Formation of Voids: Unclear Picture

[hellfire]

I think this question was addressed many times in the literature but I was not able to make a clear picture for an answer. It is known that peculiar velocities of galaxies are too small to explain the formation of voids. Voids must have existed already before galaxies were in place with their usual speeds. However, I do not understand how this is consistent with two facts: (i) we do not expect to find clusters beyond z > 2. Clustering is also evolutionary, so why should the formation of voids follow a different history? (ii) Quasar distribution at z > 2 shows no clustering nor voids.

 

[Wallace]

I think you're right. I don't think there is really much of a void problem, N-body simulations using the best fit parameters from observations match the structure seen pretty well. Of course the match is never perfect, that just tells us we don't know everything!

I think there is a lot of confusion in the literature about exactly what constitutes a void. A void in an underdense region, but how underdense? Clusters, the other end of the non-linear growth spectrum are easier to understand, since you can see them, rather than seeing them by not seeing them. However even clusters are poorly understood on many levels. You get a problem with voids when one group defines a void one way when say matching to simulations, then another group defines it differently when looking at data, apparently uncovering a 'void problem'.

For instance, I can't remember the exact reference but there was a paper that found a void in (I think) the NVSS galaxy catalogue. This was widely reported in the media and described an empty region as being so many millions of light years across etc. LCDM models don't predict empty regions anywhere near that big, so there was hints of a problem. However, this 'void' was far from empty, there were plenty of galaxies seen within it, just far fewer galaxies than elsewhere. Counting galaxies is a very uncertain method for determining the mean underdensity of a large region since galaxy formation is not well enough understood.

 

[hellfire]

I guess one of the conclusions about structure formation that can be inferred from the peculiar speeds is that there was no, or not substantial, structure formation in voids. Otherwise those structures should have moved away from the voids towards filaments or superclusters, and there is not enough time for that. The point is then why don't we observe voids in the distribution of quasars, for example?

 

[jonmtkisco]

Hi hellfire,

... we do not expect to find clusters beyond z > 2.

Do you mean "galaxy clusters" or "superclusters"? I assume it's the latter.

Jon

 

[hellfire]

Yes, I had superclusters in mind, considering them on the same hierarchy than voids somehow. I think we should expect some galaxy clusters at z > 2, but no superclusters. However, after some reflection I do not think that this argument about superclusters is correct, because we can actually have voids without superclusters. This may happen even in the z = 0 universe I think, if voids are sepparated by sheets and filaments of galaxies. In such a pattern superclusters are especially dense clumps or nodes. More interesting seemed to me the fact that the quasar distribution does not show voids. However, even this is now unclear to me, because after some search I found some references that describe quasar clustering and weak patterns beyond z > 2. At the end of the day I think that Wallace addressed it in the right way: it depends on the definition of void and the relative densities. At z >~ 2 definitions that hold for z = 0 may be difficult to apply.

 

[Wallace]

The higher the redshift the less the clustering you expect to see, since the Universe starts smooth and gets increasingly lumpy. I'm not sure that I've heard of any major disagreements with this principle in the data? Voids, clusters etc are just ways of simplifying the description of structure in human terms. Analysis of the density field power spectrum at various redshifts, using different types of galaxies and quasars as tracers of the density field give a result consistent with predictions, at least as far as I'm aware.

 

[Chronos]

Voids are not a problem, filamentary structure is more the issue. Is the universe 'old' enough to display the observed distribution of matter? I believe that is the essential question. The short answer is . . . not without something that looks a lot like inflation.

 

[Allday]

just a minor contribution: its hard to try to compare the evolution of underdense regions with overdense regions (a la voids vs quasars) because the density distribution is not symmetric. What I mean is, looking at the over density rho/rho_c - 1 the lowest value you can possibly get is 0, but the overdensity can be much greater than 1.

 

[Allday]

just to correct my post above, the lowest value you can get is -1 not 0