<font size=-1>[ This Message was edited by: asciirock on 2003-01-26 10:26 ]</font>

It's not just distant bodies. It's also nearby galaxies, asciirock.

<font size=-1>[ This Message was edited by: asciirock on 2003-01-26 10:27 ]</font>

Pretty much.

Is the deferential in stellar orbits in galaxies all inclusive? Does every galaxy show a need for a halo of dark matter to explain the speed of rotation? And is it consistant across all samples taken? If that's true, then it includes our own Milky Way, does it not?

In fact, since it's easier to measure the rotations of nearer galaxies than farther ones, then the evidence for dark matter should be even clearer nearby than far away.

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...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

How about JS or someone giving a good rundown on just what observational evidence there is for the existance of dark matter? I know of galaxy rotation (above), and the motions of galaxies in clusters. What else?

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...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

Dark matter:

1) EVERY Spiral galaxy that has had a rotation curve meausured exhibits a flat rotation curve out to the edge of the galaxy. This curve actually continues out farther than even the visible light as it can be tracked with 21 cm emmission features. Because we don't see the rotational velocities slowing down in spiral galaxies, we know that there is some sort of "dark matter" interior to the rotational velocity as what is expected simply by Kepler's Law (or the virial theorem, more precisely).

Right now, we know that a rapidly spinning disk, like we see in the spirals, is stabalized by a spherical halo. This means that we could have a uniform density halo of dark matter that extends all the way around the galaxy. If you meausure the motions of the orbits of Globular Clusters and other halo objects you find that they are moving fast enough to account for Dark Matter too.

2) In Elliptical galaxies and in clusters of galaxies, a velocity dispersion is calculated and the width of this dispersion acts similarly to the rotational curve of the spiral. This width represents the virialization and is calculated out to a given radius. Again, the amount of mass that is observed interior to this is way too high. It looks like there is some dark matter in ellipticals and in clusters too. Unfortunately, this doesn't tell us anything about the density profiles: just how much stuff we expect to see.

3) X-Ray Emmission in clusters. In case you're a MOND fan, you now have to deal with this piece of evidence. X-ray studies of hot cluster gas show evidence for how much mass is in the cluster. How? By realizing that it is gravitational infall that heats up the cluster gas. There are other sources of heat too (radiative transfer, among other things), but astronomers take these into account carefully and find that the hot cluster gas predicts a similar amount of dark matter. This is independent of rotation curves and isn't well accounted for by MOND. However, there's a bigger whale of a problem for mond.

4) Gravitational Lensing. Strong and weak lensing of clusters gives us a completely independent measurment of the masses of cluster. Such measurements in the last few years have finally piled up enough to give us a statistical sampling of clusters. What is found is that we have the same amount of mass as was predicted by the rotation curves, velocity dispersions, and X-Ray emmission. This is completely independent of any velocity or Kepler's Law or virialization argument, so it's very hard for me to see how MOND might approach this. It looks like the Dark Matter is very real and is very problematic.

How do we know that the Dark Matter isn't just some extra baryons that we just haven't detected? Well, first of all, we know how many baryons to expect from Big Bang nucleosynthesis models. The parameter Omega_b can be determined using various constraints that are given us by the CMB, the abundance of light elements, large scale structure, and the Hubble Constant. There does appear to be some baryonic matter missing, but not much. at the very most it looks like we might see half of the baryonic matter (but it looks like we've found more and more of it in the form of ionized intra-cluster gas and other components). In any case, right now it looks like roughly 90% of all matter isn't accounted for. This means that there must be a component of the dark matter that is non-baryonic.

Moreover, we know it's not neutrinos now because we have an upper bound on their mass from various neutrinon observatories that observed the transformation of neutrinos from one flavor to another.

What does this leave us? Not much I'm afraid. There is the primordial black hole theorem and the gravitino idea, but BHs have the problem of generally causing a lot of havoc wherever they go (and we don't see the havoc) and gravitinos and other weird non-baryonic massive particles have some problems messing up the universe at Early Times and causing disasterous consequences for the creation of structure or the development of the universe as we know and love it.

An encouraging candidate is the so-called axion or the lightest supersymmetric particle. This particle is supposed to be stable, would not interact with light or baryonic matter, and would be fairly difficult to detect other than through sUSY-breaking coupling that makes neutrino cross-sections look downright jagnormous. Needless to say, no one has yet seen this particle which they say might be the particle that makes up most of the mass of the universe. It is by far the best theory that we've had yet as to what on Earth this stuff is, but it's still anybody's guess. We're pretty sure that MACHOs (MAssive Compact Halo Objects) are out from various considerations {including lensing surveys) that are trying to look for their effects that should be visible. We're also having some issues with WIMPS (Weakly Interacting Massive Particles), which is basically what the axion is, in getting it to behave with our Big Bang Cosmology. There are problems of "cuspy" distributions and galactic substructure. Simulations of dark matter don't match observations. Yes, it's alternatively a mess or progress is being made. So, let's summarize.

1) Dark matter is everywhere. It's probably around you in some fashion, not interacting.

2) It looks like Dark Matter was the "seeds" for galaxy and cluster and structure formation. That is, the Dark Matter clustered around the density peaks and allowed the baryons, after they decoupled, to fall into these halos and form the pretty, pretty galaxies we know and love today.

3) We have not found the Dark Matter.

4) We're further along on this problem than on Dark Energy which is a real whopper of a mystery.

Hope this helps!

Thank you JS! I knew you wouldn't let us down. That was a most thorough rundown. [img]/phpBB/images/smiles/icon_smile.gif[/img]

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...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere