3.9.3.3
Light from distant galaxies is redshifted because space itself is expanding
Astrophysics | AQA A-Level Physics
Key Definition
Galactic redshift: The shift of spectral lines from a distant galaxy towards longer wavelengths (the red end of the spectrum), caused by the expansion of space stretching the light waves as they travel.
Observing galactic redshift
- When spectral lines from a distant galaxy are compared to a reference spectrum (from a laboratory source, a nearby star, or the Sun), the lines are shifted towards longer wavelengths.
- This shift can be observed by comparing the absorption line pattern: the same pattern of lines appears, but displaced towards the red end of the visible spectrum.
- Fundamentally, this tells us the galaxy is moving away from us.
An expanding Universe
- Astronomers discovered that almost all distant galaxies show redshift. This was a remarkable finding: it means nearly every galaxy is receding from us.
- This led to the idea that space between galaxies is expanding. The galaxies themselves do not move through space; rather, the space between them stretches.
- A useful analogy is dots on an inflating balloon. As the balloon inflates, every dot moves away from every other dot, not because the dots are moving on the surface, but because the rubber between them is stretching.
- Crucially, the expansion stretches the light waves as they travel through space, increasing their wavelength and shifting them towards the red end of the spectrum.
Distance and redshift
- The key observation is: the greater the distance to a galaxy, the greater its redshift.
- This means more distant galaxies are receding faster than nearby ones.
- This relationship between distance and recession speed is the foundation of Hubble's law (covered in the next note).
Common Mistake
Students often say "galaxies are moving away through space." The key part is that the galaxies are not flying through space like rockets. Space itself is expanding, carrying the galaxies with it. The light is redshifted because the space through which it travels stretches while the light is in transit, increasing its wavelength.