Cosmology deals with some of the trippiest ideas in science, but the notion of vacuum bubbles that can potentially create and destroy universes is pretty mind-boggling even for this field. Yet, these hypothetical bubbles of nothing in spacetime are a major topic of research and scientists are honing in on possible avenues to observationally confirm their existence.
The bizarre bubbles are manifestations of a concept in quantum field theory known as false vacuum decay. Consider a vacuum that is in a stable state, but that still contains slightly more energy than a “true” vacuum that occupies the minimum energy state in the universe. This “false” vacuum could be somehow kicked into the true vacuum state of minimum energy, and this transition—or decay—would cause a bubble of true vacuum to form (or nucleate) and expand out into space, consuming everything around it.
Some models suggest that our universe was born as a true vacuum bubble that exists within a multiverse containing many similar bubbles, while others posit that we currently inhabit a false vacuum that might one day be destroyed by the advance of a true vacuum bubble.
Now, scientists led by Dalila Pîrvu, a doctoral student at the Perimeter Institute for Theoretical Physics, have shed light on the formation and interactions of these hypothetical bubbles by running a new type of decay simulation that includes variables that other models omit.
The results reveal that bubbles might form in clusters, as opposed to popping up at random positions in spacetime, a finding that could help scientists figure out how to look for real observational signatures to confirm the existence of these bubbles. The team’s research is also “the first time biasing has been investigated in the context of bubble formation in vacuum decay,” according to the study, which was recently posted on the preprint server arXiv.
“No one has really treated this problem this way,” said co-author Jonathan Braden, a postdoctoral fellow at the University of Toronto’s Canadian Institute for Theoretical Astrophysics (CITA), in a joint call with Pîrvu. The code can “simulate really complex dynamics and apply it to this problem in a way that had not been done before.”
The new study tested an assumption, embedded in past research, that bubbles form randomly. By modeling vacuum fluctuations in space, the researchers showed that bubble nucleations can be correlated with one another. The team’s simulations are the first to model many bubbles at once, in contrast to previous studies that were limited to one-bubble configurations.
This process, which the team compared to galaxy formation, “goes against the assumption that [bubbles] will be uniformly distributed” across spacetime, said Pîrvu.
The team’s findings could help hunt down signs of false vacuum decay that might be embedded in the most ancient epochs of the cosmos. Scientists have proposed that the rapid inflation of the early universe may have been driven by an expanding bubble that we now all live in.
This form of bubble nucleation in a cosmological context is called the false vacuum eternal inflation scenario. It posts that our bubble universe may be one of many bubbles that are part of a larger multiverse: if these bubbles formed in clusters, it’s more likely that our universe collided with its neighbors.
“In that picture, the Big Bang for our universe is actually like a bubble forming,” Braden said. “This is not universally accepted but it is a possibility for how we have to think about the universe.”
“In that case, our observational signature is basically two of these bubbles colliding, so we want to know at what rate that happens,” he continued. “If you make one bubble and are more likely to nucleate another one, or less likely, that will change what these bubble rates look like.”
Another possibility is that bubbles emerged and collided within the early universe, potentially producing gravitational waves, which are ripples in spacetime that humans have recently learned to capture with observatories such as LIGO and Virgo.
While these detectors are probably not attuned to the stochastic waves from these speculative bubble collisions, instruments such as the European Space Agency’s Laser Interferometer Space Antenna (LISA), which is due for launch in the 2030s, might be able to capture them, if they exist. Scientists using LISA, or other next-generation detectors, will need to know what type of signals they should be searching for, which is why it’s important to simulate and constrain the properties of bubble clusters, which is one aim of the new study.
Understanding false vacuum decay has immense consequences for unraveling the mysteries of our universe, but its cosmic role will remain unclear in the absence of observations. Pîrvu and her colleagues hope their research will inform these future searches for one of the weirdest concepts in science, which cannot be easily tested in other ways.
“If you want it to reproduce this process”—meaning bubble nucleation
in a cosmological context—”technically you would have to make a new universe,” Braden concluded, “which is rather non-trivial.”