String theory is physicists’ best guess at a unified theory of all interactions but it comes with some strange predictions. One of these is that spacetime consists of 10 dimensions rather than just the four we’re familiar with. And that raises some interesting questions.

One of them is what shape singularities can form in this higher dimensional space. In four dimensions, the only solution is spherical and that’s the type of black hole cosmologists have imagined all over the universe.

But in higher dimensions, there are all kinds of other solutions. We’ve looked at the possibility of black rings but today Maria Rodriguez at the Max Planck Institute for Gravitational Physics in Golm, Germany, compiles a catalogue of all know species of black hole.

It turns out there’s a whole menagerie of other black hole solutions. Here are just a few: the black saturn, the black helical ring, the di-ring, the black bowtie, and the bicycling black ring as well as the more general blackfolds.

While these solutions may exist mathematically, they may or may not exist in the real universe. In fact, Rodriguez is able to work out certain criteria that a solution must meet for it to have a hope of existing in the real world. For example, a black ring can only exist if there is enough centrifugal repulsion to prevent it from collapsing.

Rodriquez points out that the list is incomplete. “The catalog of different species (exact solutions) of black holes shows a very rich structure but seems far from being complete.”

That makes it an interesting topic for ambitious cosmologists. But be warned: there’s a good reason the list is incomplete. The solutions in this higher dimensional space are fiendishly difficult to find.

Nevertheless, it would be good to either rule out the possibility of their existence or work out if and how they can be distinguished observationally from common or garden spherical black holes.

Ref: arxiv.org/abs/1003.2411: On the Black Holes Species (By Means Of Natural Selection)