What evidence do astronomers have that the Universe is expanding?
A) There is a measurable redshift in the spectral lines of distant galaxies.
B) The disks of galaxies are getting smaller over time.
C) Distant galaxies are dispersing, pulled apart by the expansion of space.
D) Galaxies disappear when they pass the cosmic horizon.
E) Nearby clusters of galaxies are smaller than more distant ones.

Question

Quizplus · Accepted Answer

The redshift in the spectral lines of distant galaxies indicates that they are moving away from us, which is evidence of the expansion of the Universe. This phenomenon is known as the Doppler effect, where the wavelength of light stretches as objects move away, shifting towards the red end of the spectrum.

What Evidence Do Astronomers Have That the Universe Is Expanding