The Universe is thought to have begun some 13.7 thousand million years ago with an enormous explosion which we call the Big Bang. The temperatures at that time were unimaginably huge but as time passed since the Big Bang the Universe cooled. The temperature in deep space dropped and dropped. The background radiation moved into the infrared and the cooling continued.
The photons produced in the Big Bang have continued ‘cooling’ ever since. The temperature of deep space has now reached 2.725 K and the temperature will carry on falling as long as the universe continues expanding.
In 1948 George Gamow, Ralph Alpher, and Robert Herman predicted that the ‘remains’ of this radiation should still be observable even after such an enormous time.
In 1965 two American astronomers, Arno Penzias and Robert Wilson were using the antenna at the Bell laboratories in New Jersey (see photo credit Bell Laboratories) for scanning the sky when they found that there was a background "noise" (like static in a radio). This uniform signal was in the microwave range with a wavelength of about 7 cm and seemed to come from all parts of the sky. It has since been called the Cosmic Microwave Background or CMB. Penzias and Wilson tried to get rid of this annoying background interference. They ejected the pigeons living in the horn of the antenna and even cleaned out their droppings but still the signal persisted. (Modern estimates for the peak intensity of this radiation give a wavelength around 2mm).
By pointing the ‘telescope’ in a variety of directions they concluded that the interference wasn't radiation from our galaxy or extraterrestrial radio sources and because it remained constant throughout the year it couldn't have come from the solar system or even from a 1962 above-ground nuclear test, because in a year that fallout would have shown a decrease.
Finally they realised that it was not random noise causing the signal but something that pervaded the whole Universe. This was the cosmic background radiation with a ‘temperature’ of around 2.7K and was given an evocative name the ‘echo of the Big Bang’. It is the residual radiation predicted by Gamov and others and is the result of the Universe cooling from the unimaginably hot state over the intervening 13000 million years. The detection of the CMB supports the Big Bang idea of the Universe because the cooling of the Universe after the Big Bang would suggest an expansion over many millions of years.
We can detect the radiation produced by this temperature in our homes. It has been estimated that about 1% of the background hiss on your television set is due to the after effects of this enormous fireball.
Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery.