In 2002, Dr. Vera Cooper Rubin was awarded the Gruber Prize in Cosmology for her role in discovering dark matter. Through her measurements and analyses of the motions of galaxies relative to each other, as well as their distribution throughout the universe, it became clear that there was more matter in the universe than that which we can see—indeed, the matter we can’t see greatly outweighs the stuff we can see. The stuff we can’t see is called dark matter, and it appears to make up 85 % of all the matter in the universe. Its presence can only be deduced by its gravitational effects.
One particular set of observations performed in the late 1960s by Rubin, in collaboration with W. Kent Ford, was on the arms of the Andromeda galaxy; at a mere 2.5 million light-years away, it’s one of the closest neighbors to our own Milky Way Galaxy. At that time, technology had progressed to the point where it was possible to obtain the spectral measurements from particular regions of the galaxy, rather than having to settle for the spectrum of the entire galaxy. By focusing their observations on the arms of the Andromeda Galaxy, they were able to use the Doppler effect on known spectral features to measure the speed of rotation of the stars in those arms, at various distances away from the center of the galaxy. The expectation was that the speed of rotation would drop as distance from the center of the galaxy increased. After all, this is how it works in our solar system: Mercury, closest to the Sun, speeds around in its orbit, while the speed of planetary revolution around the Sun drops for each successive planet going out, until you get to Neptune, lumbering about its orbit at about one-tenth the speed of Mercury.
But that’s not what Rubin and Ford observed for the arms of the Andromeda Galaxy. The speed of rotation of stars in the arms was pretty much the same whether it was close to or distant from the galactic center. The only way to account for this observation was to assume that there was a substantial amount of invisible matter within the galactic disc, and a whole lot more outside its visible edge. Either that, or Newton’ law of gravity was wrong (and there have been those who have suggested that possibility). It was suggested that the Andromeda Galaxy was a fluke, but similar measurements on other galaxies bore out that this constancy in the rotational speed of stars with respect to distance from the center was universal. So then dark matter appears to be everywhere. But what is it?
The answer to that question does not lie on the other side of the fold, but there’s plenty of speculation. Please make your way past the galaxy-sized top below...
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The Bullet Cluster
As mentioned earlier, one possible explanation for the different behavior of galaxies and planetary systems is that Newton’s gravitational law no longer works on the much larger distance scales of galaxies. If this were true, it might not be necessary to invoke the existence of dark matter, or at least reduce the amount needed to explain observations. This idea is called MOND (modified Newtonian gravity). However, a clever observation from about ten years ago, reproduced many times since on other systems, affirms that dark matter is real and present in large quantities. The observed object is a collision between two galaxy clusters, called the Bullet Cluster. Two separate observations are made. In this kind of collision, the ordinary (not-dark) matter heats up to very high temperatures and emits x-rays, which can be detected using the Chandra x-ray telescope orbiting the Earth. Hence, the x-rays indicate the presence of ordinary matter. The dark matter, on the other hand, we can only detect from its gravitational influence. From Einstein’s general theory of relativity, we know that large amounts of matter warp space, and that warping is capable of bending light passing nearby. Indeed, the warping of space around the matter can act like a lens, focusing the light from a source behind it. This effect is called gravitational lensing, and it can be used to estimate the total mass of an object. The results of these two measurements were superimposed onto one image (above). The pink portion is the x-ray image (ordinary matter). The blue portion is the mass distribution determined from gravitational lensing (all matter, but mostly dark). What we see is that the blue portion extends beyond the pink. This is because, in the collision, the ordinary matter is slowed down, what with running into other bits of ordinary matter, while the dark matter, which doesn’t interact at all (except gravity), just continues moving. This image (and others like it) prove the reality of dark matter.
So what sorts of objects could dark matter consist of? Early on, it was thought that dark matter could consist of massive astronomical compact halo objects (MACHOs). These are planet-sized objects, brown dwarfs (somewhat larger than the planet Jupiter), and small black holes made up of ordinary matter that don’t emit light and move independent of any planetary system. However, the observation described in the previous paragraph pretty much eliminates the possibility of objects made of ordinary matter from causing this effect. The alternative to MACHOs are, of course, WIMPs (weakly interacting massive particles). These are massive elementary particles that have no electrical charge and, at best, interact only very weakly with other matter. One example of such particles that is known to exist is the neutrino. When the idea of the neutrino was originally conceived, it was thought to have no mass, but more recent theories have required that the neutrino have a small mass, though this mass is not great enough to account for the amount of dark matter known to exist in the universe.
None of the particles currently considered as candidates for dark matter have actually been observed; as yet, they are still the products of theorists’ fevered imaginations. One of these is called the neutralino, predicted to exist by a theory called supersymmetry. However, supersymmetry predicts the existence of many particles, none of which has been observed. Another candidate is a particle called the axion, which has been invoked to patch up a problem with the standard model, (the current theory that explains the behavior of all known particles). But nobody has observed that particle either.
So dark matter makes up 85 % of the matter in the universe, and nobody knows what it is! We know it’s out there, that its gravity influences celestial motion on a galactic scale, and that it’s undetectable by means other than looking for its gravity effects. There are some experiments in process designed to detect one or another of these exotic particles, and if one of them exists, I sure hope it gets discovered while I’m still around to appreciate the discovery.
I know this is something of a heavy topic for a TC diary on Thanksgiving Eve, but it’s what I’ve got. Thanks for joining me.
TOP COMMENTS November 25, 2015
Thanks to tonight's Top Comments contributors! Let us hear from YOU when you find that proficient comment.
From Jon Stizman:
This one by rugbymom took my breath away. All about why we need to show the world we ARE NOT a nation of Trump fans, why we are getting better, why we are not a glut of ignorant, heavily-armed racists with chubby thumbs twitching near a big red nuke-launch button. More eyes should see this. Please consider it!
My concern is that “inhumanity and atrocity” are rapidly getting more popular in the United States too, which undoubtedly feeds the same dynamic in the Muslim world. When US politicians at the highest level talk about waterboarding Muslim suspects, closing mosques, treating Muslims like rabid dogs (shooting on sight, one presumes), mob violence as a justified response (Trump), and full-out making the US a theocratic Christian nation (Cruz, at least), it’s hard to claim any moral high ground or criticize other groups for equally abhorrent proclamations and behavior. I have no way to influence what’s going on in Syria or Iraq. I’d like to find some way to influence the equally offensive rhetoric within the US.
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TOP MOJO November 24, 2015
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