State the essential difference between cosmological red shift and Doppler red shift.

In a Doppler red shift the object is moving through space, while in a cosmological red shift the object is essentially at rest in space and it is the space in between that is expanding.

Which type of red shift accounts for the observed shifts of very distant galaxies?

The cosmological red shift, because at such distances the stretching of space during the light's journey completely dominates any motion the galaxy has of its own.

Which type of shift accounts for the movement of a star orbiting within our own galaxy?

The Doppler shift, because the star really is moving through space towards or away from us and the distance is far too small for the expansion of space to matter.

CCEA A-Level Physics 1210

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Astronomy

Cosmological red shift and Doppler red shift

3 things to know
Unit AS 2

Overview

Recall questions on what causes each kind of shift and the essential difference between them, where in the light's journey the stretching happens, which applies to a distant galaxy and which to an orbiting star, why Andromeda is blue-shifted, the inflating balloon analogy, and why the Earth is not at the centre.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What causes a Doppler red shift?

Key term

What causes a cosmological red shift?

Key term

At what point in the light's journey does the stretching that produces a cosmological red shift occur?

Questions from this subtopic

Each one with the model answer underneath.

1
What causes a Doppler red shift?
Model answer
A Doppler red shift is caused by the source physically moving away through space from the observer, so successive wavefronts are emitted from points further away and arrive further apart.
2
What causes a cosmological red shift?
Model answer
A cosmological red shift is caused by the expansion of space itself between the source and the observer, which stretches the wavelength of the light while it is in transit.
3
State the essential difference between cosmological red shift and Doppler red shift.
Model answer
In a Doppler red shift the object is moving through space, while in a cosmological red shift the object is essentially at rest in space and it is the space in between that is expanding.
4
At what point in the light's journey does the stretching that produces a cosmological red shift occur?
Model answer
Continuously throughout the journey, so the longer the light has been travelling the more it has been stretched by the time it arrives.
5
Which type of red shift accounts for the observed shifts of very distant galaxies?
Model answer
The cosmological red shift, because at such distances the stretching of space during the light's journey completely dominates any motion the galaxy has of its own.
6
Which type of shift accounts for the movement of a star orbiting within our own galaxy?
Model answer
The Doppler shift, because the star really is moving through space towards or away from us and the distance is far too small for the expansion of space to matter.

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