Black holes have fascinated scientists and the public alike for decades. There is special appeal in the idea that the universe contains regions of space so dense that light itself cannot escape and so extreme that the laws of physics no longer apply. What secrets can these extraordinary objects hide?

Today, we get an answer thanks to the work of Carlo Rovelli at the University of Toulon in France, and Francesca Vidotto at Radboud University in the Netherlands. These guys say that inside every black hole is the ghostly, quantum remains of the star from which it formed. And that these stars can later emerge as the black hole evaporates.

Rovelli and Vidotto call these objects “Planck stars” and say they could solve one of the most important questions in astrophysics. What’s more, evidence for the existence of Planck stars may be readily available, simply by looking to the heavens.

Black holes arise naturally from Einstein’s theory of general relativity which predicts that gravity influences the trajectory of photons moving through space. Indeed, when gravity is strong enough, light shouldn’t be able to escape at all. That region is then a black hole.

Astrophysicists have long believed that black holes form when stars a little bigger than the Sun run out of fuel. No longer supported by thermal energy, the star collapses under its own weight to form a black hole. Since there is no known force that can stop this collapse, astrophysicists have always assumed that it eventually forms a singularity, a region of space that is infinitely dense.

But this has never been entirely satisfactory. The laws of physics break down in a region of infinite density, leaving physicists scratching their heads over what must be going on inside a black hole.

Even worse, many physicists believe black holes slowly evaporate and disappear. That raises problems because the information that describes an object must fully determine its future and be fully derivable from its past, at least in principle. But if black holes disappear, what happens to this information?

Nobody knows, a problem known as the “information paradox” and one of the hottest mysteries in astrophysics.

Now Rovelli and Vidotto have the answer. They begin by revisiting some ideas about what might happen should the universe end in a big crunch, the opposite of a big bang. Their key insight is that quantum gravitational effects prevent the universe from collapsing to infinite density. Instead, the universe ”bounces” when the energy density of matter reaches the Planck scale, the smallest possible size in physics.

That’s hugely significant. “The bounce does not happen when the universe is of planckian size, as was previously expected; it happens when the matter energy density reaches the Planck density,” they say. In other words, quantum gravity could become relevant when the volume of the universe is some 75 orders of magnitude larger than the Planck volume.

Rovelli and Vidotto say the same reasoning can be applied to a black hole. Instead of forming a singularity, the collapse of a star is eventually stopped by the same quantum pressure, a force that is similar to the one that prevents an electron falling into the nucleus of an atom. “We call a star in this phase a “Planck star”,” they say.

Planck stars would be small— stellar-mass black hole would form a Planck star about 10^-10 centimetres in diameter. But that’s still some 30 orders of magnitude larger than the Planck length.

An interesting question is whether these Planck stars would be stable throughout the life of the black hole that surrounds them. Rovelli and Vidotto have a fascinating answer. They say that the lifetime of a Planck star is extremely short, about the length of time it takes for light to travel across it.

But to an outside observer, Planck stars would appear to exist much longer. That’s because time slows down near high-density masses. For such an observer , a Planck star would last just as long as its parent black hole.

It then becomes possible for the black hole to interact with the Planck star it contains. Rovelli and Vidotto point out that as the black hole evaporates and shrinks, its boundary will eventually meet that of the Planck star as it expands after the bounce. “At this point there is no horizon any more and all information trapped inside can escape,” they say.

That immediately solves the information paradox. The information isn’t lost or trapped inside an unimaginably small region of space but eventually re-emitted into the universe.

There’s yet another exciting consequence of these ideas. Rovelli and Vidotto say this release of information would generate radiation with a wavelength of about 10^-14 cm. In other words, gamma rays.

The universe is filled with a foggy background of gamma rays that astrophysicists have already observed in considerable detail with orbiting telescopes. Could it be that they have already detected the signature of Planck stars releasing their information into the cosmos?

There will certainly be no shortage of astrophysicists willing to comb through the data to find out. Worth watching in the near future.

Ref: http://arxiv.org/abs/1401.6562 : Planck Stars