Redshift and blueshift describe the change in frequency of light waves depending on whether an object is moving forward or away from us. When an object moves away from us, the light from that object is called redshift, and when an object moves towards us, the light from that object is called blueshift.
Astronomers use redshift and blueshift to infer how far an object is from Earth, a concept that is key to charting the expansion of the universe.
To understand redshift and blueshift, you first need to remember that visible light is a spectrum of colors, each with a different wavelength. According to NASA, purple has the shortest wavelength around 380 nanometers, and red has the longest wavelength around 700 nanometers. When an object (e.g. a galaxy) moves away from us, it is ‘redshifted’ because the wavelength of light is ‘stretched’, so the light is considered to be ‘shifted’ about the red end of the spectrum, according to ESA.
Related: What is a light year?
Redshift, blueshift and Doppler effect
The concepts of redshift and blueshift are closely related to the Doppler effect – which is the apparent change in frequency of a sound wave to an observer depending on whether the source is approaching or moving away from them, according to the educational website Physics Classroom. The Doppler effect is was first described by Austrian physicist Christian Doppler in 1842 and many of us experience the first Doppler effect almost every day without even realizing it.
We’ve all heard how sirens change as police cars pass, with high-pitched sirens on approach, shifting to lower pitch as the vehicle accelerates. This change in pitch apparent to the observer is due to the sound waves gathering together or effectively spreading out. All is relative because the frequency of the siren does not change. As the police car moves towards you, the number of waves compressed into a decreasing distance, the increase in frequency of the sound waves you hear making the pitch appear higher. While the ambulance then passes you and moves away, the sound waves are propagated over an increasing distance thus reducing the frequency you hear so the pitch sounds lower.
This principle of the Doppler effect applies to light as well as to sound.
American astronomer Edwin Hubble (Whose name is the Hubble Space Telescope) was the first to describe redshift and associate it with an expanding universe. His observations, revealed in 1929, showed that nearly all the galaxies he observed were moving away, NASA said.
This phenomenon is observed as a redshift of the galactic spectrum. “This redshift appears to be larger for faint, perhaps more distant galaxies. Therefore, the farther a galaxy is, the faster it is receding from Earth.”
Galaxies are moving away from Earth because the fabric of space itself is expanding. While the galaxies themselves are moving – For example, the Andromeda Galaxy and the Milky Way are on a collision course – there is an overall redshift that occurs as the universe gets larger.
The redshift and blueshift terms 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 dimmed or shifted towards higher frequencies. Gamma rays converted to radio waves mean a lower frequency shift or redshift.
The red shift of an object is measured by examining 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 towards or away from us, lines can be found at wavelengths different from where they would be if the object were not moving (relative to us).
Three types of redshift
At least three types of redshifts occur in the universe – from the expansion of the universe, from the motions of galaxies relative to each other, and from the “gravitational redshift”, which occurs when light is shifted displacement due to the large amount of matter within the galaxy.
This second redshift is the smallest of the three, but in 2011 scientists were able to determine it on a cosmic scale. Astronomers performed a statistical analysis of a large catalog known as the Sloan Digital Sky Survey and discovered that gravitational redshift does occur – completely consistent with Einstein’s theory of general relativity. This work has been published in the journal Nature.
“We have independent measurements of the cluster mass, so we can calculate the expectation for a gravitational redshift based on relativity,” said astrophysicist Radek Wojtak of the University of Copenhagen. How wide is it?” “It agrees exactly with the measurements of this effect.”
The first detection of gravitational redshift occurred in 1959 after scientists discovered it occurred in gamma-ray light emitted by a laboratory on Earth. Prior to 2011, it was also found in the sun and in nearby white dwarfs, or dead stars left over after sun-sized stars stop nuclear fusion at the end of their lives. .
How does redshift help astronomers?
Redshift helps astronomers compare the distances of distant objects. In 2011, scientists announced that they had seen farthest object ever seen – a gamma-ray burst known as GRB 090429B, emitted by an exploding star. At the time, scientists estimated the explosion took place 13.14 billion years ago. For comparison, the Big Bang happened 13.8 billion years ago.
The most distant known galaxy is GN-z11. In 2016, the Hubble Space Telescope determined it existed only a few hundred million years after the Big Bang. Scientists measured GN-z11’s redshift to see how its light is affected by the expansion of the universe. GN-z11’s redshift of 11.1 is much higher than the next highest redshift of 8.68 measured from the galaxy EGSY8p7.
Scientists can use redshift to measure the structure of the universe on a large scale. An example of this is Hercules-Corona Borealis Great Wall; Light takes about 10 billion years to pass through the structure. The Sloan Digital Sky Survey is an ongoing redshift project attempting 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.
An emerging area of research concerns how to extract redshift information from gravitational waves, the perturbations in space-time that occur when a massive object 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 indicating their redshift mass, extracting redshift from that requires some calculations and estimations, according to a 2014 paper. in the peer-reviewed Physical Review X magazine.
looking for more information about the Doppler Effect with NASA and exploring the Doppler Shift with University of California, Los Angeles. You can also read the characteristics of waves with the educational website BBC Bitesize.
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