Our Expanding Universe: 3 Facts About Redshift

It took scientists and thinkers centuries to start to understand the true nature of the universe in which we live. One of the crucial steps forward came with measurements of the ‘redshift’ of distant galaxies.

Whirlpool Galaxy

Whirlpool galaxy [photo: NASA/ESA]

1. Redshift is an Example of the Doppler Effect

In 1842, Austrian mathematician and physycist Christian Doppler developed the idea that the observed wavelength and frequency of a wave depends on the relative motion of the source and the observer. This was tested and confirmed by a Dutch meteorologist, Christophorus Buys-Ballot 3 years later using musicians sitting in an open wagon of a train. He observed that the sound of their instruments changed depending on whether they were approaching or moving away and how fast they were going. As they approached, the notes sounded higher but as they moved away, the notes sounded lower. When the train was moving faster, the change in the frequency and therefore the pitch of the note was greater.

The reason this happens is that as the source of the sound approaches, each successive sound wave arrives a little sooner than if the train was stationary. The forward movement of the train ‘squashes’ the waves closer together. As the train recedes, successive waves are a little further apart. To help you to visualise this, if you look carefully at the picture of the swan below, you can see that the waves in front of the swan are loser together.

Doppler effect on water waves.

Swan [photo:”Doppler hattyu” by Zátonyi Sándor ]

Since sound is a wave, Doppler guessed correctly that it would apply to all waves. It is easy to observe the Doppler effect using sound because sound only travels at around 330m/s. Light travels considerably faster at 299,792,458 metres per second so at speeds that you are used to, you don’t notice redshift. It was only another 3 years until Doppler was proved right when French physicist Hippolyte Fizeau spotted it in 1848 in starlight. Fact 2 explains how he did it.

2. Redshift is Measured from Spectra

When you pass starlight or the light from a distant galaxy through an instrument called a spectrometer, the light is split up into its component colours. But that’s not all, individual chemical elements in the stars create a series of dark and light lines called spectral lines. When these are compared with their normal position which can be measured in a laboratory, they are seen to be in a different position, in other words, they have been shifted. This is exactly how Fizeau first measured redshift.

Stars and galaxies are moving at staggering speeds – hundreds or even thousands of km per second! At these speeds, the doppler effect that creates the redshift can be measured for light. For stars and galaxies moving away from the observer, the frequency is seen to be lower as the waves are stretched by the motion so they have a longer wavelength and therefore lower frequency. Longer wavelengths and lower frequencies of light are redder, hence the name ‘redshift’. Ths two spectra below show this:

Redshifted spectrum.

The lower spectrum shows redshift relative to the upper one [image:Georg Wiora]

If the star or galaxy is moving towards the observer, the spectral lines are shifted to higher frequencies i.e. towards the blue end. Guess what this is called … blueshift of course! Some of the nearby galaxies are blueshifted meaning that they are approaching us. Redshift and blueshift can also be used to discover the motion and rotation of stars in our own galaxy.

3. Redshift Offers Evidence for the Big Bang Theory

Most scientists now believe that our universe is expanding and that it began from what is called the ‘big bang‘. The ‘big bang’ theory states that originally all the matter and energy in the universe was concentrated into a single tiny point. For whatever reason, about 13.7 billion years ago, this tiny point suddenly expanded in a massive explosion and the universe has continued to expand today. If the theory is right, it means that distant galaxies would all be receding from us here on Earth.

The theory started to take shape in the 1920s when a Belgian physicist, Georges Lemaître, put forward the idea including evidence that the universe was expanding. American astronomer Edwin Hubble measured the redshift of many distant galaxies. His readings verified that they were receding from us and he also discovered that the further away they were, the greater the redshift and therefore the faster they are moving. This has been called Hubble’s Law.

And Finally…

Hubble wasn’t the first to systematically measure the redshifts of Galaxies. In 1912, Vesto Slipher of the Lowell Observatory measured the redshift of 15 spiral galaxies and found that 12 had redshift and 3 had blueshift. The galaxies with blueshift are part of a group of nearby galaxies that we call our ‘Local Cluster’.

Taught science for 16 years at a secondary school in the East Midlands.