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Cosmological redshift
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Space expansion stretches light.
In an expanding universe light's wavelength stretches with the scale factor: 1 + z = a_now/a_then. It's not a Doppler shift through space.
z = 1 means the universe was half its present size.
Used to date distant objects.
In more detail
The cosmological redshift is caused by the expansion of space, which stretches the wavelengths of light travelling through it. It is the main reason distant galaxies appear redshifted.
Real-world uses
- Redshift measurements give the distances and expansion history of the universe.
- Cosmologists use redshift to date distant events.
- Cosmologists use the stretching of light from distant galaxies to measure how the universe has expanded.
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Frequently asked questions
Is redshift a Doppler effect?
Not exactly, since galaxies are not moving through space; space itself is expanding.
What does a high redshift mean?
The light has travelled a long way, so it comes from an early epoch of the universe.
Does redshift mean galaxies are moving through space?
Mostly no. The space between galaxies expands, which stretches the light as it travels to us.
Related topics in Relativity
Gravitational waves detection · Frame dragging · Light bending by the Sun · Wormholes