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Redshift

Redshift, often denoted by the letter z, is an increase in the wavelength of light coming from distant objects in space. This shift happens for three main reasons: when objects move away from us (Doppler redshift), when light escapes from strong gravity (gravitational redshift), or when the universe itself expands (cosmological redshift). Redshift matters enormously in astronomy because it reveals how fast objects are moving and how far away they are. By measuring redshift, astronomers discovered that distant galaxies are moving away from us, leading to Hubble's law—the foundation of our understanding that the universe is expanding. This single measurement has transformed cosmology from guesswork into a science based on observation.

Sources

• en.wikipedia.org: “In physics, a redshift is an increase in the wavelength, or equivalently, a decrease in the frequency, of electromagnetic radiation (such as light).”
• en.wikipedia.org: “Three forms of redshift occur in astronomy and cosmology: Doppler redshifts due to the relative motions of radiation sources, gravitational redshift as radiation escapes from gravitational potentials, and cosmological redshifts caused by the universe expanding.”
• en.wikipedia.org: “The value of a redshift is often denoted by the letter z, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio 1 + z (which is greater than 1 for redshifts and less than 1 for blueshifts).”
• en.wikipedia.org: “In 1929 Hubble combined his distance estimates with redshift data from Slipher's reports and measurements by Milton Humason to report an approximate relationship between the redshift and distance, a result now called Hubble's law.”