and see if it can be stated more clearly (",galaxy wall observers," left me cold).

In the model comparison, does the "No" mean that the theory fails to explain the observation? If so, the new theory is impressive. What does "Tension" mean?

-S

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#17
In reply to #16

Re: Fractal Hubble Bubbles?

04/14/2011 12:44 AM

Hi S, yes there seem to be stray words in my sentence. The offending ", galaxy wall observers," should not be there. I have changed the OP. Thanks for the heads-up.

You asked: "In the model comparison, does the "No" mean that the theory fails to explain the observation?"

AFAIK, a "No" means that there is no wide agreement on how to explain it using the ΛCDM model, with its homogeneity averaging assumptions. It obviously does not mean that there is 'wide agreement' on Wiltshire's 'timescape' model either. This is just part of his 'change proposal', but the 'cosmic configuration committee' (CCC?) seems to be quite slow in reacting.

I found an interesting private mail, copied with Wiltshire's consent into this Physics Forums thread. It gives some informal views from the man. Here is one paragraph (which I chopped into more than one paragraph for readability):

"A number of other people (Buchert, Carfora, Zalaletdinov, Rasanen, Coley, Ellis, Mattsson, etc) have (in some cases for well over a decade) looked at the averaging problem - most recently with a view to understanding the expansion history for which we invoke dark energy. But given an initial spectrum of perturbations consistent with the evidence of the CMB these approaches, which only consider a change to the average evolution as inhomogeneity grows, cannot realistically match observation in a statistical sense.

The clock effect idea is my own "crazy" contribution, which the others in the averaging community in mathematical relativity have not yet subscribed to. But with this idea I can begin to make testable predictions (which the others cannot to the same degree), which are in broad quantitive agreement with current observations, and which can be distinguished from "dark energy" in a smooth universe by future tests.

My recent paper 0909.0749, published in PRD this month, describes several tests and compares data where possible. The essay, which summarises the earlier PR D78 (2008) 084032 is an attempt to describe in non-technical language why this "crazy idea" is physically natural."

Good stuff!

-J

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#19

Dr. Prochaska's Modern Cosmology

04/17/2011 11:53 AM

Hers is a very good 2010 lecture from Dr. Xavier Prochaska, UCO/Lick/Santa Cruz.

Quite entertaining, although he ran out of time and did not touch dark energy.(a) The lecture starts around 15m 40s into the 1h 15m movie(b), with a supercomputer simulation of dark matter forming galaxies - fascinating stuff. The rest of the video is standard, easy stuff, but his presentation is quite enlightening.

Enjoy, Jorrie

(a) I have the feeling the prof. is of the 'protect the canon school' and probably would not have touched Wiltshire's work. I will look out for his promised future talk on dark energy - maybe a member of the audience will ask him about it.

(b) Movie made by Lecturemaker LLC. The initial parts, especially the 'walking shots' are a little amateurish, but the lecture part was done quite well, although I could not see all of the projector screen at crucial times; quite frustrating, but it could have something to do with my media player setup, though. The video is also 'unduly' large, almost 0.5 G-byte, so it does not work on slowish internet like mine. I had to download and save it first.

-J

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#20
In reply to #19

Re: Dr. Prochaska's Modern Cosmology

05/26/2011 3:55 PM

Yeah, I'm pretty open to the idea of an inhomogeneous universe.

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#22
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Re: Dr. Prochaska's Modern Cosmology

11/17/2011 9:23 AM

The Hubble Constant is a numerical measure, of the rate, of the expansion, of our space-time fabric. Qualitatively, SNIa observations seem to show an acceleration, of that expansion -- i.e. the Hubble Constant, as measured at present epoch, seems to be larger, than it was, in the past.

Now, to make those measurements, of the Hubble Constant, astronomers observe the light, from distant SNIa. Since light is always moving, through our space-time fabric, measurements of the Hubble Constant back 'then', necessarily are measurements way over 'there'. And, similarly, measurements of the Hubble Constant 'now', necessarily are measurements 'here'.

Therefore, light-like, line-of-sight measurements, of the Hubble Constant, indicate that its value, was larger, "there-and-then", as compared to "here-and-now". And, there are two ways of interpreting these results. First, 'Dark Energy' proponents, appeal non-flexibly to the 'Cosmological Principle', i.e. they extrapolate all observations non-flexibly, according to the assumption, of 'uniformity / isotropy / homogeneity'. Thus, they demand, that the HC "there-and-then" must have been the same, as the HC "here-and-then"; and, that the HC "here-and-now" must be the same, as the HC "there-and-now". Er go, evolution of the HC, observed along light-like LOSs, are extrapolated, non-flexibly, to the rest of our space-time fabric, i.e. the HC is increasing; and, our space-time fabric is accelerating in its expansion.

However, 'Hubble Bubble' proponents interpret the same results, differently. They appeal, to telescopic observations, of non-uniform, non-isotropic, non-homogeneous Large-Scale Structure, i.e. 'Cosmic Web'; and, of our galaxy's entrainment, into the same (e.g. Virgo Flow); as justification, to dispense with the Cosmological Principle, on "cosmologically small" spatial scales (<100 Mpc). Accordingly, LOS measurements, of the HC, "there-and-then" need not reflect the value, of the HC, "here-and-then"; and, similarly, measured values for the HC "here-and-now" need not reflect the value, of the HC, "there-and-now", e.g. in the middle of vast inter-galactic voids, outside of the CW (which is where most of space-time resides).

Instead, observed variations, in the HC, along SNIa LOS, are interpreted as spatial inhomogeneities / non-uniformities / anisotropies, i.e. evolutions, in the value of the HC, through space; and, not as evolutions, in the value, of the HC, through time. Specifically, the HC is larger "here-and-now", due entirely to space "here" having a higher-than-cosmic-average density, because we reside in the CW; and, the HC is lower "there-and-then", due entirely to space "there" having a lower-than-cosmic-average density, because "out there" is mostly voids. Time is alleged a non-issue, spatial inhomogeneities are attributed the entire (anomalous) variation in values, of the HC.

Observations, of SNIa, along static LOS, employing ultra-long ultra-deep exposures, cp. HDF / HUDF, would compare "apples with apples", i.e. SNIa whose light had threaded through the same nearby spatial structures. Er go, such "single sight-line" SNIa observations would disentangle space-like, from time-like, variations in the value of the HC.

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#23
In reply to #22

Re: Dr. Prochaska's Modern Cosmology

11/17/2011 10:49 AM

"Qualitatively, SNIa observations seem to show an acceleration, of that expansion -- i.e. the Hubble Constant, as measured at present epoch, seems to be larger, than it was, in the past."

Not quite, if I understood you correctly. The value of the time varying Hubble parameter H is proportional to (da/dt)/a, where a is the expansion factor. In the case of the ΛCDM model, even with accelerated expansion, H is, and always has been decreasing. It may eventually become a constant (in trillions of years), but it will never decrease.

Maybe because of this uncertainty, I do not understand the rest of your post.

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#24
In reply to #23

Re: Dr. Prochaska's Modern Cosmology

11/17/2011 11:18 AM

You're imposing upon me, not improperly, more required precision, than I can accommodate.

I merely mean, that SNIa show that the expansion, of our space-time fabric, is faster, nearby ("here-and-now"), than far away ("there-and-then"). And, without appealing to 'Dark Energy', that evolving variation, in Hubble expansion rate, can be attributed, to spatial inhomogeneities (i.e., local relative over-density, in CW; vs. remote relative under-density, in Voids); and, not to spatially homogenous, i.e. pan-cosmic universe-wide, time evolution.

In summary, 'Dark Energy' alleges time variation, amidst spatial uniformity; whereas, 'Hubble Bubble' alleges spatial inhomogeneity. Yes?

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#25
In reply to #24

Re: Dr. Prochaska's Modern Cosmology

11/17/2011 11:33 AM

OK, I see what you meant: nearer space seem to be expanding slightly faster than distant space, simply because we observe distant space at an earlier time.

Yes, I think you summarized the 'Fractal Hubble Bubble' case well. Wiltshire's model is not actually the standard 'Hubble Bubble', but one with time variation, so it may mimic dark energy.

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#26
In reply to #25

Re: Dr. Prochaska's Modern Cosmology

11/18/2011 2:38 AM

According to a related thread:

Wiltshire-and-the-Cosmic-Bell

"The ratio fv0 = 0.762 represents the present volume of the voids to the total volume of observable space, i.e. 76% voids by volume"

Now, 'Dark Energy' comprises ~74% of our universe's matter-and-energy density. Does this not imply, that 'voids' comprise ~76% by volume; and, 74% by mass; of our universe's matter-and-energy 'budget' ? If so, then our cosmos is 3/4ths 'voids', "plus a little by volume"; and, "minus a little by mass"; presumably b/c 'voids' have expanded a little more (than structure-bearing space); and, have 'deposited' matter, into the CW, e.g. filaments.

Meanwhile, space-spanning 'structure' -- i.e. luminous matter, e.g. gas & stars; and, non-luminous 'Dark Matter', traced by the former -- comprises ~26% of our universe's matter-and-energy density, whilst occupying ~24% of our universe's volume. If so, then our cosmos is 1/4 'structure', "plus a little by mass"; and, "minus a little by volume"; presumably b/c structure-bearing space has expanded a little less (than surrounding voids); and, has 'gained' matter, into the CW, from the voids.

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#21
In reply to #19

Re: Dr. Prochaska's Modern Cosmology

05/26/2011 10:06 PM

Thanks Jorrie:

Lick will be having a summer visitor program again this year.

www.ucolick.org/public/sumvispro.html

Best regards,

Ron

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#27

Re: Fractal Hubble Bubbles?

11/20/2011 12:02 PM

On-line links regarding the 'Hubble Bubble' hypothesis:

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#28
In reply to #27

Re: Fractal Hubble Bubbles?

12/05/2011 3:31 AM

From universe review of Canada:

Illusion - This explanation asserts that the apparent acceleration is just an illusion. Sky survey reveals that matter is distributed unevenly on large scales with gigantic super-clusters and huge voids in between. Because we live in a gravitationally bound system (the Milky way), our clocks run more slowly than they would in a void. In addition, space is negatively curved in the void, so the volume for a given radius is larger than in the relatively flat space. In effect, our estimate of volume is too small and the estimate of time is too slow giving the wrong impression of acceleration

Void - If the Earth is located in a vast cosmic void (with less matter than the average) of the size between 300 million to 3 billion light years, then object outside the void would be further away (than envisioned from a homogeneous universe) because the void would expand faster with less gravitational retardation

Citing some seemingly inexplicable anomalies in astronomical observations, it is suggested that the "Void Theory" would remain viable even the Earth is slightly off center (up to 50 million light years). It has been shown that the CMBR is a bit lopsided - hotter in one direction than in the other. This asymmetry is usually attributed to the motion of the Solar system through space but could also be a sign of a lumpy universe. Furthermore, small fluctuations in the CMBR appear to align in the specific direction (the "axis of evil"). This alignment picks out a preferred direction in the sky, which, though hard to imagine in a Copernican universe, might be explained in terms of displacement of the Earth from the center of a void. A preferred direction would also have other effects, such as large-scale coherent motions of galaxies and galaxy clusters. Several observations have claimed detection of such "dark flow"

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#29
In reply to #28

Re: Fractal Hubble Bubbles?

12/05/2011 4:03 AM

Hi Widdekind,

You are apparently copying from your own website (universe review of Canada). If so, it might be more appropriate to start your own Blog, rather than hiding it deep inside some other Blog.

I would prefer sticking to answering questions related to my Blog entry here.

-J

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#30
In reply to #29

Re: Fractal Hubble Bubbles?

12/15/2011 8:23 AM

(I came across what looked like reasonably relevant, helpful links, sorry if they somehow detract from the discussion.

-W)

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