One of the most spectacular discoveries of the 20th century was that the Universe itself was expanding. When Einstein put forth his general theory of relativity, he swiftly recognized that there was a consequence he was unhappy about: a Universe that was filled with matter in all directions would be unstable against gravitational collapse.
Einstein’s fix for this was to make up an invisible, outward-pushing force that prevented this collapse from occurring, a cosmological constant. But if you didn’t include this cosmological constant, others soon realized, you’d wind up with a Universe that wasn’t static in time, but where the fabric of space itself was either expanding or contracting with time.
In the 1910s, Vesto Slipher noticed that the spiral nebulae in the sky were redshifted, consistent with the interpretation that they were moving away from us. In the 1920s, Hubble discovered that these nebulae were in fact galaxies, and determined their distance from us.
If you combined these two facts — that there were distant galaxies that rapidly moved away from us — you noticed an interesting trend: the farther away a galaxy was from us, the faster it appeared to be moving away!
While this could have been due to a number of factors, including:
•The light from these distant galaxies getting “tired” and losing energy as they travel through space,
•A rapid motion, where the faster-moving galaxies wind up farther away over time,
•An initial explosion, which pushes some galaxies farther away from us by the present,
•Or the fabric of space itself expanding,
only the last option was validated by the full suite of data supporting both the general theory of relativity and the astrophysical distribution and properties of all the galaxies observed. It became apparent very quickly — as early as the 1930s — that there are no two ways about it: the Universe is, in fact, expanding.
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