1. Light from distant galaxies: Galaxies emit light, which contains a spectrum of different wavelengths.
2. The Doppler Effect: As a galaxy moves away from us, the light waves it emits get stretched out, similar to how the sound of a siren changes pitch as it moves away from you. This stretching makes the wavelengths of light appear longer, shifting them towards the red end of the electromagnetic spectrum. This is known as redshift.
3. Measuring Redshift: Astronomers use spectrographs to analyze the light from distant galaxies. They look for specific spectral lines – like fingerprints of elements – and compare their positions to the same lines in the lab. The amount of shift towards the red end of the spectrum tells us how much the light has been stretched.
4. Calculating Velocity: The amount of redshift is directly proportional to the galaxy's recessional velocity. This relationship is described by Hubble's Law:
v = H₀ * d
Where:
* v is the galaxy's velocity
* H₀ is the Hubble Constant, a value that represents the expansion rate of the universe (approximately 70 km/s/Mpc)
* d is the distance to the galaxy
In summary: By measuring the redshift of a distant galaxy, astronomers can determine its velocity relative to Earth and understand how the universe is expanding.
Important Note: Redshift also occurs when light passes through a strong gravitational field, due to the bending of spacetime. This is called "gravitational redshift" and is a separate phenomenon from the Doppler redshift used to measure galaxy velocities.
