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Is the Universe expanding?

  1. Nov 23, 2007 #1
    I know it's been proven beyond all doubt that the universe is expanding but one thing is bugging me. Why isn't the earth (and the other planets for that matter) moving away from the sun due to this expansion? Maybe this is a daft question but I can't think of an answer. Can somebody please explain?
  2. jcsd
  3. Nov 23, 2007 #2
    They don't because that's what General Relativity says (more or less). :smile:

    OK, that was a joke. Seriously, it has been discovered not so many years ago
    that the Astronomical Unit (AU) is increasing, so at least the Earth's distance from
    the Sun is increasing secularly at this epoch. See

    G.A. Krasinsky and V.A. Brumberg, Celestial Mechanics & Dynamical Astronomy 90, 287 (2004).

    It must be stressed that the result reported here (an increase of the AU of about 15 m per century)
    is indirect, inasmuch as this increase is inferred from an analysis of radiometric measurements of distances between the Earth and the major planets. So while it seems that the reported increase
    of the AU is far too small to have anything to do with the Hubble expansion, it is possible that this
    discrepancy can be blamed on incorrect theory-dependent assumptions made when analyzing the data.
  4. Nov 23, 2007 #3

    Chris Hillman

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    Science Advisor

    What expands?

    It's a good question, and somebody has. See [thread=153958]this PF thread[/thread] and references therein.

    The keyword is "pennies on an expanding balloon", but at a higher level of approximation, clearly we should model gravitationally bound solar systems and galaxies in a more sophisticated way than as rigid pennies. For advanced students, some relevant arxiv eprints are
    http://www.arxiv.org/abs/gr-qc/0102103, http://www.arxiv.org/abs/astro-ph/0609271, http://www.arxiv.org/abs/astro-ph/0605611, and (often cited!) http://www.arxiv.org/abs/astro-ph/0310808
  5. Nov 24, 2007 #4
    Thanks for pointing me in the direction of some answers. Very interesting stuff.
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