| What if reality is infinitely vaster than it seems? After all, Earth is but one of many planets, and the sun is but one of many stars. Perhaps the pattern repeats, and our universe — the unknowably vast expanse of space and time around us — is one of many universes, each a bubble of stability in a frothy and violently expanding multiverse.
The multiverse is one of the most controversial theories in physics. The hypothesized other universes would be too far away to observe or detect, so talk of their existence strikes some physicists as inherently unscientific. Still, many of the field's leading lights are convinced by the arguments in the multiverse's favor.
The idea that many bubble universes exist in far-apart regions of space (which, incidentally, is distinct from the concept of parallel universes that comes up in the "many worlds" interpretation of quantum mechanics) is an unavoidable consequence of the leading theory of the universe's birth: inflation.
In the 1980s, Alan Guth, Paul Steinhardt and Andrei Linde pioneered the theory that our universe began with a brief, rapid growth spurt, doubling in size many times in a split second. Then this process of inflation ceased, producing the state of cosmic calm that surrounds us today. (Our modern universe continues to expand, but slowly.) Physicists have sought — and failed to find — conclusive proof that inflation really happened, but most cosmologists consider it the origin story to beat, because it elegantly explains a handful of key features of the distribution of matter throughout the universe.
Yet inflation comes with an awkward side effect: Once it starts, it can't stop. The exponential expansion halts here and there, creating stable bubbles of space like ours, but it continues overall, creating space for new bubble universes to form. The result is an eternally inflating reality featuring a growing multitude of cosmoses.
In 1987, the Nobel laureate Steven Weinberg used the multiverse to make a fateful prediction. Quantum physicists had been puzzling over a paradox. They reasoned that quantum fluctuations should infuse empty space with a huge amount of energy, which would tear the universe apart through violent expansion. And yet no such "vacuum energy" had been detected. Weinberg proposed that in a multiverse, each bubble universe might have a different density of vacuum energy, causing it to expand at a different rate. Those that expand too quickly would be hostile to matter, stars and life. Static universes would be vanishingly rare. Weinberg hypothesized that we live in a mildly energized but still habitable universe, and he estimated the density of energy that's likely to infuse our vacuum.
Ten years later, observations of supernovas revealed that the universe is in fact expanding under the influence of some "dark energy" quite close in strength to what Weinberg had predicted. The multiverse hypothesis thereby became plausible to many physicists as an explanation for dark energy.
But the win felt like a loss to many, since it implied that the laws of physics in our universe might be random. Universes, like planets, could be all over the map.
What's New and Noteworthy
A key aspect of the multiverse is that the vacuum might be able to take on various states corresponding to different laws of physics. In other words, there could be many different types of nothing. I explored this concept in a 2022 article for Quanta.
One driver of interest in the multiverse hypothesis over the past decade or so has been the mass of a particle called the Higgs boson. Like the density of dark energy, the Higgs mass seems much, much smaller than theorists would naïvely expect it to be, and no one has found a convincing natural explanation yet for its peculiar smallness. Some theorists suspect that the Higgs mass, like the vacuum energy, varies from universe to universe, as the physicist David Kaplan explained on Quanta's podcast, The Joy of Why.
A common criticism of the multiverse theory is that since other bubble universes would be constantly swept away from us by eternal expansion, they would never be directly detectable. But some physicists wonder whether a past collision between our universe and another one just as they were bubbling up could have left an imprint on the sky. It's a long shot, but that hasn't stopped researchers from trying to work out in detail how such a collision might have played out.
An even more fundamental challenge to the multiverse lies in the theory's mathematics. If the number of bubble universes is truly infinite, how can you say what percentage have a large or small vacuum energy? Physicists discussed this "measure problem" a lot in the 2010s but never reached a satisfactory solution.
Due to its essentially untestable nature, the multiverse has become a flash point in physics. Can an untestable theory be scientific? If so, what does "scientific" even mean? Philosophers and physicists gathered in Germany to debate such questions in 2015, but the debate will likely rage on for a long time to come. | 
