Black holes possess gravitational tugs so powerful that not even light, the fastest thing in the universe, can escape them. The invisible spherical boundary surrounding the core of a black hole that marks its point of no return is known as its event horizon.
A black hole is a prediction from Einstein’s general theory of relativity. Another is known as a white hole, which is like a black hole in reverse: while nothing can escape the event horizon of a black hole, nothing can enter the event horizon of a white hole.
Previous research has suggested that black holes and white holes are connected, and matter and energy fall into a black hole potentially emerging from a white hole, either elsewhere in the cosmos or in another universe altogether. In 2014, Carlo Rovelli, a theoretical physicist at the University of Aix-Marseille in France, and his colleagues suggested that black holes and white holes might be connected in another way: when black holes die, they could become white holes.
In the 1970s, theoretical physicist Stephen Hawking calculated that all black holes should evaporate mass by emitting radiation. Black holes that lose more mass than they gain are expected to shrink and eventually disappear.
However, Rovelli and his colleagues suggested that the reduction of black holes could not disappear if the fabric of space and time were quantum, that is, made of indivisible quantities known as quants. Space-time is quantum in research that seeks to unite general relativity, which can explain the nature of gravity, with quantum mechanics, which can describe the behavior of all known particles, in a single theory that can explain all the forces of the universe.
In the 2014 study, Rovelli and his team suggested that, once a black hole evaporated to a degree that it could not be further reduced because space-time could not be squeezed into something smaller, the dying black hole would bounce off to form a white hole.
Particles from the black hole escaping.
"We come across the fact that a black hole becomes a white hole at the end of its evaporation," Rovelli told Space.com.
Black holes are now believed to form when massive stars die in giant explosions known as supernovae, which compress their corpses into the infinitely dense spots known as singularities in the hearts of black holes. Rovelli and his colleagues previously estimated that it would take a black hole with a mass equal to that of the sun about a billion times the current age of the universe to become a white hole.
However, earlier work in the 1960s and 1970s suggested that black holes might also have originated within a second after the Big Bang, due to random density fluctuations in the hot and rapidly expanding newborn universe. The areas where these fluctuations concentrated matter together could have collapsed to form black holes. These so-called primordial black holes would be much smaller than stellar-mass black holes, and could have died to form white holes within the lifetime of the universe, Rovelli and colleagues noted.
Even white holes with microscopic diameters could still be quite massive, just as black holes smaller than a grain of sand can weigh more than the moon. Now, Rovelli and co-author of the study Francesca Vidotto, of the University of the Basque Country in Spain, suggest that these microscopic white holes could constitute dark matter.
Although dark matter is thought to constitute five-sixths of all matter in the universe, scientists do not know what it is made of. As its name suggests, dark matter is invisible; does not emit, reflect or even block the light. As a result, dark matter can currently only be traced through its gravitational effects on normal matter, such as that of stars and galaxies. The nature of dark matter is currently one of the greatest mysteries of science.
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