Redshift and blueshift: What do they mean?

Redshift and blueshift are used by astronomers to work out how far an object is from Earth.

Redshift and blueshift describe the change in the frequency of a light wave depending on whether an object is moving towards or away from us. When an object is moving away from us, the light from the object is known as redshift, and when an object is moving towards us, the light from the object is known as blueshift. 

Astronomers use redshift and blueshift to deduce how far an object is away from Earth, the concept is key to charting the universe's expansion. 

To understand redshift and blueshift, first, you need to remember that visible light is a spectrum of color each with a different wavelength. According to NASA, violet has the shortest wavelength at around 380 nanometers, and red has the longest at around 700 nanometers. When an object (e.g. a galaxy) moves away from us it is 'red-shifted' as the wavelength of light is 'stretched' so the light is seen as 'shifted' towards to red end of the spectrum, according to ESA.

The concept of redshift and blueshift is closely related to the Doppler effect — which is an apparent shift in soundwave frequency for observers depending on whether the source is approaching or moving away from them, according to the educational website The Physics Classroom. The Doppler Effects was first described by Austrian physicist Christian Doppler in 1842 and many of us experience the Doppler effect first hand almost every day without even realizing it. 

We've all heard how a siren changes as a police car rushes past, with a high pitch siren upon approach, shifting to a lower pitch as the vehicle speeds away. This apparent change in pitch to the observer is due to soundwaves effectively bunching together or spreading out. It is all relative as the siren's frequency doesn't change. As the police car travels towards you the number of waves are compressed into a decreasing distance, this increase in the frequency of sound waves that you hear causes the pitch to seem higher. Whereas then the ambulance goes past you and moves away, the sound waves are spread across an increasing distance thus reducing the frequency you hear so the pitch seems lower.

This principle of the Doppler effect applies to light as well as sound. 

American astronomer Edwin Hubble (who the Hubble Space Telescope is named after) was the first to describe the redshift phenomenon and tie it to an expanding universe. His observations, revealed in 1929, showed that nearly all galaxies he observed are moving away, NASA said.

"This phenomenon was observed as a redshift of a galaxy's spectrum," NASA wrote. "This redshift appeared to be larger for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth."

The galaxies are moving away from Earth because the fabric of space itself is expanding. While galaxies themselves are on the move — the Andromeda Galaxy and the Milky Way, for example, are on a collision course — there is an overall phenomenon of redshift happening as the universe gets bigger.

The terms redshift and blueshift apply to any part of the electromagnetic spectrum, including radio waves, infrared, ultraviolet, X-rays and gamma rays. So, if radio waves are shifted into the ultraviolet part of the spectrum, they are said to be blueshifted or shifted toward the higher frequencies. Gamma rays shifted to radio waves would mean a shift to a lower frequency or a redshift. 

The redshift of an object is measured by examining the absorption or emission lines in its spectrum. These lines are unique for each element and always have the same spacing. When an object in space moves toward or away from us, the lines can be found at different wavelengths than where they would be if the object were not moving (relative to us).

At least three types of redshift occur in the universe — from the universe's expansion, from the movement of galaxies relative to each other and from "gravitational redshift," which happens when light is shifted due to the massive amount of matter inside of a galaxy.

This latter redshift is the subtlest of the three, but in 2011 scientists were able to identify it on a universe-size scale. Astronomers did a statistical analysis of a large catalog known as the Sloan Digital Sky Survey and found that gravitational redshift does happen — exactly in line with Einstein's theory of general relativity. This work was published in a Nature paper.

"We have independent measurements of the cluster masses, so we can calculate what the expectation for gravitational redshift based on general relativity is," said University of Copenhagen astrophysicist Radek Wojtak at the time. "It agrees exactly with the measurements of this effect."

The first detection of gravitational redshift came in 1959 after scientists detected it occurring in gamma-ray light emanating from an Earth-based lab. Previous to 2011, it also was found in the sun and in nearby white dwarfs, or the dead stars that remain after sun-sized stars cease nuclear fusion late in their lives.

HOW DOES REDSHIFT HELP ASTRONOMERS?

Redshift helps astronomers compare the distances of faraway objects. In 2011, scientists announced they had seen the farthest object ever seen — a gamma-ray burst called GRB 090429B, which emanated from an exploding star. At the time, scientists estimated the explosion took place 13.14 billion years ago. By comparison, the Big Bang took place 13.8 billion years ago.

The farthest known galaxy is GN-z11. In 2016, the Hubble Space Telescope determined it existed just a few hundred million years after the Big Bang. Scientists measured the redshift of GN-z11 to see how much its light had been affected by the expansion of the universe. GN-z11's redshift was 11.1, much higher than the next-highest redshift of 8.68 measured from galaxy EGSY8p7

Scientists can use redshift to measure how the universe is structured on a large scale. One example of this is the Hercules-Corona Borealis Great Wall; light takes about 10 billion years to go across the structure. The Sloan Digital Sky Survey is an ongoing redshift project that is trying to measure the redshifts of several million objects. The first redshift survey was the CfA RedShift Survey, which completed its first data collection in 1982.

One emerging field of research concerns how to extract redshift information from gravitational waves, which are disturbances in space-time that happen when a massive body is accelerated or disturbed. (Einstein first suggested the existence of gravitational waves in 1916, and the Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected them directly in 2016). Because gravitational waves carry a signal that shows their redshifted mass, extracting the redshift from that requires some calculation and estimation, according to a 2014 article in the peer-reviewed journal Physical Review X.