The theory of cosmic inflation makes the extraordinary assertion that within the first 10−35 seconds or so, the universe underwent a dramatic quasiexponential expansion, doubling in size more than 60 times, before ending in an episode of “reheating” that produced the basic stuff that evolved into the universe as we see it today.

There are good reasons for believing that seemingly outrageous proposition. As Alan Guth pointed out when he introduced the idea,1 it solves several problems that otherwise plague Big Bang cosmology. Furthermore, quantum fluctuations are magnified by inflation and lead to inhomogeneities that seed the observed structure of the universe.

Among the quantum fluctuations are those of the gravitational field itself. They are predicted to imprint a unique signature on the polarization of the cosmic microwave background (CMB), the light emitted when the universe was 380 000 years old and cool enough for atoms to form....

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