Tuesday, November 14, 2006

PS on Dark Matter

Two weeks ago, I reported on Stefan Hofmann's work about the influence of dark matter on the small scale structure of our universe. Last week, his collaborator Anne Green from the University of Nottingham gave a very good follow-up seminar on their work, and thanks to the new PI-websites, I can actually give you a link:

    Dark matter: from the early Universe to the Milky Way
    Speaker: Anne Green
    Date: 11/07/2006, 2:00 PM CST
    Length: 1 Hours, 5 Minutes, 46 Seconds
    Abstract: The initial conditions for structure formation, and hence the dark matter distribution on sub-galactic scales, depend on the microphysics of the dark matter in the early Universe. I will focus on WIMPs and explain how collisional damping and free-streaming erase perturbations on comoving scales k> ~1/pc. Consequently the first structures to form in the Universe are mini-halos with mass of order the Earth. I will then describe the status of calculations of the subsequent dynamical evolution of these mini-halos. Finally, if time permits, I'll briefly overview the microphysics of axions.


If you have the time and an interest in dark matter, you should invest the hour to look at it! During her talk, I noticed that in my earlier post I forgot to point out the important thing to notice in the figure below: the absence of even smaller structures in the magnification.


(picture from astro-ph/0501589)



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13 comments:

QUASAR9 said...

Hi Bee,
Haven't had a chance to look in depth at Stefan's & Ann Green's papers, but in simple terms ...

we are talking here of dark matter as dust, meteors and possible debri from collisions between planets, stars, or even galaxies, which do not emit light ... but are evidently physical particles (even bodies of particles) with their relative 'gravity' or gravitational force in 3D+T space

and not the more 'obscure' dark matter that some attribute to be gravitational forces with no physical constituents, simply hidden perhaps in another (other) dimension(s)

QUASAR9 said...

PS - Just been watching on tv about how some of the 'glass' found in the Sahara might have been formed 30 million years ago by a 120 metre (or thereabouts) meteor which entered earth, started burning in the atmosphere and finally exploded above ground (not on impact) producing heat 10,000 times that of the first man made atomic mushrooms - creating some very exotic Egyptian glass found in a radius over several tens of kilometres.

Not wholly conclusive, no definite impact crater can be seen from space - but this particular Egyptian glass required more heat than lava (volcanic) glass.

Not related, just interesting - similar to meteor in remote Russia in the begining of the 20th Century

Bee said...

Hi Quasar,

we are talking here of dark matter as dust, meteors and possible debri from collisions between planets, stars, or even galaxies, which do not emit light ... but are evidently physical particles

The dark matter does not form such dense and compact objects as planets or stars. It remains, even today, a very dilute dust with low energy density, and homogeneous on scales of about the solar system. No smaller structures have ever formed. But yes, evidently physical particles. In the above simulation some kind of WIMPs, I think neutralinos.

Meteors are spooky. I have a very small one which was found somewhere in Mexico. It's kind of weird to hold it in the hand, and to imagine where it has been before.

Best,

B.

Arun said...

"The dark matter does not form such dense and compact objects as planets or stars. It remains, even today, a very dilute dust with low energy density, and homogeneous on scales of about the solar system. No smaller structures have ever formed."

For some variety of dark matter, is it possible to convince oneself of this by back-of-the-envelope calculations?

QUASAR9 said...

Bee,
is the Universe according to the laws of physics allowed to produce more matter - or just recycle the existing matter into new (different) density matter?

if that is the case then all matter must have existed at the beginning - all it did is either implose and become incresingly dense
Or it was 'massively' dense and exploded into galaxies, stars, planets, meteors, debris, dust, ... and dark matter.

Or neither, in which case the universe is creating more matter, or more matter is entering 'this' universe thru ... you know where

But this still doesn't resolve is our universe and all the matter their in a bubble inside higher dimensional matterless or massless bubble
or is the matter of the universe we are (perhaps I should say perceive) only a portion of the total matter existing in the 'real' outer universe, the universe beyond our hypothetical, theoretical and artificial periphery - the one called the cosmic event horizon.

Back to the elephant - if a cosmic body (planet, star, galaxy) near the fringes of the cosmic event horizon obey different gravitational laws and laws of physics than we do in our solar system. Will the light from their star not be visible equally by us and others equidistant on their other 'side'

Bee said...

Hi Arun,

For some variety of dark matter, is it possible to convince oneself of this by back-of-the-envelope calculations?

In the case above it's some kind of WIMPs, I think they used neutralinos, but the only important criterion about the type of dark matter (besides being cold and weakly interacting) here is that it was in chemical and thermal equilibrium with baryonic matter in the early universe.

If that was not the case (e.g. axions), the small scale structures might look significantly different. This is the exciting thing about their work: that the small scale structures might carry an imprint of the microphysis that underlies the dark matter dynamics. The axions are work in progress, see e.g. Anne's talk, second half.

I doubt it is an back-of-the-envelope calculation, as is hardly anything that involves cosmological perturbation theory (not to mention the non-pert. part). At least I can't be very helpful in this regard. You find a very readable introduction and summary in the paper

The First WIMPy halos

Best,

B.

JJD said...

I hope this is not a stupid question, but is it possible that "dark matter" might be purely geometric? I am thinking of wrinkles or dents in space-time which could be remnants of a crumpled-up primordial space-time. The wrinkles would tend to accumulate puddles of baryonic matter, and they would also have some energy content which presumably would have mass corresponding to the gravitational effect of the wrinkles.

Bee said...

Hi JJD,

I hope this is not a stupid question, but is it possible that "dark matter" might be purely geometric?

Well, you know... there are no stupid questions, only stupid answers...

It's an interesting question actually, but I am afraid I don't quite understand it. According to General Relativity, space-time curvature and its matter content are directly related. How would you get 'purely geometric' effects without having something (alias dark matter) that causes these effects (curvature)? I mean, purely for the sake of getting an potential that allows the baryons we observe to cluster in a way that we see, in principle the potential itself is probably sufficient. But were does it come from?

Best,

B.

JJD said...

Well, you know... there are no stupid questions, only stupid answers...

The www.despair.com poster for "Cluelessness" says "There are no stupid questions, but there are a lot of inquisitive idiots."

How would you get 'purely geometric' effects without having something (alias dark matter) that causes these effects (curvature)?

The "How would you get" question applies also to an unknown "dark" kind of matter, but I thought that since the vacuum is already considered to have "dark energy" driving accelerated expansion (a geometric effect), perhaps it could also be considered whether the vacuum might bear some kind of localized residual energy content as a remnant, perhaps, of an intensely crumpled initial state just after the Big Bang and before the appearance of ordinary matter. I realize that this is very informal language, but I suppose the underlying technical question (which I cannot formulate, not being a physicist) is whether it would not be reasonable to try to model the vacuum itself as an elastic medium which can carry something like localized stress, the origin of which might be e.g. the jumbly structure of the "quantum foam" before cosmic inflation.

Bee said...

Hi JJD,

whether it would not be reasonable to try to model the vacuum itself as an elastic medium which can carry something like localized stress, the origin of which might be e.g. the jumbly structure of the "quantum foam" before cosmic inflation.

If the space-time carries stress-energy, then it's not a vacuum solution. What do you mean with 'vacuum'? Or what do you mean with 'stress-energy'. I.e. the space-time itself does not have anything that contributes as a source. If you want an effect similar to that of dark matter, you need a source term for the field equations, but you can not get this source term from the gravitational field itself.

Nevertheless, I think I know roughly what you mean. You have something in mind like topological defects or so? The problem here, is that you still need these 'leftovers' to behave appropriately to cause the observed structures. Dark matter like the one mentioned above does this fairly well. If you have some 'leftovers' they'll have to behave similarly. A huge problem I see is that you need to explain why these 'wrinkles' weren't all smoothened out during the expansion of the universe, how they form appropriate structures, and why they wouldn't be observable in our solar system on the other hand.

Best,

B.

PS: There are attempts to explain dark matter and dark energy with one and the same thing, but so far it's not so really convincing.

Arun said...

Since you're encouraging questions, does any theory/string theory help us understand why we live in a universe dominated by matter rather than anti-matter?

Is there any conceivable connection between the dark matter and matter/anti-matter problems?

Bee said...

Hi Arun,

Since you're encouraging questions,

That's what this blog is here for :-)

...does any theory/string theory help us understand why we live in a universe dominated by matter rather than anti-matter?

Ah. I'm not sure I understand the question. If our universe was full with anti-matter, we'd probably just call it matter (though the universe wouldn't be an exact copy). The problem is why there is so much less anti-matter than matter, or why matter and anti-matter didn't completely annihilate in the early univese. As far as I am aware, this is still an unsolved problem, and one that neither string theory nor LQG have very much to say about.

Is there any conceivable connection between the dark matter and matter/anti-matter problems?

Not that I know of. The matter/anti-matter problem is probably due to some kind of symmetry breaking in the early universe. The microphysics of that process could, but must not necessarily be, connected to dark matter.

A nice overview is e.g.

The origin of the matter-antimatter asymmetry

Best,

B.

Arun said...

Thanks for the link to the overview article! Sorry I wasn't very precise in my question, but you were kind enough to figure it out anyway.

Before I even read the article let me just say I had a vague notion like - if axions (dark matter candidate!) were inoperative for a while somehow then we might have strong CP violation - and that is needed for the matter/anti-matter symmetry. :)

Anyway, off to read, talk to you later, I hope!