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VACETS Regular Technical Column

"Science for Everyone"

"Science for Everyone" was a technical column posted regularly on the VACETS forum. The author of the following articles is Dr. Vo Ta Duc. For more publications produced by other VACETS  members, please visit the VACETS Member Publications page or Technical Columns page.

The VACETS Technical Column is contributed by various members , especially those of the VACETS Technical Affairs Committe. Articles are posted regulary on vacets@peak.org forum. Please send questions, comments and suggestions to vacets-ta@vacets.org

Mon, 20 Mar 1995

Dark Matter

For decades, astronomers have been increasingly puzzled by what might be called the "hidden mass" problem, according to which most of the matter comprising the Universe is apparently invisible. They discovered this problem while observing the rotation of spiral galaxies like our own. If the mass of a galaxy is concentrated where its light is, in the bright core, then just as the sun's gravity pulls distant planets around much more slowly than it pulls the inner planets, stars in the outer reaches of a galaxy should orbit much more slowly than stars in the center. But they don't. The conclusion is that there must be some form of matter surrounding the galaxy, in a spheroidal halo, that doesn't show up in telescopes. The problem is that we just don't know what form it takes.

In principle, the "hidden mass" could be made of diffuse gas, collapsed stars such as white dwarfs, neutron stars, and black holes, or faint red dwarfs. Some suggest that it is composed of neutrinos and the neutrinos have finite rest masses. Some other suggest the "massive compact halo objects (MACHOs)", the planet-size clumps of ordinary matter, are the carriers of the hidden mass. Other scientists says that the hypothetical "weakly interacting massive particles (WIMPs)", magnetic monopoles, or cosmic strings are the carriers.

Let's survey the candidates one by one starting with the collapsed stars. Collapsed stars are dead stars. All stars are in equilibrium under the influence of the inward force form gravity and the outward force resulting from the gas pressure and radiation due the thermonuclear furnace at the center of the star. When the furnace goes out, the star collapses. In a small or medium-sized star, the collapse is slow, and the result is a white hot dwarf, only slightly larger than the earth. If the star is about four solar masses, it ends its life with a tremendous explosion, the supernova. A large fraction of the star is blown off into space, but the explosion compresses the core with a tremendous pressure and the result is tiny neutron star only about 10 or so kilometers across. If the original star is even bigger, about eight or more solar masses, the remaining core after the supernova explosion may be greater than 3 solar masses and the result is a black hole. (Stellar corpses are so interesting, especially black holes. I'll discuss them in greater details later in an other article.)

Several hundred white dwarfs have been observed, the best known being the companion of the bright star, Sirius. Also, we have identified several hundred neutron stars, which are called pulsars nowadays. It is much harder to identify stellar-collapse black holes. There are several good black hole candidates, which all are binary stars. White dwarfs, neutron stars, and stellar-collapse black holes are the remnants of stars like our sun or larger and it's unlikely that a large number of such stars (enough to account for all the dark matter) have died in the last 10 or 15 billion years. Therefore, stellar corpses are not good candidates for dark matter.

The next candidates are red dwarfs. Red dwarfs are small stars, ranging in mass from about one-half to one-tenth that of the sun. Large numbers have been observed but the numbers don't seem any where near to be enough to account for all the dark matter. It is argued that due to their brightness (only about 0.01% that of our sun) that most of them are effectively hidden beyond the present telescopes. Even if most of them are unseen and can be accounted for all the dark matter, however, there are problems associated with the idea. Suppose that there were enough red dwarfs in the halo to account for the dark matter, there should also be many stars about the mass of our sun mixed in with them. Yet we see no evidence for solar mass stars in the halo. On the basis of this, we think that red dwarfs are not good candidates.

The next candidates are the "massive compact halo objects (MACHOs)", which sometimes called brown dwarfs. They are mainly Jupiter size or larger objects that are not quite massive enough to trigger nuclear reaction in their core and shine as stars. We know that red dwarfs are abundant, accounting for about 80% of the seen mass in the universe, and brown dwarfs are only slightly smaller than red dwarfs. Therefore, it is reasonable to assume that a large number of brown dwarfs exist. The method of detecting MACHOs or brown dwarfs is based on a discovery of Einstein's, that concentrations of mass bend light. If a MACHO in the Milky Way's halo were to pass directly between the earth and a star in another galaxy, this "gravitational microlensing" effect would cause the starlight to brighten gradually and fade over the course of a month or so. By 1992, two groups of astronomers, one American and one European, were searching for MACHOs. By the end of last year, the number of MACHOs observed by both groups were few, too small to make up any significant fraction of the dark matter in our galaxy's halo.

The next candidates are the neutrinos. We expect the number of neutrinos to be similar to that of the background photons which are about 10^9 times more abundant than nucleons. If the neutrinos were to have a mass of about 10 electron-volts (eV) (for comparison, an electron has mass of 5*10^5 eV, the mass of a proton or neutron is 10^9 eV), then their contribution would be enough to make the universe close. A few years ago, the announcement of the detection of the 17 eV rest mass of electron-associated neutrinos generated a flurry of excitement. Unfortunately, the excitement did not last. Today, it is generally believed that the rest mass of electron-associated neutrino is much smaller (if not zero) than the 17 eV reported. That would eliminate electron-associated neutrino as candidate for dark matter. There are still two other kinds of neutrinos, muon-associated neutrinos and tau-associated neutrinos. Their possibilities as candidates for dark matter are not completely eliminated. However, most scientists working in the area are convinced that interest in it will eventually fade away.

It now seems that none of the known-to-exist objects (collapsed stars, red dwarfs, MACHOs, neutrinos) are good dark matter candidates. That would make the WIMPs or some other hypothetical particles the winners by default, wouldn't it?

NEXT: WIMPs and other exotic particles.


Duc Ta Vo, Ph.D.
ducvo@lanl.gov

For discussion on this column, join vacets-tech@vacets.org


Copyright © 1996 by VACETS and Duc Ta Vo

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