Wing V · the map roomRoom III of IV
Inv. no. V.3 · the horn
The afterglow of the Big Bang
Point a radio horn at any patch of empty sky, day or night, and it picks up a faint hiss. It comes from every direction alike, and it is the oldest light we can reach: light set free when the universe was 380,000 years old and had cooled enough to turn clear. It left glowing orange. In the 13.8 billion years since, the stretching of space has pulled it out into microwaves, at less than three degrees above absolute zero.
The fog clears
Fig. I. A box of the young universe, not to scale: protons in red, free electrons in blue, light in gold. The background glows with the colour of the gas at that temperature.
- Temperature
- Electrons still free
- Stretched since then
- The glow peaks at
For its first few hundred thousand years the universe was a hot soup of nuclei and electrons, too hot for them to stay together. Light could not get far: every few steps it hit a free electron and bounced off somewhere else, the way light gets lost in fog. Watch the gold streaks in the box: they zigzag.
As the universe grows it cools. Slide on, and between about 3,500 and 3,000 kelvin the electrons are caught by the protons and settle into hydrogen atoms. With no free electrons left to bounce off, the light suddenly flies straight, and it has been flying ever since. That moment, about 380,000 years after the start, is what the microwave sky shows. The Big Bang itself stays hidden behind it; what we see is the fog clearing.
From ember to microwave
Fig. II. The glow's spectrum as space stretches it: every wavelength grows by the same factor, so the curve keeps its shape and slides from visible light into microwaves.
- Stretched so far
Hot things glow with a spectrum whose shape is set by their temperature alone: a black body. At 3,000 kelvin that glow peaks in the near infrared and looks orange to the eye. Since the light was set free, the universe has grown about 1,100 times in every direction, and every wave in it has been stretched by the same factor. A stretched black body is again a black body, only colder: 3,000 divided by 1,100 is 2.7 kelvin, peaking at a wavelength of about a millimetre. So the afterglow reaches us as a radio hiss. The light is still there; it has only moved out of the colours our eyes can see.
In 1990 the COBE satellite measured that spectrum and found it on the black-body curve; by 1996 its measurements matched the curve to within 50 parts in a million. Of all the ideas about the early universe, only a hot, dense beginning makes light like that.
The sky in microwaves
Fig. III. The whole sky as one oval, the Milky Way across the middle, filled in by a spinning beam the way a satellite scans it. An imitation with the right ingredients, made here; not the measured map.
- Colours run from
As the horn sees it, the sky is a little warmer on one side and a little cooler on the other, by three thousandths of a degree. That is our own motion. The Sun moves at about 370 km/s relative to the glow, towards the border of the constellations Leo and Crater, so the glow ahead of us is Doppler-shifted warmer and the glow behind us cooler. Take our motion out and the Milky Way shows across the middle, glowing with its own dust and gas.
Take that out too and what is left are the ripples: spots a degree or so across, warmer or cooler by about a hundred-thousandth. They are the denser and thinner patches of the young universe. The denser ones pulled in more matter over billions of years and became the clusters and galaxies we live in. Measured carefully, their sizes give the age of the universe and how much matter it holds, dark matter included. They also give how fast it grows.
Who worked it out
Where it already matters
A hiss in a horn, 1964. Arno Penzias and Robert Wilson at Bell Labs in New Jersey wanted to use a big horn antenna for radio astronomy and could not get rid of a faint hiss, the same in every direction. They even cleaned out the pigeons nesting in it. Down the road at Princeton, Robert Dicke's group was building an instrument to look for exactly that glow; the two papers came out side by side in 1965. Penzias and Wilson shared the Nobel Prize in 1978.
Predicted, then forgotten. Ralph Alpher and Robert Herman had worked out in 1948 that a hot Big Bang should leave a glow of a few kelvin. Hardly anyone went looking.
Snow on the television. On an old analogue TV tuned between channels, a small part of the snow is this afterglow, picked up by the aerial along with everything else.
COBE, WMAP, Planck. COBE measured the perfect black body (1990) and the first ripples (1992); John Mather and George Smoot shared the 2006 Nobel Prize for it. WMAP and then the European Planck satellite mapped the ripples ever finer. From them: the universe is 13.8 billion years old, about 5% ordinary matter, 26% dark matter and 69% dark energy (Planck 2018).
A disagreement. The growth rate worked out from the afterglow, about 67 km/s per megaparsec, does not match the rate measured nearby with stars and supernovae, about 73. Either one of the two measurements is off, or something is missing from the universe's history. Nobody has settled which yet.