In those first seconds of the Universe, there might have been another ingredient in the primordial soup: black holes.
These black holes from the very beginning of time, known as primordial black holes (PBHs), could still be lurking around today – and some scientists believe they could solve the problem of dark matter.
But in the first second of the Universe, stars didn’t yet exist – so how could black holes have formed back then.Kusenko and some others believe primordial black holes are now the most promising candidate for dark matter that we have.
The real appeal of PBHs is their simplicity – unlike other options for dark matter, which require making theories to describe new particles, we already have evidence that black holes are real.
Their story begins back in 1960s Russia, when two physicists – Yakov Zeldovich and Igor Novikov – first considered the possibility of extremely dense objects like black holes forming in the very early Universe.
A black hole about the mass of a mountain and the size of a proton, formed billions of years ago when the Universe began, should be showing the explosive end of its life right about now.
The key to making a primordial black hole is somehow making a small region of that hot early Universe slightly denser so that it can collapse into a black hole.
Another possibility is that inflation, the process that quickly expanded the early Universe just after it formed, created more fluctuations in density that could then go on to become black holes.One model, which Carr is currently working on, proposes that primordial black holes formed at a time known as the QCD transition, an absolutely minuscule 100,000th of a second after the Big Bang.Kusenko, on the other hand, is working on a model that adds just one new bit of physics – a new kind of interaction between particles known as ‘Yukawa forces’ – to squeeze together enough matter to form just the right amount of primordial black holes to account for dark matter.
One theory suggests that cosmic strings – cracks in the fabric of the early Universe – might sometimes loop around on each other to create black holes.
Another theory proposes that ‘baby universes’ from the multiverse could appear in our own Universe as primordial black holes.
Even if primordial black holes weren’t glowing from Hawking radiation, they would be emitting light as they eat up matter, heating up the area around them.One of the most dramatic ways primordial black holes can interact with matter, though, is by colliding with other objects in the Universe, like stars and planets.The other method for detecting primordial black holes uses gravitational waves generated when massive objects (like black holes) disrupt spacetime.Although scientists can’t agree on whether nearby black hole mergers are from stars or primordial black holes, finding evidence of merging black holes sufficiently far away would tip the scales towards primordial black holes.
Gravitational waves also can’t travel faster than light – so, if we see a black hole merger so far away that it’s from before stars formed, that would have to be a merger of primordial black holes.There is one more observation we could make with gravitational waves that would be surefire evidence of primordial black holes: finding a black hole the size of the Sun.
Stars couldn’t produce a black hole that small, so it would have to be from the early Universe.This leads us back to our original question – if primordial black holes do exist, can they solve the problem of dark matter.
Research based on Hawking radiation and microlensing rules out quite a few sizes of primordial black holes, but not all of them.
There are three (or four) masses of primordial black holes that could exist and play nice with current observations, and maybe even explain the mystery of dark matter, depending on whom you ask.Carr thinks that there are four masses of primordial black holes that could exist: asteroid-sized, moon-sized (about ~1/10th the mass of Earth), Sun-sized, and mind-bogglingly huge (more than a billion times the mass of our Sun).
Although the huge primordial black holes are interesting, and may explain what we see with gravitational waves, they’re not really a candidate for dark matter?Even then, quite a few scientists are sceptical about primordial black holes, which are sometimes dismissed as a bonkers fringe idea.However, even he agrees that there are some cons to primordial black holes as dark matter.
These small black holes are quite difficult to detect, and it’s looking somewhat unlikely that primordial black holes could make up all of dark matter given the current constraints.So even if primordial black holes can’t explain all of dark matter, they’d certainly have interesting effects on the Universe from the start.
Beyond forming dark matter, scientists think that giant primordial black holes could help explain how the biggest black holes in the centres of galaxies – like our own Sagittarius A*, recently imaged by the Event Horizon Telescope – got started.Constantly improving technology, moreover, should help us investigate primordial black holes.
Kusenko and collaborators, such as the physicist Misao Sasaki at the Kavli Institute for the Physics and Mathematics of the Universe in Japan, are searching for small black holes with the Subaru Telescope’s Hyper Suprime-Cam on Hawai‘i.
When a small primordial black hole passes in front of a star in our nearest neighbour galaxy, Andromeda, they should be able to spot how the starlight bends around the PBH.
Gravitational wave detectors could find ancient black hole mergers, black holes smaller than the Sun, or even signatures of when black holes first formed in the primordial soup of the Universe’s first second of existence