abstract and link to paper
discussion stimulated by Lubos Motl (more focused on white than black holes)
more discussion (on the entropy of isolated / exploding white holes)
I should note that I have no idea whether these "eternal" black holes are stable against perturbations.
Eternal black holes are the ultimate cosmic safes
18:59 03 September 2010 by Stephen Battersby
If you wanted to hide something away for all eternity, where could you put it? Black holes might seem like a safe bet, but Stephen Hawking famously calculated that they leak radiation, and most physicists now think that this radiation contains information about their contents. Now, there may be a way to make an "eternal" black hole that would act as the ultimate cosmic lockbox.
The recipe for this unlikely object was discovered by looking at an even more abstruse entity, the white hole. White holes are black holes that run backwards in time, throwing out matter instead of sucking it in. Where a black hole might form from a collapsing star, a white hole would explode and leave a star in its place. White holes have never been observed, though general relativity predicts they could exist in principle.
Stephen Hsu of the University of Oregon in Eugene wanted to caculate whether a white hole would emit radiation like a black hole. He considered the special case of a white hole sitting in a perfect vacuum, and calculated that when it spits out its contents, there is a burst of radiation essentially identical to a black hole's Hawking radiation (arxiv.org/abs/1007.2934).
Hsu realised that running the process backwards would be equivalent to a black hole forming and then existing in a perfect vacuum, with no Hawking radiation. "It becomes a black hole that's not radiating, which is a very weird thing," Hsu says.
The snag is that to run this process backwards and make the eternal black hole, you would need to send in a precisely crafted burst of radiation as the hole forms. The radiation would have to be "exactly tuned to interfere with the Hawking radiation that would otherwise come out", says Hsu.
"Maybe in a highly advanced civilisation, physicists could create a black hole that didn't evaporate," he told New Scientist. "It would be exquisitely difficult, but mathematically you can do it."
If one did build an eternal black hole, it would be the perfect place to store sensitive information.
Normal black holes are thought to gradually release information about their contents through Hawking radiation. "Most theorists have come to the conclusion that black hole evaporation is analogous to the burning of a book," says Martin Einhorn of the University of California, Santa Barbara. "All the information in the book must be encoded in special properties of the outgoing radiation." In principle, it would be possible to recreate the original book – if you could collect all the outgoing radiation and understood the quantum properties of gravity.
But with eternal black holes, "it's as if you just put the information inside a box and at the end you still have the box", says Hsu.