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