# Physics - Waves from Space (AQA Syllabus A)

In GCSE Science students will look at waves, both mechanical and electromagnetic. This is the last of six quizzes on that subject and it looks at how scientists are using waves coming from space to study the Universe.

The only way that we can learn about our universe is by using waves which come to Earth from space. The whole electromagnetic spectrum is represented - from mysterious gamma ray bursts, all the way to radio waves. But whichever waves you look at, you are always looking back in time. The light and heat from the Sun takes just over 8 minutes to reach the Earth so if the sun was to somehow be extinguished we on Earth would not find out until 8 minutes later. Light from the stars takes even longer. The closest star to our Solar System is Proxima Centauri - light takes just over 4 years to reach us from this star. When astronomers look at the most distant galaxies, they are seeing them as they were thousands of millions of years in the past.

So how can astronomers study the Universe using these waves from space? There are many advanced techniques but one of the fundamental ones is to use the Doppler effect. If a wave source is moving relative to an observer there will be a change in the observed wavelength and frequency. You can hear this when you are standing next to a main road with fast moving traffic. As the vehicles approach, they sound slightly different to when they are moving away. The pitch of the sound is higher as they approach because the waves are 'bunched up'. This shortens their wavelength and increases their frequency so they sound higher pitched than they do when they are not moving. As the vehicle passes and moves away, the waves get left behind and their wavelength is longer and their frequency is lower.

This happens with light too but is only detectable if something is moving at high speed as light waves have such a high velocity. Shorter wavelengths of light are bluer and longer ones are redder. Astronomers studying distant galaxies have observed an increase in the wavelength of light from most of them. The further away the galaxies are, the bigger the observed increase in wavelength. This effect is called red-shift and shows that the most distant galaxies are moving away from us at the fastest rates - the Universe is expanding.

Microwaves have played an important part in understanding the formation of the Universe. Cosmic microwave background radiation (CMBR) is a form of electromagnetic radiation filling the Universe. It comes from radiation that was present shortly after the big bang that has been cooling ever since. The ‘Big Bang’ theory is currently the only theory that can explain the existence of CMBR but who knows, tomorrow, or a hundred years from now, there could be new discoveries that change our understanding completely.

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1. Which of the following waves can be changed by the Doppler effect?
Any wave whose source is moving is subject to the Doppler effect
2. When the source of a wave moves towards the observer, which of the following is not true?
Light from a galaxy that is approaching ours would be blue-shifted, not red-shifted
3. Galaxies emit (give out) many different types of wave. Which of the following would not be emitted by a galaxy?
Sound waves are longitudinal waves and cannot travel through the vacuum of space
4. Scientists measuring radio waves from a distant galaxy find the wavelength of the waves they have detected is longer than it should be. What is this called?
It doesn't matter what type of electromagnetic radiation you are looking at, if the wavelength has been increased it is still called red-shift
5. Scientists discovered that all of the distant galaxies they looked at showed red-shift. What does that tell us about the direction that most galaxies are moving?
Scientists talk about the 'recession of the galaxies'. Recession is an alternative way of saying that the galaxies are moving away from our galaxy
6. What information does the red-shift of a galaxy give us?
Galaxies often occur in clusters. Our Galaxy is called the 'Milky Way' and is a part of what is called the 'Local Cluster'. Only galaxies in the Local Cluster are blue-shifted and are therefore moving towards us
7. What conclusion can be drawn from the fact that nearly all galaxies are moving away from us?
American astronomer Edwin Hubble is credited with the discovery that the Universe is expanding. It is possible that a Belgian mathematician, Georges Lemaitre, came to the same conclusion some years before Hubble. Hubble came up with the law named after him. Hubble's Law states that the further away a galaxy is, the faster it is moving
8. Which theory about the formation of the Universe is supported by its expansion?
The term Big Bang was coined by British astronomer Sir Fred Hoyle who actually preferred the Steady State theory
9. In 1964, American radio astronomers Arno Penzias and Robert Wilson discovered the cosmic microwave background radiation (CMBR). Why was this an important discovery?
In science, theories are rarely proved beyond any doubt. In time, the Big Bang Theory may even be dropped completely in favour of another that fits the evidence better
10. As the Universe gets older, what might happen to the cosmic microwave background radiation?
It is thought that its wavelength will increase as the Universe expands further and cools down even more. The CMBR is currently at a temperature of about 3 K - that's a pretty chilly minus 270oC