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 ...
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