Could our massive universe be just one of many, like a bubble in a frothy stream of cosmos-spawning stuff? It sounds like something out of a 1970s British scifi novel, but it's become a popular explanation for the origin of our universe. But how can we test this hypothesis, when we're stuck in just one universe?

*Image by Olena Shmahalo / Quanta Magazine*

Physicists who were once wary of the multiverse hypothesis have started to come around to this radical new way of thinking. This is partly because it helps explain why our universe just happens to have the right physical ingredients to make life possible.

In a fascinating two-part article about the multiverse over at *Quanta*, Natalie Wolchover and Peter Byrne write:

Many physicists loathe the multiverse hypothesis, deeming it a cop-out of infinite proportions. But as attempts to paint our universe as an inevitable, self-contained structure falter, the multiverse camp is growing.

The problem remains how to test the hypothesis. Proponents of the multiverse idea must show that, among the rare universes that support life, ours is statistically typical. The exact dose of vacuum energy, the precise mass of our underweight Higgs boson, and other anomalies must have high odds within the subset of habitable universes. If the properties of this universe still seem atypical even in the habitable subset, then the multiverse explanation fails.

But infinity sabotages statistical analysis. In an eternally inflating multiverse, where any bubble that can form does so infinitely many times, how do you measure "typical"?

Guth, a professor of physics at the Massachusetts Institute of Technology, resorts to freaks of nature to pose this "measure problem." "In a single universe, cows born with two heads are rarer than cows born with one head," he said. But in an infinitely branching multiverse, "there are an infinite number of one-headed cows and an infinite number of two-headed cows. What happens to the ratio?"

For years, the inability to calculate ratios of infinite quantities has prevented the multiverse hypothesis from making testable predictions about the properties of this universe. For the hypothesis to mature into a full-fledged theory of physics, the two-headed-cow question demands an answer.

**Read the rest at Quanta**