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Message 2873 - Posted: 19 Sep 2007, 20:37:32 UTC

Hi Ben,

as a Seti-cruncher I just wondered if Dr. Kevin Douglas works on GALFA and SETHI are also used/helpful to check/verify what we calculate here?

I doug out a post of Kevin on a galaxie at high velocity...
Velocity is one point we're interested in here also, right?

Do we use any data from Arecibo here?


mic.


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Message 2880 - Posted: 20 Sep 2007, 0:38:21 UTC - in response to Message 2873.  

Hi mic -

Here at Cosmology@Home we are mostly interested in the global properties of the Universe, while even the extragalactic work done at Arecibo tends to be very "local" (well, at least on cosmological scales).

Still, some work that is done there will open up new ways of probing the Universe on large scales. For example their ALFALFA survey is detecting neutral clumps of hydrogen gas in the (relatively) nearby Universe. Future instruments like the Square Kilometer Array, would be able to do the same for a large fraction of the observable Universe.

By the way, it's not like I am only interested in superlarge scales - I have actually done some research on the relation between the physical properties of dark matter particles and the density profiles of galaxies.

So, I just took your message as an opportunity to look at their website and found something quite interesting. One of the surveys going on at Arecibo is called ALFALFA. That team have found that there seem to be some hydrogen clumps that do not have a known optical counterpart. In plain English, they look for star light coming from the direction of the hydrogen clumps they find and for some of them they do not find any light. This is interesting, because hydrogen clumps are normally associated with galaxies. For example, our very own Milky Way also harbors neutral hydrogen.

This is very interesting for a cosmologist, for the following reason: in the standard model of how structure forms in the Universe, dark matter is the main clumpifying agent (not a technical term, but I think it gets the point across...). For example, all galaxies we see sit inside dark matter clumps. These dark matter clumps actually extend beyond the edge of where the stars are in the galaxies, so the dark matter clumps are called "halos". So far, with only one exception, the only way of finding dark matter halos, has been by virtue of the fact that they usually harbor (one or more) galaxies whose starlight we can see. For the aficionados: gravitational lensing is the exception.

If Arecibo is finding dark hydrogen clumps without galaxies, then "bingo" - you have a new way of finding dark matter halos, because you need dark matter to produces the gravitational force that keeps the hydrogen clump from flying apart (and therefore clumped!). All kinds of questions ensue: why do some dark matter halos apparently not have galaxies in them? Can we find dark matter halos this way that are much smaller than those that harbor galaxies? Can this help us learn about the smallest scale density fluctuations in the Universe? Hmmm, sounds like we are talking about cosmology again... :)

All the best,
Ben
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Kenneth Larsen
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Message 3122 - Posted: 5 Oct 2007, 9:28:59 UTC

Hi Ben,

would this disovery throw the Modified Newtonian Dynamics theory out the window, or am I missing something? If you have a dark halo without visible matter, the gravity would only be coming from the dark matter, right? Or was the MOND theory even realistic to begin with?

Interesting thread!
Kenneth
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Message 3313 - Posted: 18 Oct 2007, 10:20:19 UTC - in response to Message 3122.  

Hi Ben,

would this disovery throw the Modified Newtonian Dynamics theory out the window, or am I missing something? If you have a dark halo without visible matter, the gravity would only be coming from the dark matter, right? Or was the MOND theory even realistic to begin with?

Interesting thread!
Kenneth


Yes, that's right!

MOND advocates would probably say that since there is hydrogen in the hydrogen clumps, it's not completely free of normal matter...

I wouldn't want to start a flame war, but I would say that MOND by itself is not really a complete theory, but a hack of Newtonian dynamics (ND) that gives surprisingly good fits of a bunch of galaxy rotation curves. For example there is no way to predict the cosmic microwave background anisotropy using MOND. You need general relativity, which reduces to ND on small scales (much smaller than the size of the Universe) and for weak fields (far away from black holes or similar compact objects).

To me MOND seems to have some pretty glaring logical flaws, but I should confess that this is not something I have spent a long time thinking about.

There have been attempts (e.g. by Bekenstein) to embed MOND in a proper relativistic theory, ie a theory that can be applied to the whole Universe and that reduces to MOND similar to the way general relativity reduces to ND. This allows checking the predictions for the cosmic microwave background as well. Bekenstein's theory has several other properties which can be tested - e.g. the fundamental constants should vary with time. The comparison to actual observations is ongoing.

All the best,
Ben



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Forums : Cosmology and Astronomy : Does Seti or Arecibo help us here?