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The Question

(Submitted February 17, 2002)

Could dark matter be composed of mini-blackholes formed at the time of the Big Bang?

The Answer

This is a very good question. We at "Ask an Astrophysicist" know that it's currently not considered to be a major constituent of dark matter, but we're not sure if we know all the reasoning behind this.

Here is what we do know.

Stephen Hawking proposed primordial (i.e., created just after the Big Bang) black holes in connection with his work on black hole evaporation:

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/010703a.html

This process (Hawking radiation) should have destroyed the lowest mass primordial black holes by now, and should currently be destroying ones that started out with the mass of a small asteroid. However, no mini black holes have been discovered to date, through Hawking radiation:

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970519a.html

or through other means.

Note that the number and the masses of primordial black holes depend sensitively on the nature of density fluctuation in the early universe. We have learned a lot on this topic since Hawking made the original suggestion: the Cosmic Microwave Background as measured by COBE is extraordinarily smooth on the one hand, while we now know of large-scale clustering of galaxies, on the other hand. As we understand it, current models of density fluctuations that account for these observations do not predict a large number of primordial black holes. Therefore, they are probably not a major component of dark matter.

You might want to fallow this up with the Dark Matter FAQ site at Berkeley:

http://cdms.berkeley.edu/Education/

Hope this helps,

Koji & Ilana
for "Ask an Astrophysicist"

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