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.
