Physicists are still hunting primordial black holes to solve the dark matter problem

For a while now, physicists have been hunting for primordial black holes, exotic objects that could have formed in the early Universe and spawned a whole range of cosmic shenanigans.

Using a giant 8.2-metre-wide (that's 27 feet) telescope, physicists from the University of California, Los Angeles, and the Kavli Institute for the Physics and Mathematics of the Universe in Japan are searching for signs of these objects; discovering them could even suggest our Universe was breeding baby universes when it was a wee youngster itself.

What they hope to see won't exactly be as scandalous as peeping into alternative realities. But if their new models are correct, and they're patient enough, they might find a primordial black hole (PBH) floating between us and a nearby galaxy.

Discovering such an object has the potential to fill in several gaps in our knowledge on a whole range of phenomena, from the nature of dark matter to the distribution of heavy elements throughout space.

More tantalisingly, it could also be a clue as to whether our own Universe is just one of many in a branching family tree of multiverses once spawned as its babies during the cosmic inflation - although plenty of debate would still remain on the latter point.

Primordial black holes have a lot in common with run-of-the-mill black holes formed by collapsing stars. They are both intense concentrations of matter that pinch surrounding space-time into a singularity, for example.

Singularities are themselves curious objects, comprising of points where the space-warping physics of general relativity meet the more granular metrics of quantum mechanics. Unfortunately these two master theories don't agree on certain crucial details of reality, so nobody is precisely sure what a singularity is.

Even the surrounding warping of space and time makes a mess of our intuitions, leaving room to speculate that each black hole is an umbilicus to an entirely separate universe.

It's not as far-fetched as it sounds. There are plenty of good reasons to think once a tumbling observer crosses the event horizon – a line of no return – space and time become indistinguishable from an expanding universe like ours.

That would mean that every time a star collapses to form a singularity, our Universe becomes a parent. Mazel tov!

Where PBHs differ is that they would have been spawned back when our Universe was maybe around a second old, a time when radiation dominated (and not much else).

Given enough of a shove in any one area, that concentrated sea of light could tip over the edge into a singularity. And because conditions were already extreme, the amount of mass required would be far below that needed for even the smallest stellar black holes.

Primordial black holes are interesting ideas in desperate want of solid evidence. Unfortunately smaller holes would have long since evaporated in a puff of Hawking Radiation. And anything large enough we'd surely have noticed by now.

But there are possibilities researchers have yet to rule out.

In this new model, the team returned to a theory where quantum effects in empty space could create something of a vacuum bubble, providing a seed for collapse.

Their maths shows these conditions during a period of rapid inflation could reasonably create primordial black holes of a range of masses. Interestingly, some would match what we'd expect of dark matter.

It's an old idea that's been kicked around for a while, to the extent that it's looking increasingly unlikely as a candidate. If a population of these itty-bitty black holes does behave like dark matter, it'll probably only account for a proportion of it.

Just to add to the scepticism, the method the team wants to use to search for these objects has also been attempted before.

Last year, researchers used the Subaru Telescope's Hyper Suprime-Cam to collect nearly 200 snapshots of our neighbouring galaxy Andromeda over the course of seven hours, just to see if a PBH with the mass of our own Moon might float by.

Aside from a single 'maybe', the experiment didn't find anything overly exciting.

But with this new model, the researchers argue if we wait a little longer – like around 88 hours – we just might get lucky this time. Or at least rule out their prediction.

Identifying a primordial black hole of this size would provide cosmologists with an object that could help to explain a range of perplexing problems. Not only might it contribute to our understanding of dark matter, their collisions with neutron stars might explain fast radio bursts.

We might have already seen a smash-up between these light-weight black holes in a signature of a gravitational wave event that had all the hallmarks of a neutron-star merger, without the flash.

As to whether these ancient black holes truly house the babies of our own Universe, we'd need some pretty revolutionary physics to confirm. But the kinds of black holes produced in this scenario would be just what we're looking for.

Fingers crossed Hyper Suprime-Cam just might contribute a little something to the family album.

This research was published in Physical Review Letters.

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