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Title: Dark Matter Exists
Source:
cosmicvariance.com
URL Source: http://cosmicvariance.com/2006/08/21/dark-matter-exists/
Published: Sep 9, 2006
Author: Sean
Post Date: 2006-09-09 12:46:20 by Neil McIver
Keywords: None
Views: 108
Comments: 6
Sean at 11:52 am, August 21st, 2006 The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twenty-first-century cosmology will actually be to understand the nature of these mysterious dark components. A beautiful new result illuminating (if you will) the dark matter in galaxy cluster 1E 0657-56 is an important step in this direction. (Here’s the press release, and an article in the Chandra Chronicles.) A prerequisite to understanding the dark sector is to make sure we are on the right track. Can we be sure that we haven’t been fooled into believing in dark matter and dark energy? After all, we only infer their existence from detecting their gravitational fields; stronger-than-expected gravity in galaxies and clusters leads us to posit dark matter, while the acceleration of the universe (and the overall geometry of space) leads us to posit dark energy. Could it perhaps be that gravity is modified on the enormous distance scales characteristic of these phenomena? Einstein’s general theory of relativity does a great job of accounting for the behavior of gravity in the Solar System and astrophysical systems like the binary pulsar, but might it be breaking down over larger distances? A departure from general relativity on very large scales isn’t what one would expect on general principles. In most physical theories that we know and love, modifications are expected to arise on small scales (higher energies), while larger scales should behave themselves. But, we have to keep an open mind — in principle, it’s absolutely possible that gravity could be modified, and it’s worth taking seriously. Furthermore, it would be really cool. Personally, I would prefer to explain cosmological dynamics using modified gravity instead of dark matter and dark energy, just because it would tell us something qualitatively different about how physics works. (And Vera Rubin agrees.) We would all love to out-Einstein Einstein by coming up with a better theory of gravity. But our job isn’t to express preferences, it’s to suggest hypotheses and then go out and test them. The problem is, how do you test an idea as vague as “modifying general relativity”? You can imagine testing specific proposals for how gravity should be modified, like Milgrom’s MOND, but in more general terms we might worry that any observations could be explained by some modification of gravity. But it’s not quite so bad — there are reasonable features that any respectable modification of general relativity ought to have. Specifically, we expect that the gravitational force should point in the direction of its source, not off at some bizarrely skewed angle. So if we imagine doing away with dark matter, we can safely predict that gravity always be pointing in the direction of the ordinary matter. That’s interesting but not immediately helpful, since it’s natural to expect that the ordinary matter and dark matter cluster in the same locations; even if there is dark matter, it’s no surprise to find the gravitational field pointing toward the visible matter as well. What we really want is to take a big cluster of galaxies and simply sweep away all of the ordinary matter. Dark matter, by hypothesis, doesn’t interact directly with ordinary matter, so we can imagine moving the ordinary stuff while leaving the dark stuff behind. If we then check back and determine where the gravity is, it should be pointing either at the left-behind dark matter (if there is such a thing) or still at the ordinary matter (if not). Happily, the universe has done exactly this for us. In the Bullet Cluster, more formally known as 1E 0657-56, we actually find two clusters of galaxies that have (relatively) recently passed right through each other. It turns out that the large majority (about 90%) of ordinary matter in a cluster is not in the galaxies themselves, but in hot X-ray emitting intergalactic gas. As the two clusters passed through each other, the hot gas in each smacked into the gas in the other, while the individual galaxies and the dark matter (presumed to be collisionless) passed right through. Here’s an mpeg animation of what we think happened. As hinted at in last week’s NASA media advisory, astrophysicists led by Doug Clowe (Arizona) and Maxim Markevitch (CfA) have now compared images of the gas obtained by the Chandra X-ray telescope to “maps” of the gravitational field deduced from weak lensing observations. Their short paper is astro-ph/0608407, and a longer one on lensing is astro-ph/0608408. And the answer is: there’s definitely dark matter there! Poster Comment: Photos & mpegs are on the source site. Seems a relatively laymen friendly description of the evidence.
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#1. To: Neil McIver (#0)
I can't quite wrap my brain around it either, but this write up seems a lot friendlier than some of the others. There's a series of graphics that show why they say this is very strong evidence. Basically they say that this collision between two clusters of galaxies has caused the dark matter to separate from the visible matter, and the gravity of dark matter is detectable because of the effect it has on the light shining from behind it. They claim it's essentially proof that the previously theorized dark matter is really there. Dark matter was previously theorized to exist to explain where the extra gravity comes from to keep galaxies from breaking up as they spin. But until now (at least) the existance of dark matter only been a convenient theory. Without dark matter, they'd have to pretty much rewrite the books on how gravity works, and such a basic rewrite would substantially impact much of known physics, probably including much of Einstein's work.
So the inverse square law would be modified by the addition of factor so small at short distances as to be so far undetectable, but which grows exponentially at astronomical distances? What force grows at a distance?
To every individual in nature is given an individual property by nature not to be invaded or usurped by any. ["An arrow against all tyrants," Richard Overton, 12 October 1646]
Hypocrisy does, for sure. :^) Good point.
The above article gives evidence of dark matter in a galaxy far, far away. But there is an experiment being designed here on earth to detect the particle of dark matter called an axion, which is theorized to be less than a millionth the mass of an electron and electrically neutral. Since axions are so small and neutral, they pass right through walls. But photons of light don't. Now, since it is theorized that strong magnetic fields can turn photons into axions and vice versa, what you do is, shine a laser beam through a magnetic field, thus turning some of the photons into axions, then have the beam hit a wall. The photons get blocked, but the axions pass through the wall into a dark room beyond. Then, in the dark room you have another magnetic field to turn the beam of axions back into photons, and you should get a faint spot of light on the far side of the dark room.
This is so far over my head that I cannot tell you...
#2. To: lodwick (#1)
#3. To: Neil McIver (#0)
Could it perhaps be that gravity is modified on the enormous distance scales characteristic of these phenomena? Einstein’s general theory of relativity does a great job of accounting for the behavior of gravity in the Solar System and astrophysical systems like the binary pulsar, but might it be breaking down over larger distances?
#4. To: leveller (#3)
What force grows at a distance?
#5. To: Neil McIver (#2)
Thanks for bringing down to my level. Makes sense.
#6. To: Neil McIver (#0)
I've already said too much.
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