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Origins of the Universe

Evidence of the Big Bang

There are two key pieces of evidence for Big Bang theory. These are red shift and the Cosmic Microwave Background radiation.

Red shift

You may have noticed that when an ambulance or police car goes past, its siren is high-pitched as it comes towards you, then becomes low-pitched as it goes away. This effect, where there is a change in frequency and wavelength, is called the Doppler effect. It happens with any wave source that moves relative to an observer.

This happens with light too. Our sun contains helium. We know this because there are black lines in the spectrum of the light from the sun, where helium has absorbed light. These lines form the absorption spectrum for helium.

spectrum of light with black verticle lines

Spectrum of the sun

When we look at the spectrum of a distant star, the absorption spectrum is there, but the pattern of lines has moved towards the red end of the spectrum, as you can see below.

as before, but the lines are shifted towards the left and right ends of the spectrum

Spectrum of a distant star

This is called red shift. It is a change in frequency of the position of the lines.

Astronomers have found that the further from us a star is the more its light is red shifted. This tells us that distant galaxies are moving away from us, and that the further a galaxy is the faster it is moving away. Since we cannot assume that we have a special place in the universe this is evidence for a generally expanding universe. It suggests that everything is moving away from everything else. The Big Bang theory says that this expansion started billions of years ago with an explosion.

Cosmic Microwave Background radiation

Scientists discovered that there are microwaves coming from every direction in space. Big Bang theory says this is energy created at the beginning of the universe, just after the Big Bang, and that has been travelling through space ever since.

A satellite called COBE has mapped the background microwave radiation of the universe as we see it. Big Bang theorists are still working on the interpretation of this evidence.

Interpreting the evidence

A summary of some of the evidence of the Big Bang and its interpretation

The light from other galaxies is red-shifted.The other galaxies are moving away from us.
The further away the galaxy, the more its light is red-shifted. The most likely explanation is that the whole universe is expanding. This supports the theory that the start of the universe could have been from a single explosion.
Cosmic Microwave Background The relatively uniform background radiation is the remains of energy created just after the Big Bang.

Back to Radiation and the Universe index

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