The Cosmic Microwave Background

A hiss from everywhere#

Point a sensitive radio antenna at any patch of empty sky — no star in the field, no galaxy, just darkness between them — and you will pick up a faint, steady hiss. Turn the antenna to a different patch and the hiss is still there, essentially unchanged. It does not come from the Sun, or the Milky Way, or any object you can name. It comes from every direction at once, with the same faint whisper of energy.

That hiss is the oldest light there is. It is the glow of the entire universe when it was just 380,000 years old — light set free the moment the cosmos first became transparent, and then stretched and cooled by billions of years of expansion into microwaves barely three degrees above absolute zero. It is the cosmic microwave background (CMB), and it is the single most important observational pillar under the hot-dense-beginning picture that the expanding universe arrives at by running the film backward. That article predicts an afterglow; this one is the afterglow.

The early universe was a glowing fog#

To see where the CMB comes from, rewind the expansion. As space was smaller in the past, the same energy was packed into less volume, so the early universe was not only denser but far hotter. Push back far enough and it was hot enough that atoms could not hold together at all.

In that state, matter was a plasma: bare atomic nuclei — mostly protons, plus the helium built in Big Bang nucleosynthesis — mixed with a sea of free electrons that no nucleus could keep. And a plasma of free electrons is opaque to light. A photon travelling through it barely moves before it slams into an electron and scatters off in a new direction, then hits another, and another. Light did not stream through the early universe; it diffused through it, endlessly redirected, exactly like sunlight lost in a thick fog. The whole universe was a uniform, glowing, opaque fog — bright as the surface of a star, but everywhere.

Crucially, this fog was not sitting in the middle of empty space with an edge you could see from outside. It filled all of space. There was no outside, no centre, no special point it radiated from — the same uniform hot plasma stretched in every direction as far as there was universe to fill.

Recombination: the fog clears#

The fog could not last, because the universe was expanding and therefore cooling. As it cooled through about 3000 kelvin, a threshold was crossed. Below that temperature, a passing photon no longer carried enough energy to knock electrons loose, so the electrons could finally settle into orbits around the nuclei. Free electrons and bare nuclei combined into neutral atoms — a moment cosmologists call recombination.

The instant the free electrons were bound up into atoms, there was nothing left to scatter the light. Neutral gas is transparent. The fog cleared, and the photons that had been trapped, bouncing endlessly, suddenly streamed away in straight lines — from everywhere, all at once. The sphere around us from which that final release reaches us today is called the surface of last scattering: not a real wall, but the moment in every direction when the light we now see stopped scattering and set out toward us.

Drag the slider — or press Cool — to lower the temperature and watch the plasma recombine. While the gas is hot and ionised, the box is a reddish fog: the gold photons zig-zag frantically, trapped, scattering off the blue free electrons that surround every proton, and the box stays opaque. As you cross ~3000 K, watch the electrons fall onto the protons and bind into neutral atoms — the blue free electrons vanish into atomic shells — and the photons abruptly straighten out and stream free in every direction. That release, happening throughout all of space at once, is the light we now call the CMB. Notice the readout: this crossing happens at roughly 3000 K, about 380,000 years after the Big Bang.

Two things are worth pinning down here. The light was not emitted from a point and it does not come from a special direction — the hot plasma filled all of space, so the surface of last scattering surrounds every observer. And it is not starlight: recombination happened long before the first star existed, so the CMB is not made by galaxies. It is the universe itself glowing, then going transparent.

From a 3000 K glow to a 2.725 K whisper#

When it was released, this light was the glow of gas at about 3000 K — a dull red-and-infrared thermal radiation, the kind of near-perfect blackbody spectrum that any hot dense body emits (the same idea used in spectroscopy to read a source's temperature from the shape of its continuous spectrum). Yet today we measure it as microwaves at just 2.725 K. The bridge between the two is expansion.

Recall from the expanding universe that light riding through expanding space has its wavelength stretched in step with the scale factor $a$. Every wavelength in the spectrum grows by the same factor, and a blackbody stretched uniformly stays a blackbody — just a colder one, because its temperature scales inversely with the stretch:

$$T \propto \frac{1}{a}$$

So the temperature we observe relates to the temperature at emission exactly as the wavelengths do:

$$1 + z = \frac{a_\text{obs}}{a_\text{emit}} = \frac{T_\text{emit}}{T_\text{obs}} = \frac{3000\ \text{K}}{2.725\ \text{K}} \approx 1100$$

The CMB reaches us from a redshift of about $z \approx 1100$: the universe has expanded roughly a thousandfold since the light was released, stretching every wavelength — and lowering the blackbody temperature — by that same factor of a thousand.

Where the spectrum peaks follows from Wien's displacement law, $\lambda_\text{peak} = b / T$ with $b \approx 2.898 \times 10^{-3}\ \text{m·K}$. At 3000 K the peak sat near

$$\lambda_\text{peak} = \frac{2.898 \times 10^{-3}}{3000} \approx 9.7 \times 10^{-7}\ \text{m} \approx 970\ \text{nm}$$

in the near-infrared, just past visible red. Cool the same blackbody to 2.725 K and the peak slides to

$$\lambda_\text{peak} = \frac{2.898 \times 10^{-3}}{2.725} \approx 1.06 \times 10^{-3}\ \text{m} \approx 1.06\ \text{mm}$$

a wavelength squarely in the microwaves. Same light, same blackbody shape, redshifted from a hot glow to a cold whisper.

The faint gold curve is the original 3000 K blackbody at recombination, fixed for reference. Press Expand — or drag the slider — to run cosmic expansion forward and watch the live indigo curve stretch and cool: its peak slides steadily to the right, out of the visible band, through the infrared, and into the microwaves, while the temperature readout falls from 3000 K toward 2.725 K and the redshift climbs toward $z \approx 1100$. The shape never changes — it is always a blackbody — it only shifts and cools. On the right is the other half of the story: a patch of the sky's temperature ripples, the roughly one-part-in-100,000 variations we turn to next.

Discovered by accident, mapped with precision#

The CMB was predicted before it was found. In the late 1940s Ralph Alpher and Robert Herman, working with George Gamow's group on the physics of a hot early universe, estimated that its afterglow should survive today as thermal radiation at a few kelvin. The prediction was largely forgotten.

Then, in 1965, Arno Penzias and Robert Wilson at Bell Labs were calibrating a large horn antenna and could not get rid of a persistent background noise. It came from every direction, day and night, in every season; it was not the Sun, not New York City, not their equipment. Famously, they even cleared out pigeons nesting in the antenna, suspecting their droppings — and still the hiss remained. Only after talking to physicists at nearby Princeton, who were preparing to search for exactly this radiation, did they realise what they had: the afterglow of the Big Bang. Penzias and Wilson shared the 1978 Nobel Prize in Physics for the discovery.

A steady, unchanging universe predicts no such thing. A hot, expanding, cooling universe predicts a uniform thermal bath at a few kelvin, arriving from all directions — precisely what the antenna heard. The CMB is the observation that decisively favoured the Big Bang picture over its rivals.

The story since then has been one of ever-sharper maps. In 1992 the COBE satellite showed two things at once: the CMB spectrum is a blackbody to extraordinary precision — one of the most perfect blackbodies ever measured — and, superimposed on its uniformity, there are faint anisotropies, temperature variations of about

$$\frac{\Delta T}{T} \sim 10^{-5}$$

roughly one part in a hundred thousand, or tens of microkelvin against the 2.725 K background. WMAP (2003) and then the Planck satellite (2013–2018) mapped these ripples across the whole sky in exquisite detail.

Those ripples matter enormously. They are the imprint of slightly denser and slightly rarer patches in the early universe — and the denser patches, pulling on their surroundings by gravity, are the seeds from which galaxies and clusters of galaxies later grew. Every structure in the cosmos traces back to that faint pattern. And the precise statistics of the ripples — how much power sits at each angular scale — form a kind of treasure map: fit them and you read off the age of the universe, its geometry, and how much of it is ordinary matter, dark matter, and dark energy. The CMB is at once the oldest photograph of the universe and one of our most powerful cosmological measuring instruments.