Redshift
In astronomy, redshift refers to the phenomenon where light or other electromagnetic radiation from an object is shifted toward longer (redder) wavelengths. This effect results in a decrease in frequency and energy and occurs in three primary contexts: Doppler redshift, gravitational redshift, and cosmological redshift.
Doppler redshift happens when a light-emitting object moves away from the observer. Similar to how a siren sounds lower in pitch as it moves away, the light’s wavelength is stretched. Gravitational redshift, predicted by Einstein’s General Theory of Relativity, occurs when light escapes a strong gravitational field and loses energy, resulting in longer wavelengths. Cosmological redshift is due to the expansion of the universe itself: as space expands, it stretches the light traveling through it, increasing the wavelength.
Redshift is measured using the dimensionless parameter z, calculated as z = (λ_observed − λ_emitted) / λ_emitted. For relatively nearby galaxies, this can be directly related to velocity via v ≈ z × c, where c is the speed of light. In very distant galaxies, redshift indicates how long the light has been traveling and, by extension, how far away the galaxy is.
Redshift is central to modern cosmology. Edwin Hubble’s discovery in 1929 that more distant galaxies exhibit greater redshifts led to the formulation of Hubble’s Law, which describes the expanding universe. Measuring the redshifts of galaxies and supernovae has also revealed that the universe’s expansion is accelerating, suggesting the presence of dark energy.
Astronomers detect redshift by comparing the known spectral lines of elements (like hydrogen or calcium) to those observed in light from distant sources. The greater the shift, the farther and older the object is. Today, some galaxies have been observed with redshifts greater than 13, meaning their light has traveled for over 13 billion years, offering a glimpse into the early universe.
Sources
APODs including "Redshift"
A Redshift Lookup Table for our Universe
8 April 2013
How far away is "redshift six"? Although humans are inherently familiar with distance and time, what is actually measured for astronomical objects is redshift, a color displacement that depends on exactly how energy density has evolved in our universe. Now since cosmological measurements in recent years have led to a concordance on what energy forms pervade our universe, it is now possible to make a simple table relating observed cosmological redshift, labeled "z", with standard concepts of distance and time, including the extrapolated time since the universe began. One such table is listed above, where redshift z is listed in the first and last columns, while the corresponding universe age in billions of years is listed in the central column. To find the meaning of the rest of the columns, please read the accompanying technical paper. Although stars in our galaxy are effectively at cosmological redshift zero, the most distant supernovae seen occur out past redshift one, which the above chart shows occurred when the universe was approximately half its present age. By contrast, the most distant gamma-ray bursts yet observed occur out past redshift six, occurring when the universe was younger than one billion years old, less than 10 percent of its present age.