9.3.3 Galactic Redshift

Galactic Redshift

The lines have shifted towards a longer wavelength, or towards the red end of the spectrum

This shift can be observed by comparing the spectra from the distant galaxy to a spectrum produced by a nearby object, such as our Sun, or a laboratory sample

Comparing the light spectrum produced from the Sun and a distant galaxy, downloadable IGCSE & GCSE Physics revision notes

Comparing the light spectrum produced by the Sun with light from a distant galaxy

After the discovery of redshift, astronomers began to realise that almost all the galaxies in the Universe are receding
- This led to the idea that the space between the Earth and the galaxies must be expanding

This expansion stretches out the light waves as they travel through space, shifting them towards the red end of the spectrum

Expanding Universe Balloon

As the balloon is inflated, the dots all move away from each other
In the same way, as the rubber stretches when the balloon is inflated, space itself is stretching out between galaxies
Just like the dots, the galaxies move away from each other, however, they themselves do not move

Another observation from looking at the light spectra produced by distant galaxies is that the greater the distance to the galaxy, the greater the redshift
- This means that the greater the degree of redshift, the faster the galaxy is moving away from Earth

5-12-5-redshifts-of-galaxies_ocr-al-physics

The further a galaxy is from Earth, the greater its redshift tends to be

Hubbles-law, IGCSE & GCSE Physics revision notes

The furthest galaxies appear to be redshifted the most and are receding the fastest

The spectra below show dark absorption lines against a continuous visible spectrum.

Redshift, downloadable AS & A Level Physics revision notes

A particle line in the spectrum of light from a source in the laboratory has a frequency of 4.570 × 10¹⁴ Hz.

The same line in the spectrum of light from a distant galaxy is observed to have a frequency of 4.547 × 10¹⁴ Hz.

Calculate the speed of the distant galaxy, and state whether it is moving towards or away from the Earth.

Answer:

Step 1: Write down the known quantities

Step 2: Write down the Doppler redshift equation

$\frac{∆ f}{f} = \frac{v}{c}$

Step 3: Rearrange for speed v, and calculate

$v = \frac{c ∆ f}{f}$

$v = \frac{(3.0 \times 10^{8}) \times (- 2.3 \times 10^{12})}{4.570 \times 10^{14}}$ = −1.5 × 10⁶ m s^–1

Step 4: Write a concluding sentence

The relative velocity is negative, so the source is moving away from the Earth

The observed frequency is less than the emitted frequency (the light from a laboratory source), therefore, the source is receding, or moving away, from the Earth at 1.5 × 10⁶ m s^–1

Keep track of the minus signs in your calculation, as this gives you information about whether the object is moving away or towards the observer.

The speed of light is given in your data booklet, you will not need to memorise this value.