Question

The mass of dark matter in the Milky Way is measured to be about 2×1042 kg. Black holes
emit no light, so perhaps they may be responsible for the unseen dark matter? To evalu-
ate this possibility, determine how many black holes must exist to explain the dark matter
and what process created them. As far as we know, every black hole was formed as a re-
sult of a supernova explosion (though not every supernova creates a black hole).
A. Find the number of black holes in the Galaxy by dividing the mass of all the dark mat-
ter by the mass of a single stellar-mass black hole (5 times the mass of the Sun).
B. Since we believe black holes are created in supernovae, the number of black holes
must be equal to the number of supernovae that have occurred. On average, humans
have observed about one supernova per century in the Milky Way. At this rate, how
long would take to create all enough black holes to explain all the dark matter?
C. If the current rate of supernovae does not appear feasible, consider an alternative hy-
pothesis that nearly all the black holes in the galaxy were formed long ago, and we
were not around to witness them. What signs of this ancient “burst” of star formation
might astronomers look for in our galaxy?

Answer 1

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