In 1964 two radio astronomers at Bell Labs in New Jersey, Arno Penzias and Robert Wilson, were trying to get rid of a faint hiss in their horn-shaped antenna. It came from every direction, day and night, whatever they did. They checked the electronics and cleared out pigeon droppings. The hiss remained. They had found the cosmic microwave background, the oldest light in the universe, and in 1978 they shared the Nobel Prize in Physics for it.
Light from the beginning
For its first few hundred thousand years, the universe was a hot, glowing fog of charged particles. Light could not travel far before bouncing off an electron. About 380 000 years after the Big Bang, the universe cooled enough for electrons and nuclei to combine into atoms, and the fog cleared. The light released at that moment has been travelling ever since.
As the universe expanded, that light was stretched from visible and infrared light into microwaves. Today it is a bath of radiation with a temperature of just 2.725 degrees above absolute zero. Every cubic centimetre of space holds about 411 of its photons. Some of the snow on an old analogue television tuned between channels came from it.
Its existence had been predicted. In 1948 Ralph Alpher and Robert Herman calculated that light left over from a hot early universe should still fill space at about 5 K (-268 °C).
The ripples
At first the background looked perfectly smooth. In 1992 NASA’s COBE satellite found tiny differences in temperature from place to place, about one part in 100 000. These ripples are the imprint of slight differences in density in the young universe: the seeds from which gravity later built galaxies and the cosmic web. John Mather and George Smoot shared the 2006 Nobel Prize for COBE’s measurements.
NASA’s WMAP satellite and then ESA’s Planck mapped the ripples ever more precisely. From their pattern cosmologists measure the age of the universe, 13.8 billion years, and how much of it is ordinary matter, dark matter and dark energy.
The Cold Spot
One feature stands out. In the constellation Eridanus there is a region about 5 degrees across that is about 70 millionths of a degree colder than average, larger and colder than expected. Planck confirmed it is real. Whether it is a rare fluke, a huge void in front of it, or something stranger is still unknown.