How do scientists know the universe is expanding?
Quick Answer
They measure how light from distant galaxies is stretched to longer wavelengths, a phenomenon called redshift. The farther a galaxy is, the faster it usually appears to be receding.
The Full Story
The expansion of the universe is inferred mainly from redshift measurements of galaxies and supernovae. Edwin Hubble showed in 1929 that more distant galaxies generally have larger redshifts, meaning their light is stretched as space expands. Later observations of distant supernovae and the cosmic microwave background confirmed and refined this picture. The rate of expansion is described by the Hubble constant, which astronomers continue to measure with improving precision.
Key Facts
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Suggested video title for this topic:
"The stretching light that proved the universe is expanding."
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