I was surprised to pick up Sunday's newspaper and read an article about miniature black holes that may be swimming through our solar system. I wasn't surprised by the idea, as I've heard various versions before, but I recognized one of the scientists' names. Charles Keeton is a collaborator of mine; we work on gravitational lensing together. This is where some source of gravity, like a galaxy, bends the light from a galaxy or quasar in the background.
Anyway, the idea is this. Many people have suggested that dark matter, which makes up 30% of the universe, could be miniature black holes formed during the Big Bang. The problem with this theory is that black holes actually radiate light and particles (this is the "Hawking Radiation" that Stephen Hawking determined). The smallest black holes would by now have radiated all of their mass away into space, and we should see these events as miniature nuclear explosions throughout space. We don't see such explosions. So, it seemed that this theory was dead.
Now we go from the bizarre to the realm of pure speculation. There is a currently-popular hypothesis called the "braneworld theory." The idea is that there are many more dimensions than the four we live in (three space dimensions, plus time). If the theory is true, then our Universe is just part of a larger "braneworld" existing in multiple dimensions, though we are stuck in our four. Are you lost yet? I sure am, so don't ask me to try and explain this further! :)
Keeton and his collaborator, Arlie Petters of Duke University, calculated that, in braneworld theory, black holes may survive much longer than Hawking radiation would suggest, and that the Universe might be full of black holes that have about 200 million metric tons of mass each (if I did my math right, this is about the mass of a cube of granite one quarter of a mile on a side, or the weight of a small hill), but are only about three millionths the size of an atom!
While these sound giant, these are really tiny black holes. They could pass through you and not do anything to you! This is because black holes are not cosmic vacuum cleaners -- they only can eat what they pull in by gravity. And the gravity of a small hill is pretty weak, or else we would all be pulled off of our feet anytime we walked past a mound of dirt. Just remember -- black holes are cool; they don't suck. And imagine trying to find something the size of an atom in the solar system!
Anyway, Keeton and Petters figured out that a new satellite about to be launched to look at giant cosmic explosions, the GLAST satellite, might be able to detect such black holes, if they even exist.
Personally, I remain unconvinced that these little black holes exist. But this is why astronomers keep physics theorists around -- they come up with many ideas, many of which are wrong, but some of which prove correct. And it would be really neat if a co-worker ends up predicting what would be a very exciting discovery -- miniature black holes!