Dark matter questions

Factual Questions
1

http://news.bbc.co.uk/1/hi/wales/south_east/4288633.stm

So scientists think that they’ve found signs of the first dark matter galaxy. It’s my understanding that dark matter is supposedly normal matter which simply does not radiate or reflect radiation, is that correct?

Seemingly, there seems to be a lot of hydrogen just floating around in this galaxy. Why is it that the dark matter has not clumped like normal matter does to form “dark stars” which would attract this hydrogen unto themselves?

I’ve a very rudimentary understanding of the issues involved here, so my post may seem to be muddled.

2

I don’t know; my initial impression, when people started talking about dark matter some years ago, was that it wasn’t ordinary matter, but something a bit more exotic - kind of ‘dark’ in the sense of invisible and not interacting with ordinary matter (except by gravity). Maybe I just got the wrong end of the stick, but it does seem like ‘dark matter’ doesn’t mean what it used to mean.

3

Even if it simply interacted with ordinary matter solely via gravitational forces, the hydrogen would still be attracted to it, wouldn’t it?

4

Heh heh, I actually know the people involved with this. The postdoc himself, Robert Minchin, looks exactly like the bearded scientist Griff Rhys Jones played in those Vauxhall adverts. The main Professor (Mike Disney) isn’t mentioned because he’s on holiday in Jordan and forbade the release of the story until he got back next week, but due to the massive media pressure (it’s even in the Sun for crying out loud! And I love their photo caption!), Cardiff Uni Physics Department just went ahead and released it anyway. Disney has quite a temper and Minchin is characteristically absent-minded with regards to social etiquette (he once, in mid discussion, wiped a bogey on Disney;s whiteboard. Disney was so furious he had to go for a bike ride to calm down.)
[quot]It’s my understanding that dark matter is supposedly normal matter which simply does not radiate or reflect radiation, is that correct?
[/quote]
No, the point is that the normal hydrogen cannot account for the gravitational ineractions in the galaxy, and between that galaxy and others: the galaxy is mainly made of something else completely (or, at least, it behaves as though it is made of vastly more stuff than simple hydrogen).

5

Looks like it’s a massive cloud (galaxy sized!) of hydrogen with no stars in it. I take that to mean that “Dark Matter” is non-radiating matter. Cold clouds of hydrogen would be very hard to see. Apparently they’ve seen these before but only due to the interaction of bright galaxies which have thrown off the gas cloud. Hit the Cardiff link from the BBC and read the paper if you want to.

I’m still a little confused by the idea of dark matter though. I was under the impression it was anything from neutrino mass to exotic baryonic particles to “stuff”. Maybe Chronos or Angua will drop by.

6

My point is why the masses of hydrogen present in the galaxy aren’t being attracted to “dark stars”. If the reason for this is that it’s impossible for a “dark star” to form, why is that the case?

Surely the scientists who found this dark galaxy could just check where the concentrations of hydrogen are present in the galaxy and infer from there as to whether there are “dark stars” in the vicinity?

7

Dark matter comes in a variety of forms.

Black holes, brown dwarfs (MAssive Compact Halo Objects, or MACHOs) and particles like neutrinos (Weakly Interacting Massive Particles, or WIMPs), plus possibly other stuff we haven’t thought of yet could all be dark matter. There’s a lot of debate about which of these is dominant, and the answer may be different in different galaxies (and regions of galaxies!)

There is a distinction between hot dark matter (where the particles have high random velocities) and cold dark matter (where the particles have low velocities.)

I’m WAGing, but if the dark matter in the “dark galaxy” is hot dark matter, the particles move fast, so they would not clump into clouds that would then collapse into stars. If the hydrogen in that galaxy is too thin, it won’t reach the critical density required to create a cloud that will collapse under its own gravity.

This, though, is just weird: “The astronomers say it is hard to study the universe’s dark, hidden objects because of the Earth’s proximity to the Sun.” Uh, yeah. That’s why we would be doing most of the optical astronomy at night.

8

Griff Rhys Jones-a-like and bogey wiper.
Furious professor who’ll go beserk when he finds out this has been released.

It’s not just hydrogen. From the bogey wiper himself:

9

I’m no expert (I did acoustics at Cardiff, not astronomy), but I gather that this is why the galaxy is so important. We don’t know what the dark matter is - whether it is normal bayonic stuff compressed into ‘dark stars’ or something else entirely.

10

Dark matter Wiki-article.

I’d be interested what the atronomers here have to say about what this “dark galaxy” signifies. If it is just normal baryonic stuff, how come it didn’t form stars like every other galaxy in the sky?

11

This stuff is beyond me. I love the Sun’s representation of what a dark galaxy looks like through a telescope, however.

12

As I understand it from the cosmology course I took at Uni, some dark matter may be compressed baryonic matter, but not very much of it. This is largely due to the fact that it is possible to estimate the amount of baryons in the universe and there just are not enough to account for the missing mass. I don’t have my notes with me right now so I can’t quote the exact figures, but from memory all of the “actual” matter accounts for about 3 percent of the total mass of the universe. The rest being dark matter and dark energy (about 27 and 70 percent each). One thing I don’t get though, is if matter and energy are interchangeable why do cosmologists differentiate between dark matter and energy.

Podkayne - Neutrinos are not WIMPS, they are weakly interacting, but neutrinos are low mass weakly interacting particles. WIMPS are a different animal entirely and can have a mass of many MeV.

13

It was perhaps a bad choice of terminology for just this reason, but dark matter and dark energy are distinct concepts. Dark matter is the unidentified stuff out there that, because it gravitates much like un-dark matter, is trying to slow down the expansion of the universe. Dark energy is whatever it is that’s causing the expansion to accelerate.
With two big unanswered puzzles like this out there, it’s obviously attractive to think there might be some connection between them, but that’s pure speculation at this stage.

14

Thanks, I stand corrected. I thought “massive” just meant “having mass,” but looks like I was wrong.

15

Having read the astro-ph article, it appears that what they’ve discovered is a massive atomic hydrogen cloud, greater than 16kiloparsecs in diameter (4.9x10[sup]20[/sup] metres). It appears to be gravitationally bound, i.e. its sitting in an underlying dark matter potential, and the spectra suggest that its moving in a way that’s not producing any shocks, so probably a ‘flat’ rotating disk.

They’re unable, even with very deep observations, to find any sort of optical counterpart to the cloud, which is unusual, as atomic hydrogen is normally associated with star formation. They argue that if it did have an optical counterpart, we would have found it by now, as it has an absolute magnitude in the B band of -19, which would correspond to a 12th magnitude galaxy, if the cloud’s at the distance of the Virgo cluster, but that said, its Hubble distance is about 30Mpc, which is also reasonably close, and would mean that we’s see an optical counterpart.

Their argument as to what’s going on is that this hydrogen cloud isn’t as dense as what we’d expect from a star forming cloud, in fact, its an order of magnitude less than the proposed theoretical upper limit for the density of a star forming cloud of atomic hydrogen., hence why we’re not seeing any stars forming.

16

Some of this has already been posted by others, but a brief primer on dark matter:

First of all, dark matter is any matter which doesn’t emit light. This, in itself, is not too mysterious: You and I are both examples of dark matter. However, we can measure the total amount of matter in a galaxy or the Universe by its gravitational effects, and based on the relative abundances of various isotopes of hydrogen, helium, and lithium (all of which were formed in the Big Bang), we can calculate about how much of that matter should be ordinary (or baryonic) matter. When we compare the two numbers, we find that there’s about ten times as much matter in the Universe as can be accounted for by baryonic matter. We don’t see any non-baryonic matter in the Universe, so whatever the rest of it is, it must be dark. Not all dark matter is weird, since we know that some normal matter is dark, too (again, like you and I), but most of the dark matter must be the weird stuff. Dark matter is loosely classified as “hot” and “cold”, which reflects how fast the particles are moving: Hot dark matter particles would travel at the speed of light, or close to it, while cold dark matter particles would be much slower. Current observations suggest that most of the dark matter is cold.

In addition to the dark matter, there’s also dark energy, which is even weirder and less well-understood. In some sense, dark energy (also called cosmological constant, quintessence, or a few other names) counts towards the total amount of “stuff” in the Universe; in fact, it appears to make up about 70% of all the “stuff” there is. Dark energy is not believed to form any sort of structures at all, but is instead believed to pervade all of space uniformly. The only reason it’s not apparent in our laboratories is that we have so incredibly much normal matter here, compared to the average in the Universe as a whole. The main difference between dark energy and matter (normal or dark) is its pressure. Gravity is produced by both energy density (most familiar to us as mass) and by pressure, but for most “stuff” (normal matter and cold dark matter), the contribution of pressure is much, much smaller than that of density. For radiation, such as light or hot dark matter, the contribution from energy density is about the same sixe as that from pressure, but both are positive, so they add up. But for dark energy, not only is the pressure significant, but it’s also negative, so the negative gravitational contribution from the pressure is actually greater than the positive gravitational contribution from the density, so dark energy produces a sort of anti-gravity.

17

I realise this, but what special property of dark matter precludes it from clumping?

18

If dark matter only interacts through gravity, it’ll have a tough time transferring momentum.

19

Excellent post. This bit caused me to wonder a question that I didn’t feel worthy of its own thread, and if it’s too much of a PITA to answer, don’t bother.

Anyone know roughly how much matter/mass/whatever the sun contains compared to the rest of the solar system, including all the planets, the asteroid belt, and that other belt of rocks of which Pluto is the most famous?

I never considered it, and if I had to WAG, I’d guess the Sun accounts for about a tenth of the matter in the solar system. Anyone have a clue, or even another WAG?

20

Actually, I believe the sun accounts for just about 99% of the matter in the solar system. Most of the rest of that other 1% is Jupiter, with the other little bits of dust we like to call planets being pretty much negligible. Dark matter only has presence on a galactic scale.