Black holes are not eternal. They slowly lose mass and energy in the form of heat, a phenomenon known as Hawking radiation. This process would eventually lead to black holes evaporating and disappearing over unimaginable time scales.
But what if black holes are not the only things that can evaporate in this way? What if the whole universe is doomed to the same fate?
This is the intriguing possibility raised by a new theoretical paper by physicists at Radboud University. They argue that Hawking radiation is not exclusive to black holes, but can also occur far away from them, due to the curvature of space-time and the tidal forces of gravity.
“We demonstrate that, in addition to the well-known Hawking radiation, there is also a new form of radiation,” co-author Michael Wondrak said in a statement. “We show that far beyond a black hole the curvature of spacetime plays a big role in creating radiation. The particles are already separated there by the tidal forces of the gravitational field.”
The paper, published in Physical Review Letters, shows that this new form of radiation can happen around any massive object in the universe, not just black holes. This means that stars, planets, and even galaxies could emit Hawking-like radiation and slowly evaporate over time.
“That means that objects without an event horizon, such as the remnants of dead stars and other large objects in the universe, also have this sort of radiation. And, after a very long period, that would lead to everything in the universe eventually evaporating, just like black holes. This changes not only our understanding of Hawking radiation but also our view of the universe and its future,” third co-author Heino Falcke added.
The temperature of Hawking radiation is inversely proportional to the mass of the object, so the bigger the object the smaller the emission. For example, a black hole with the mass of the Sun would have a temperature of about 60 nanokelvins, much colder than the cosmic microwave background radiation. Therefore, it would actually gain more mass from absorbing this radiation than it would lose from emitting Hawking radiation.
The same would apply to other objects in the universe, such as stars and galaxies. They would be too massive and too cold to emit significant amounts of Hawking-like radiation. However, as the universe expands and cools down, these objects would eventually become isolated and start to lose mass and energy.
The authors estimate that this process would take much longer than the age of the universe so far. For example, a neutron star with a mass of 1.4 times that of the Sun would take about 10^106 years to evaporate completely. That’s a one followed by 106 zeros, or a million times longer than a googol years.
So while this scenario is not something we need to worry about anytime soon, it is a fascinating implication of quantum mechanics and gravity that could have profound implications for our understanding of the ultimate fate of the cosmos.