For a few hundred thousand years after its explosive birth, the universe was too hot a place for atoms to survive. High-energy radiation filled the young cosmos. But as the universe cooled and expanded, protons, electrons, and other particles eventually combined into atoms of hydrogen and helium. Given the amount of time the universe has been expanding and cooling since the Big Bang--and the amount of hydrogen and helium theorists predict was created--there seems to be a shortfall of the two elements. Fifty percent is missing.
Astrophysicist Shin Sasaki at the Tokyo Metropolitan University and his colleague Masayuki Umemura at the University of Tsukuba believe they have an explanation for the discrepancy. Energy unleashed by a primordial population of black holes could, they say, explain the problem.
How? Many cosmologists believe that a high-energy event some time after the Big Bang set back the accumulation of hydrogen atoms by making protons and electrons too energetic to stay in stable groups. By this argument, some of those particles wouldn’t have cooled enough to form atoms to this day. We call this the cosmological missing link, explains Sasaki.
What provided that energy? According to Sasaki and Umemura’s calculations, an enormous population of primordial black holes, each with the mass of 100,000 suns. About 2.5 million years after its birth, the universe would have been one three-hundredth of its current size, and radiation given off by atoms falling into the black holes would have heated the universe enough to prevent the formation of hydrogen and helium.
The theory is controversial because cosmologists have doubted that black holes could form at that stage in the universe’s existence. Particles of matter would have been streaking around so fast that gravity wouldn’t make them clump together. But the Japanese astronomers calculate that electrons and protons could have been slowed by interacting with photons--particles of energy--and felt a drag akin to air friction. The protons and electrons could then have formed the seeds of massive black holes, which would have been ten times as numerous as the number of galaxies in the universe today.
So where are those black holes now? Once the gas near a black hole is consumed, the hole ceases to emit radiation and becomes difficult to detect. Umemura and Sasaki say that a careful study of the motions of distant galaxies may reveal the effects of these quiescent black holes--and bolster their theory.