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.
15 June 2016
A new sky is becoming visible. When you look up, you see the sky as it appears in light -- electromagnetic radiation. But just over the past year, humanity has begun to see our once-familiar sky as it appears in a different type of radiation -- gravitational radiation. Today, the LIGO collaboration is reporting the detection of GW151226, the second confirmed flash of gravitational radiation after GW150914, the historic first detection registered three months earlier. As its name implies, GW151226 was recorded in late December of 2015. It was detected simultaneously by both LIGO facilities in Washington and Louisiana, USA. In the featured video, an animated plot demonstrates how the frequency of GW151226 changed with time during measurement by the Hanford, Washington detector. This GW-emitting system is best fit by two merging black holes with initial masses of about 14 and 8 solar masses at a redshift of roughly 0.09, meaning, if correct, that it took roughly 1.4 billion years for this radiation to reach us. Note that the brightness and frequency -- here mapped into sound -- of the gravitational radiation peaks during the last second of the black hole merger. As LIGO continues to operate, as its sensitivity continues to increase, and as other gravitational radiation detectors come online in the next few years, humanity's new view of the sky will surely change humanity's understanding of the universe.