Objectives: - Describe red shift - Explain evidence for the Big Bang
The Doppler Effect
As a car drives past you, the sound sounds high-pitched. This is because the waves are being compressed together.
As the car drives away from you the sound sounds more low-pitched. This is because the waves are now spread out
Now how does this relate to galaxies and red shift?
The diagrams above demonstrate sound waves, however the same applies to light and you need to know about light waves. Light waves show a similar shift. If a source of light, for example a star or galaxy, comes towards us the wave lengths are compressed. This means that the light is more blue shifted on the spectrum. However, if a star or galaxy seems to be moving away from us the wavelengths are stretched and longer. This means that the light would be more red-shifted in a spectrum.
GET THIS INTO YOUR HEAD
The further away a galaxy is, the faster it is moving away from the Earth
The further away a galaxy is the more red shifted it is.
Just checking.
QUESTION TIME!
The diagram shows a teacher using a loudspeaker to demonstrate an important effect. The loudspeaker, which produces a note of constant frequency, is swung around in a circle.
(a) As the loudspeaker moves towards the students, the frequency of the note heard by the students increases.
What happens to the note heard by the students as the loudspeaker moves away from them? (1 mark)
The teacher is using the demonstration to model the red-shift in light that is observed from most distant galaxies.
(b) (i) Which one of the following statements gives the main reason why models are used in science?
Put a tick () in the box next to your answer (1 mark)
(b) (ii) Explain how this demonstration can be used as a model for red-shift (2 marks)
(c) Red-shift provides evidence to support the theory that the Universe began from a very small initial point.
What name is given to this theory? (1 mark)
According to scientists, the Big Bang, is one of the theories that explain the origins of the UNIVERSE. The Big Bang occurred around 13-14 billion years ago, so quite a long time ago. But how did this all happen? Scientists believe that all matter in the universe was concentrating into a tiny initial point. This point began to expand rapidly in a hot explosion. We are still expanding today.
The Big Bang hasn't been proven yet however we do have evidence which suggests that the Big Bang did occur. The evidence you need to be aware of for the exam, is red shift and cosmic background radiation. You should also be aware one of the main reasons why scientists haven't yet proved theories is because of the lack of evidence that we currently have.
Red Shift
Have you ever noticed that when an ambulance drives past you the siren sounds more high pitched and when it drives away the pitch decreases? This can happen to light and it is known as the Doppler effect. The Doppler effect is when there is a change in frequency and wavelength. Look at the diagrams on the left which explain the Doppler effect in more detail.
When we collect light from other galaxies, we can look at their spectrum. Light with stretched and long wavelengths are shifted more to the red end of the spectrum. Smaller wavelengths are shifted towards the blue end.
What is red-shift?
This is a popular question that creeps its way into exams year after year. It is usually a one mark question.
Answer: Red-shift is the increase in wavelength. The longer the wavelength the more red-shifted it is on the spectrum.
Below is a table that helps you with the evidence
You need to remember that:
- The further away a galaxy is, the faster it is moving away from the Earth
- The further away a galaxy is the more red shifted it is. - Scientists believe that all matter in the universe was concentrating into a tiny initial point. This point began to expand rapidly in a hot explosion
Cosmic Microwave Background Radiation
Another key part of evidence is cosmic microwave background radiation. This is a microwave that was emitted during the Big Bang and is still here today. BECAUSE it is a microwave this means that it is also an electromagnetic wave SO it travels at the same speed of light.