LONG before the first stars were born, something else may have lit up the darkness – gamma rays from a swarm of microscopic “primordial” black holes, each weighing no more than a small asteroid and forged in the violence of the big bang.
Physicists predict that primordial black holes would have spewed out radiation, including high-energy gamma rays, while shrinking and becoming even hotter.
Katherine Mack of Princeton University in New Jersey and Daniel Wesley of Cambridge University think that such gamma rays would have warmed the soup of hydrogen gas that filled the universe at the time. Future radio observatories may be able to measure this temperature signature indirectly. That’s because the hydrogen gas would also have been absorbing some of the radiation left over from the big bang, which astronomers can observe today. If the gas had been warmed by the gamma rays, it would have become less able to absorb this relic radiation – a signature which we may one day see if highly sensitive observatories are built. This may provide the first evidence that primordial black holes existed, say the researchers.
Advertisement
Of course other things could have warmed the hydrogen gas at this time, such as dark matter particles, which some physicists reckon would have been decaying and emitting radiation. But if the black holes were all born with the same mass, as some models predict, they would all evaporate at the same time, producing a sudden spike in temperature after a period of steady warming, rather than the long-term warming expected from dark matter decay.
“The dark ages are such a great time to look for exotic physics because very little else is happening,” says Mack. The pair presented the idea at the Fifth Harvard-Smithsonian Conference on Theoretical Astrophysics in Cambridge, Massachusetts.
Avi Loeb of Harvard University says that the existence of primordial black holes is still speculative. “But nevertheless, it’s a legitimate possibility that one should explore the consequences of,” he says. Spotting the gamma-ray signature could also confirm that black holes do evaporate, which has yet to be observed.
Cosmology – Keep up with the latest ideas in our .
