Redshift is a key concept in astronomy that describes the stretching of light to longer, redder wavelengths. When the light from an object is observed to have a longer wavelength than when it was emitted, it is said to be redshifted.

It is one of the most important tools astronomers have, as it provides crucial information about the motion and distance of celestial objects like stars and galaxies.

There are three main causes of redshift: the Doppler effect (motion through space), cosmological expansion (the expansion of space itself), and gravitational redshift (the effect of gravity on light).

This is the same phenomenon as the Doppler effect for sound. When a light source is moving away from an observer, the waves it emits are stretched out, increasing their wavelength and causing a redshift.

Conversely, if a light source is moving towards an observer, the waves are compressed, decreasing their wavelength and causing a blueshift.

This effect is used to measure the radial velocity (motion towards or away from us) of nearby stars and galaxies.

For distant galaxies, the primary cause of redshift is not their motion *through* space, but the expansion of space itself.

As the universe expands, the fabric of spacetime stretches. A photon traveling through this expanding space is also stretched, causing its wavelength to increase.

The farther away a galaxy is, the longer its light has been traveling through expanding space, and therefore the greater its redshift. This relationship is known as Hubble's Law.

Predicted by Albert Einstein's theory of General Relativity, gravitational redshift occurs when light escapes from a strong gravitational field.

A photon must expend energy to climb out of a 'gravity well'. Since a photon cannot slow down, it loses energy by decreasing its frequency and increasing its wavelength, causing a redshift.

This effect is very small and only becomes significant near extremely massive objects like neutron stars and black holes.

Redshift is arguably the most important measurement in modern cosmology.

Hubble's Law (v = H₀d): By measuring a galaxy's redshift, astronomers can calculate how fast it is receding from us. This law then allows them to estimate the galaxy's distance, which is fundamental for mapping the large-scale structure of the universe.

Looking Back in Time: Because light from very distant galaxies has taken billions of years to reach us, their large redshifts allow us to study the universe as it was in its infancy.

Discovery of Dark Energy: The precise measurement of redshift in distant supernovae led to the surprising discovery that the expansion of the universe is accelerating, an effect attributed to a mysterious force called dark energy.