Dark Matter - Bunch of Questions

I have a few naive questions about dark matter that I hope someone can shed some light on for me. I’ve checked a bit around the net and these questions either weren’t answered or were answered vaguely. And, besides, the SDMB knows everything.

  1. If a lot of dark matter were baryonic, would it not tend to coalesce (under the force of gravity) to eventually form stars (and thus become visible)?

1a. Is there any evidence, say, for the existence of populations of young stars living in otherwise apparently dark and empty regions of space?

  1. Regardless of the nature, or natures, of dark matter would it not tend to coalesce under gravity to form black holes; so many black holes in fact that signs indicating the presence of BH’s would have to be pretty common (after all, there’s what, maybe ten times as much dark matter as regular matter?)

  2. Do we see evidence of dark matter falling towards other objects (or vice versa) by giving off radiation as it heats up during the fall?

As always, thanks.

For those of us interested in the question, but have no idea what you’re talking about, could you explain what dark matter is?


Dark matter is a catch-all phrase to describe any matter which do nor shine with a light of their own (stars, quasars, nebulae) or reflect light from a nearby source (planets, asteroids, comets). This ranges from fundamental particles which MAY have mass, like neutrinos, to rogue planets drifting between stars and black dwarves: stars which have burned out and cooled into a frozen cinder.

One candidate for dark matter is simply clouds of hydrogen (ionized hydrogen, actually–basically protons). While I don’t know much about the dynamics, I would guess that a diffuse cloud might not self-attract enough to coalesce when it also has gravitational forces from distant objects pulling it apart. I don’t really know about that, though.

In any case, another major (baryonic) dark matter candidate is brown dwarfs–objects a little heavier than Jupiter, but not large enough to begin fission, in which case they wouldn’t be easily detectable. There are some major ongoing surveys to look for brown dwarfs in the dark matter “halo” around our galaxy (mostly by looking for how they gravitationally distort light from objects behind them).

I don’t know the particulars and don’t have any references with me at the moment, but I think that various sizes of black holes have been pretty constrained as major dark matter constituents.

I’m not sure that gravitational acceleration is significant enough to result in radiation for most baryonic dark matter candidates. The existence of dark matter is inferred from a few major things:

[li] Dyanamics of galaxies (objects in the disks of galaxies behave as if they are embedded in a “halo” of non-luminous matter, and simulations show that spiral galaxies would not be stable without it).[/li][li] Dynamics of clusters of galaxies (again, galaxies in clusters behave as if there’s more matter there than what is seen).[/li][li] Universe geometry (there is fair evidence that the universe is flat, and the luminous matter is only about 1% of what’s necessary to make it so).[/li][/ul]

Whether a cloud of hydrogen (I’m approximating that all baryonic matter is hydrogen) will coalesce depends both on its mass and its temperature. It’s quite possible to have stable clouds of hydrogen, and many of these have been observed. Of course, since they’ve been observed, they’re not dark matter. Most baryonic dark matter probably has coalesced, to brown dwarfs or smaller objects. On the other hand, the relative abundances of hydrogen, deuterium, lithium, and helium 3 and 4 in the Universe tells us a lot about the number of baryons in the Universe, and there’s not nearly enough to account for all of the dark matter, so there’s definitely some non-baryonic component.

As for black holes, the densities involved are far, far, too low to collapse to holes. Sure, there’s a lot of the stuff, but space is deep, Excellency. Dark matter, whatever it is, seems to be pretty evenly distributed, not clumped up like bright matter so it can form holes. This doesn’t necessarily say anything interesting about DM: The main reason that bright matter is bright in the first place is that it is clumped.

Primordial black holes (formed directly in the Big Bang, not produced from other matter) have been proposed, and we can’t rule them out completely, but most models for primordial black holes would produce big holes and small ones, both, and small black holes can make themselves very noticeable via Hawking radiation.

Thanks all. That was illuminating. Seriously!

Chronos is right (as usual). Most “clouds” of hydrogen that we can detect are too hot to collapse. Their internal temperature provides enough pressure to resist the gentle tug of gravity. Only very cold clouds (a few tens of degrees above absolute zero) have much of a chance to build stars. And even then it depends a lot on the internal structure of the cloud. If it is very homogeneous, then most of the molocules are tugged in every direction equally. Only the edges of the cloud would have more gravity tugging them toward the cloud center. And by no coincidence, it is the edges of the cloud that tend to have the highest temperatures and therefore tend to maintain their distance. So most gas clouds are pretty stable.

One thing that can change this equation is if another galaxy passes through. (Some say “collide”, but that’s misleading because when two galaxies come together, there are very few literal collisions between stars. They pretty much just pass through each other.) When that happens, the gravity of the passing galaxy will disturb and set up shock waves in the gas clouds. These shock waves consist of areas of higher density surrounded by areas of lower density. These (slight) density differences is all that a cold gas cloud needs to start collapsing and forming stars.

Many of the so-called “star burst” galaxies which feature tremendous swaths of star formation have “recently” interacted with another galaxy.

Near-by super novas can also trigger star formation, although on a smaller scale. In this case it’s the shock wave caused by outward flow of material instead of gravitational disturbance.

P.S. - a collapsing gas cloud will not directly form black holes (at least not at this point in the universe’s evolution). When the density and pressure of a coalescing spot reach a certain point, nuclear fusion starts which gives off energy and heats up the surrounding areas of the cloud. It isn’t until the star has used up most of it’s hydrogen fuel and goes through the collapse-explode cycle of a supernova that the density in the middle of the star is high enough to form an event horizon.

I always had the impression that dark matter meant “We don’t know why the universe behaves the way it does and current theory says that to behave this way there has to be lots more matter. We don’t see lots more matter, so there must be some stuff that we’ll call ‘dark matter’ that has special properties like not being obervable.” Kind of a catch-all/fudge-factor like the Lambda cosmological constant was originally intended to be. This definition makes more sense than what I had read previously, and seems less like grasping at straws.

So what’s the deal? Am I just “people unclear on the concept”, or did I accidentally read some sci-fi rag and remember it as Scientific American?

I agree, Joe. That was my impression, and there have been two theories regarding the dark matter: WIMPs and MACHOs. I’m sure Chronos and can fill in all the details.

Thanks, Joe_Cool… I knew there was something I was forgetting to mention before!

It turns out that most of the “dark matter” in the Universe is, in fact, lambda. Approximately 70% of the “density” of the Universe (as it relates to curvature, at least) appears to be due to the Cosmological Constant. Of course, you can call lambda modification of the laws of physics or a weird “substance”, but it doesn’t really matter what you call it; the point is that there’s something going on there that we can’t see directly.

Of course, not all of the “dark matter” is explained by lambda: There’s definitely a good bit of perfectly ordinary matter in the Universe that just happens to not be very bright. It’d be very surprising if all matter were visible from Earth.

IMHO, ‘Dark Matter’ is a fiction. It’s existence is necessitated because it has been ‘discovered’ that there is not enough matter in the universe to reverse the ‘big bang’. Since the ‘big bang’ is the current MOST favorite theory of the beginning of the universe, and it is not believed that the universe will expand infinitely, there must be some ‘stopping point’ which, when reached, will cause the universe to contract again into a, what, ‘little point’?

Unfortunately for this theory, scientists have been busy weighing all of the matter in the universe (this is quite a job since they don’t have the proper equipment and haven’t actually been ‘out there’), and have found it to be insufficient to prove the theory.

‘Dark Matter’ to the rescue. By postulating something which cannot be seen or measured (or proven), and assigning ‘weight’ to this substance, the aforementioned scientists can now justify the theory that the universe is indeed, NOT infinite, and will at some point (5 or 10 years from now) begin shrinking again.

IMHO, ‘Dark Matter’ will no longer exist, except as obsolete theories in 10 to 15 years time.

Interested readers of this post may also replace the term weight with the term mass, if they so desire.

Basically, IMHO (again), ‘Dark Matter’ was invented so scientists won’t have to admit that they don’t know everything (which we already knew :)).

Sure dark matter has some cosmic implications that physicists and mathematicians alike enjoy playing with, but it also has direct and measureable effects on the orbits of the stars and gases in our very own galaxy.

The universe is flat? :confused:

Yeah, what’s with the “fair evidence” that it’s flat? I thought most scientist were riding the fence on this one.

This isn’t really why dark matter was thought up. The geometry of the universe looks to be flat, but there isn’t enough visible matter in the universe to make it that way. Dark matter was used to explain the discrepency. Also, there is a bit of a mystery about what holds galaxies together. There isn’t enough visible matter to do it. In this case, there certainly appears to be some dark matter at work.

As Chronos said, the new data about lambda shows that the universe doesn’t need all that extra matter to have the observed geometry. However, with the issues concerning galaxies I doubt physicists will do away with dark matter entirely. I don’t think it will be very exotic, just some brown dwarfs and rogue planets probably.

Oh, please. The last thing scientists want to do is to pretend they know everything. The day they discover everything about the universe is the day they lose their jobs! “Dark matter” was a name given to a mystery, fully acknowledging the fact that it was a mystery that needed to be solved.

Whatever happened to the WIMPs versus MACHOs theories? And, as in Squink’s link, the (what I thought) is the newest theory: HALOs? Are they no longer considered valid theories as to the missing matter?

Thanks, Chronos. I’m still confused, but in a slightly different direction now. :slight_smile:

Urban1’s comments pretty well sum up what I have always heard and read about Dark Matter. Kind of an astronomical circular reasoning, using the theory to prove itself:

Scientist: The universe can’t expand forever, but we don’t detect enough mass for gravity to stop it. So there has to be lots of Dark Matter for the universe to have enough mass to stop expanding.
Regular guy: Why can’t the universe expand forever?
Scientist: Because there’s too much Dark Matter for that to happen.

Whoever said that the Universe can’t expand forever? Given the current data, we’re almost certain that it will. Even before the discovery of the cosmic acceleration, the prevailing theory was that the Universe would expand forever, but at an asymptotically decreasing speed: If there’s no cosmological constant or other such weirdness, then a flat Universe implies infinite, asymptotically-slowing expansion. In turn, the reason that many physicists believed that the Universe was flat in the first place is because that’s predicted by the Inflationary Model, which correctly accounts for many other observations. In addition, the latest observations of the Cosmic Microwave Background seem to also support a flat geometry for space. To sum up: Inflation and various observations require that the Universe be flat. A flat universe implies a bunch of stuff (of some sort) that we can’t see. We call that stuff we can’t see dark matter. You might say that dark matter was invented as a way for physicists to admit that we don’t know everything.

I haven’t heard anything recently about any of the WIMP theories, but MACHOs definitely exist. The question now is just how many of them there are.

By the way, don’t worry too much about the meaning of space being “flat”. Basically, it just means that the Euclidean geometry you learned in high school works.

Actually, until recently, most big-bang adhurants wanted the universe to collapse back into itself. For purely aesthetic reasons (I 'spose I should look up that word. I 'spose I won’t.)

It is direct observation that got in the way of that. The “mysterious” dark matter was hypothosized to make the theory agree with observation. It’s not circular.

FYI, current theory does not predict the universe’s gravitation stopping the expansion. The universe will continue to expand forever. So I’ll re-work the above discussion:

Scientist: The universe will expand forever, but just barely. But we don’t detect enough mass for it to be “just barely”. So there has to be lots of Dark Matter for the universe to “just barely” expand forever.
Regular guy: Why can’t the universe expand forever by a whole lot?
Scientist: Because there’s too much Dark Matter for that to happen. HA HA! Just kidding. We astronomers are a wild and crazy bunch, aren’t we? Actually, now that we are measuring distances by two methods - redshift and super nova brightness - the observed expansion rate indicates that the universe will “just barely” expand forever. That is of course assuming that the basic mathematical framework of the big bang theory is an accurate model of the our universe’s birth. If it’s not, then we’re in big trouble because big bang is by far the best theory we have that explains the thousands of observations that have been made to date, from background microwave radiation to the relative amounts of hydrogen and lithium that we’ve measured.