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.clair.

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Message 9488 - Posted: 17 Sep 2011, 18:58:29 UTC

A news article from the BBC website
http://www.bbc.co.uk/news/science-environment-14948730
It seems to be right up our street as to what we do here, any comments ?
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Profile Benjamin Wandelt
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Message 9498 - Posted: 4 Oct 2011, 21:23:40 UTC - in response to Message 9488.  

Hi there -
Thanks for this question.

In fact this problem, namely that the "standard" weakly interacting cold dark matter (CDM) scenario predicts too many small galaxies compared to what is observed has been around for quite some time. I worked on an early attempt to resolve it nearly 10 years ago which was to include interactions between the dark matter particles. That particular hypothetical type of dark matter was called self-interacting dark matter.

At that time there was a flurry of papers with various modifications of the dark matter physics which would give the correct number of dwarf galaxies around normal, Milky-Way type galaxies.

Others attempted to explain this phenomenon in more mundane terms. Maybe the small dark matter clumps people see in simulations and interpret as places where dwarf galaxies should form, maybe these clumps do not actually harbor galaxies. In fact, during the epoch when the first stars send out their UV light and ionize the Universe the gas in the proto-dwarf galaxies could have been heated to the point of never being able to cool enough to form stars.

If this were true, we could explain the absence of dwarf galaxies by saying that there are lots of dark matter clumps around us that just never lit up because stars did not form in them.

What appears to be new in this story (as far as I can tell from this BBC article) is that there are now more detailed simulations of both cold dark matter and gas, including a model for the process of star formation and that these simulations suggest that the small dark matter clumps should in fact harbour lots of dwarf galaxies. The problem is that we don't observe them in the real Universe. So: no CDM?

In my opinion, these simulations are so complex and the details of star formation are sufficiently ill-understood that there is still no air-tight argument to disprove CDM. And CDM has gotten lots of other things right, so I am not ready to throw it out yet.

The most fascinating aspect of this for me (and the reason that I got so excited about this that I worked on these issues about a decade ago) is that this story speaks of the promise of using galaxies as laboratories for dark matter. The number of dwarf galaxies around the Milky Way and other galaxies like it suddenly is a way to learn about the particle physics of dark matter: the mass of the dark matter particle and the nature of its interactions.

So galaxies, things so large that just over a hundred years ago now one could conceive of the sheer enormity of their size, have the potential to teach us about something so small and hard to grasp that even now no one has been able to detect it or produce it in a lab.

To me, that is awesome.


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Message 9500 - Posted: 5 Oct 2011, 0:38:48 UTC

Thank you Ben for a good answer,
In some ways it is nice to know that we don`t know everything yet as it give`s us something to aim at.
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Message 9606 - Posted: 28 Oct 2011, 14:55:12 UTC

I saw an article suggesting a new explanation for PART of the dark matter, based on some of the results of the Milkyway@Home project.

Apparantly, dwarf galaxies orbiting galaxies with more mass tend to get scattered into star streams during close passes to the galaxies with more mass, and therefore become less visible.

This would mean that, in the early universe, there were more dwarf galaxies that had not been scattered yet.

I thought I had saved a link to this article, but when I looked for that link, I couldn't find it.

The article did not claim that this explained all dark matter, and did not mention dark energy at all.
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Message 9807 - Posted: 6 Jan 2012, 20:26:22 UTC

Dark matter is aptly named; it doesn't seem to interact electromagnetically. We're going to have a tough time actually finding anything of this nature, other than in a mathematical model. We'll see. I should probably be examining this very problem right now, except that I'm an idiot so instead I get to inspect dirt. Don't ask. :*(
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Message 9936 - Posted: 30 Jan 2012, 0:17:22 UTC - in response to Message 9807.  
Last modified: 30 Jan 2012, 0:17:47 UTC

A number of recent discoveries seem to be related to dark matter and dark energy in one way or another. I have reported on some of these discoveries here.

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