Redshift and blueshift describe the change in the frequency of a light wave depending on whether an object is moving toward 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 moving 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 moving, according to ESA.
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As objects move away from us, their light gets shifted into longer wavelengths or the red end of the spectrum — that’s redshift. Blueshift is the opposite, when light is shifted to shorter wavelengths on the blue side of the spectrum moving as an object comes towards us. These give essential clues on things like distance — and when staring at a distant galaxy, it lets you know how close you are to staring back at the dawn of time.
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This is important for telescopes like the James Webb Space Telescope, which its stakeholders have tasked with learning about the earliest galaxies in the infant universe.
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 Effect was first described by Austrian physicist Christian Doppler in 1842 and many of us experience the Doppler effect firsthand almost every day without even realizing it.
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To learn more about redshift and blueshift, Inverse spoke with Salvatore Vitale, an assistant professor of physics at the Massachusetts Institute of Technology. Vitale does data analysis on gravitational waves (ripples in space-time) charted by the Laser Interferometer Gravitational-Wave Observatory (LIGO) following huge events like black hole mergers.
LIGO physicists work with astrophysicists to chart the distance to the gravitational waves they measure using redshift and blueshift — relying on multiple paths to understand the merger of some of the most potent forces in the universe.
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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 sound waves 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 is compressed into a decreasing distance, this increase in the frequency of sound waves that you hear causes the pitch to seem higher.
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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.
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.
Usually, redshift is discussed when talking about the expansion of the universe. An event 13.8 billion years ago, dubbed the Big Bang, caused the rapid inflation and expansion of space-time. Astronomers still see the echoes of that Big Bang, as objects in the universe are all receding from one another, thus experiencing some degree of redshift.
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Those objects that are furthest away have the highest redshift. We know the universe is accelerating thanks to measuring the redshift of a particular type of star explosion (supernovas), called Ias. Astronomers have nicknamed these types of supernovas “standard candles” as they have a consistent luminosity. Since we know these supernovas’ inherent brightness, we can then chart their brightness in association with distance.
“This phenomenon was observed as a redshift of a galaxy’s spectrum moving,” 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.”
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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 moving, including radio waves, infrared, ultraviolet, X-rays and gamma rays. So, if radio waves are shifted into the ultraviolet part of the spectrum moving, 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 moving. 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).
