Astronomy
Cosmological Redshift: Measuring Cosmic Distances and Expansion
Quick fact
The most distant galaxies have their light redshifted by a factor of over 8, meaning the universe has expanded dramatically since that light was emitted.
Why this is interesting
When you look at distant galaxies, their light is stretched like a slinky being pulled apart. Could that stretching be a cosmic ruler?
Read the full explanation
Understanding Cosmological Redshift: Measuring Cosmic Distances and Expansion
Imagine a star in a distant galaxy emitting light. As it travels through space, the universe itself expands, stretching the light's wavelength. This is not because the galaxy is moving through space, but because the space between us and the galaxy is growing. This stretching makes the light appear redder (longer wavelength) than it was emitted, which we call cosmological redshift. It is different from the Doppler effect you hear with a moving ambulance siren (which is due to relative motion), though both are named redshift. Astronomers measure how much the light is shifted, which tells them how much the universe has expanded since that light left the galaxy. Because the expansion is roughly uniform, a greater redshift means a more distant light source. Thus, by measuring the redshift of a galaxy, we can estimate its distance and how long ago its light left it.
A deeper explanation
Cosmological redshift arises because of the expansion of space. As a photon travels from a distant galaxy to us, the universe's scale factor (the 'size' of space) increases. This stretches the photon's wavelength, so redshift z = (λobserved - λemitted) / λemitted. The redshift is directly related to the scale factor a(t) at emission: 1 + z = 1/a(t). Thus, redshift is a direct measure of how much the universe has expanded since emission. It is not a velocity, but a cosmic expansion effect. The most distant galaxies have redshifts of z 8, meaning the universe was much smaller then. This redshift-distance relationship, encapsulated by the Hubble-Lemaitre law (v ≈ H₀ × d, where v is recessional velocity from redshift), provides a powerful tool to measure distances and the expansion rate. By observing many galaxies and their redshifts, astronomers map the expansion history, revealing that the expansion is accelerating—a discovery that led to the concept of dark energy. This is why cosmological redshift is foundational for modern cosmology.