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The Question
(Submitted January 03, 1998)
There has been a lot of debate over the nature of missing matter
in the universe and people have been putting forward many candidates
for this missing matter. My question is simply given the equivalence
of mass & energy couldn't the so called dark matter simply be the energy
left and floating around from the big bang ?
I am a banker by profession however have a keen interest in relativity
and astrophysics through reading books without any formal education.
The Answer
You are absolutely correct about the contribution of the energy left from
the big bang to the energy of the universe. Unfortunately, this cannot be
the missing mass. It is certainly taken into account and in fact it makes
the dominant contribution to the energy density of the universe at early
times. However, since its energy density drops faster than that of ordinary
matter it is not important today. It simply constitutes the 2.7 degree K
cosmic background radiation. Furthermore, because it consists totally of
photons, i.e. it moves at the speed of light it cannot be clustered in the
gravitational potential of a galaxy, which cannot trap particle traveling
faster than about 300 km/sec. For that we indeed need particles with non-zero
inertial mass.
Sincerely,
Demos Kazanas
for the Ask a High-Energy Astronomer Team
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