Cosmological redshift is the increase in the wavelength of light from distant galaxies and other astronomical objects. It is caused by the expansion of the universe, which causes the distance between objects to increase over time. As the distance increases, the light waves from the objects are stretched out, resulting in a shift towards the red end of the spectrum.

The amount of cosmological redshift is proportional to the distance of the object, so more distant objects have a higher redshift. By measuring the redshift of an object, astronomers can determine its distance from Earth and study the expansion history of the universe.

Cosmological redshift is one of the most important observations in cosmology and has profound implications for our understanding of the universe. It provides evidence for the Big Bang theory and the expansion of the universe, and it has been used to measure the age of the universe and the rate at which it is expanding.