Dark Matter (part one)
Published on Dec 18, 2005 at 7:48 pm.
4 Comments.
Filed under cosmology, dark matter, physics.
A while back, Tom had suggested dark matter as a blog topic. So, here goes with part one. It is a pretty intense subject, and no one really understands all about dark matter, so it will be the subject of more than one entry.
The story goes back over 70 years. Fritz Zwicky and Walter Baade were studying the Coma galaxy cluster. This is a very large cluster of galaxies in the direction of the border between the constellations Coma Bernices and Virgo. By studying the number and distribution of galaxies, along with measurable motions, they found that the galaxies were moving too fast. What do I mean by too fast? Well, all matter attracts other matter by gravitational forces. With a lot of stars or galaxies moving around in a cluster, the gravitational attraction of the bodies will pull any body moving out of the cluster back into the cluster. Occasionally, some bodies will be going fast enough to achieve escape velocity, and they will leave. Enough of the members doing this leads to evaporation of the cluster. Open star clusters tend to evaporate, while globular clusters are compact enough and have enough mass that they last a long time. Zwicky and Baade used these principles to study the Coma Cluster, and they found that the galaxies were moving so fast that the entire cluster would evaporate in a very short time. In fact, the motions were fast enough that they didn’t see any way that there could even be a galaxy cluster. Clearly there was a cluster, so something had to be going on. They proposed that there must be more matter in the cluster than we could see. Zwicky postulated that this “dark matter” was the source of the additional gravitational forces holding the cluster together. No one knew what the dark matter was composed of, though.
Within a decade, another astronomer, Robert Trumpler, was studying star clusters. He found that the more distant clusters were dimmer than they should be. What was going on? Were distant stars somehow different than nearby ones? Did light get “tired” and fizzle out on the way to Earth? These and several other ideas were suggested and deemed unworkable. Eventually, Trumpler suggested that all of space was filled with a kind of “fog” that is now termed the interstellar medium. We have now found that the interstellar medium is composed mostly of hydrogen and helium. Also, we have found that in spiral galaxies such as the Milky Way, the interstellar medium has more mass than the stars themselves. Is this the source of the dark matter? Unfortunately, no.
By observing the orbits of stars around the galaxy, we can devise a velocity profile that shows the orbital speed of stars at different distances from the center of the galaxy. Such a profile should show that the more distant stars orbit slower than the inner stars. This is not what the velocity profiles of galaxies, show, though. They show that for most of the galaxy, the orbital speeds are the same. This can only be true if there is substantially more mass being orbited. We can determine the amount of interstellar medium in the galaxy. This does not come even close to the required mass to account for velocity profiles that we see.
More recently, we have found hot plasma between the galaxies in most galaxy clusters. This intergalactic medium is also mostly hydrogen and helium. It has substantial mass, but it, too, is far too little to account for the motions that we see among the members of galaxy clusters.
Over the years, we have found gas, dust, and all sorts of other forms of matter that we could not see before. Much of this material does not shine with visual light. However, it can still be detected by its absorption of visual light, or its emission of non-visual forms of light, such as X-rays, infrared, etc. Astronomers don’t consider this to be “dark matter,” though. That term is reserved for the undetected mass that seems to exist in abundance in the universe. These dark forms of matter that we detect by other means are often lumped together as “dim matter,” to differentiate it from the dark matter. All told, the luminous matter and the dim matter account for only about 10% or so of the mass that seems to exist in the universe. Galaxies and galaxy clusters seem to be composed of nearly 90% dark matter. What is this stuff? Well, that is still a mystery. Since we’ve been unable to detect it, we can only guess.
Numerous suggestions have been made as to the source of the dark matter. These range from ordinary objects (things that technology may eventually detect, making them dim matter) to rather bizarre particles, or even strange substances. I’ll save some of these suggestions for a later posting. There have even recently been suggestions that there is no dark matter at all, but rather that the gravitational effects that we see which lead us to speculate that there is dark may actually be artifacts of the nature of space — a sort of leaking of gravitational energy into other dimensions. These are the more exotic theoretical proposals, and again I’ll leave them for later.
Though we don’t know what dark matter is, we can make some guesses as to its nature, assuming that we don’t hold to the most exotic explanations. It must be fairly weakly interacting with ordinary matter, other than through gravitational forces, or we’d have detected it already. Studies of the clumping of galaxies show that the dark matter must be slow moving enough to clump up to form galaxy clusters within the time frame of the early universe. However, studies of the cosmic background radiation show that the whole universe is uniform in temperature, and this suggests that the dark matter must move at near the speed of light in order to smooth out the universe during the few years available after the big bang. Both suggestions can’t be right! Well, perhaps they can — if there were more than one type of dark matter. We call the slow moving, low energy form cold dark matter, and we call the fast, high energy form hot dark matter. These must be different forms of dark matter. They can not be just the same stuff with two different energies, otherwise we’d expect a continuum of energies. Also, it would be hard to imagine how the processes at work in the creation of the universe would give two entirely different energies to groups of the same type of matter.
So, to fit our observations, we postulate that there must be this unseen form of stuff out there that does not interact with anything else, except by gravity. Making it more exotic, we contend that the bulk of the universe is made of this unseen stuff. Then, we go one step farther, and we say that there must be at least two different kinds of stuff out there! This gets to be a bit much to accept for some people, so you can imagine the conundrum that dark matter causes. The fact is that we have no idea what dark matter is. There are suggestions (later post), but even our best suggestions seem to fall short of what we need.
So, that sets the stage. Unfortunately it does not answer the question of what the dark matter may be. No one knows that. If anyone were to really figure it out, then there’s a Nobel Prize waiting for them!
-Astroprof
Astroprof’s Page » AAS Meeting — Dark Matter on January 8, 2007 at 12:40 am: 1
[…] Well, quite simply, we look for its gravitational effects. Dark matter, like all matter, interacts via the gravitational force. Einstein told us (and it was later verified experimentally) that gravity can bend light. Massive galaxies can have so much gravity that they bend light in a phenominon called gravitational lensing. This, in fact, is how we know that there is dark matter out there. We see far too much gravitational lensing going on. There is more gravitational lensing than can be accounted for with normal matter. So, “dark matter” was proposed as solution to that problem. I blogged about dark matter before. By measuring the amount of excess gravitational lensing, you can figure out how much dark matter is helping to bend light. When you do that, you get a plot like this: […]
Astroprof’s Page » What is the Matter? on November 7, 2007 at 12:48 pm: 2
[…] Some time back, I had a post about dark matter. So, I thought that I’d say a bit more about matter. First of all, just what is matter? After all, if we are going to talk about dark matter, we should have some idea what non-dark matter is, right? This is going to be a bit more technical post than most that I have done. Long time readers will note that I once in a while toss in things like this. […]
Trueblue on December 15, 2007 at 3:58 pm: 3
http://news.nationalgeographic.com/news/2007/01/070108-dark-matter.html .
Tom’s Astronomy Blog » Blog Archive » “Missing” Matter Found on May 6, 2008 at 4:27 pm: 4
[…] That’s “normal matter” and amazingly that is only about 5 percent of what makes up the universe. Pretty hard to imagine isn’t it? 95 percent of the universe is made up of stuff we can’t see, touch, or detect. It’s a combination of “dark matter (23%) and dark energy (72%). Strange stuff! Astroprof has done a few great posts on dark matter, here’s a like to the first one. […]