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SESK: Stuff Everyone Should Know · sesk.info/astro-hubbles-law

Hubble's law says a galaxy's recession speed is proportional to what?

Its distance from us: v = H x d.

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  1. Its distance from us ✔
  2. Its mass
  3. Its age

Why: In 1931 Hubble and Humason showed that farther galaxies recede faster, in direct proportion. The constant H is the Hubble constant. For galaxies following smooth cosmic expansion, recession speed can estimate distance.

Huh, didn't know that: Georges Lemaitre proposed an expanding universe in 1927, two years before Hubble's first paper on it.

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The universe is expanding. That explains why I can never find my keys.
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References

Answer: Its distance from us. In 1931 Hubble and Humason showed that farther galaxies recede faster, in direct proportion. The constant H is the Hubble constant. For galaxies following smooth cosmic expansion, recession speed can estimate distance.

  1. OpenStax Astronomy 2e, 26.5 The Expanding Universe openstax.org/books/astronomy-2e/pages/26-5-the-expanding-universe
    were able to show that the recession velocities of galaxies are directly proportional to their distances from us
    Checked 2026-10-10.

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