“If our universe is just one of an infinite number, then once inflation stopped in ours, pockets within it that had been inflating would have then collapsed into black holes. The longer each pocket inflated, the more massive the black hole,” suggested Alexander Vilenkin, professor of physics and director of the Institute of Cosmology at Tufts University and author of Many Worlds in One: The Search for Other Universes.

Many of these universes collapsed and formed black holes. If the black holes are big enough, they may have inflating universes inside of them, and these expanding universes would be connected to the visible universe by wormholes. Infinite ‘bubble universes,’ filled with alternate versions of ourselves or nothing at all, might exist right alongside our own, according to Vilenkin, who introduced the idea of quantum creation of the universe from a quantum vacuum in 2015.

Cosmologists like Vilenkin have painted a vivid picture of the origin of cosmic structures such as galaxies, stars, planets, and life where everything around us arose from random quantum fluctuations during the first 10-35 seconds after the birth of the universe. Galaxies like the Milky Way grew from these faint wrinkles in the fabric of spacetime. The density of matter in these wrinkles was slightly greater compared to surrounding areas and this difference was magnified during inflation, allowing them to attract even more matter. From these dense primordial seeds grew the cosmic structures we see today.

Inflation would thus leave behind a population of black holes with a telltale range of masses. In principle, by measuring the ripples in space and time produced in black hole collisions—like the gravitational waves discovered by LIGO last year—astronomers can take a census of black hole masses and see if they were created by inflation, which would imply the multiverse.

But there is also a universe that is limited by what people can see: the visible universe. Around 13.8 billion years ago there was a Big Bang, a really hot, dense phase when the universe started expanding. Everything in the visible universe has to be younger than that. But, theoretically, people suspect that the universe seen with telescopes – what we see within the horizon of the Big Bang – may not be all there is.

The theory of eternal inflation, proposed in part by Vilenkin, could begin to answer these questions. Eternal inflation says that space is always expanding overall, but some pockets of space will expand and create universes while others stop expanding. The universes that form are called “bubble universes” because they bubble up where energy is being concentrated.

“We used to think that beyond the visible universe there was simply more of the same,” said Vilenkin. “More planets and stars and galaxies. But other universes may have different physical laws.”

Not all bubble universes are created equally. If the mass of a proton or electron were tweaked, the universe might not have stars, planets or life. Some of the universes expand, contract and collapse in a very short time before forming everything. Some universes are like ours.

“Once a reasonable idea comes, you can never say it’s wrong,” affirmed Vahe Petrosian, Professor of Physics and Applied Physics at Stanford University who died in 2018. “And this is not too crazy. We will probably never have answers to these questions, but it is important to ask them. So we do ask, and sometimes we are successful. It’s possible that sometimes these bubbles interact, and that one will interact with our bubble and produce observational evidence. “Say, what are the possibilities, what happens if these bubbles interact, what sort of information will it give us.”

The Daily Galaxy, Sam Cabot, via Tufts University, MIT, and NPR Science Friday

Image at the Top of the page: The new version of Hubble’s deep field image. In dark grey you can see the new light that has been found around the galaxies in this field. That light corresponds to the brightness of more than one hundred billion suns. It took researchers at the Instituto de Astrofísica de Canarias almost three years to produce this deepest image of the Universe ever taken from space, by recovering a large quantity of ‘lost’ light around the largest galaxies in the iconic Hubble Ultra-Deep Field.