The Big Bang and the Cosmic Microwave Background

The universe is expanding. Wind the film backwards and everything must once have been hot, dense, and crushed together. That guess left a fingerprint — a faint hiss of light — and in 1964 two engineers stumbled into it by accident.

For most of human history, the sky was assumed to be a fixture. The stars were where they had always been. The universe simply was. In the twentieth century, in the space of about forty years, that picture was destroyed and replaced by something stranger: the entire cosmos is expanding, it has a finite age, and if you point a sensitive enough antenna at the sky in any direction you will hear the whisper of its birth.

The story runs from a mathematician's equations to a farm in New Jersey.

Hubble's law — the universe is not still

In the 1910s and 1920s the astronomer Vesto Slipher, working at Lowell Observatory, noticed that the light from most distant “spiral nebulae” was redshifted — its wavelengths stretched toward the red end of the spectrum, as if the objects were moving away. Edwin Hubble, working with a bigger telescope on Mount Wilson, went further. He measured distances to those nebulae (which turned out to be galaxies in their own right), and in 1929 he plotted distance against velocity.

The plot was a straight line. The farther a galaxy was, the faster it was receding. Double the distance, double the speed. This is Hubble's law, and it does not describe a universe of galaxies flying outward from some centre. It describes a universe whose space itself is stretching. Every galaxy sees every other galaxy retreating, with no privileged point of origin. The classic analogy is a raisin loaf rising in the oven: every raisin moves away from every other raisin, but no raisin is at the centre.

The Belgian priest and physicist Georges Lemaître had actually derived this expansion from Einstein's equations of general relativity two years before Hubble published his data. Einstein himself had initially resisted the idea, inserting a “cosmological constant” into his equations to keep the universe static, and later called it his greatest blunder. The equations were trying to tell him something, and he had refused to listen.

Winding the film backwards

Once you accept that the universe is expanding, an obvious question follows: run time in reverse. If everything is moving apart, then earlier, everything was closer together. Earlier still, closer still. Extrapolated to its limit, all of the matter and energy in the observable universe was concentrated in a state of enormous density and heat, some 13.8 billion years ago. Lemaître called this the “primeval atom.” A rival astronomer, Fred Hoyle, who preferred a steady, eternal universe, dismissed the idea in a BBC broadcast in 1949 with the mocking phrase “big bang.” The name stuck.

Crucially, the Big Bang is not an explosion in space, radiating outward from a point. It is an expansion of space, everywhere at once. The early universe was not a small blob sitting in a vast emptiness; it was a hot, dense state that filled all of space, and space itself has been growing ever since.

In the 1940s, the physicist George Gamow and his students Ralph Alpher and Robert Herman made the model quantitative. If the early universe was hot and dense enough, they reasoned, it would have been a plasma of protons, electrons, and radiation, all locked together. Light could not travel far without scattering. But as the universe expanded, it cooled. Eventually — around 380,000 years in — things got cool enough (about 3,000 K) for electrons and protons to combine into neutral hydrogen. Suddenly, light was free.

That freed light, Alpher and Herman predicted in 1948, should still be around today. Stretched by 13.8 billion years of cosmic expansion, it should now appear as faint microwaves reaching us uniformly from every direction — a background glow at roughly 5 K above absolute zero. It was a bold, specific, testable prediction. And for about sixteen years, almost no one paid any attention to it.

The accidental discovery

In 1964, two Bell Labs radio engineers, Arno Penzias and Robert Wilson, were trying to use a large horn antenna in Holmdel, New Jersey, to do sensitive radio astronomy. They kept picking up a persistent hiss — a signal at about 4 K, coming from every direction, at all times of day, in every season. They checked the antenna. They checked the cables. They famously climbed inside and cleaned out pigeon droppings (“a white dielectric material”). The hiss remained.

They mentioned the problem to a colleague, who mentioned it to Robert Dicke's group at Princeton, who were just then building an antenna specifically to search for exactly this signal, having independently rediscovered Alpher and Herman's prediction. Dicke put down the phone and told his team: “Boys, we've been scooped.”

The oldest light in the universe had been leaking into human antennas for as long as there had been human antennas. It just took someone with a good enough instrument, and an inconvenient enough amount of noise, to notice.

Two papers appeared back-to-back in the Astrophysical Journal in 1965. Penzias and Wilson described the signal. Dicke's group explained what it meant. It was the cosmic microwave background — the CMB — a faint bath of radiation whose spectrum is the near-perfect blackbody curve of a body at 2.725 K, exactly as expected from a universe that had once been hot and had been expanding ever since. Penzias and Wilson shared the Nobel Prize in 1978.

What it tells us

The CMB is a photograph of the universe as it was 380,000 years after the Big Bang — the moment light escaped — stretched and cooled by the expansion of space over 13.8 billion years. Three features of it, taken together, changed cosmology from a speculative field into a precision science.

First, it exists at all, with exactly the temperature and spectrum a hot Big Bang predicts. That single fact killed the rival steady-state theory outright.

Second, it is astonishingly uniform. Look in any direction and the temperature is 2.725 K, with variations of only about one part in one hundred thousand. The early universe was very close to perfectly smooth. (This uniformity is so extreme it needed its own explanation, which led in 1980 to the theory of cosmic inflation — a brief burst of exponential expansion in the first split second, which flattened out any earlier lumpiness.)

Third, those tiny one-part-in-a-hundred-thousand ripples are not noise. Precision satellites — COBE in 1992, WMAP, and Planck — mapped them in detail. They are the seed patterns of everything: the slight overdensities that gravity would spend billions of years pulling together into galaxies, stars, planets, and, eventually, the observers now looking back and puzzling over the fossil light of their own beginning.

A hiss in a New Jersey antenna, mistaken at first for pigeons, turned out to be the oldest signal in existence. Everything that has ever happened has happened inside the afterglow of it.