Cosmic Microwave Background
Cosmology · The oldest light
The cosmic microwave background is the afterglow of the Big Bang: light released when the universe first became transparent, about 380,000 years after it began. It has been stretched by the expanding universe into faint microwaves that come from every direction.
- EverywhereIt is all around you, in every direction.
- ~380,000 years after the Big BangReleased
- 2.7 degrees above absolute zeroTemperature
Take a closer look
Almost Perfectly Smooth
The afterglow is the same temperature in every direction to about 1 part in 100,000. Those tiny ripples are like waves one millimetre high on a lake 100 metres deep – yet they were enough for gravity to grow them into every galaxy we see today.
Hot as a Star
If you could stand in the universe when this light was released, every direction would glow orange-hot – about 2,700 °C, like the surface of a cool red star. Since then, the expansion of space has stretched that light about 1,100 times, turning it into faint microwaves.
Telescopes in Dry Places
Water in the air soaks up microwaves, so CMB telescopes are built in very dry places like the South Pole and Chile’s Atacama Desert, or launched into space. Measuring the pattern of tiny ripples is how scientists worked out the universe’s age and its recipe.
How we found out
- 1948
Ralph Alpher and Robert Herman predict a leftover glow from the hot early universe.
- 1964–65
Arno Penzias and Robert Wilson find a mysterious hiss in their antenna. They even clean out pigeon droppings! It turns out to be the Big Bang’s afterglow.
- 1978
Penzias and Wilson win the Nobel Prize in Physics.
- 1989–2013
The COBE, WMAP and Planck satellites map the tiny ripples in the glow – the seeds of all galaxies.
Why can’t we see further back than the CMB with light?
Before then, the universe was a hot, glowing fog: light could not travel freely. The CMB is the moment the fog cleared.
See it yourself
It’s all around you, all the time – but only special microwave telescopes can detect it.
From the encyclopedia
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