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Does the Universe expand by stretching or creating space?

It’s been almost 100 years since humanity first reached a revolutionary conclusion about the nature of our Universe: space itself cannot and does not remain static, but rather evolves with the passage of time. One of the most unsettling predictions of Einstein’s General Relativity is that any Universe — so long as it’s uniformly (or almost uniformly) filled with one or more species of matter, radiation, or energy — cannot remain the same over time. Instead, it must either expand or contract, something initially derived independently by three separate people: Alexander Friedmann (1922), Georges Lemaitre (1927), and Howard Robertson (1929), and was later generalized by Arthur Walker (1936).

Right at around the same time, starting in 1923, observations began to show that the spirals and ellipticals in our sky were actually galaxies: “island universes” that were well outside of our own Milky Way. With new, more powerful measurements, we could determine that the farther away a galaxy was from us, the greater the arriving light in our instruments was redshifted.