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 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:

“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.

Image by
olena shmahalo / quanta magazine

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