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Cosmic Microwave Background Radiation: Definition & Overview

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson defines cosmic microwave background radiation, including the history behind its discovery and what this tells us about the universe as a whole. A short quiz will follow.

Definition of Cosmic Microwave Background Radiation

Scientists tell us that the universe began in a Big Bang, but it wasn't until the discovery of the cosmic microwave background that we knew this for sure. The cosmic microwave background is radiation coming from space in every direction that originated in the Big Bang. As often happens in science, its discovery was completely accidental, and yet it had huge implications for the past and future of the universe.

Big Bang Theory

The Big Bang Theory is the idea that the universe is expanding, and therefore was once much denser than it is today. If the universe is expanding in every direction, it follows that everything we see in the universe today was once compressed to a single point. The expansion of that single point is called the Big Bang.

The Expansion of the Universe
Universe Expansion

The Big Bang was first theorized in 1922 by Alexander Friedmann, who showed, using Einstein's general relativity equations, that it was mathematically possible for the universe to be expanding. There was very little evidence at the time that the universe truly was expanding, but the work done by Friedmann did eventually lead to some predictions that we could test.

One of the biggest predictions of the Big Bang Theory was that there would be background radiation left over and detectable today. This prediction was made by Ralph Alpher and Robert Herman in 1948 but went largely unnoticed since so few people thought that the universe was expanding. Only in the 1960s was the prediction revisited, and a group of scientists at Princeton began work to look for the cosmic microwave background.

Accidental Discovery

In 1965, Penzias and Wilson detected something they didn't expect. At Bell Laboratories, they were testing a microwave radiation detector. Expecting to hear nothing, they instead found a small amount of background noise - like the 'snow' on an old-fashioned television, or the hum of a radio that isn't tuned to a station.

Microwave Detector
Microwave Detector Horn

Penzias and Wilson tried pointing the antenna in different directions, but whatever they did, the noise wouldn't go away. They assumed that it was a problem in their detector, and tried cooling it down, but nothing they did helped. Finally they realized that it must be coming from space - a background noise of microwave radiation that was the same in every direction.

Only after getting in contact with the group of scientists at Princeton did they realize what their discovery meant. When one of the scientists at Princeton, Robert Dicke, heard about the discovery, he responded by saying, ''Boys, we've been scooped.''

Penzias and Wilson had accidentally confirmed the Big Bang Theory, and in 1978 they received the Nobel Prize for their work.

Black Body Curves

A black body is an object that is a perfect absorber and emitter of radiation. Such an object would absorb all radiation that hit it and re-radiate that energy in the same way no matter the source of the original radiation. A black body would always produce radiation in a characteristic black body curve, a curve that would only vary if the temperature of the object changed.

Black Body Curves
Black Body Curves

The Sun is an example of a black body and produces a black body curve. But if the Big Bang Theory is correct, then the entire universe was once squashed together, just like the matter in the Sun. If that was the case, then at the time of the Big Bang, a signature of black body radiation would have been produced, just like it is in the Sun.

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