# Dark matter - observational evidence

1. Mar 6, 2012

### sri sharan

Sorry if this has already been discussed. I just couldn't find it.

So the main proof for dark matter comes from the anomalies of the gravitational interaction between galaxies and not being able to explain the near constant velocities of the rotating stars observed in a galaxy, independent of radius. However how does it directly imply that dark matter exists. Why cant be it a flaw in our theory. I see that many people are confident about its existence. Do we have any other proof for its existence other that this

2. Mar 6, 2012

### juanrga

There is not proof of the existence of dark matter but several proofs that it does not exist.

3. Mar 6, 2012

### sri sharan

For example...

4. Mar 7, 2012

### Chalnoth

Well, certainly main people have tried to use a different law of gravity instead of dark matter. But ultimately these attempts have failed.

Here is a nice blog post describing one of the stronger pieces of evidence for dark matter:
http://blogs.discovermagazine.com/cosmicvariance/2006/08/21/dark-matter-exists/

To me, even stronger is the evidence of the Cosmic Microwave Background from WMAP, as the light from the CMB was emitted before any stars or planets had formed, and yet we see the evidence of dark matter clear as day in the CMB.

5. Mar 7, 2012

### clamtrox

They claim that you can explain CMB and galaxy power spectra using modified gravity theories as well, for example in http://arxiv.org/abs/1104.2957.

6. Mar 7, 2012

### Chalnoth

Scalar-Vector-Tensor gravity is distinctly uninteresting, as it adds far, far more complexity than $\Lambda$CDM. I also have severe doubts that their model can possibly work, as it requires approximately ten times as much baryonic matter as is observed.

7. Mar 7, 2012

### Constantin

http://en.wikipedia.org/wiki/Scalar-tensor-vector_gravity
STVG theory is actually quite interesting. And it is still claimed, at least on wikipedia, that it explains all the observed phenomena.

I see dark matter as just the easy solution:
1. If there's a gravitational effect we don't understand the source of, then there's must be matter causing it. As we can't see that matter, let's call it dark matter.
2. Next get a large number of physicists to start looking for proof that dark matter exists.

Of course they'll start finding evidence dark matter exists. But that evidence can also be interpreted using Modified Gravity.
As there are much fewer physicists trying to build a Modified Gravity theory than those looking for dark matter, more glory to those few ones (if they succeed)!

8. Mar 7, 2012

9. Mar 7, 2012

### Constantin

10. Mar 7, 2012

### Chalnoth

I really, really doubt this is the case, especially for matter-poor galaxies (many low-mass galaxies are almost entirely made of dark matter, as they don't have enough mass to keep the normal matter from being expelled in their violent beginnings).

But looking back at their claim that SVT gravity fits the CMB posted by clamtrox, well, I don't buy it. First, if they don't use any dark energy, it's just plain wrong that SVT fits. They use some hand-waving argument to claim that they SVT should replicate an early universe with $\Omega_M = 0.3$ and $\Omega_b = 0.035$, but even if this is true, this statement doesn't work without $\Omega_\Lambda = 0.7$ (basically, without the dark energy component, you end up with an absolutely absurd value for the expansion rate, which is flatly contradicted by nearby estimates of the rate of expansion).

Second, I think their hand-waving argument that they can simply replace $\Omega_M = 0.3$ is just plain insufficient, and almost certainly wrong. They claim, in particular, that they can estimate the effect of cold dark matter simply as a "gravity enhancer," increasing the effective gravitational constant $G$. But this is simply false.

The thing that makes the CMB such powerful evidence for dark matter is the fact that the baryonic matter in the early universe experienced pressure and was able to bounce, while dark matter did not. So in the early universe, the dark matter and the normal matter were not in sync, and we see this degree of mismatch in the CMB power spectrum very clearly. In the paper posted by clamtrox, they simply skated over this fact and flatly assumed that the power spectrum could be replicated in detail just by picking the right amount of enhancement of the gravitational constant. I don't think this is remotely likely, and would only be convinced if somebody actually ran the simulations to demonstrate it (and these simulations were subsequently checked and validated).

There's a reason why there are many fewer: the evidence today that it is dark matter and not modified gravity is extremely strong.

11. Mar 8, 2012

### clamtrox

I have to say I don't find the current calculations compelling either, but it seems pretty interesting to see if it's possible.

12. Mar 8, 2012

### sri sharan

Thanks for that Dave, the bullet cluster observation is very convincing.On a side note, has anyone worked on a modified gravity theory to explain this(although I guess that they would have to invoke gravity at skewed angles, it doesn't really sound elegant)

13. Mar 8, 2012

### Chalnoth

Yes, but they can't seem to explain it without having extra neutrinos (i.e., some dark matter).

14. Mar 8, 2012

### Constantin

Modified Gravity theory is quite developed and its proponents have explanations for all known phenomena. Whether you accept those explanations is another matter, but they do exist. So the current observational evidence can't conclusively solve the debate.

This is the exact link to the paper about the Bullet Cluster: http://arxiv.org/abs/astro-ph/0702146 .
And that has been posted previously just below Dave's post.

15. Mar 8, 2012

### Chalnoth

I'm pretty sure they don't.

I have yet to see an explanation for the Bullet cluster that does not include some dark matter (that is, heavy neutrinos).

I have yet to see an explanation for the CMB that holds up to the lightest scrutiny.

I have yet to see an explanation for the behavior of dark-matter-rich galaxies.