What is "Red - shift" ?

To answer this question and fully understand "red-shift" and its importance in the world of Space Physics, we must first understand something known as "the Doppler effect".

Imagine you are standing by the side of the road and you hear an ambulance or a police car approaching (eg from your left).
As it gets closer to you the sound gets louder (obviously) but as it gets really close then quickly passes you, you hear a change in the pitch of the sound.
It rises higher in pitch just before it passes you, then it drops lower in pitch once it rushes past you.

Do you recognise this description? I hope so.

This is the effect first described by Christian Doppler in 1842 and since then it is known as "the Doppler effect".

Applied to sound, as in this example, the Doppler effect is:

Notice, the actual pitch of the sound as heard by the driver of the police car would not change at all because there is no change in the movement between this observer and the police car (since the driver is sitting in the car !!). So the Doppler effect is quite a weird thing, isn't it. The police car driver hears no change at all in the pitch of the siren but you do as it whizzes past you.

So, can we explain this strange Doppler effect?

First, let's look at a situation where there will not be a Doppler effect:

Consider a stationary police car with its siren sounding.

An observer in front of the car at A hears the sound at a pitch given by the wavelength λ.
An observer behind the car at B hears the sound at the same pitch, also given by the wavelength λ.
Also, the police man will hear the same pitch because the wavelength of the sound is also λ at the car.

We can see that the pitch is the same for both observers and the police man because the distance between the wavefronts, λ, is the same for those near A or B or the car.

So, when there is no movement of a sound source relative to an observer, the observer measures no change in the pitch (or wavelength) and there is no Doppler effect.

Now we will look at a situation where there will be a Doppler effect:

Consider the police car moving to the right with its siren sounding.

The police car literally catches up with the sound waves that have moved out infront of it such that the wavefronts appear closer together than when the car was stationary.
So, the observer at A hears a pitch of wavelength λ_A, which as you can see, is smaller than the original pitch observed when the police car was stationary, λ. Hence the observer at A hears a pitch shifted higher than normal.

The police car is also literally dashing away from the sound waves that have moved out behind it such that those wavefronts appear further apart than when the car was stationary.
So, the observer at B hears a pitch of wavelength λ_B, which as you can see, is larger than the original pitch observed when the car was stationary, λ. Hence the observer at B hears a pitchshifted lower than normal.

The police man in the police car hears the exact same pitch, λ, as he did when he was standing beside the stationary car; its neither higher or lower.

For a single observer, the pitch will shift to higher than normal as the car approaches, then it will shift to lower than normal when the car passes and moves away.

This is a great example of the Doppler effect. Its one we hear regularly when we are out on the side of a main road.

You should now be able to agree that the Doppler effect when applied to sound is:

Important NOTE:
From the above explanation we can learn an important fact about the Doppler effect:
When the wave source is moving towards the observer its wavelength will decrease, and
when the wave source is moving away from the observer its wavelength will increase.

Does the Doppler effect work with waves other than sound?

Yes, it will work with any type of wave, but our main concern in this section on Space Physics is with light waves, so lets consider the Doppler effect with regard to light.

Imagine our police car has an Orange warning light on its roof. Light waves will move forwards and backwards from the moving car, towards observer A and away from observer B, just like the sound waves.

And just like the sound waves, the light waves in front of the car will be squashed together causing their wavelength to decrease according to observer A, and stretched apart according to observer B.

But what will the two observers see ?
The observer at A will "see" the light shifted towards a shorter wavelength since from his perspective the wavefronts are closer together than normal; so the Orange light will shift towards Yellow.
The observer at B will "see" the light shifted towards a longer wavelength since from his perspective the wavefronts are further apart than normal; so the Orange light will shift towards Red. (a Red - shift!)

Important NOTE:
So, from this little example we can learn another important fact about the Doppler effect:
When a light wave source is moving towards the observer, he/she will see a shift in the wavelength of the light towards a shorter wavelength eg orange to yellow or even further all the way to blue or violet.
When a light wave source is moving away from the observer, he/she will see a shift in the wavelength of the light towards a longer wavelength eg orange to red; we call this a Red - Shift.

Although in our real world, the Doppler effect with light for a moving police car would occur, it would NOT be noticeable because for the Doppler effect to be noticeable the speed of the wave source has to be a fair fraction of the speed of the wave itself.

In the case of sound this condition is met because the speed of sound waves is about 300 m/s and the speed of a fast police car is about 25 m/s, which is a fair 8% of the speed of sound.

But since the speed of light is 300,000,000 m/s, a police car even moving at 100 m/s (way too fast) is only 0.0000003 % of the speed of light!

So, where then do we see examples of the Doppler effect and Red - shift?

Red - shift and distant galaxies

In 1919 the astronomer Edwin Hubble used the best telescope of his time, the 100 inch reflecting telescope on the top of Mount Wilson in California, to observe nebulae. At this time it was thought that our galaxy, the Milky Way galaxy, was the only one and that all of the nebulae that Hubble was cataloguing were inside the Milky Way galaxy. By 1924, Hubble had shown that many of these nebulae were not part of our galaxy (like the stellar birth places we discussed in section 4.8.1) but were much more distant galaxies themselves. This was the first time that the world of astronomy had the view that the universe was made up of more than one galaxy.

Having got his idea accepted, in 1929 Hubble began looking at these distant galaxies more closely.
He analysed the light from the galaxies.

The following diagrams show the sort of thing he was looking at:

The first shows the spectrum of the light from a typical star within our Milky Way galaxy. The dark lines are called "absorption" lines and they are like little gaps in the light from the star; they are like finger prints which tell astronomers about the star and they always occur in the same pattern, in the same place.

But when Hubble examined light from a star in one of the distant galaxies, this is what he saw:

He could see that all of the absorption lines had been Red - shifted.

After observing and analysing many distant galaxies Hubble made two conclusions:
1. The light from all the distant galaxies was Red - shifted.
2. The further away the galaxy, the bigger the Red - shift.

The implications of Hubble's conclusions

Between 1919 and 1929, just 10 years, the known "universe" had gone from one galaxy to multiple galaxies and now Hubble's two new conclusions provide evidence for a radical theory proposed by a Belgian Roman Catholic priest and astronomer, Georges Lemaitre in 1927, the Big Bang Theory (although it was NOT named as such at the time).

The first conclusion suggests that if the light from all the galaxies is Red - shifted, then all the galaxies are moving away from each other! And if they are moving away from each other "now", then at some time in the past they must have been all together; so Hubble's observation is evidence for the idea that the universe began with all matter in one place when a "Big Bang" occurred and the expansion of the universe began. This is the so-called Big Bang theory.

The second conclusion suggests that the furthest galaxies are moving faster (because a bigger Red - shift occurs if a light source is moving faster) providing more evidence for the idea that the universe is expanding. Eventually Hubble was able to show from the Red - shift data that the recession speed of galaxies was proportional to their distance.

The Big Bang Theory

What it says in one sentence:

The Big Bang theory suggests that the universe began from a very small region that was extremely hot and dense about 13.8 billion year ago when expansion started and the universe that we know began.

Notice that it is called a "theory" but it is a theory with a lot of very compelling evidence and no other theory of the origin of the universe exists with such strong evidence.

There are 4 main "pillars" of evidence upon which the Big Bang Theory rests:

First: Hubble's conclusions (see above) from his observation on distant galaxies showed that they are moving away from each other and so they must have been located at a single point billions of years ago.

Second: If the universe began from a single point then that single point would, in its "early years", act like a single star; it would fuse together hydrogen to make helium. When physicists measure the amounts of hydrogen and helium in the observable universe the amounts match what you would expect to find in an early universe that was once a single big star.

Third: If a sensitive radio telescope is pointed at any space in the night sky between stars it picks up a very faint signal. No matter where in the sky the telescope is pointed this "background radiation" is detected. It is radiation in the microwave region of the electromagnetic spectrum and so was named Cosmic Microwave Background Radiation (C.M.B.R). It is explained as being a remnant of radiation from an early stage of the universe when it was much hotter and smaller; the idea is that this "hotter" radiation spread out with time and distance shifting in wavelength from very hot short wavelengths to colder longer wavelengths (ie microwave wavelengths). So, what we detect today is evidence for a once smaller, hotter, denser universe.

Fourth: The large scale structure of the observable universe is consistent with it beginning from a single mass a long time ago. The shape and distribution of galaxies at near and far distances are as expected from such a beginning.

NOTE: For AQA GCSE you don't need to know all 4 of these pieces of evidence. The first is the most important since it is the one concerning Red-shift but it would be good to know the third, the evidence from the CMBR.

Supernovae, Dark Energy and Dark Matter

You learnt about supernovae in 4.8.1.2
When a star explodes as a supernova its brightness is so great that a single supernova in a distant galaxy can be seen by a telescope on Earth!
In 1998 researchers observing these stellar explosions (using the Hubble Space Telescope, HST) found that though bright they were not quite as bright as they expected based on the assumed distance of each galaxy.
The suggested explanation - these galaxies were further away than originally measured, and this suggestion led to the conclusion that the expansion of the universe was actually speeding up, not slowing down as we should expect due to the inevitable pull of gravity (like throwing a ball up into the air; it might go up and up for a long time but gravity will inevitably slow its progress).

This led to the question - what is causing the accelerating expansion?

Scientists have not really answered this question and so have proposed an idea that there is something which, for want of a clearer understanding, they have called Dark Energy which is causing this acceleration of space.
It is not a very satisfactory answer since no one has been able to identify any Dark Energy but many scientists remain convinced that something of its kind is out there. An alternative solution is to have a new theory of Gravity, to replace the existing theory of Einstein's!

Another unsatisfactory "problem" that scientists and astronomers are facing is that of so-called Dark Matter.
When they look at stars within galaxies astronomers find that the stars at the outer edges, far from the centre, orbit at the same speed or even slightly faster than those closer to the centre and yet our knowledge of "Orbital Motion" (see 4.8.1.3) tells us that objects orbiting at larger distances from the centre should move more slowly.
So, once again gravity does not seem to be behaving as it should OR there is some unseen extra mass/matter in the galaxy that is causing the outer stars to orbit faster. At the moment scientists are proposing that the solution is some unseen extra mass, which they have imaginatively called Dark Matter, which makes up a significant proportion of the matter in our universe.

Observations and Theories

Notice how, in much of the items above, observations have led to the suggestion of theories:

• Hubble's observations of nebulae in 1924 led to the theory that these nebulae were not within our Milky Way galaxy but were themselves galaxies at great distance from our own.
• Hubble's observations of these galaxies and their spectrums in 1929 led to his Red - shift theory that galaxies were moving away from each other.
• The red - shift observations provided evidence to support Georges Lemaitres theory of the expansion of the universe from a single point (which later became known as the Big Bang Theory).
• Observations and measurements of the hydrogen/helium balance of gases in the current universe matched that expected of a single original star at the beginning of time, supporting the Big Bang Theory.
• Observations of the CMBR supported the Big Bang Theory.
• Observations of the accelerating expansion of the universe and of the orbital speed of stars within galaxies are suggesting theories of Dark Enery and Dark Matter, OR requiring a re-write to our existing theory of gravity as written by Einstein.

If you have worked through all the sections of this website in order, then you have arrived at the END. So, well done!
I hope you have enjoyed working through this Physics course and I wish you best of luck in any examination that you take.
Let me know how you get on; click on the Contact button at the top of this or any page to send me a message.