All praise Google.

This quote could be the source of the 1000-times reference:

The Telegraph: Scientists puzzle over enormous void:

There we go. It's not 1000 times larger than other known large voids. It's 1000 times larger than an expected typical void.

It's big but it's not unholy big.

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Is the Eridanus Super void an anomaly?

Voids take time to grow based on what I could find on super structure surveys and theory. They are theorized to reach a maximum size and then grow by merging with other voids. I believe the standard cosmological model has difficulties explaining large structures.

The Eridanus Super Void is estimated to be 1800 Mpc to 3000 Mpc Rc (The great distance is required to explain the 10 degree CMB cold spot. i.e. The size of the CMB cold spot requires an explanation.) as compared to roughly 300 Mpc for the super voids listed in the Wikipedia article on super voids. (See link.)

A super void at 300 Mpc has had 12.6 billion years to form. (Using Ned Wright’s calculator, link attached.)

A super void at 1800 Mpc to 3000 Mpc has had 10.3 billion to 6.5 billion years to form. Is, both, the location (time available for it to form) and the size, of the Eridanus Super void, the reason that it is considered to be an anomaly?

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

http://www.astro.ucla.edu/~wright/DlttCalc.html


The following is an excerpt from the CMB cold spot preprint that provides an estimate of the probability of the Eridanus Super Void, using an analysis based on the standard cosmological model.


http://xxx.lanl.gov/pdf/0704.0908

Comment:
1) The Wikipedia article notes that two papers concerning the Eridanus super void/CMB cold spot were submitted in August, 2007. If someone finds the second paper could they please, provide a link in the forum.

2) Is the CMB cold spot real? See Jerry's above comment, that discusses a paper that questions whether local contamination has been removed from the CMB data. It would be interesting based on Jerry's comment to find a hypothesized contamination source/mechanism that could create a 10 degree cold spot in the CMB. Any thoughts? I have not but will read the paper Jerry refers to.

Using their methods, what would you calculate for the probability of, say, the formerly largest known void, about 160 Mpc in diameter?

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It should be consistent, because those are roughly the voids they are using to constrain the distribution. Put differently, they seem to be saying that the void fraction as a function of size r should scale like exp(-r) raised to some pretty large power you can get from their formula. But more importantly, what that means is that every time you double the linear size, you square the void fraction at that size. So if they expect voids that big to have a fraction like 10^{-10}, then they'd expect voids half that big to cover a fraction like 10^{-5}. That's not too far off from what you see. So the real question is, what is the basis for expecting that a double-sized void should cover the square of the volume fraction? That's basically Poisson statistics-- if you double the amount (not linear size here) of radioactive material you have, then the probability of getting no decays in a certain fixed time goes like the square of whatever that probability was before you doubled the amount. In effect, they are treating each half-radius as an independent probability set. Why one radial half of a void can be considered to be independent of the other half, and why that's done in radius and not in volume, is not so obvious, but it's a place to start. Note it's much worse if you do it by volume rather than radius-- if you treat 8 times the volume as 8 independent voids that all have to come out empty, then the void fraction should be down by the smaller fraction raised to the 8th power, not squared, so you'd expect 10^{-40} to be the supervoid fraction. They seem to feel the "independent units" of void scale linearly with radius, I don't know why.

In reply to 01101001

Ken G. has already applied and discussed the paper's formula. Ken's comment:

I believe the formula should and does take into account that the voids must merge, hence the probability should be reduced by void probability (for a smaller void) times void probability (for a smaller void), as the void radius increases (i.e. The formula divides larger void size by smaller void size to determine a factor that is applied to the probability (frequency of occurrence) of the smaller void.) The following are different void probabilities calculated by applying paper's formula.

300 1E-20
140 4.6E-10
70 2.2E-05
35 4.6E-03
10 2.2E-01


It would be interesting to look at void size verse time, there should be a maximum void merge rate based on how quickly the large scale structures can change.

What did the astronimer mean by ""It looks like something to be taken seriously," added Brent Tully, a University of Hawaii astronomer.
"

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Please tell us you're not thinking that "taking it seriously" means being concerned that it will harm us.

It is meant to provide a weighting to the immediately previous idea, in the Telegraph article:

Take it seriously: don't consider the evidence to be a mere statistical oddity, but as an indication of a likely void.

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I am still confused

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Why?

The analysis was suggesting that there is statistical evidence, measurements that could be interpreted as indicating a void. One could take the statistics as a fluke, an error in the numbers, noise, of no real signicance.

Condon says: more likely, the numbers indicate a void.

Tully says: take the idea of the numbers indicating a void seriously. It's probably real. Do not assume the measurement data are a fluke of no matter. (Ha. Indicating nothing. Double-ha.)

Why are you confused?

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Ah, bmpbmp, the poster with no worries!

Good to see you again!

If there's an absence of matter there, then cruising through the bubble would actually be much safer than where we are today.

How do we know we're not in it? It's framed by galaxies that are closer than it is and more distant. The intrinsic versus absolute magnitudes of these galaxies then provide us an idea of its distance and dimensions.

Concerning it being something of an anomaly versus what astronomers expected, well, that happens in science all the time and leads to new objective evidence, speculations, hypotheses, and eventually new theories.

Meanwhile, based on the above, I think this phenomenon should be labeled "Void Where Prohibited".


Then again, it might be where the rain gets in.

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Hi bmpbmp on page 1: The numbers are huge, enormous, extreme large. In some countries a billion is what we call a trillion in the USA, so that confuses the topic by 1000 times. In the USA 1000 times 1000 times 1000 = one billion and one trillion is 1000 times a billion. So one trillion is a million multiplied by one million. A light year is about six trillion miles or ten trillion kilometers. So a billion light years is ten trillion kilometers times a billion kilimeters. That is 10E22. Each 3 represents multipling by 1000 so 10E22 is 1000 multiplied by it's self 7 times. Then mulipy by ten. 10E22 is also a 1 followed by 22 zeros.
If the big hole is expanding at 1% of the speed of light in our direction we will be in trouble (maybe) one trillion years from now. Since our sun will be a cold whie dwarf before that, why worry? Likely the void is expanding at less than 1% of the speed of light.
Besides devour and sucking up are likely the wrong words. We might notice nothing, if our galaxy was pulled into the great void other than the distant galaxies being dimmer than now. Neil

I am unclear about the meaning and significance of the term "diameter," in the context of supervoids - particularly as it relates to the contention that doubling the diameter multiplies the volume by 8. Of course that contention would be true if all voids were spherical or even if they all had roughly the same shape, but could one be sort of longer and thinner, while another is shorter and fatter?

Unless all voids have roughly the same shape, then a void whose longest dimension (maybe this is what "diameter" means?) is double that of the longest dimension of another void would not necessarily have double the other dimensions. Then a void with twice the "diameter" would not necessarily have anywhere near 8 times the volume.

I am struck by the fact that the team seems to have gotten all its data from public data bases - no viewing time needed.

A spherical estimate is all we've got, I think. I'd never expect them all to be the same shape. Isn't it a good estimate for comparing volumes, though? What else is there?

I used twice the diameter to imply 8 times the volume in an effort to be charitable to the -- it turns out wrong -- contention that the Eridanus void was 1000 times larger than any other voids. The only other measure we had was its diameter, no? And we had the diameters, half the size, of other large voids. I even asked about the possibility that Eridanus was cigar-shaped to try to get from a reasonable eight-fold volume estimate to the unreasonable 1000-fold.

I didn't create the volume comparison. Is there some measure I missed that indicates these void volumes are being estimated by other than cubing the measured diameters? Do they measure depths? Do voids even present a circular face to us, so that their diameters are diameters of rough circles? Or are their presented diameters their measured cross-sectional area converted to a circular area and linearized? (If they do present rough circular faces, does it make a little more sense to assume they are roughly spherical rather than that they are hiding their odd dimension from us?)

When Rudnick speaks of the Eridanus void volume being 1000 times that of a typical void, where does he get that? Does he have anything besides diameters to go by? Isn't he only saying the Eridanus void diameter is 10 times the diameter of a typical void? Maybe volumes were compared just to get attention for the work. 1000 sounds larger than 10. Can't blame him.

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1) Why is a void that has a diameter greater than 30 Mpc considered to be an anomaly?
2) What is the astronomical evidence for voids greater than 30 Mpc? Note until very recently the maximum void size was believed to be 30 Mpc.

This is what I could find to answer question 1. Can someone answer question 2? Any other comments concerning question 1?

The following is taken from “Extragalactic Astronomy and Cosmology”, by Peter Schneider. (Published 2006).

The average maximum galactic velocity (at the edge of a cluster) in a cluster is roughly 1000 km/sec. At a maximum velocity of 1000 km/sec, the fastest moving galaxy in a cluster can travel 1 Mpc in 0.95 billion years.

Assuming a near homogeneous start (Is a near homogeneous start required to explain the uniformity of the CMB?) and a universe that is roughly 14 billion years, it would be expected that the maximum void size in the universe (At R=0) would be roughly 30 Mpc in diameter. Schneider’s text book states that the voids can have a diameter up to 30 Mpc. (Introduction page 12).

01101001 and Ken G,

I was really bringing up the question of diameter and volume for two reasons. The first was just to check whether my interpretation of the term "diameter" was correct. The second was a prelude to addressing Ken's question about why Rudnick et al might have estimated their likelihood on the basis of radius instead of volume - maybe they didn't feel that they knew the volumes of the previously known voids or even of their own void. Maybe the data bases they used only listed diameters.

Am I missing something? I think the evidence is that they've found some.
Several were listed in the wikipedia link you provided in your post #122.

But you can infer a volume from a diameter. It won't be exact, as you say, but neither is the concept of "diameter" an exact concept for irregular shapes. It's all what you mean statistically, and volume still seems like a more applicable "independent variable" than diameter. The distinction is huge-- the volume contrasts are far greater and seemingly more difficult to achieve randomly. But what is really the random variable here? For birds on a wire, it's clump diameter, for people milling around a subway station, it's clump area, and for galaxies wandering around a void, I'd think it would be volume.

In reply to Fortunate's comment:

I am curious when super voids were discovered. (I provided a recent text book reference source that noted that voids were believed to have a maximum diameter of 30 Mpc. The Erindus super void is asserted to have a diameter of 140 Mpc. It is difficult to explain how a void that is significantly greater than 30 Mpc could have had time to form. See my post #122.)

Are super voids real? Could the data which indicates that there are super voids have been misinterpreted? (Let's define the term super void to be a void that has a diameter that is significantly greater than 30 Mpc.)

Are there any papers that discuss super voids? Was a super void invoked to explain the CMB cold spot data?

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