White holes, which are theoretically the exact opposite of black holes, can make up a significant portion of the mysterious dark matter that is believed to make up most of the matter in the universe. Some of the white holes may have even predated the Big Bang, the researchers said.
Black holes have a gravitational pull so powerful that even light, probably the fastest thing in the universe, cannot escape their impact. The invisible spherical boundary that surrounds the black hole's core and marks its point of no return is known as the event horizon.
The existence of black holes was predicted by Einstein's general theory of relativity. A white hole is a black hole in reverse: while nothing can leave the event horizon of a black hole, nothing can enter the event horizon of white holes.
Previous studies have shown that black and white holes are linked, with matter and energy falling into a black hole, potentially emanating from a white hole either somewhere in space or in another universe. In 2014, Carlo Rovelli, a theoretical physicist at the French University of Aix-Marseille, and his colleagues suggested that black and white holes might be related differently: when black holes die, they can become white holes.
In the 1970s, Stephen Hawking suggested that all black holes should evaporate mass by emitting radiation. Thus, black holes that lose more mass than they gain should shrink and eventually disappear.
However, Rovelli and his colleagues suggested that shrinking black holes might not disappear if the fabric of spacetime is quantum - that is, made up of indivisible quantities known as quanta. The study attempts to combine general relativity, which explains the nature of gravity, with quantum mechanics, which describes the behavior of all known particles, into a single theory capable of explaining all the forces of the universe.
According to the authors, when a black hole evaporates to such an extent that it can no longer shrink because space-time cannot be compressed into something even smaller, the dying black hole transforms into a white hole.
According to modern theories, black holes form when massive stars die in giant explosions known as supernovae that compress their corpses into infinitely dense points known as singularities. Rovelli and his colleagues previously calculated that a black hole with a mass equal to the sun's would take about four times as long as the current age of the universe to turn into a white hole.
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However, in the works of the 60-70s, it is assumed that black holes could have arisen in a second after the Big Bang due to random fluctuations in density in a hot, rapidly expanding newborn Universe. The areas where these vibrations concentrated matter formed black holes. These so-called primordial black holes could be much smaller than stellar mass black holes and die to form white holes. But even microscopic white holes can be quite massive, just as black holes smaller than a grain of sand can weigh more than the Moon. Now Rovelli and study co-author Francesca Vidotto of the University of the Basque Country in Spain believe that these microscopic white holes form part of dark matter, the nature of which is one of the greatest scientific mysteries.
In addition, Rovelli and Vidotto have suggested that some white holes in this universe predate the Big Bang. Such white holes from a previous universe may help explain why time only flows forward in the modern universe.