The Big Bang theory

Scientists have gathered a lot of evidence and information about the Universe. They have used their observations to develop a theory called the Big Bang. The theory states that about 13.7 billion years ago all the matter in the Universe was concentrated into a single incredibly tiny point. This began to enlarge rapidly in a hot explosion, and it is still expanding today.

Evidence for the Big Bang includes:

all other galaxies are moving away from us
the further away a galaxy is, the faster it is moving away

These two features are found in explosions - the fastest moving objects end up furthest away from the explosion.

Scientists have also detected cosmic microwave background radiation or CMBR. This is received from all parts of the Universe and is thought to be the heat left over from the original explosion.

Red shift and the Big Bang

You may have noticed that when an ambulance or police car goes past, its siren is high-pitched as it comes towards you, and 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, and is called 'red-shift'. 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. The first image below shows the colour 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 (or ‘shifted’) towards the red end of the spectrum, as you can see in the second image below.

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 away 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.

Interpreting the evidence

Summary of some of the evidence of the Big Bang and its interpretation.

Evidence / Interpretation

Light from other galaxies is red-shifted / 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 and this supports the theory that the start of the Universe could have begun with a single explosion

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