What is meant by background radiation?

Asked by: Kyle Burney


According to the current model of how the universe was formed, all of the universe, including all space and all the mater and energy in that space, exploded out of a tiny point in what is termed the big bang. Just as if all the heat energy in a swimming pool were condensed into a drop of water the drop would be very hot, this tiny point was VERY hot (containing as it did all the energy in the universe). As the universe expanded it cooled, and much of the energy converted into matter- the same protons, neutrons, electrons, etc. which are whizzing around today. However some of the energy remained energy, just flying around as photons. These photons are not emitted from any source, but just permeated (and continue to permeate) all of space. This energy left behind is what we call the background radiation.

Cosmic microwave background fluctuation map by NERSC

Why don't we see the whole universe as glowing if all of space is permeated by this energy? Well, in a sense we do, but as very low energy radio waves rather than the visible light you would associate with the word 'glowing.' No matter which direction we point a radio detector in space, we pick up this background radiation, but it is barely noticeable now because it has been 'stretched' so thinly as the universe has expanded to its current enormous size.
Answered by: Rob Landolfi, Science Teacher, Washington, DC

If we flash a torch (flashlight), photons of light are released in all directions. Light is electromagnetic radiation. An observer in space, many years later, might be able to detect this radiation if it hasn't been absorbed or deflected on its way.

In the first half of this century, observations of distant galaxies suggested that the universe is expanding. Perhaps, astronomers conjectured, there was a time that all of space and time and matter was condensed into one place ? The idea is that the universe began with a great eruption of energy - the 'big bang'. The question is, if there was this enormous flash of high-energy photons at the beginning of space and time, what would have happened to all that radiation by now ?

In 1948 the Russian-American physicist George Gamow predicted that we should be able to detect left-over radiation from the big bang. Because it occurred at every point of space, this radiation would have no specific direction, but would be floating around randomly. It would no longer be the same high-frequency that it was to begin with, but the expansion of the universe would have cooled it to down to a temperature of approximately 2.7 K (-270.3 Celsius).

This 'background radiation' was first observed by Penzias and Wilson in 1963. These two experimenters were listening to microwave radiation from the Milky Way, but just couldn't get rid of some unwanted noise from their recordings. The signals they were picking up continued throughout the year, despite the motion of the Earth around the Sun, which suggested that they originated outside of the solar system. They came to the conclusion that they were in fact detecting radiation that originated from the start of the universe - the background radiation predicted by Gamow.
Answered by: Sally Riordan, M.A., Management Consultant, London