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Since the discovery of the Cosmic Microwave Background (CMB, see last month's
article), one of the nagging questions for cosmologists has been how to get
from the very smooth gas at the time of "recombination" to today's highly
concentrated galaxies, stars and planets. The problem is that the density
contrast in the CMB is about one part in 100,000, while the density contrast in
today's universe is essentially infinite (black holes to near vacuum). So, what
happened in the intervening 10 billion years?
One theory that has come into vogue is that the dark matter is so-called "cold dark matter" or "CDM." CDM is essentially a collection of particles that behave something as would dust: they have mass, but they exert no pressure. Remarkably, allowed mass ranges for CDM particles can be subatomic or they can be the size of a pea all the way up to the mass of the earth; and then again from a few solar masses up to perhaps a thousand solar masses. Believe it or not, it's extremely difficult to detect particles anywhere in these mass ranges. Exotic subatomic particles that, by definition, interact very weakly with usual matter, are, by definition, very hard to detect. Peas have enough mass that, to comprise the dark matter, they could be so rare as to cause no noticeable absorption of light. Meanwhile, earth is small enough that its gravitational effects on other objects (or light rays) are difficult to detect. Only recently have we begun to rule out the mass range between earth masses and a few solar masses via gravitational microlensing, in which a distant star's light is deflected slightly by a planetary or stellar-massed object in the galaxy. However, for larger masses, one needs so few of them that they'd be hard to find by chance. So, we're left with trying to understand how the Universe: (1) created CDM particles of completely unknown mass, and (2) evolved in the presence of such particles. To solve this problem, astrophysicists have turned to computer simulations, in which a billion CDM particles can be set to interacting gravitationally. The simulation is started with a density contrast similar to that observed in the microwave background. It is then evolved up to the present day by simply integrating Newton's law of gravity for each particle's effect on every other (and vice versa). The computer power necessary to process this much information is Go To Page: 1 2 |
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