The oldest light remembers everything

Point a sensitive radio receiver at any patch of sky and you will pick up a faint hiss of microwaves with a temperature of 2.725 degrees above absolute zero. It comes from every direction, almost perfectly evenly. It is the oldest light in the universe, and it has been traveling for nearly 13.8 billion years.

It is also everywhere you are. Every cubic centimeter of space, including the one in front of your face, holds about 411 of these photons at any moment. A small part of the static on an old analog television came from them.

Where it came from

For its first few hundred thousand years, the universe was filled with a hot plasma of nuclei, electrons and light. Light could not travel far before scattering off a free electron, so the universe was opaque, like the inside of a cloud. As space expanded, the plasma cooled. At around 3,000 degrees, about 380,000 years after the Big Bang, electrons combined with nuclei to form neutral atoms, and the light was suddenly free to travel.

That released light has been stretched by the expansion of the universe ever since, by a factor of about 1,100. Multiply today’s 2.725 degrees by 1,100 and you recover almost exactly 3,000. What began as the orange glow of a hot gas is now a bath of microwaves just above absolute zero.

An accidental discovery

In 1964 and 1965, Arno Penzias and Robert Wilson were trying to use a horn-shaped antenna at Bell Labs in New Jersey for radio astronomy, and could not get rid of a persistent background noise. They checked the electronics, pointed the antenna in different directions and evicted a pair of pigeons. The noise remained. A group led by Robert Dicke at nearby Princeton, which had been preparing to search for exactly this radiation, recognized it for what it was. The two papers appeared side by side in the same journal. Penzias and Wilson shared the 1978 Nobel Prize in Physics.

What began as the orange glow of a hot gas is now a bath of microwaves just above absolute zero.

Three layers of pattern

The radiation has structure at three very different levels, and each was a discovery of its own. The first is the spectrum. NASA’s COBE satellite showed in the early 1990s that the light follows the theoretical curve of a perfect thermal emitter with astonishing fidelity, a result that left very little room for alternatives to a hot, dense beginning.

The second is a slope across the whole sky. One side is warmer than the other by a few thousandths of a degree, because the solar system is moving through the radiation at about 370 kilometers per second. The microwave background gives astronomers a practical way of saying how fast we are moving relative to the universe at large.

The third, once that motion is subtracted, is the mottling on the map above: temperature differences of about one part in 100,000. Those tiny ripples are the imprint of slightly denser and thinner regions in the early universe, the seeds from which galaxies later grew.

Reading the ripples

In the plasma before atoms formed, gravity pulled matter into denser regions while the pressure of light pushed back, setting up sound waves. When the light escaped, those waves were frozen in place. The map is a photograph of a ringing fluid, and cosmologists analyze it the way an acoustics engineer analyzes a recording: by asking how much of the signal sits at each scale.

The resulting graph has a series of peaks, and each tells you something different. The first and largest corresponds to spots about one degree across on the sky, roughly twice the width of the full Moon. We can calculate how large those spots really were, so their apparent size works as a test of the geometry of space. A curved universe would magnify or shrink them. They come out as a flat universe predicts.

The relative height of the second peak depends on how much ordinary matter there is, since atoms weigh down the oscillations and exaggerate the compressions compared with the rebounds. The third peak is sensitive to dark matter, which adds gravity without feeling the pressure of light. A single curve, measured with increasing precision by WMAP and then Planck, pins down the inventory below.

The inventory rests on a model, and that should be said plainly. These figures are what you get if you assume the standard cosmological model and ask which settings fit the map. The fit is superb, which is strong evidence for the model. It is also why a disagreement with measurements made elsewhere is taken so seriously.

What is left to learn

The temperature pattern has now been measured about as well as the sky allows. The frontier is polarization, the faint orientation of the light’s electric field. A particular swirling pattern, called B-modes, would be evidence of gravitational waves from the first instant after the Big Bang, and a direct test of the theory of cosmic inflation. So far it has not been seen. The BICEP/Keck telescopes at the South Pole, combined with Planck, have limited its strength to a tensor-to-scalar ratio below about 0.036, which has already ruled out several of the simplest inflation models. Telescopes in Chile and at the South Pole are pushing that limit lower.

The background is also the anchor for one side of the Hubble tension, and the sound waves frozen into it are the ruler behind the new questions about dark energy. If the oldest light and the nearby universe really do disagree, the resolution will have to be written in both.

Sources

• Penzias and Wilson, “A Measurement of Excess Antenna Temperature at 4080 Mc/s”, The Astrophysical Journal, 1965
• Dicke et al., “Cosmic Black-Body Radiation”, The Astrophysical Journal, 1965
• Planck Collaboration, “Planck 2018 results”, Astronomy & Astrophysics, 2020
• BICEP/Keck, “Improved Constraints on Primordial Gravitational Waves”, Physical Review Letters, 2021