To glimpse the oldest light in the universe, simply tune an old television between channels: some of the tiny specks dancing on the screen result from the antenna being bombarded relentlessly by photons that were emitted shortly after the big bang, some 13.8 billion years ago. These photons fly uniformly through space from all directions, with an average temperature of 2.7 kelvins (−455 degrees Fahrenheit), composing a cloud of radiation called the cosmic microwave background (CMB). Because these photons are so old, the familiar two-dimensional map of the CMB is often called a “baby picture” of the universe, providing a window back into the primordial conditions that created the cosmos we see around us today.