BLACK holes may be hiding other universes. A quirk of how space-time behaved in the early universe could have led to short-lived wormholes connecting us to a vast multiverse.
If borne out, the theory may help explain how supermassive black holes at the centres of galaxies grew so big so fast.
The idea that ours is just one of a staggering number of universes – what cosmologists call the multiverse – is a consequence of our leading theory of how the universe grows: eternal inflation.
The theory holds that during its early phase, space-time expanded exponentially, doubling in volume every fraction of a second before settling into a more sedate rate of growth. Eternal inflation was devised in the 1980s to explain some puzzling observations about our universe that standard big bang theory alone couldn’t handle.
But cosmologists soon realised that the inflationary universe came with caveats. Quantum mechanical effects, which normally only influence the smallest particles, played an important role in how all of space-time evolved.
One of these effects was that a small patch of space-time within the larger universe could shift into a different quantum state, forming a bubble. Such bubbles could form at random throughout our inflating universe.
“Our universe could even look like a black hole to physicists in some other universe”
That means that even after rapid expansion ended in our cosmos, a number of bubbles could keep inflating into their own baby universes. Each of these would give rise to other bubbles, spawning a sprawling multiverse.
“While inflation is going on, bubbles can pop out and expand in this inflating space,” says cosmologist Alex Vilenkin of Tufts University in Medford, Massachusetts, one of the pioneers of inflationary cosmology.
But proof has been hard to come by. Cosmologists have suggested that bubbles colliding with our universe could have left imprints in the cosmic microwave background, the leftover radiation of the big bang. However, such a signal would be very faint, and no conclusive evidence has yet been seen.
Vilenkin and his colleagues wondered if they could spot signs of the multiverse elsewhere in our universe. To investigate, they did a mathematical analysis of the fate of the bubbles formed during inflation.
They found that bubbles that form with an internal inherent energy lower than the inherent energy in our inflating universe will indeed begin to expand: the tension of space-time outside the bubble is greater than that inside, so the bubble walls are pulled outwards.
But when inflation ends in our universe, the tension dissipates, and the bubbles appear to start collapsing like deflating balloons.
A world within
That’s how it looks from the outside, from our vantage point, “but there is more to this picture”, says Vilenkin. The bubbles’ true fate depends on their size.
Bubbles that formed later would be smaller, and should collapse into standard black holes, with nothing inside apart from an infinitely dense point called a singularity.
But earlier bubbles would be bigger and would create larger black holes that conceal their own inflating universes.
For the first fractions of a second after inflation ended in our patch of space-time, when the bubbles began collapsing, we would have been connected to their interiors via wormholes. Unfortunately these wormholes would have closed almost immediately, cutting off the inflating universes within. “The opportunity has passed for us to send signals to these other universes,” says co-author Jaume Garriga of the University of Barcelona, Spain.
Even after the wormholes close, the space-time inside the black holes keeps inflating (arxiv.org/abs/1512.01819v1).
Andrei Linde of Stanford University in California, another pioneer of inflationary cosmology, is impressed. The work builds on ideas that were first thought up nearly 30 years ago, but Vilenkin and his colleagues have carried out the most detailed analysis yet of the bubbles’ fate. “It is beautiful general relativity,” says Linde. “General relativity sometimes offers you things that are extremely non-intuitive.”
The analysis provides a fresh way to look for signs of the multiverse by suggesting that our universe should have a distinctive distribution of black holes. The higher the mass of the black holes, the more of them there should be up to a critical mass, after which the number should fall. “That critical mass separates ordinary black holes from black holes that contain an inflating multiverse inside of them,” says Garriga.
This could help solve a long-standing mystery. Standard astrophysics has a hard time explaining how supermassive black holes became as big as they are today – there hasn’t been time for them to suck up sufficient matter. But in the new theory, the largest of the black holes that hide a universe within them would have started out much bigger than is otherwise possible. These giants could have grown to become the supermassive black holes we see today at the heart of galaxies, including our own Milky Way.
The work may also have implications for the black hole information loss paradox, which physicists have battled over for decades (see “Information lost and found“).
Don Marolf, who studies general relativity and black holes at the University of California Santa Barbara, points out that physicists have long wondered whether black holes conceal more than they reveal at their surfaces.
“This is essentially an extreme example of an ancient point, that black holes can have enormous interiors,” says Marolf.
Our universe could even look like a black hole to physicists in some other universe.
“This subject is really, really deep,” says Linde. “We are just starting to touch the surface and discover new things about the multiverse.”
Information lost and found
Black holes pose a conundrum: in gobbling up matter, they also gobble the information it encodes.
Quantum mechanics says that information cannot be destroyed, but Stephen Hawking famously showed that black holes emit radiation and eventually evaporate. So where does the information go?
Hawking and his colleague Jacob Bekenstein suggested that the information comes out in the radiation. But accepting this idea leads to other problems – such as the formation of a blazing firewall at the horizon of the black hole, something prohibited by Einstein’s general relativity.
Now we think black holes may hide entire universes beyond their horizons – worlds away from what we normally assume lurks beneath their surface (see main story). This may drive physicists to revisit the information loss paradox.
This article appeared in print under the headline “The hole wide multiverse”