Could our huge universe be just one of numerous, like a bubble
in a foamy river of cosmos-spawning stuff? It sounds like somewhat out of a
1970s British sci-fi novel, but it's turn out to be a general explanation for
the beginning of our universe. But how can we test this theory, when we're trapped
in just one universe? Physicists who were once wary of the multiverse theory
have started to come around to this fundamental new mode of thinking. This is
partly because it helps clarify why our universe just happens to have the right
physical requirements to make life possible. In an interesting two-part article
about the multiverse over at Quanta, Natalie Wolchover and Peter Byrne write:

Image by Olena Shmahalo / Quanta Magazine |

*“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 more here.