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Edwin Hubble
1929· Person
ProvenTested many times; part of working physics.
Edwin Hubble (1889-1953), American astronomer at Mount Wilson Observatory who proved other galaxies exist and that they are moving apart.
See it on the Science map →How it works
Using the 100-inch Hooker telescope, Hubble found pulsing stars of known brightness in the Andromeda nebula in 1923. How dim they looked gave their distance: far outside our Milky Way. So Andromeda was a separate galaxy. In 1929 he compared galaxy distances with their speeds (mostly measured by Vesto Slipher) and found the farther a galaxy is, the faster it recedes. He stayed cautious about calling this expansion.
Why it matters
He enlarged the known universe from one galaxy to countless billions, and gave the key observation behind the Big Bang. The Hubble Space Telescope carries his name.
How the work held up
Later surveys of millions of galaxies confirm the distance-speed rule. His own expansion rate was about seven times too high because early distance yardsticks were miscalibrated; the pattern itself held up.
Still a mystery
He never fully committed to the idea that space itself is stretching. Today, two methods of measuring his expansion rate disagree by about 9%, the unresolved 'Hubble tension'.
Connections (2)
▸Edwin Hubble measured The universe is expandingProven
The relationship
Hubble paired galaxy distances, from stars of known brightness, with speeds, from redshift. The result: speed rises in proportion to distance. That straight-line relation is the observable fingerprint of uniform expansion. If every gap in space grows at the same rate, galaxies twice as far away recede twice as fast.
How the link was found
Edwin Hubble in 1929, using 24 galaxies with distances and redshifts largely measured by Vesto Slipher; extended with Milton Humason in 1931.
Why it's proven
Every later survey, now millions of galaxies and supernovae, shows the same relation far beyond Hubble's reach.
Open questions and mysteries
His rate was about seven times too large due to miscalibrated distances. Today's two best methods still disagree by about 9%.
▸Edwin Hubble showed other galaxies exist, revealing Galaxies and the cosmic webProven
The relationship
Hubble found stars in the Andromeda nebula whose pulses reveal their true brightness, and so their distance. Andromeda was far outside the Milky Way: a separate galaxy. Suddenly the countless faint 'nebulae' were galaxies too. That opened the question of how galaxies are arranged, which later surveys answered with the cosmic web.
How the link was found
Edwin Hubble, using the 100-inch Hooker telescope at Mount Wilson; first Cepheid found in 1923, announced in January 1925. It settled the 1920 'Great Debate' between Harlow Shapley and Heber Curtis.
Why it's proven
His distance to Andromeda, about 900,000 light-years, was too short (today: 2.5 million), but far enough to prove the point.
Open questions and mysteries
The web itself was not mapped until the 1980s. Why its largest structures are as big as they are is still studied.