The Cosmic Microwave Background

Image: ESA and the Planck Collaboration. The whole sky in a single oval: the cosmic microwave background as mapped by ESA's Planck satellite. The mottled colors are temperature differences of a hundred-thousandth of a degree — hotter and cooler patches in the infant universe that became the seeds of every galaxy that exists today. This is the universe at 380,000 years old.

The moment the fog lifted

For its first 380,000 years, the universe was too hot to be transparent. It was a dense, glowing plasma of loose electrons and nuclei, and light couldn't travel far before bouncing off a free electron — the whole cosmos was an opaque fog, like the inside of the Sun. Then expansion cooled it below about 3,000 degrees, cool enough for electrons and nuclei to join into the first neutral atoms. Suddenly the fog cleared, and light could stream freely for the first time. That light has been traveling ever since. We call the moment it was set free the "surface of last scattering," and the light itself is the cosmic microwave background.

Two things about it are astonishing. First, it comes from every direction — because the early universe was everywhere, its afterglow fills all of space. Second, it is almost perfectly smooth, the same temperature everywhere to about one part in 100,000. Those tiny departures from smoothness — the mottling in the Planck map — are the whole story of everything that came after. The slightly denser patches had a little more gravity, so they pulled in more gas, and over billions of years grew into the cosmic web of galaxies. Every structure in the universe was seeded in that map.

The static between the stations

Here is the honest, wonderful thing about the CMB — unlike almost everything else in cosmology, you don't have to take it on faith or squint at a telescope. You have already seen it. On an old analog television tuned between channels, a few percent of that snowy static was the cosmic microwave background: real photons from the young universe, ending their 13.8-billion-year journey inside your living room. An old radio hissing between stations catches a little of it too. It is the most democratic object in astronomy — no equipment, no dark sky, no aperture required. The afterglow of creation is soaking through you right now, about 400 of those ancient photons in every cubic centimeter of the room you're sitting in.

Found by accident, blamed on pigeons

The greatest discovery in the history of cosmology was, at first, mistaken for bird droppings. In 1964, two radio engineers at Bell Labs — Arno Penzias and Robert Wilson — were trying to use a sensitive horn antenna in New Jersey and kept picking up a faint hiss they couldn't get rid of. It came from every direction, day and night, all year. They checked everything. They evicted a pair of pigeons roosting in the antenna and scrubbed out what Penzias delicately called "a white dielectric material" (the droppings). The hiss remained. Only after talking to physicists at nearby Princeton — who had been about to go looking for exactly this signal — did they realize what they had: the predicted afterglow of the Big Bang, hiding in their static. They won the 1978 Nobel Prize for a discovery they'd spent months trying to eliminate as noise.

The signal had, in fact, been predicted years earlier by George Gamow, Ralph Alpher, and Robert Herman, who calculated in the late 1940s that a hot Big Bang should leave a cold afterglow. Nobody went looking hard enough until Penzias and Wilson stumbled into it. Later missions — COBE in the 1990s, then WMAP and ESA's Planck — mapped its faint ripples in ever-finer detail, and it was those maps that pinned down the universe's age at 13.8 billion years and its recipe of ordinary matter, dark matter, and dark energy. When COBE first resolved the ripples in 1992, the lead scientist, George Smoot, reached for the biggest words he had.

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