Dark Matter Could Be Made Of Primordial Black Holes From The Multiverse

Artist impression of baby universes forming during cosmic inflation. Kavli IPMU

Dark matter is believed to be a mysterious substance surrounding galaxies, making up most matter in the universe. It is estimated to outweigh regular matter five to one. We have many hypotheses about what it is, but very few certainties. A team of researchers based in Japan has now proposed something quite radical.

Dark matter could be explained by numerous black holes, with masses smaller than the Moon. These could have formed in the first instants after the Big Bang. Then, the universe experienced an incredible moment of expansion – the so-called cosmic inflation, responsible for planting the seed of future galaxies.

The idea of Primordial Black Holes (PBHs) explaining dark matter is not new, but in a paper published in Physical Review Letter, the team discusses a very intriguing hypothesis on how they formed. They hypothesized that they are leftovers of a budding multiverse that was branching off from our Big Bang.

The first thing that we need to know is that in the fraction of a second just after the Big Bang, the universe was extremely dense. So dense that a positive fluctuation in the local density of just 50 percent would lead to the formation of a black hole. This is large compared to what’s necessary to form galaxies and everything else we see, but it is not out of the realm of possibilities. Several processes could possibly generate this fluctuation.

The one that got the team excited is the idea of a Big Bang cascade. The inflation was so incredible that it is possible that offspring universes formed following the birth of our own. And now here's the kicker: these baby universes might appear different if you are inside them or outside them. This is sanctioned by Einstein’s theory of gravity. So, what do these baby universes look like? You might have guessed that they would appear from the outside as small black holes.

The paper details calculations suggesting that it is possible for inflation to have created so many PBHs, it explains all of the dark matter in the Universe. So the next step was to actually look for the possible presence of these black holes. Not something easy, since these black holes are as small as a human hair is wide.

But taking that into consideration, there’s a way to do it. The Hyper Suprime-Cam (HSC) of the Subaru Telescope is capable of imaging the entire Andromeda Galaxy every few minutes. If one of these black holes happened to be right in front of a star in andromeda, its light would be magnified.

One candidate for such an event was already detected, but one possibility obviously doesn’t confirm a theory. Many more observations are necessary to start looking at this idea like a very promising scenario for dark matter.

An impression of how a tiny PBH could be spotted by HSC. KAVLI IPMU/HSC Collaboration
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