First look
The idea, in plain words
In 1964 two radio astronomers in New Jersey, Arno Penzias and Robert Wilson, were using this antenna, built for talking to early satellites. It picked up a faint steady hiss that they could not get rid of. It was the same by day and by night, and the same wherever in the sky they pointed. They had found the oldest light there is.
For its first 380,000 years the universe was a glowing fog. It was so hot that atoms could not hold together, and light could not travel any distance without bumping into a loose particle. Then it cooled to about 3,000 degrees, atoms formed, and the fog cleared. The light that was there at that moment has been travelling ever since.
It no longer looks like light. While it travelled, space itself stretched, and the waves were stretched with it, about 1,100 times over. What set out as the glow of a hot gas, mostly just beyond red, now arrives as microwaves about a millimetre long. Its temperature has fallen from 3,000 degrees to 2.7 degrees above absolute zero.
This is a map of that glow over the whole sky, flattened out like a map of the Earth. It is astonishingly even. The red patches and the blue patches differ in temperature by about two ten-thousandths of a degree. A picture with honest contrast would be one flat colour.
The glow comes from every direction because the fog was everywhere. Look far enough any way you like and you are looking back to the time it cleared. So the map is the inside of a shell with us at the middle, and it is as far as light can ever show us.
Those faint patches matter more than they look. The slightly denser spots pulled in more matter and grew into galaxies and clusters of galaxies. And the sizes of the patches fix the numbers: the universe is 13.77 billion years old, and ordinary atoms make up only about 5 per cent of it.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Cosmic microwave background
- The oldest light there is: a faint glow of microwaves coming from every direction in the sky.
- Recombination
- The time, about 380,000 years after the Big Bang, when the universe cooled enough for electrons to settle onto nuclei and make the first whole atoms.
- Big Bang
- The hot, dense early state from which the universe has been expanding and cooling for 13.8 billion years.
- Plasma
- Gas so hot that its atoms have lost electrons, so it carries electric current and is steered by magnetic fields.
- Wavelength
- The distance from one crest of a wave to the next.
- Kelvin
- The temperature scale that starts at absolute zero, the coldest anything can be.
- Dark matter
- Matter that gives out no light at all and is known only by the pull of its gravity.
- Dark energy
- The name for whatever is making the expansion of the universe speed up.
Where this comes from
- ESA, Planck and the cosmic microwave background
- NASA, WMAP overview
- Nobel Prize in Physics 1978: discovery of the cosmic microwave background
- NASA photograph GPN-2003-00013, the horn antenna at Holmdel (1962)
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.