Help me understand Dark Matter

[No1Mensan]

I keep hearing about all the mysteries of the Universe and this is certainly one of them.

My understanding of it is that for Galaxies, the stars orbit the Super-Massive Black Hole in the centre of the Galaxy with the same angular velocity regardless of distance unlike Solar Systems where planets like Mercury would have a much greater angular velocity than planets like Neptune.

To explain this we add some Dark Matter which effectively increases the mass of everything. This is where it sounds like a square peg in a round hole. Wouldn't we need a significantly higher concentration of Dark Matter around the Stars in the outer reaches of the Galaxy so that they don't fly off into space and the inner ones don't get sucked up by the Black Hole?

 

[SagansHeroes]

I don't know an exceptionally large amount about dark matter. But basically the calculations for how much gravity would be needed to keep galaxies together compared with how much gravity there should be (using actual reading from matter in them) is significantly lacking. Although tbh until kind of recently we were rather guessing with planets, as we find more and more maybe it will help bring the effect of dark matter down... but it has been seen through "gravitational lensing".. that is gravity bends light, and we can see light bending around area's where there is no matter.

 

[sgrunterundt]

I keep hearing about all the mysteries of the Universe and this is certainly one of them.

My understanding of it is that for Galaxies, the stars orbit the Super-Massive Black Hole in the centre of the Galaxy with the same angular velocity regardless of distance unlike Solar Systems where planets like Mercury would have a much greater angular velocity than planets like Neptune.

To explain this we add some Dark Matter which effectively increases the mass of everything. This is where it sounds like a square peg in a round hole. Wouldn't we need a significantly higher concentration of Dark Matter around the Stars in the outer reaches of the Galaxy so that they don't fly off into space and the inner ones don't get sucked up by the Black Hole?

It is not just the velocity distribution, but also the absolute magnitudes of the velocities. The mass of galaxies calculated from the orbits of the stars (and other galaxies for that matte) turns out to be much higher than the mass we see, so we conclude that there is a lot of mass that we don't see.

The assumption is that this dark matter, interacting much less than ordinary matter, is spread out much more than the stars and gas which cluster in the center.

Although tbh until kind of recently we were rather guessing with planets, as we find more and more maybe it will help bring the effect of dark matter down.]

Planets are utterly insignificant in this respect. Brown dwarfs might not be however.

 

[SagansHeroes]

Planets are utterly insignificant in this respect. Brown dwarfs might not be however.

Ah ok, well I just thought if you took our solar system as a standard one for example, we have 8 planets, 100's of moons, an asteroid belt, the Kuiper (icy/comet) belt, then the Ort cloud... that all that matter's gravity would contribute, and seeing as we can only see stars/gas in other solar systems/galaxies for the moment that might account for a portion of it.
But that's just my thoughts, I have no real idea as to the whole dark matter/energy situation, just some background info and am happy to find out more.

 

[Nelson]

Ah ok, well I just thought if you took our solar system as a standard one for example, we have 8 planets, 100's of moons, an asteroid belt, the Kuiper (icy/comet) belt, then the Ort cloud... that all that matter's gravity would contribute, and seeing as we can only see stars/gas in other solar systems/galaxies for the moment that might account for a portion of it.

There are a few problems with this. One is that, although we can not resolve all of the planets in the galaxy, we would still be able to detect large scale effects if there were the number out there that you are suggesting. There would be a huge infrared excess of light everywhere we looked that could not be explained by reasonable amounts of dust and gas alone.

The reason for this is the shear amount of planets that would be necessary to explain the rotation curves.

The ratio of DM to regular matter is about 4:1 universally and somewhere around twice this within galaxies. To generate this ratio would require you to assume that the average number of planets around each star in the galaxy is ~8,000 Jupiter masses worth. Keep in mine, our solar system has < 2 Jupiter masses worth. Each star needs to have about 9x it's own mass in planets. This sort of hypothesis does not fit with the results of any planet search to date, and we have surveyed a large enough portion of the sky that the statistical likelihood of this actually being the galactic average is so small that it is not even worth considering.

While people usually like to focus on the rotation curve because it is the most intuitive bit of evidence for DM, it is by no means the strongest. I would suggest you read the wikipedia page: http://en.wikipedia.org/wiki/Dark_matter. There is a whole section on observational evidence, and the page is filled with references to the original journal papers on the subject.

The strongest case against it being any sort of planet or asteroid is that current big bang nucleosynthesis models predict very specific primordial ratios of the elements, and these models are wildly successful, but only after incorporating in the observed fraction of dark matter. Dark matter is not just regular matter that is hard to see, it is fundamentally different. It doesn't interact through the electromagnetic force at all, light waves just pass through it without even noticing.

So that is what we know about DM so far. It is the best explanation for what we observe about the universe. People have been working really hard on this stuff for 30 years now. I assure you, you won't be able to come up with a quick solution that no one else has thought of yet.

If you are interested on catching up on the problem, you only have a few papers to read through...
http://arxiv.org/find/all/1/ti:+EXACT+dark_matter/0/1/0/all/0/1
(arxiv search for the exact match "dark matter" in the tittle)

 

[SagansHeroes]

So that is what we know about DM so far. It is the best explanation for what we observe about the universe. People have been working really hard on this stuff for 30 years now. I assure you, you won't be able to come up with a quick solution that no one else has thought of yet.

Oh don't get me wrong, I wasn't saying this was in any way a solution or a "ha, I proved dark matter isn't real" sort of thing. I was just thinking that there MIGHT possibly be a slight re-calculation (again this is without knowing the nitty gritty details). I know that there is supposed to be ~12% dark matter to the ~4% matter we know and observe and I wasn't thinking/expecting planets/asteroids etc. to make much of a dent... I was just throwing ideas out there, more or less in that it might make a slight change in the ratio's... slight as in like hundredths to thousandths of a percent, which may or may not impact the future thinking on dark matter... or maybe it has no baring what-soever... We just haven't known much about exo-planetary systems until recently, and are learning more and more as technology improves.

But yeah, I'll check out some of those papers over the weekend... Just stayed up to watch NZ painfully tie their way out of the world cup and then off to work tomorrow >.<.

Cheers for the in depth info.