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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.

Demos Kazanas
for the Ask an Astrophysicist Team

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