Hawking radiation causes black holes to eventually evaporate. This is because particle pairs are spontaneously created near the event horizon (the position of the last ray of light that can escape the black hole’s gravitational pull). A particle and its antiparticle are created for a short moment in time and disappear immediately afterwards. But sometimes a particle falls into the black hole, allowing the other particle to escape: this is Hawking radiation. According to Hawking, this would ultimately mean that no black holes would remain in the Universe.
Rigorous model
Astronomer Heino Falcke, physicist Michael Wondrak and mathematician Walter van Suijlekom had previously demonstrated that the event horizon plays a subordinate role in the origin of the radiation. They have now also provided a mathematical proof for a similar problem. Van Suijlekom: ‘We wanted to formulate a mathematically rigorous model as precisely as possible. We wanted hard mathematical proof in the case that only a temporal horizon exists and that the Universe in the end resembles its initial state.’
19th-century mathematics
To this end, the mathematician proposed a cosmological model in which he first switched the gravitational field on and then off again. ‘You start with nothing, then there is a period with a strong gravitational field, and after you ‘switch it off’, it turns out that something remains.’
To achieve this, the researchers used methods developed for another well-known effect: the Schwinger effect, albeit slightly more complex as it had to apply to gravity. ‘In the Schwinger effect, matter is spontaneously created by a strong electric field. Interestingly, it is precisely here that switching the electric field on and off plays a key role in the calculations. So we looked for a way to incorporate gravity into a similar model.’ That was no easy feat mathematically, but one of Van Suijlekom’s colleagues had the answer. ‘During a chat by the coffee machine, a colleague mentioned a 19th-century equation that had laid the foundations for what I was looking for. That was exactly what we needed.’
Beauty of maths
After Van Suijlekom simulated this equation on the computer, he saw that it really did work. ‘That’s the beauty of it. The further you get into math, the fewer moments like that you have. It’s great when it all comes together.’
‘The link between these two long-standing theories – the Schwinger effect and Hawking radiation – has already yielded many new insights,’ says Michael Wondrak, ‘but there is still much to discover in this interplay between astronomy, quantum physics and mathematics. So we’re going to continue.’