The Solar System - 8.2 Red-Shift (Physics Only) (GCSE Physics AQA)

Red-Shift (Physics Only)

Red-Shift

Wavelength and Frequency

We can look at the wavelength and frequency of waves, such as sound waves and light waves:

• The wavelength tells you the length of a single wave.
• The frequency of a wave tells you the number of waves per second.

The higher the wavelength, the lower the frequency, because if you have a short wavelength then many waves can be present in one second (hence a high frequency), and vice versa.

Wavelengths of Light in Space

From Earth, we can observe light from other galaxies. When we look at light from other galaxies, the wavelength we observe can tell us how far the light is coming from. The longer the wavelength, the further the light is from the earth.

The wavelengths we see are continuously increasing, which means that the galaxies are getting more and more ‘distant’ from Earth. To put this more simply, if a galaxy has a very large increase in wavelength, it means that the galaxy has moved very far away from Earth.

We can describe the increasing wavelength of light as ‘red-shift’. If you look at this diagram below, it shows the different wavelengths of lights in the electromagnetic spectrum. The highest wavelength lights are red. The lights we see from space tends to move towards the red end of the spectrum because their observed wavelength is getting higher and higher(and frequency is therefore lower).

Red Shift and The Big Bang Theory

Expansion of the Universe

As we learnt above, red-shift shows that galaxies must be moving away from each other. Therefore red-shift can provide evidence for certain scientific theories. For example, if the galaxies are moving away from each other, this must mean that the universe is expanding, which means that it must also support the Big Bang theory. We will be exploring this idea more in the next section.

The Big Bang Theory

At the beginning of the universe, scientists think that all matter was condensed into a tiny area. This area was very hot and very dense.

At some point, there was a big bang, when the hot, dense area of matter exploded. The matter all began to move away from each other, meaning that the ‘universe’ at that point was expanding.

From our observations of the wavelengths of light from distant galaxies (red-shift), we know that the universe is still expanding today. This provides evidence to support the Big Bang theory.

Red Shift and Receding Galaxies

Evidence for Receding Galaxies

We have learnt that red-shift provides evidence for galaxies moving away from each other. ’Receding’ is simply another way of saying this.

• The further away the galaxies are, the more quickly they are receding. We can use red-shift to determine how fast a galaxy is moving away from us. The larger the red-shift of a galaxy, the faster it is moving away from us. Distant galaxies will have a large red-shift, since they are receding most quickly.

Evidence for Expansion

The speed of a galaxy will be determined by its distance from us. The closer a galaxy is to us, the slower it will move. The further away it is, the faster it will move.

We have just mentioned that galaxies that are furthest away from us will move the fastest. Since they are moving, space (and therefore the universe) must be expanding.

Forming Theories from Observation

Using Observations for Theories

Through conducting experiments, or simply observing natural phenomena, we can gather evidence. This evidence can be in the form of numbers, graphs or simply descriptions.

Now we know that red-shift provides evidence for the Big Bang theory. This is a real example of how scientific evidence has contributed to the development of theories.

Remaining Question Marks

Sometimes, scientists will come up with theories, such as those involving dark mass and dark energy. However, sufficient evidence may not be found to back up these theories. Currently, we still do not understand very much about the universe.