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Expansion of space: Earth - Moon

  1. Aug 18, 2012 #1

    I made a (rough) calculation of how far the Moon would be moving away from Earth according to Hubble's law = 72.6 (km/s)/Mpc


    1 megaParsec = 3.08 × 1022 m ≈ 4 × 1022 m

    Distance between Moon and Earth ≈ 4 x 108 m

    Expansion rate ≈ 7.2 104 (m/s)/4 × 1022 m = 7.2 (m/s)/4 × 1018 m

    Thus expansion for Earth - Moon ≈ 7.2 x 10-10 m/s


    Is this calculation correct? If so, than that it would be about one picometer per second, or is Earth's gravity force simply keeping the Moon in it place?


  2. jcsd
  3. Aug 18, 2012 #2


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    Hubble's law applies to intergalactic distances. It does not apply to stars within our own galaxy, much less distance between planets in a single solar system not distance between a planet and it moons.
  4. Aug 18, 2012 #3
    ok, thanks.

    Than how can we calculate the expansion of the Universe on a local scale, or is there no expansion locally because gravity keeps it all together?
  5. Aug 18, 2012 #4


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    Correct. Gravitational bound objects are essentially immune to the effects of expansion. The force of expansion is overwhelmed by gravity over 'short' distances. By 'short' distances we are talking the level of galactic clusters.
  6. Aug 20, 2012 #5
    FWIW, the Moon *is* receding, but due tidal friction, currently ~ 38 mm / yr.

    IIRC, this is measured by laser ranging using the retro-reflector panels positioned by Apollo astronauts or mounted on Russian rover.
  7. Aug 21, 2012 #6
    Why do I keep hearing that the expansion of space will eventually rip even atoms apart, if it's too weak to affect anything below intergalactic distances?
  8. Aug 21, 2012 #7


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    The so-called "Big Rip" scenario that you describe is hypothetical and not widely believed to be likely. There is no evidence for it, just speculation.

    I SEEM to recall (sorry I can't give a link, so this is hearsay) that it has even been disproven, but that may be a stretch. Call it unlikely.
  9. Aug 21, 2012 #8


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    The influence of the cosmological expansion on local systems
    F. I. Cooperstock, V. Faraoni, D. N. Vollick

    They calculate this kind of thing. The effect is extremely small. The predicted general-relativistic effect on the radius of the earth's orbit since the time of the dinosaurs is calculated to be about as big as the diameter of an atomic nucleus.
  10. Aug 22, 2012 #9
    Here's an interesting blogpost by a theoretical astrophysicist on this topic, and somewhere on it there is also a link to that paper:

  11. Aug 22, 2012 #10
    Thanks, this was the kind of information I was curious about.
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