The Doppler Effect and Redshift

When something making waves moves, the waves in front get bunched and the waves behind get stretched. This is called the Doppler effect.

The Doppler Effect (everyday idea)

An ambulance siren sounds higher as it comes towards you and lower as it moves away. The sound waves are squashed in front (shorter wavelength, higher pitch) and stretched behind (longer wavelength, lower pitch).

Redshift and Blueshift in Light

Light is also a wave. If a star or galaxy moves away from us, its light waves are stretched. Longer wavelengths are towards the red end of the spectrum. This is called redshift. If it moves towards us, wavelengths are shorter and shift towards blue (blueshift).

Astronomers look at dark or bright lines in a spectrum made by known atoms. If those lines are shifted compared with lab values, we measure how much the light has been stretched.

Finding speed from redshift

For galaxies moving much slower than the speed of light, the speed away from Earth, v, is linked to the change in wavelength:

$$\frac{v}{c} = \frac{\Delta \lambda}{\lambda_{\text{lab}}}$$

• c is the speed of light ($3.0 \times 10^8$ m/s)
• $\Delta \lambda$ = observed wavelength − lab wavelength
• Positive $\Delta \lambda$ means redshift (moving away)

What redshift tells us about the Universe

Light from most distant galaxies is redshifted. This shows they are moving away and the Universe is expanding. The speed increases with distance, described by Hubble’s law:

$$v = H_0 \, d$$

$H_0$ is the Hubble constant. This supports the Big Bang idea.

Common misconceptions

• Objects do not “turn red.” The whole spectrum shifts to longer wavelengths.
• The Doppler effect applies to all waves. Sound needs a medium; light does not.
• Redshift here is due to motion/expansion, not because dust simply reddens light.