Of all the patterns that could possibly be preserved in the post–Big Bang radiation, the one we see is surprisingly smooth on large angular scales.

Sitting by a campfire on a dark night, looking up at the Milky Way, a curious child asks, “What does the sky tell us? Where does it all come from? Does space go on forever?” A caring adult might share a little awe and humility about humanity’s place in the grand scheme or perhaps relate a traditional creation story. A scientist like me, who came of age soon after the discovery that the sky is not actually dark but awash in primeval radiation, might instead relate the still-unfinished scientific story of the boundaries and origins of time and space. That tale is displayed in nature’s own record of the structure of the early universe, a mosaic of temperature and density fluctuations preserved in the primordial light that astronomers call the cosmic microwave background (CMB).

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