WE HAVE stellar-mass, intermediate and supermassive black holes. Might they also come in "dwarf" sizes too?
The controversial proposal by two researchers suggests that the lightweight objects might form when gas and dust is compressed from without rather than collapsing from within, and could account for some of the universe's dark matter.
A stellar-mass black hole forms when gravity overwhelms all other forces and crushes the core of a massive star down to a "singularity". Gravity is strong enough to do this only if the core is at least twice the mass of the sun.
But black holes of much lower mass could form in the turbulence of a supernova explosion, say Andrew Hayes and Neil Comins of the University of Maine in Orono. In the swirling cauldron of matter ejected in the blast, matter could be crushed to densities great enough to form black holes with the mass of a planet, they say (arxiv.org/abs/1104.2501).
If they exist, these black holes could contribute to the mysterious dark matter that appears to hold galaxies and galaxy clusters together. They could have evaded discovery so far if their parent supernovae pushed them out of the Milky Way's central region, where astronomers have searched for such massive dark matter candidates, says Katherine Freese of the University of Michigan in Ann Arbor.
Other researchers are not convinced that dwarf black holes could form in the first place. Stephen Hawking has suggested that the violence of the big bang created densities large enough to form "microscopic" black holes weighing as little as 10 millionths of a gram. But Hans-Thomas Janka of the Max Planck Institute for Astrophysics in Garching, Germany, doubts that supernovae could eject matter forcefully enough to create lightweight black holes today.
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