The terms redshift and blueshift describe how the frequency of a light wave changes based on whether an object is travelling towards or away from us.When an object moves away from us, the light it emits is known as redshift; when an object moves towards us, the light it emits is known as blueshift. Astronomers use redshift and blueshift to determine how far an object is from Earth; the notion is critical in documenting the expansion of the cosmos.
To comprehend redshift and blueshift, recall that visible light is a spectrum of colours, each with its own wavelength.
According to NASA (opens in new tab), violet has the shortest wavelength at approximately 380 nanometers while red has the longest at approximately 700 nanometers.According to ESA, when an object (such as a galaxy) moves away from us, the wavelength of light is’stretched’ and the light is viewed as’shifted’ towards the red end of the spectrum.
According to the educational website The Physics Classroom, the notion 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 (opens in new tab).The Doppler Effect was initially described (opens in new tab) in 1842 by Austrian physicist Christian Doppler, and many of us experience it on a daily basis without ever realising it.
We’ve all heard how a police car’s siren changes pitch as it approaches, with a high pitch on approach and a lower pitch as the vehicle accelerates away.The observed variation in pitch is caused by soundwaves effectively bunching together or dispersing apart.Because the frequency of the siren does not change, everything is relative.As the police car approaches, the quantity of waves is compressed into a shorter distance; this rise in frequency of sound waves that you hear causes the pitch to appear higher.When the ambulance passes you and moves away, the sound waves are spread out over a greater distance, reducing the frequency you hear and making the pitch appear lower.
This Doppler effect idea applies to both light and sound.The redshift phenomenon was initially described and linked to an expanding cosmos by American astronomer Edwin Hubble (after whom the Hubble Space Telescope is named).
According to NASA, his 1929 observations revealed that nearly all galaxies he viewed are travelling away (opens in new tab).”This phenomena was noticed as a redshift in the spectra of a galaxy,” NASA reported.”This redshift looked to be greater for fainter, more distant galaxies.As a result, the farther a galaxy is from Earth, the faster it recedes.”
Because the fabric of space itself is expanding, galaxies are moving away from Earth.While galaxies are moving — the Andromeda Galaxy and the Milky Way, for example, are on a collision course (opens in new tab) – an overall phenomena of redshift is occurring as the universe expands.Redshift and blueshift are phrases that refer to any section of the electromagnetic spectrum, including radio waves, infrared, ultraviolet, X-rays, and gamma rays.
Radio waves are considered to be blueshifted or shifted toward higher frequencies if they are moved towards the ultraviolet section of the spectrum.Shifting gamma rays to radio waves would imply a redshift to a lower frequency.
The absorption or emission lines in an object’s spectrum are examined to determine its redshift.These lines are unique to each element and are always spaced the same.When an object in space moves toward or away from us, the lines can be found at different wavelengths than if the object was stationary (relative to us).
There are three forms of redshift.There are at least three types of redshift in the cosmos: redshift caused by the expansion of the universe, redshift caused by the movement of galaxies relative to each other, and “gravitational redshift,” which occurs when light is shifted due to the huge amount of matter inside a galaxy.
This last redshift is the most faint of the three, yet scientists were able to identify it on a universe-wide scale in 2011.
Astronomers conducted a statistical examination of a vast catalogue known as the Sloan Digital Sky Survey and discovered that gravitational redshift occurs — exactly as Einstein’s theory of general relativity predicted.This work was published in the journal Nature.”We have independent measurements of the cluster masses, so we can compute the expectation for gravitational redshift based on general relativity,” said Radek Wojtak of the University of Copenhagen at the time.”It precisely agrees with the observations of this effect.”
The first observation of gravitational redshift occurred in 1959, when scientists discovered it in gamma-ray photons emerging from an Earth-based laboratory.It was previously discovered in the sun and adjacent white dwarfs, or the dead stars that remain when sun-sized stars stop nuclear fusion late in their lives.
How may astronomers benefit from redshift?
Redshift assists astronomers in comparing the distances of distant objects.Scientists revealed in 2011 that they had discovered the farthest object ever seen – a gamma-ray burst termed GRB 090429B, which came from an exploding star.Scientists estimated the explosion occurred 13.14 billion years ago.By contrast, the Big Bang occurred 13.8 billion years ago.
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Redshift can be used by scientists to determine how the universe is organised on a big scale.The Hercules-Corona Borealis Great Wall is an example of this; light travels across the structure in around 10 billion years.The Sloan Digital Sky Survey is an ongoing redshift study that aims to calculate the redshifts of millions of objects.The CfA RedShift Survey was the first redshift survey, with the first data collected in 1982.
One developing area of study is how to extract redshift information from gravitational waves, which are disturbances in space-time caused by the acceleration or disruption of a huge entity.(Einstein proposed the presence of gravitational waves in 1916, and the Laser Interferometer Gravitational-Wave Observatory (LIGO) was the first to directly discover them in 2016).According to a 2014 research published in the peer-reviewed journal Physical Review X, obtaining the redshift from gravitational waves needs some math and estimation because they contain a signal that reflects their redshifted mass (opens in new tab).
Additional materials
Learn more about the Doppler Effect with NASA and Doppler Shift with the University of California, Los Angeles (opens in new tab).You can also learn about wave characteristics by visiting the BBC Bitesize website (opens in new tab).
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