| The story of the multiverse starts like this: In the beginning, an unimaginably small speck of space flashed outward far faster than the speed of light. An instant later, that expansion stopped. Space then grew at a far slower pace for the next 13.8 billion years, producing the universe we see today. This theory of rapid expansion, known as inflation, fits all the cosmological evidence collected to date.
The price of explaining the universe in this way, however, is that inflation also predicts the likely existence of a vast multiverse extending far, far beyond our cosmological horizon. In most versions of inflation, rapid expansion would be the default state of reality, with bubbles of calm (like the one we would inhabit) forming randomly here and there. These bubbles might be infinite in number, but they would be constantly swept away from each other by the faster-than-light expansion of space between them. There would be no hope of the inhabitants of one bubble ever directly detecting the presence of another bubble. (This "inflationary" multiverse would be distinct from the collection of coincident universes seen in Marvel movies and Everything Everywhere All at Once, for instance, which has more in common with the "many worlds" interpretation of quantum mechanics).
To some physicists, this multiverse idea is unacceptable, even unscientific. While small in number, these maverick theorists seek to develop alternative ways of explaining the large, smooth, relatively featureless universe that we see, to avoid the philosophical baggage that a multiverse brings. In recent years, their competing proposals have grown more sophisticated. The theories can't yet match inflation's predictive power and are generally viewed as underdogs, but underdogs with tremendous potential to rewrite our understanding of our origins.
"There are some questions I consider so important that even if you have only a 5% chance of succeeding, you should throw everything you have at it and work on them," said Raman Sundrum, a physicist at the University of Maryland, in 2018 of the effort to develop inflation alternatives.
What's New and Noteworthy Inflation does a great job of ironing out cosmic irregularities, but it's not the only way this could happen. Perhaps the universe is cyclic, expanding and contracting, expanding and contracting, for all eternity. In that case, the lengthy contractions could have also created the uniform conditions we see today, without making a multiverse. Such "bouncing" universes didn't initially seem so plausible. In the 1960s, Roger Penrose proved that contracting universes tend to crunch all the way down to a dead end in space-time known as a singularity; he later received a Nobel Prize in part for doing so. But in 2017, two teams found loopholes in Penrose's proof just big enough to allow a collapsing universe to bounce back.
One of the theorists involved, Anna Ijjas, built her bounce theory by extending previous work with her adviser Paul Steinhardt of Princeton University. Steinhardt played a key role in the development of inflation decades ago but had a change of heart after it spawned the multiverse — an idea he calls "hogwash." In 2020, Steinhardt and Ijjas joined forces with experts who specialize in computer simulations of space-time and showed that their flavor of bounce could indeed smooth out a wide variety of space-time variations, much as inflation would.
Another possibility is that the Big Bang spawned two universes joined at the hip, each the mirror image of the other, and we live in one of them. Neil Turok of the Perimeter Institute in Canada developed this theory along with Latham Boyle and Kieran Finn in 2018. Turok and other collaborators had just mounted a controversial attack on a position championed by Stephen Hawking that time had no beginning, and Turok had been thinking about new ways of accounting for old observations.
While some theorists debate the finer points of mirror universes, bouncing universes and multiverses, others look ahead to the next generation of astronomical observatories for answers. In the 2030s, for instance, a trio of satellites known as LISA will listen for a hum of ripples in space-time that could clarify what happened during the early moments of our universe. | 
