Redshift
What causes redshift in the spectrum of an astronomical object?
What is the Doppler effect?
What is Hubble’s law?
What is the cosmic microwave background (CMB)?
Redshift, displacement of an astronomical object’s spectrum toward longer (red) wavelengths (lower frequencies), which occurs when the object moves away from the observer or vice versa. This shift is usually caused by the Doppler effect, a change in wavelength that results when a source of waves (such as light or radio waves) and an observer move with respect to each other. In the case of gravitational redshift, the change occurs because light loses energy in a strong gravitational field.
Types of redshift
Doppler shift
As noted above, the redshift of light spectra arises from the relative motion of an object and the observer. As the source moves away from the observer, the distance between each successive wave crest grows. The faster an object moves away, the more its light is redshifted. (The converse shift, in which the object moves toward the observer or vice versa, is called a blueshift, since the object’s spectrum is shifted toward shorter [blue] wavelengths.)
Because the velocity v of a wave is the wave’s frequency f multiplied by its wavelength λ—that is, v = fλ—and the speed of the light wave does not change, the corresponding increase in wavelength is accompanied by a corresponding decrease in frequency. The redshift z is quantified by seeing how much the observed wavelength of a known spectral line differs from the wavelength of that same line at rest. The formula is z = (λo − λr)/λr,where λo is the observed wavelength of the spectral line and λr is the wavelength of that line at rest.
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The Doppler effect is used to study binary stars, which are two stars orbiting their common center of mass. Some binary stars are called spectroscopic binaries because they look like a single star, but observing the spectrum reveals the true nature of the system because it has both redshifted and blueshifted spectral lines.
The Doppler effect was also used by Swiss astronomers Didier Queloz and Michel Mayor in their Nobel Prize-winning discovery of 51 Pegasi b, the first known extrasolar planet orbiting a Sun-like star. They found the planet by observing the star’s spectral lines being alternately redshifted and blueshifted as the planet moved in its orbit.
Cosmological redshift
American astronomer Edwin Hubble found in 1929 that distant galaxies are receding from the Milky Way Galaxy (in which Earth is located) and that their redshifts increase proportionally with their increasing distance. This relation is called Hubble’s law, which states that the recessional velocity of a galaxy equals its distance from Earth times a constant (now known as the Hubble constant). That is to say, the greater the distance of the object, the larger its recessional velocity, and thus the greater the redshift manifested by light emanating from such an object.
Hubble’s law was the first evidence of the expansion of the universe. This redshift is called the cosmological redshift. The universe began 13.8 billion years ago with rapid expansion from a highly compressed initial state. The expansion has continued to the present time and is not outward from a single point but an expansion of all space that results in every galaxy moving away from every other galaxy.
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About 380,000 years after the big bang, the universe became transparent to light when it was cool enough for protons and electrons to form hydrogen atoms. Before this time, called recombination, photons were scattered by the free electrons, and the universe was opaque. The temperature of the universe at that time was about 3,000 K. As the universe has expanded, that “first light” at the time of recombination has redshifted to microwave wavelengths and has a blackbody spectrum with a temperature of 2.7 K. This light is called the cosmic microwave background (CMB) and is further evidence of the expansion of the universe. The CMB has a redshift z of about 1,089.
Gravitational redshift
Gravitational redshift is a consequence of general relativity. As light escapes from a gravitational field, it loses energy working against that field. The speed of light is constant, and the energy of light is E = hν, where h is Planck’s constant and ν is the frequency of the light. Since the light cannot lose energy by slowing down, it loses energy by decreasing its frequency—that is, undergoing a redshift.
This redshift is seen in objects with strong gravitational fields such as white dwarf stars, and the first detection of such a redshift was in 1954 in observations of the white dwarf 40 Eridani B. This redshift has also been measured on Earth in the laboratory. In 1960 American physicists Robert Pound and Glen Rebka measured the gravitational redshift of gamma rays in a 23-meter (74-foot) tower.