Hubble Shows the Universe Is Expanding
Redshifted galaxies reveal that the universe itself is growing, not just full of moving stars
Quick facts
- Astronomer
- Edwin Hubble
- Key paper
- "A relation between distance and radial velocity among extra-galactic nebulae," 1929
- Key relationship
- Hubble's Law: recession speed increases with distance
- Distance method used
- Cepheid variable stars
What happened
By 1929, Edwin Hubble had shown that the universe was home to millions of galaxies beyond the Milky Way, and that the universe itself was expanding. Studying the light from distant galaxies, Hubble found that it appeared displaced toward the red end of the spectrum, a redshift indicating that those galaxies were receding from Earth. Hubble demonstrated that galaxies farther away recede faster than those nearby, a relationship now known as Hubble's Law, using measurements of Cepheid variable stars to establish galaxy distances and comparing those distances against each galaxy's measured redshift. His 1929 paper, A relation between distance and radial velocity among extra-galactic nebulae, laid out the observational evidence for cosmic expansion.
Why it matters
Hubble's discovery overturned the long-held assumption of a static, unchanging universe and gave the idea of an expanding cosmos, and by extension the Big Bang theory of its origin, direct observational grounding for the first time. Hubble's Law remains the basic tool astronomers use to measure distances across the observable universe and to reconstruct its expansion history.
How we know
Hubble's 1929 paper survives in full and lays out both his galaxy-distance measurements using Cepheid variables and his redshift data directly, and the relationship he identified has been repeatedly confirmed and refined by later astronomers using ever more distant and precise galaxy surveys.
Sources
- NASA Science. Edwin Hubble · Reputable sourcescience.nasa.gov · The domain "science.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- NASA Science. The History of Hubble · Reputable sourcescience.nasa.gov · The domain "science.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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