This story is part of our Cosmic Perspective series, in which we confront the staggering vastness of the cosmos and our place in it. Read the rest of the series here.
We think our universe contains everything that exists, has ever existed and will exist in the future. But this might not be the case: there are many ways other universes could exist.
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One is that we could be a single part of a branch of infinite universes known collectively as the multiverse. These universes might have appeared shortly after the big bang, they might be hiding in extra dimensions or they could pop into existence whenever a quantum property goes from a cloud of possible states to a single reality.
Multiverse ideas gained scientific weight in the 1980s with the invention of inflation, a period when the early universe suddenly expanded. Inflation explains why the cosmos is so flat and smooth, but it also predicts the creation of a multitude of independent bubble universes.
Cyclic universes
Yet inflation is just one route to a multiverse, and it has its critics. In recent years, many cosmologists have turned to alternatives like cyclic universe theories, which say the universe is on an unending cycle between ballooning and compressing. These theories still invoke multiple universes, but at different times.
“What I didn’t like about inflation was that there are very few genuine predictions – you don’t get out much more than you put in,” says , a physicist at the University of Edinburgh, UK, who helped develop a , published in 2001, as a rival for inflation. “It just struck me that there has to be a better explanation.”
The cyclic universe has its benefits. Dark energy, the mysterious force causing our universe’s expansion to accelerate, appears much weaker than predictions, which is a problem. It has been suggested since the 1980s that dark energy could in a series of “jumps”. Then, in 2006, Turok and at Princeton University combined this idea with a cyclic universe. They showed that, as dark energy got smaller, the time between jumps would get longer and longer, so a cyclic universe would naturally spend the most time with low dark energy, making what we see today more likely.
Finding other universes
In the hunt for evidence for a multiverse, one place cosmologists are looking is the cosmic microwave background (CMB), the light left over from the big bang. Cosmologist at the University of Cambridge and her colleagues have shown that colliding bubble universes, like those predicted by inflation, circle-shaped imprints in the CMB. In 2011, they discovered four patches of sky that may one day reveal such scars. Now, Peiris is involved in a quantum experiment to establish how bubbles of vacuum form and collide – insights that could improve the multiverse predictions. “The thing that needs to be sharpened up is the theoretical predictions,” she says. This could strengthen the evidence for the imprints or rule them out.
When it comes to cyclic models, Roger Penrose, a mathematical physicist at the University of Oxford, has suggested signs of these could also pop up in the CMB. Others have said black holes from previous cycles could potentially survive into this universe and be detectable.
For his part, Turok now favours a newer concept called a mirror universe – where time reverses at the big bang – a model he believes simpler than the cyclic universe. And it may soon be put to the test.
A mirror universe could be the answer to the mystery of dark matter, the unknown substance that makes up some 85 per cent of the mass in the universe. The idea is that dark matter could be made up of as-yet undiscovered massive neutrinos, which could be in abundance in a mirror universe. In 2022, a group of researchers showed that this would mean the lightest neutrino has exactly zero mass – a prediction currently being tested in large-scale galaxy surveys such as the Vera C. Rubin Observatory, due to start scanning the universe next year. One hundred years on from Edwin Hubble’s proof that other galaxies are out there, we could be on the cusp of finding evidence of another realm beyond our universe.
