← Did You Know?
📻Space8 min read

The Cosmic Microwave Background

Two engineers spent a year trying to get rid of an annoying hiss in their antenna. They cleaned out pigeon nests, taped over rivets, and checked for interference from New York City. The hiss was the oldest light in the universe.

In plain English

Space is not cold and empty in the way it looks. Everywhere you point a sensitive enough radio antenna, in any direction, a faint microwave glow comes back. It is the same brightness from every direction to within about one part in 100,000.

That glow is light released when the universe was 380,000 years old.

Here is why it exists. For the first few hundred thousand years the universe was hot enough that atoms could not hold together. Electrons roamed free, and free electrons scatter light. Any photon travelled a short distance before bouncing off an electron and heading somewhere else. The universe was opaque, like the inside of a fog bank, or the inside of the Sun.

As the universe expanded it cooled. At around 3,000 degrees Kelvin, it became cool enough for electrons to be captured by nuclei and form neutral hydrogen. The free electrons vanished into atoms. Suddenly there was nothing left to scatter light.

The fog lifted, all at once, everywhere.

The light that was flying around at that instant has been travelling ever since, with almost nothing in its way. It has been travelling for 13.8 billion years. In that time the expansion of the universe has stretched its wavelength by a factor of about 1,100, dragging it down from the visible glow of something at 3,000 K to microwaves at 2.7 degrees above absolute zero.

That stretched light is arriving at your location right now, from every direction, all the time. It has been passing through you throughout this sentence.

Five things to file under "wait, what?"

  • It was discovered by accident, by people trying to eliminate it. In 1964 Arno Penzias and Robert Wilson were using a horn antenna at Bell Labs in Holmdel, New Jersey, and could not get rid of a persistent excess noise of about 3.5 K. They checked everything. They found pigeons roosting in the horn and evicted them. They scrubbed out the droppings, which Penzias memorably referred to in print as "white dielectric material". The noise stayed. It was the same in every direction, day and night, all year.

  • Another team was actively hunting for it a half-hour's drive away. At Princeton, Robert Dicke, Jim Peebles, Peter Roll and David Wilkinson had worked out that a hot early universe should have left exactly this residue, and were building a detector to find it. Penzias phoned Dicke about the mysterious noise. Dicke listened, hung up, turned to his team and said, "Boys, we've been scooped." The two groups published side-by-side papers in 1965. Penzias and Wilson's is titled, with tremendous understatement, "A Measurement of Excess Antenna Temperature at 4080 Mc/s".

  • It had been predicted sixteen years earlier and everyone forgot. Ralph Alpher and Robert Herman, working with George Gamow, calculated in 1948 that a hot early universe should leave a background at around 5 K. The prediction was published, then essentially ignored for over a decade, partly because nobody thought such a faint signal could ever be detected. Penzias and Wilson had not read it.

  • It is the most perfect blackbody ever measured, and nothing else comes close. A blackbody is an idealised object that emits a precise mathematical spectrum determined only by its temperature. Real objects only approximate it. When the COBE satellite measured the CMB spectrum in 1990, the data matched the theoretical blackbody curve so exactly that the error bars were smaller than the width of the printed line on the graph. The audience at the meeting where it was presented gave it a standing ovation.

  • You have seen it as static. On an old analogue television tuned between channels, a small percentage of the snow on screen was the cosmic microwave background being picked up by the aerial. A slightly humbling thought: for decades, the primary way most people encountered the oldest light in the universe was as the thing they were trying to tune away from. Digital broadcasting has quietly retired the experience.

The full story

Why a hot beginning leaves a residue

If the universe is expanding now, then running the clock backwards means it was smaller, denser and hotter. Compress any gas and it heats up. Push this far enough and you reach conditions where matter is fully ionised: a plasma of bare nuclei and free electrons.

Photons cannot travel far in a plasma. They scatter off free electrons constantly, which is why the interior of the Sun is opaque despite being made of the brightest thing most people can name.

The critical event is called recombination, which is a slight misnomer since the electrons and nuclei had never previously been combined. At about 380,000 years after the beginning, the temperature dropped to roughly 3,000 K and neutral hydrogen became stable. The scattering stopped. Photons that had been ricocheting endlessly suddenly found the universe transparent and simply carried on in whatever direction they happened to be pointing.

Astronomers call the moment the surface of last scattering. It is not a place, it is a time, and because light takes time to reach us we see it as a shell surrounding us in every direction, at the edge of the observable universe.

What the temperature tells you

The CMB temperature is 2.72548 K, measured to within about half a thousandth of a degree by the FIRAS instrument on COBE. That is the most precisely known temperature in cosmology.

The uniformity is what makes it strange. Two patches of sky in opposite directions have the same temperature to within a hundred-thousandth of a degree, despite the fact that, in a simple expanding universe, they were never in causal contact and could not have exchanged heat. This is the horizon problem, and it is the main reason cosmic inflation was proposed: a brief phase of exponential expansion would take a tiny, thermally equalised region and stretch it far beyond the observable universe.

The ripples, and why they matter more than the uniformity

Perfect uniformity would be a disaster. A perfectly smooth universe has nothing to build with. Gravity needs somewhere denser than average to start pulling material together.

In 1992 the COBE satellite's DMR instrument found the anisotropies: variations of about one part in 100,000. Slightly hotter patches, slightly cooler patches. George Smoot and John Mather shared the 2006 Nobel Prize for the work.

Those ripples are density variations in the early universe, and they are the seeds of everything. Every galaxy, cluster and supercluster grew by gravitational collapse from those tiny initial over-densities. The map of the CMB is, in a real sense, a photograph of the initial conditions from which all subsequent structure emerged.

Later missions sharpened the picture enormously. WMAP mapped it through the 2000s, and the European Planck satellite produced the definitive full-sky map from data taken between 2009 and 2013. The detailed statistics of those ripples pin down the age, composition and geometry of the universe: 13.8 billion years old, spatially flat to within measurement error, and roughly 5% ordinary matter, 27% dark matter and 68% dark energy.

The dipole, and what it says about us

The single largest feature in the raw CMB map is not cosmological at all. One side of the sky is slightly hotter and the opposite side slightly cooler, by about one part in 1,000.

That is us moving. Our motion relative to the CMB blueshifts the light ahead and redshifts the light behind, and the size of the effect gives the speed: the Solar System is travelling at roughly 370 kilometres per second relative to the frame in which the CMB looks uniform.

This is as close to a universal reference frame as physics offers. It does not violate relativity, since there is no preferred frame for the laws of physics. But there is a frame in which the contents of the universe are, on average, at rest, and you can measure your speed with respect to it from your back garden with sufficiently good equipment.

Go deeper

For the curious:

On YouTube:

Keep going

More things worth knowing

All topics →