A Doppler shift is defined as a change of frequency of light or sound when an object is moving toward or away from an observer. Edwin Hubble observed in 1929 that galaxies appear on average to be ...

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Cosmological redshift depends upon a galaxy's distance. Credit: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC) In 1929 Edwin Hubble published the first solid evidence that the universe is expanding. Drawing ...

General relativity is the basis of our understanding of gravity. But twenty-first-century work in cosmology and particle theory strengthens the belief that it is an incomplete description. Although ...