Slide 1
Redshift means light arrives at longer wavelengths
Redshift means that observed wavelengths are longer than their emitted values. Astronomers measure it by comparing recognizable spectral patterns with laboratory wavelengths.
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The name does not require an object to look visibly red. A feature originally in ultraviolet light can be shifted into visible or infrared wavelengths while still being described by the same concept.
Slide 2
The fractional wavelength increase measures the shift
Redshift measures the fractional increase in wavelength. If a feature arrives at twice its emitted wavelength, its redshift is one.
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The calculation is observed wavelength divided by emitted wavelength, minus one. For a hypothetical shift from 500 to 1,000 nanometers, the result is 1,000/500 − 1 = 1; these are illustrative values.
Slide 3
Expansion stretches light during its journey
For distant galaxies, cosmic expansion stretches wavelengths while light travels through space. This cosmological redshift differs from a simple local Doppler shift.
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Motion through space can also shift light, and gravity can affect wavelengths too. An observed shift needs physical context rather than automatically being assigned to one mechanism.
Slide 4
Several shifted lines make the identification stronger
Several spectral features can be matched at a common shift. Their shared displacement is more informative than an isolated unexplained peak.
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A single line can sometimes be identified in more than one way. Additional lines, broad spectral shape and other observations help test whether the proposed redshift interpretation is consistent.
Slide 5
Turning redshift into distance needs an expansion model
A cosmological redshift can be related to distance and lookback time using an expansion model. The measured shift itself is not already a distance in kilometers.
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At very large distances, definitions such as present separation and light-travel distance differ. Reporting a redshift preserves an observational quantity while the conversion carries the chosen cosmological assumptions.
Sources
- What is red shift?
ESA
- The Expanding Universe
OpenStax, Rice University
- First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole
European Southern Observatory