What critical observation did astronomer Vesto Slipher first notice regarding light from remote galaxies?
Answer
Light from remote galaxies was systematically redshifted.
Slipher used a spectrograph and noted that light from distant galaxies was redshifted. This occurs because as a light source moves away, its wavelengths stretch toward the red end of the spectrum, which in cosmology is primarily a consequence of spacetime expansion during the light's travel.
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