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Redshift Explained Simply
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The light of distant galaxies carries a hidden message. It looks redder than it should. This small shift tells the greatest story of the cosmos.
What redshift is
Light can be described as a wave. Red light has longer wavelengths, blue light shorter ones. When light shifts to longer wavelengths, researchers call it redshift.
For distant galaxies this is exactly what we observe. Known lines in the spectrum all appear shifted to longer wavelengths. From this the shift can be measured precisely.
Why the light stretches
Cosmic redshift does not simply arise from motion. It arises because space itself expands. The light is stretched along its long journey.
The longer the light has traveled, the stronger the stretch. Redshift is therefore a direct measure of the expansion since the Big Bang.
Hubble’s great discovery
Edwin Hubble noticed a pattern in 1929. The farther a galaxy lies, the stronger its redshift. So the universe is expanding.
This discovery was a revolution. It showed that the cosmos has a history and emerged from a dense, hot beginning.
How we measure distances with it
From the redshift we can infer the distance. A stronger shift means a greater distance and an earlier time.
So redshift becomes the measuring tape of the universe. It links what we see with the moment the light set out.
What it reveals about the early universe
The most strongly shifted light is the cosmic microwave background. It comes from the early era and is extremely stretched; more under cosmic inflation.
So redshift provides the evidence for expansion and a hot beginning. It is a cornerstone of modern cosmology, as the Big Bang section shows.
Frequently asked questions
Is cosmic redshift the same as the Doppler effect?
Not quite. The Doppler effect arises from motion. Cosmic redshift arises because space itself expands and stretches the light along the way.
Does redshift mean everything flies away from us?
It looks that way, but we are not a center. From every galaxy the others recede. Space expands the same everywhere.
What is redshift in simple terms?
Redshift means that light shifts to longer wavelengths. Known lines in the spectrum of distant galaxies therefore appear shifted toward the red.
How do we know the universe is expanding?
Edwin Hubble found in 1929 that more distant galaxies are more strongly redshifted. This implies that the space between galaxies is expanding.
How do you measure distances with redshift?
A stronger redshift means a greater distance and an earlier time. So the shift becomes a measuring tape for distances in the cosmos.
How strongly is the background radiation redshifted?
Its redshift is about 1100. The light comes from the early universe and was stretched enormously on its long journey.
Sources and further reading
- Redshift and the Expanding Universe — NASA
- Hubble's Law — ESA
Update note (as of: 06/04/2026)
First publication of the redshift spoke.
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