Scientists at the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo, in Japan, have proposed a new theory to explain the mysterious phenomenon of dark matter. Dark matter makes up some 27% of the universe, whereas ‘normal’ matter is responsible for less than 5%. (The remaining 68% of the universe comprises dark energy.) Dark matter is so-called since we can detect it only because of its gravitational influence on normal matter.
The theory now advanced by the researchers at Kavli is that dark matter can be explained, in part or in whole, by primordial black holes (PBHs). These would have formed in the early universe, before the birth of stars and galaxies. This would have been possible because the early universe was sufficiently dense that, in the words of the institute’s press release, “any positive density fluctuation of more than 50 percent would create a black hole”.
However, the density perturbations that resulted in the creation of stars and galaxies have been determined to be much smaller than those needed to create black holes at such an early stage of the universe’s existence. But there were other processes under way at that time which could have created PBHs. These would have been consequences of the period of cosmic inflation that followed immediately after the Big Bang, the event that marked the start of our universe.
Cosmic inflation is the period of about 10-33 to 10-32 seconds after the Big Bang, during which the fabric of our universe expanded at a speed faster than that of light. One consequence of the theory of cosmic inflation is that it allows the creation of other universes, budding off from ours – a phenomenon known as the multiverse.
PBHs could have been created from such ‘baby’ or ‘daughter’ universes which branched off from ours during the period of cosmic inflation, but were too small and subsequently collapsed. The energy released in such a small volume would have been enough to create a black hole.
There is an even more exotic phenomenon that could have created PBHs during cosmic inflation. Daughter universes above a certain size would not collapse. Albert Einstein’s theory of gravity would allow these to continue and, to an observer inside such a daughter universe, appear to be a normal expanding universe. But to an observer in our universe, it would appear to be a black hole. Such a daughter universe would be invisible to us because it would be hidden by its ‘event horizon’ – the invisible boundary beyond which everything, including light, is trapped and cannot escape from the black hole.
PBHs can be very small, with masses as small as that of our Moon. They must not be confused with the much better known supermassive black holes, usually found at the centre of galaxies, including our own Milky Way.
An essential aspect of the Kavli researchers’ theory is that it can be tested. And a suitable instrument to test the theory already exists. It is the Hyper Suprime-Cam (HSC) of the 8.2 m diameter Subaru Telescope. This is an optical-infrared telescope operated by the National Astronomical Observatory of Japan and located on the summit of Mauna Kea on the island of Hawaii in the US state of the same name. The HSC is a 870-megapixel ultrawide-field digital camera.
“The HSC has a unique capability to image the entire Andromeda galaxy every few minutes,” explains the Kavli press release. “If a black hole passes through the [HSC’s] line of sight to one of the stars [in Andromeda], the black hole’s gravity bends the light rays [from that star] and makes the star appear brighter than before for a short period of time. The duration of the star’s brightening tells the astronomers the mass of the black hole.”
The HSC can simultaneously observe 100-million stars. This means that it has a real chance of detecting a PBH passing in front of a star. Already, a “very intriguing candidate event consistent with a PBH from the ‘multiverse’” has been detected. It had a mass similar to that of the Moon. A new round of observations is now under way, extending the search for PBHs, to confirm their existence and determine if there are enough of them to explain dark matter.