Garriga et al, arXiv:1512.01819v2
It's hard to say what's the most exciting element of this new paper on parallel universes, the inflationary multiverse, and black holes, by Tufts cosmologist (and FQXi member) Alex Vilenkin and colleagues. Is it the idea that black holes hide baby universes inside them -- inflating their own spacetimes -- connected to our universe by wormholes? Could it be that, according to the authors, astronomers may soon be able to find evidence to confirm this crazy notion? Perhaps it's the fact that this paper could be presenting the first way to find definitive evidence that an inflationary multiverse of parallel worlds exists. Oh yes, and the authors also say that such black holes could have seeded supermassive black holes -- the origin of which remains a mystery -- *and*, in some of the scenarios they've looked at, they could comprise dark matter, the invisible stuff that makes up most of the matter in the universe.
Phew! No wonder the paper by Vilenkin along with Jaume Garriga, at the University of Barcelona, and Jun Zhang also at Tufts, is almost 50 pages long! ("Black Holes and the Multiverse" arXiv:1512.01819v2.)
Let's take this piece by piece. Vilenkin sent me the paper, which he has just posted to the physics preprint server, arXiv, because, for him, what's exciting is that it provides a "new way to test multiverse models observationally." Their analysis is based on inflation theory -- the idea that our universe underwent a phase of rapid expansion, or inflation, in its early history. This is now a pretty mainstream notion, which serves to solve a number of mysteries about the state of our universe today. It has also had good observational backing since various satellites have now measured the slight temperature differences in the afterglow of the big bang -- the cosmic microwave background radiation -- and found patterns that match those predicted by inflationary models. (There are still alternative proposals out there to explain these features, however. See Sophie Hebden's "Faster than Light" for an example.)
Slightly more controversial is the idea that inflation forces us to accept that we live in a multiverse of neighbouring universes with potentially very different physical parameters than our cosmos. This stems from the realisation, by Vilenkin and others, that inflation is unlikely to have been a one-off event. Just as the patch of space that we now call home once inflated to create an entire cosmos for us to wonder at, other neighbouring patches are probably inflating all around us, creating parallel bubble universes nearby.
The multiverse idea has been criticised because it's tough to test. Almost by definition, parallel bubbles are spacetimes that are divorced from ours, and so we can't interact with them directly. That hasn't stopped cosmologists like Vilenkin, and our own Anthony Aguirre, from coming up with inventive ways we might be able to detect them. For instance, two neighbouring bubbles might collide and leave a scar on our universe, which we could pick out of the cosmic microwave background data. (See "When Worlds Collide" by Kate Becker.)
In their new paper, Garriga, Vilenkin, and Zhang have investigated another possible consequence of inflationary cosmology -- providing a new mechanism for the formation of black holes in our universe. We often talk about stellar mass black holes that were formed from the collapse of stars. There are also supermassive black holes that can be found at the centre of galaxies, which can have masses up to a billion times that of the Sun. Astrophysicists aren't quite sure how those latter behemoths are formed.
According to Garriga, Vilenkin and Zhang, black holes could also have been formed by little bubbles of vacuum in our early universe. These would have expanded during our universe's inflationary phase (as the cosmos they were embedded in was also growing around them). When inflation ended in our cosmos, these bubbles would -- depending on their mass -- have either collapsed down to a singularity (an infinitely dense point that we think lies at the core of a black hole) -- or, if they were heavier than some critical mass, the bubble interior would continue to inflate into an entirely new baby universe. This universe would look to us, from the outside, like a black hole, and would be connected to our universe by a wormhole. (See the image, taken from the paper, at the top of this post.)
The team has also examined another mechanism in which black holes are formed inside spherical "domain walls" that are thought to be created during inflation. A domain wall is like a fracture or defect in space, created as the universe cools. You can think of it like a defect created in a cube of ice, where the crystal structure in the solid has misaligned as the water froze.
The paper takes a detailed look at some of the possible properties of such black holes formed by these novel processes, including the masses they might have, and the sort of observable signs they might give out that astronomers could pick up. They caution that they would need to carry out comprehensive computer simulations to work out all possible signatures and the possible effects of, for instance, energy being siphoned off from our universe through the wormhole. But a preliminary analysis suggests that these novel black holes could provide noticeable signatures, in the form of gamma rays given out by the black holes, or distortions induced on the cosmic microwave background spectrum created by radiation that was emitted as gas accreted onto large black holes in the early universe.
By looking at observational evidence that is already out there, the team can rule out inflationary black holes with certain parameters, but others are still allowed. Those that remain viable could have seeded today's supermassive black holes, the team says. And for certain model parameters they have investigated, the number and mass of black holes they expect to see suggests that these black holes could make up the missing dark matter in the universe.
The authors also calculated that the baby universe could contain very different physical parameters from each other. Thus the network of baby universes within black holes, linked by wormholes, would create an inflationary multiverse.
"We note that the mass distributions of black holes resulting from domain walls and from vacuum bubbles are expected to be different and can in principle be distinguished observationally," the teams writes in their paper. "If a black hole population produced by vacuum bubbles or domain walls is discovered, it could be regarded as evidence for the existence of a multiverse."
It's worth noting here that this isn't the first time that physicists have suggested that black holes lead to parallel universes. For example, FQXi members Lee Smolin and Jorge Pullin have independently had similar ideas in the past. On the podcast, on the June 2013 edition, you can hear Pullin talking about how loop quantum gravity predicts that black holes are tunnels to parallel worlds. (Smolin is also on that edition, talking about his book.) But this is the first analysis carried out using inflationary theory.
You can also read about Nobel Laureate Frank Wilczek's ideas for detecting quantum parallel worlds by looking for energy leaking between worlds. Plus, you can listen to Howard Wiseman on the podcast talking about tests for interacting parallel worlds.