According to a new paper, white holes, the theoretical opposite of black holes, may account for dark matter, and may even predate the universe. They may even, according to Carlo Rovelli, explain the direction of time:
A black hole is one prediction of Einstein’s theory of general relativity. Another is known as a white hole, which is like a black hole in reverse: Whereas nothing can escape from a black hole’s event horizon, nothing can enter a white hole’s event horizon.
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In the 2014 study, Rovelli and his team suggested that, once a black hole evaporated to a degree where it could not shrink any further because space-time could not be squeezed into anything smaller, the dying black hole would then rebound to form a white hole.
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These white holes would not emit any radiation, and because they are far smaller than a wavelength of light, they would be invisible. If a proton did happen to impact one of these white holes, the white hole “would simply bounce away,” Rovelli said. Charles Q. Choi, “‘White Holes’ May Be the Secret Ingredient in Mysterious Dark Matter” at Charles Q. Choi
If white holes predated the universe, Rovelli suggests, future research might show that they explain the direction of time. Here’s the paper (open access):
White Holes as Remnants: A Surprising Scenario for the End of a Black Hole
Eugenio Bianchi, Marios Christodoulou, Fabio D’Ambrosio, Hal M. Haggard, Carlo Rovelli
(Submitted on 12 Feb 2018 (v1), last revised 17 Mar 2018 (this version, v2))
Quantum tunneling of a black hole into a white hole provides a model for the full life cycle of a black hole. The white hole acts as a long-lived remnant, solving the black-hole information paradox. The remnant solution of the paradox has long been viewed with suspicion, mostly because remnants seemed to be such exotic objects. We point out that (i) established physics includes objects with precisely the required properties for remnants: white holes with small masses but large finite interiors; (ii) non-perturbative quantum-gravity indicates that a black hole tunnels precisely into such a white hole, at the end of its evaporation. We address the objections to the existence of white-hole remnants, discuss their stability, and show how the notions of entropy relevant in this context allow them to evade several no-go arguments. A black hole’s formation, evaporation, tunneling to a white hole, and final slow decay, form a unitary process that does not violate any known physics.
It’s an interesting premise for science fiction:
When black holes devour massive amounts of matter and energy, it is thought that everything which appears to vanish forever actually emerges from a white hole. Exactly where the victims of a black hole come out could be anywhere from another place in this universe to another universe entirely. Theoretical physicist Carlo Rovelli theorized something even stranger linking the two. Black holes result from collapsed stars, but when these astral corpses die, they may actually turn into white holes. Elizabeth Rayne, “Bye, Black Holes: White Holes Are Even Weirder” at SyFyWire
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See also: Black holes do not behave as string theorists say they should
and
And now, black holes can be ghosts