The Clear Up

In the universe, where the temperature was very high, the light immediately bumped into particles and could not travel in a straight line. Then, when the atomic nucleus attracted electrons and formed an atom, there were no particles with an electric charge in the universe. Light is one of the electromagnetic wave and electromagnetic wave is dispersed into vibrations of particles with electric charges. If there were no charged particles in the universe, the light wouldn't be emitted or be absorbed by particles. The light, which could not travel straight until then, became able to travel in a straight line. This is called the clear up of the universe because it is just like when light divides through the heavy clouded sky and the sky brightens and clears up.

Cosmic Background Radiation

CBR of whole sky
(Hotter in the red areas)
Credit: AstroArts

The remains of the light rays that began traveling during this clearing up, are electromagnetic waves called cosmic background radiation. This was accidentally discovered by two researchers named Penzias and Wilson in 1965. They were investigating the origins of various electric waves by experimenting with a communication satellite. It was then that they found one electric wave with an unknown source.

If it was an electric wave sent from the earth, you can receive its waves just by pointing the antenna in the direction of the source. But with that mysterious wave, the electric wave came from every direction for 24 hours continuously.

A Spectrum

Rainbow
Credit: Phil Jude/Science Resource

In the sky after a rainfall, you can sometimes see a rainbow. In the rainbow, the seven colors, purple, indigo, blue, green, yellow, orange, and red, are all in a row. All seven colors came from disintegrating one ray of sunlight. In other words, sunlight is a mixture of these seven colors. The differences in colors of light is due to difference of their wavelength. The sun's ray is made from a mixture of various lights. Not only sunlight but also every light, including electromagnetic waves, is a mixture of lights of different wavelengths.

A spectrum depends on how many waves there are, with what kind of wavelength, and their degrees of mixing, in an electromagnetic wave. We mean, this much electromagnetic wave of this wavelength is, this much mixed in this electromagnetic wave. If the part shows that red light's wavelength is stronger than others, it means that the light will seem reddish.

The mysterious spectrum discovered by Penzias and Wilson was the strongest where its wavelength was 2mm, and any other wavelengths shorter or longer than 2 mm were weaker.

Planck Distribution (Decoupling Epoch)

Planck Distribution

One area is strong and others are weak. If you make this into a graph, it would form a round mount-like shape with a peak. This is called as the planck distribution. This cosmic background radiation found by Penzias and Wilson was the spectrum distribution of this planck distribution, but what can we find out from this?

At first, this kind of spectrum distribution is unique to electromagnetic waves that radiate from matter. This is already known from experiments and observations. If the spectra of electromagnetic waves could be distributed in the Planck way, this would mean that that electromagnetic waves radiated from some kind of matter.

In the terms of cosmic background radiation, you can say that electromagnetic waves radiated from matter. This time, they were emitted from particles that were in a thermal equilibrium condition. The light that became able to travel in a straight line when the universe cleared up, now enters this earth.

Next, we are able to recognize the temperature by observing the longest wavelength, that is, the wavelength of the peak of the mount-like graph. The difference in the wavelength of the electromagnetic waves was due to the difference in the energy. Temperature is one form of energy, so it changes together with the change in the wavelength. So if we knew the spectrum distribution of the electromagnetic waves, then we would also know the temperature. By observations, although not quite accurate, cosmic background radiation was 2.7k (about -270 degrees Celsius), but this was after the wavelength elongated due to the universe expansion. The wavelength was really much shorter, which means the temperature was higher.

To the Big Bang Theory

Actually, because this Cosmic Background Radiation was discovered, the theory of big bang became the most likely. Background radiation was observed from various directions, so you could say that the universe was in the beginning, a small ball. Also, by estimating from the present temperature, we can say that there used to be a condition of high temperature and high density.

By the discovery of background radiation, the theory that the universe began with a high temperature and high density fire ball, was proved.

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• COBE Homepage - The COBE satellite was developed by NASA's Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early universe. (Change and Delete)

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