Dark Matter

What exactly is dark matter in laymans terms if that is possible?

Dark matter is matter that cannot be visibly here on Earth seen but is theorized to exist in some theories of the ultimate nature/destiny of the universe. For example, a very dim or distant star would be an example of dark matter, as would just about any planet outside the solar system. There are also other forms of theoritical “exotic” dark matter, which is why it sometimes shows up in science fiction as something more grand than dark matter really is.

You see if there is sufficient matter in the universe than eventually the universe will recollapse on itself. However, if there isn’t it will expand forever.

There’s more to dark matter than its effect on the ultimate fate of the universe. We can detect that the dark matter is there because of its effect on the rotation of galaxies. See Is dark matter theory or fact? in Scientific American’s “Ask The Experts” online feature.

It’s still an open question of exactly how much is out there, and the answer to that question will tell us the fate of the universe.

This may be incredibly stupid, but why couldn’t some of the unexplained mass be something simple like a gas, for example oxygen. I mean we wouldn’t see it with our telescopes right? It wouldn’t give off elecromagnetic radiation would it? I am not proposing this or anything. I would just like to understand better.

Actually, a cloud of oxygen (or any other molecule) would be easily detectable, in that it would result in absorption lines in the spectra of objects seen through the cloud. Heavy elements (i.e. anything above helium, from a cosmologist’s perspective) are also pretty much ruled out by their comparative rarity in the universe. Absorption from hydrogen clouds is in fact seen quite a bit.

Really it all comes down to a balancing act. It would be nice to say that “all dark matter could be ionized hydrogen” or “all dark matter is brown dwarfs” or “all dark matter is supermassive black holes”, except that none of these works in the theoretical calculations, and people are just trying to find a combination that works (made even tougher when many are reaching to “exotic” non-yet-observed particles as possibilities). AFAIK, currently it’s thought that baryonic matter (stars, gas clouds, etc–the kind of stuff we’re fairly familiar with, built out of protons and neutrons) can’t make up more than about 10% of the “stuff” in the universe (assuming a flat universe). And if you have some new hypothesis of just what the mix of baronic and non-baryonic dark matter might be, it has to stand up to theoretical dynamical calculations and comparison to what we see. For example, neutrinos enjoyed a period of favored status as dark matter candidates a couple decades ago; there were a lot of problems that a “heavy neutrino” could solve. But the computer simulations just didn’t match up with what was observed, so the idea was fairly discredited even before a pretty good upper limit was placed on neutrino mass (a couple of years ago).

Forgive me if I’m not completely up with the current details; I’ve read some recent review articles, and about the only impression I get is that pretty much everyone has their own opinion, and only very vague limits can be placed on various types (luminous and non-luminous baryonic and non-baryonic) of dark matter candidate. As with most major topics in cosmology, it’s still a topic of incredible debate.


Why do they assume that the universe is flat?

I’ve wondered, occasionaly, wether some of that matter might just be rocks and the like floating around.

Any reason why not ?

Well, to get a feel for what kinds of dark matter candidates there are, it probably helps to know what the evidence is…

The “missing mass” problem was first brought up by an astronomer named Fritz Zwicky back in the 1930s. Basically he examined clusters of galaxies, and discovered that given their relative velocities and the amount of luminous matter the cluster should fly apart. In fact, the amount of matter he could see was only about 10% of what was needed to keep the cluster together. Other clusters were examined by other astronomers later, and the same problem found.

Then in the 1960s, Vera Rubin and [somebody] Ford realized that their spectrograph, which they had been using for redshift surveys, was sensitive enough to detect the difference in rotation velocities in different parts of a galaxy. That is, on one side of a galaxy seen edge-on the stars and such would be redshifted (headed away from us) relative to the center of the galaxy, and on the other side they would be blue-shifted (headed towards us). By measuring the precise velocities Rubin and Ford could examine the dynamics of galaxies. What they expected to find was that away from the core of a galaxy the velocity would fall off with the inverse 3/2 power of the radius. This is expected in any situation where an object is orbiting a central object with little else around (so to speak); this is the relationship, for example, seen in the solar system.

Instead, Rubin and Ford found that the rotation curves were “flat”; that is, away from the core of the galaxy the velocity stayed roughly constant with increasing radius, out as far as luminous matter extended. This was, to say the least, surprising. At roughly the same time, computer simulations were being done by Jim Peebles and a couple of other physicists at Princeton. They discovered that if matter was distributed in a galaxy in the manner of just the observed luminous matter, the galaxy would fall apart (e.g. no pretty spiral arms) on a (relatively) short time scale. But if the luminous matter were embedded in a sphere, or “halo”, of non-luminous matter, it was stable and would remain as we see. Eventually someone put the observations and the calculations together, and it became fairly obvious that there was, in fact, something else out there.

So then the question becomes, is the dark matter “normal stuff” like we’re familiar with, or is it something more exotic? One limit on the amount of baryonic (protons, neutrons) dark matter comes from nucleosynthesis calculations, i.e. calculations of just what could have (and should have) come out of the big bang. By the generally accepted (if anything is ever really accepted in cosmology) nucleosynthesis models, the total amount of baryonic stuff that could have come out of the big bang is only about 10% of the matter needed to close the universe (i.e. make a flat universe with density parameter Omega = 1; and more on why a flat universe, or at least why Omega should be greater than 0.1, in a minute). How do we know that the nucleosynthesis models are right (or at least plausible)? Well, they’re very successful in predicting the relative abundances of isotopes of elements (hydrogen, helium, lithium, and on up) that we see, among other things.

You could ask ten cosmologists what the value of Omega is, and you could get ten different answers (or more, depending on the day of the week and relative humidity). However, even if Omega is not equal to 1, there is pretty good evidence that it’s greater than 0.1. (actually, since a universe with Omega not equal precisely to 1 very quickly moves to higher or lower values, many astrophysicists take the view that since we’re finding it within an order of magnitude or so, it really is 1, and the observationalists just need to get their act together and figure out what they’re doing wrong. But again, different people, different answers.) Also, inflation theory predicts a flat universe.

So, it seems that not only can we not see most of the baryonic matter, baryonic matter as a whole is quite possibly the minority constituent of the universe!

So, many possibilities that have been proposed (many have been fairly definitively shot down or at least had upper limits imposed, but I’m not up on the latest). One possibility is something referred to as “MACHOs”, or MAssively Compact Halo Objects. These are non-luminous objects in the halos of galaxies. There are several ongoing MACHO surveys, and their existence has been pretty well established (if you’re curious, they’re generally detected via gravitational microlensing; that is, as a MACHO passes in front of a more distant object, that object appears to change in brightness as the light is lensed around the MACHO. Cool stuff.). However, last I heard the upper limit placed on the number of MACHOs is low enough to rule them out as the sole constituent of halo dark matter.

What are MACHOs? There are a few possibilites. They might be brown dwarfs–stars that just never quite made it (kind of like Jupiter, but bigger; I think that .08 solar masses is about the upper limit before fusion starts). They might be black holes of one of many different sizes. There are additional arguments that limit each of these.

As far as gas goes, limits are placed on the amount of neutral hydrogen by examination of its effects on quasar spectra (the Gunn-Peterson effect, as it’s called), and on the amount of ionized hydrogen by X-ray data. What the exact limits are, I don’t know, but the upshot of it all is…

Most dark matter is probably non-baryonic. Neutrinos enjoyed their day in the sun as possibilities, but have been ruled out as the sole dark matter constituent by both dynamical arguments and limits placed on their mass. Which brings things down to various “exotic” particles, generically termed WIMPs (Weakly Interacting Massive Particles–those crazy creative physicists–the “weakly interacting” because, well, we don’t see them, yet). These are crazy hypothetical particles with names like photino and gravitino and about which I know not a thing and probably don’t have the education to understand.

And now I realize I never really addressed the question. Why not rocks and such? Well, rocks and such have to be composed of elements heavier than hydrogen and helium, and since that limits it to less than 1% of the baryonic matter out there (from nucleosynthesis calculations, and not much changed by stellar production of heavy elements), it makes Omega[sub]rocks and such[/sub] pretty small.

Thanks for the reply Philbuck the universe just got a little more complicated for me but I’m less ignorant about why.

Don’t know why this suddenly flashed into my head while doing homework, but the velocity of an orbiting body should fall off as the inverse 1/2 power of the radius, not 3/2.

Not that anyone probably would’ve noticed, but I hate being corrected on simple things…

Why would the Universe collapse into a Big Crunch only if there is sufficient dark matter?

If there wasn’t any dark matter, wouldn’t the Universe collapse anyway? All the matter in the Universe has a gravitational force between all other matter. And since there is no force pushing the matter away form the point of the Big Bang, wouldn’t this force eventually bring it all back together?

There’s not much left in Philbuck’s excellent posts to expand upon, but I’ve two points: First of all, as to the fate of the Universe, remember that gravity gets weaker the farther away you get, and the Universe is expanding, so gravity is becoming less significant. If the density is low enough, then gravity is insignificant enough that it never re-collapses. It’s a similar concept to escape speed from the surface of a body.

Back more onto the OP: It currently looks like most of the dark matter in the Universe is not only non-baryonic, but not even matter at all. There’s something called the cosmological constant (denoted lambda) which can also curve space and cause it to be flat, and current measurements all seem to be agreeing that lambda accounts for approximately 70% of omega. This has very important implications on the fate of the Universe, since unlike matter, a cosmological constant will actually cause the Universe to expand faster the larger it gets, not slower.

It’s mass we know must be out there but we cannot see. It may be normal matter that is too small/too far/too dim/too non-reflective to see, or it could be something exotic & new to physics.

Recent measurements of the universe’s background radiation seem to confirm this.

Some of it certainly is. But considering how much is “missing” it could not all be rocks because that much matter would be visible.

Chronos got this already, but I’ll just add that the expansion has been coasting since the Big Bang and it would take a certain amount of gravity (i.e., matter) to stop that inertia. Then, there’s the cosmological constant that is like an anti-gravity force that is pushing on the expansion of space.

It will also suck for deep-sky viewing & intergalactic travel/communication! :slight_smile:

Well, they were briefly mentioned earlier, but I am hoping for further clarification on the subject.

How is the mass of a black hole determined? Is there any reason to suspect that a good portion of this dark matter hasn’t already been sucked into some of the more reputedly massive black holes?

And if there’s nothing within the gravitational range of a black hole to get sucked in, is that black hole detectable?