What Is Dark Matter and How Do We Know It Exists?

[LSulayman]

There is a picture on this site that I don't understand.

http://csep10.phys.utk.edu/astr162/lect/cosmology/darkmatter.html

It is used as evidence that dark matter exists, but what is it? Is it a MACHO, a WIMP or something else?
Can someone also explain what a MACHO or a WIMP is without using much astronomical terms?

 

[Calimero]

What exactly you don't understand, because it is well written, and there is no point repeating it all here? That cloud should disperce by now if it is held only with visible matter (luminous mass)

 

[LSulayman]

What exactly you don't understand, because it is well written, and there is no point repeating it all here? That cloud should disperce by now if it is held only with visible matter (luminous mass)

So that cloud is just hot gas? Isn't it something like a MACHO? My teacher told me that that picture wasn't hot gas, but something else and I'm trying to find out what.
Well then, can someone explain what a MACHO or a WIMP is anyway without using much astronomical terms?

 

[fillipeano]

From my understanding, WIMPs (Weakly Interacting Massive Particles) are theoretical/hypothetical particles that are thought to explain dark matter. They interact through the weak nuclear force and gravity. I don't know much about them, only what I've read about possible solutions for dark matter.
As far as I know it's still hypothetical.

 

[Calimero]

So that cloud is just hot gas? Isn't it something like a MACHO? My teacher told me that that picture wasn't hot gas, but something else and I'm trying to find out what.

Yes, it is x-ray image of hot gas in false pink color.

MACHO is massive compact halo object, which suppose to explain dark matter as being ordinary matter that emits little, or no light. MACHOs can be detected only through gravitational lensing, because they should be very dark, so that cloud, for sure, is not MACHO.

 

[Chalnoth]

That link seems very old. It looks like it's quoting the status of the field from the mid-90's. This is a rather more recent blog post regarding one particularly striking piece of evidence for dark matter:
http://blogs.discovermagazine.com/cosmicvariance/2006/08/21/dark-matter-exists/

 

[DevilsAvocado]

https://www.youtube.com/watch?v=mRtGUCLjQ3w

https://www.youtube.com/watch?v=2DoPAeU3a6Y


http://en.wikipedia.org/wiki/Bullet_cluster"

 

[Chalnoth]

That link seems very old. It looks like it's quoting the status of the field from the mid-90's. This is a rather more recent blog post regarding one particularly striking piece of evidence for dark matter:
http://blogs.discovermagazine.com/cosmicvariance/2006/08/21/dark-matter-exists/

Since this thread was resurrected, I thought I'd point out that soon after I posted this, Sean Carroll made a new post regarding the evidence for dark matter, in response to some bruhaha in the popular media:

http://blogs.discovermagazine.com/cosmicvariance/2011/02/26/dark-matter-just-fine-thanks/

As a side note, we had a journal club where we talked about the paper referenced in the blog post, and it looks like the author made some bad assumptions with respect to the behavior of MOND, and it is basically pure accident that the data line up with the MOND prediction. The problem is that the author assumes that the rotation curve of galaxies is flat, but not only do the data not support this, but MOND doesn't predict it. So their use of one single parameter is just invalid, and will bias the results dramatically depending upon how the parameter is selected.

There's also the annoyance that there is a plot with a line that is labeled "$\Lambda$CDM", but that line simply isn't a prediction of $\Lambda$CDM in any way, shape, or form.

 

[Nabeshin]

As a side note, we had a journal club where we talked about the paper referenced in the blog post, and it looks like the author made some bad assumptions with respect to the behavior of MOND, and it is basically pure accident that the data line up with the MOND prediction. The problem is that the author assumes that the rotation curve of galaxies is flat, but not only do the data not support this, but MOND doesn't predict it. So their use of one single parameter is just invalid, and will bias the results dramatically depending upon how the parameter is selected.

There's also the annoyance that there is a plot with a line that is labeled "$\Lambda$CDM", but that line simply isn't a prediction of $\Lambda$CDM in any way, shape, or form.

We also had a journal club type situation about the paper, and were similarly annoyed with the mysterious $\Lambda$CDM line on the key graph. Even within the context of what the author assumes about $\Lambda$CDM he is making some very bad assumptions. For one, he assumes that the ratio of baryonic matter to dark matter in the clusters is equal to the cosmological ratio, roughly 0.17. This is quite bad since we know that the ratio is markedly higher, especially for small galaxies (which is precisely the data he is considering! Masses ranged from 10^8 to 10^11 solar masses). For some of these, we know the ratio can get as high as 1:100 in favor of dark matter.

Since we don't really know that much about small galaxy formation, I'm not terribly surprised that the $\Lambda$CDM line isn't that great of a fit for the low mass galaxies! And yes, this is in a regime where you can make MOND work but my understanding is as you go to more massive galaxies MOND starts being dramatically wrong.

 

[Chalnoth]

Since we don't really know that much about small galaxy formation, I'm not terribly surprised that the $\Lambda$CDM line isn't that great of a fit for the low mass galaxies! And yes, this is in a regime where you can make MOND work but my understanding is as you go to more massive galaxies MOND starts being dramatically wrong.

It also becomes even worse for clusters, and, of course, cannot explain the CMB anisotropies or things like the Bullet cluster.

I should note that he does mention that the $\Lambda$CDM line isn't the prediction, but claims it is a fine tuning problem for the theory to exactly reproduce what just falls out of MOND. The problem is, that line isn't a prediction of MOND, because galaxy rotation curves aren't flat even in MOND, and the data points themselves are bogus because they depend upon how the "flat curve velocity" is determined!"