For some time now, physicists have been hunting primordial black holes, exotic objects that could have formed in the early Universe and spawned a whole host of cosmic regions.
Using a gigantic telescope 8.2 meters wide (that’s 27 feet), physicists at the University of California, Los Angeles, and the Kavli Institute for the Physics and Mathematics of the Universe in Japan search for signs of these objects; discovering it might even suggest that our universe was breeding baby universes when it itself was a small pup.
What they hope to see isn’t exactly as scandalous as a peek into alternate realities. But if their new models are correct and they are patient enough, they can find a primordial black hole (PBH) floating between us and a nearby galaxy.
Discovering such an object has the potential to fill in various gaps in our knowledge about a range of phenomena, from the nature of dark matter to the distribution of heavy elements throughout space.
Even more tantalizingly, it could also indicate whether our own universe is just one of many in a branching family tree of multiverses once spawned as babies during cosmic inflation – although the latter point would still be much debated.
Primordial black holes have a lot in common with regular black holes formed by collapsing stars. They are both intense concentrations of matter that, for example, squeeze the surrounding space-time into a singularity.
Singularities are themselves curious objects, consisting of points where the space-shifting physics of general relativity and the more granular metrics of quantum mechanics converge. Unfortunately, these two master theories disagree on certain crucial details of reality, so no one knows exactly what a singularity is.
Even the surrounding curvature of space and time messes up our intuitions, leaving room to speculate that every black hole is an umbilical cord of a completely separate universe.
It’s not as far-fetched as it sounds. There are many good reasons to think that once a tumbling observer crosses the event horizon – a line with no return – space and time are 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 turned off when our universe was perhaps about a second old, a time when radiation dominated (and not much else).
Given enough of a push in any given area, that concentrated sea of light could tip over the edge into a singularity. And since the conditions were already extreme, the amount of mass required would be well below the mass required for even the smallest stellar black holes.
Primordial black holes are interesting ideas in the absence of solid evidence. Unfortunately, smaller holes would have evaporated long ago in a puff of Hawking Radiation. And we would certainly have noticed anything big enough by now.
But there are possibilities that researchers have yet to rule out.
In this new model, the team returned to a theory in which quantum effects in empty space could create something like a vacuum bubble, forming a seed for collapse.
Their math shows that during a period of rapid inflation, these conditions can reasonably create primary black holes of a range of masses. Interestingly, some would match what we would expect from dark matter.
It’s an old idea that has been lying around for a while, to the extent that it looks increasingly unlikely as a candidate. If a population of these tiny black holes behaves like dark matter, that will probably only make up part of it.
To add to the skepticism, the method the team wants to use to search for these objects has also been tried before.
Last year, researchers used the Subaru Telescope’s Hyper Suprime Cam to collect nearly 200 snapshots of our neighboring Andromeda galaxy over the course of seven hours, just to see if a PBH could float past the mass of our own moon.
Other than a single “maybe” the experiment was nothing overly exciting.
But with this new model, the researchers argue that if we wait a little longer – like about 88 hours – we 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 explain a series of confusing problems. Not only could it contribute to our understanding of dark matter, their collisions with neutron stars could also explain fast radio bursts.
We may have already seen a smash-up between these lightweight black holes in a signature of a gravitational wave event that had all the hallmarks of a neutron star fusion, without the flash.
Whether these ancient black holes really house the babies of our own universe, we need some pretty revolutionary physics to confirm. But the kind of black holes produced in this scenario would be exactly what we’re looking for.
Fingers Crossed Hyper Suprime-Cam might add something to the family album.
This research is published in Physical Review Letters.