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Cold dark matter may yet be able to explain the structure of the Universe,
according to recent observations of the cosmic background radiation, a relic
of the big bang.

The observations, made from the South Pole, convincingly rule out so-called
hot dark matter, in the form of particles such as neutrinos, moving at or
near the speed of light. The technique used is almost sen sitive enough
to detect distortions in the radiation that are predicted by the cold dark
matter (CDM) theory.

Nonluminous, or dark, matter is thought to make up between 90 and 99
per cent of the mass of the Universe. Astronomers know that dark matter
exists because of its gravitational influence on the visible stars and galaxies.

Cold dark matter would be composed of slow-moving particles that clump
just like normal matter. The clumps would have been the mass centres about
which the visible galaxies formed. Unfortunately, no CDM particles have
ever been discovered, although there are theoretical reasons for believing
that they should exist. These hypothetical particles have been given names,
such as axions and photinos.

Recently, the frontrunner among theoretical models of the dark matter
has been CDM. Computer simulations show that if bright galaxies are embedded
in a sea of CDM, then, as the Universe expands and evolves, they will form
clusters and chains very similar to those seen in the sky. But the match
between the simulations and the real sky is not quite perfect.

Earlier this year, reports that the CDM model was dead appeared in the
journal Nature and even the daily newspapers. These dramatic stories were
based on the discovery of a small disagreement between the predictions of
a pure CDM model and the distribution of real galaxies.

Recently, two of the principal CDM theorists, Carlos Frenck at the University
of Durham and Nick Kaiser of the University of Toronto, have taken the writers
of those dramatic stories (largely based on a press release from Nature)
to task, pointing out that the observations suggest only that some additional
factor must be at work, as well as the gravitational influence of cold dark
matter, and that the new observational information ‘does not automatically
rule out the existence of cold dark matter’ (Nature, 2 May, vol 351, p 22).

For example, they say, the Universe may contain loops of cosmic string,
left over from the big bang, exerting a gravitational influence on the distribution
of galaxies. Cold dark matter, say the two astronomers, is far from dead.

The case of Frenck and Kaiser is strengthened by a new report in Physical
Review Letters (vol 66, p 2179) from researchers at the Canadian Institute
for Theoretical Physics, the University of Oxford, and the University of
California, Santa Barbara. They have interpreted observations of the cosmic
background radiation made at the South Pole, and compared them with the
predictions of both the hot and cold dark matter models.

The distribution of matter across the Universe should leave an imprint
on the cosmic background radiation, making its temperature slightly different
in different directions in the sky. The average temperature of the radiation
is -270 °C, just 3 degrees above absolute zero.

Dark matter models are often described by a number called the biasing
factor, which is equal to 1 if the distribution of bright galaxies is the
same as the distribution of dark matter, and greater than 1 if (as CDM models
require) galaxies are more clustered than the dark matter.

In hot dark matter models, this biasing factor must be much less than
1. But the new cosmic background studies show that it is in fact greater
than 0.86. This effectively rules out all the hot dark matter models, and
is close to the value predicted by CDM models.

Observations of the cosmic microwave background radiation made by the
Soviet satellite RELICT 1 had already shown that the biasing factor is greater
than 0.4, but this had not been enough to rule out hot dark matter completely.
However, NASA’s cosmic background explorer (COBE), now in orbit around the
Earth, should soon be measuring the anisotropy of the background radiation
at a level where the CDM effects should show up if, indeed, CDM does dominate
the Universe gravitationally.