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Does maybe someone know if there was ever the attempt to count the number

of voids in the (observable) universe?

I assume that is not the case, but would be very interested in any work that

gives some estimation.

Many thanks,

Martin

- Thread starter voidx
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- #1

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Does maybe someone know if there was ever the attempt to count the number

of voids in the (observable) universe?

I assume that is not the case, but would be very interested in any work that

gives some estimation.

Many thanks,

Martin

- #2

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What does that even mean? Why would there not just be one void, interspersed here and there (at VERY long distances) with galaxies and galactic clusters, and even strings of clusters?

Does maybe someone know if there was ever the attempt to count thenumberin the (observable) universe?

of voids

I assume that is not the case, but would be very interested in any work that

gives some estimation.

Many thanks,

Martin

- #3

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thanks for your reply and sorry for not expressing my question correctly enough!

If you consider a voids of specific size, like here

http://en.wikipedia.org/wiki/List_of_voids

Is there any study that says:

There are roughly about N voids with diameter M in the observable universe.

I searched the net and found... nothing. Also I assume noone, except me is interested in that question ;-)

Thanks,

Martin

- #4

jimgraber

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The power spectrum of the cosmic microwave background shows a big bump at around half a degree or slightlly larger. According to the (broadly accepted) acoustic theory, clumps and voids should occur at roughly this scale, on average. This scale is approximately 100 megaparsecs today. You wil notice that the voids in your wikipedia list are mostly within a factor of a few bigger or smaller than one hundred parsecs. One parsec is about three and a quarter light years, so a typical void is about 325 million light years. the observable universe has a radius of about 13.7 billion light years, so, the typical void is 1/40th or 1/50th of the radius or around 1/100th of the diameter of the observable universe. So for a cubical universe, there should be approximately 100^3 or about a million voids. For a spherical universe (pi r^3 and all that), you get a slightly smaller result, but we are nowhere near that accuracy. Also, a more correct calculation would do a more accurate treatment of the cosmic expansion, etc.

But roughly a million expected voids is correct to an order of magnitude. We've known this since WMAP, which lead to Nobel prizes for this discovery.

- #5

jimgraber

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Take a look at the WMAP picture here in Wikipedia. The smallest blue dots are typical voids at the edge of the observable universe. They should be about 1/360 th of the height or 1/720 th of the width.

That one big blue splotch near the middle is so big people worry about it.

- #6

jimgraber

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- #7

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thank you very much for your precise answer!

That helped me a lot. I was playing with the *idea* that it could

be around roughly a million, but I could not find any evidence.

Many thanks again,

Martin

- #8

Chronos

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- #9

jimgraber

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- #10

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If I understand what you are saying, it is nonsensical. EVERY void is very far away from some frame of reference. Are you trying to make us the center of the universe?I just thought of another important point. The far away voids aren't very empty. Just slight underdensities, surrounded by slight overdensities. As time goes on, the voids empty out, and the clusters contract.So the closer voids are much emptier than the far away voids.

- #11

BillSaltLake

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What might be informative here is a plot of observations (or a plot derived from the millennium simulation) which shows the distribution of energy density. The x axis would be space volume, with the least dense on the left ("x"=0) and the most dense on the right ("x"=1 or 100%-- this is a histogram format). The y axis is energy density, although it would not include "dark energy". In early times, the plot would just be a horizontal line (= uniform density). Then as clumping occurs, the left side drops while the right side of the curve rises.

I've been looking for a set of curves (for several values of z) like this from the millennium simulation, but haven't found it yet. With this set of plots, you could get a feel of at least what fraction of space is void. In other words, what fraction of space contains < 1% (for example) of the average density, at (for example) z=3.

I've been looking for a set of curves (for several values of z) like this from the millennium simulation, but haven't found it yet. With this set of plots, you could get a feel of at least what fraction of space is void. In other words, what fraction of space contains < 1% (for example) of the average density, at (for example) z=3.

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- #12

jimgraber

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Well, yes. We are the center of our *observable* universe. Far away is also backwards in time, and the voids we see at the edge of the (our) observable universe, ie in the CMB underdensities, are very "young". When they are as old as the nearby voids, they will look similar to the nearby voids, but we won't see that for another thirteen billion years or so.

Best, Jim Graber

- #13

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Ah ... now I get what you mean.

Well, yes. We are the center of our *observable* universe. Far away is also backwards in time, and the voids we see at the edge of the (our) observable universe, ie in the CMB underdensities, are very "young". When they are as old as the nearby voids, they will look similar to the nearby voids, but we won't see that for another thirteen billion years or so.

Best, Jim Graber

By the way, the quote button is very easy to use.

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Have you considered the Eridanus Supervoid or Great Void?

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It's actually out there just in astrophysics language.I searched the net and found... nothing. Also I assume noone, except me is interested in that question ;-)

The term that astrophysicists use to ask that question is "power spectrum".

See figure 6

http://universe-review.ca/R05-04-powerspectrum.htm

What that diagram says is that the size of a void is about 1 degree, which gives you 1/60th of the radius of the universe which gives you the numbers that jimgraber gives you.

Also these numbers don't tell you how "deep" the voids are.

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I think this might be what you are looking for.I've been looking for a set of curves (for several values of z) like this from the millennium simulation, but haven't found it yet. With this set of plots, you could get a feel of at least what fraction of space is void. In other words, what fraction of space contains < 1% (for example) of the average density, at (for example) z=3.

http://www.lanl.gov/projects/cosmology/sf09/carlson_sf09_pt.pdf

Also structure formation near the big bang is something that seems to be simple. Structure formation now is something that we don't totally quite understand.

One non-trivial part of cosmology is how do describe the distribution of the universe. The language that people end up using is identical to the language people use to describe sound.

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http://adsabs.harvard.edu/abs/2009ApJ...699.1252F

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