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Can Gravity Really Slow Down the Expansion of the Universe?

  1. Jun 30, 2009 #1
    Okay, for all I know space itself is expanding, carrying the matter along with it (I think). And gravity applies to matter (and energy), so even if we (meaning the matter) stop expanding because of gravity space can keeping expanding because gravity doesn't slow it down (maybe warps it, but doesn't slow it down). Am I right?

    Please really think of what I mean before you answer.
    Thanks in Advance!
  2. jcsd
  3. Jun 30, 2009 #2


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    Until about ten years ago it was generally thought that gravity would slow down the expansion. However at that time two independent observation studies showed that the expansion was speeding up. To explain it, physicists have come up with the idea of dark energy. What it all means is very much an open question.
  4. Jun 30, 2009 #3
    Well can the space expanding be the force driving matter apart from eachother and it just happens to be stronger than the force of gravity.
  5. Jun 30, 2009 #4


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    The fact that galaxies are moving away from each other is no mystery; they're moving away because gravity either cannot, or has not yet, halted the universe's expansion. The mystery is that this expansion is accelerating.
  6. Jun 30, 2009 #5
    I have a question about the halting of the universe because of gravity. Has anyone actually calculated that gravity should or shouldn't halt the expansion?
  7. Jul 1, 2009 #6


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    There's a parameter called [itex]\Omega[/itex] that characterizes that: gravity will eventually halt the expansion of the universe if and only if [itex]\Omega > 1[/itex]. Basically, [itex]\Omega[/itex] represents the density of the universe. The thing is, nobody's quite sure exactly what the value of [itex]\Omega[/itex] is - all we know is that it's very close to 1. It's based on astrophysical measurements, and right now those measurements can't be made precisely enough to tell whether [itex]\Omega[/itex] is actually greater or less than 1.
  8. Jul 1, 2009 #7
    Just to clarify one more item [tex]\Omega[/tex] actually consists of a bunch of varieties including but not limited to: [tex]\Omega_M[/tex] for ordinary matter and including Dark Matter, [tex]\Omega_R[/tex] for radiation density, and [tex]\Omega_\Lambda[/tex] for the Cosmological constant and Dark Energy. The total of all these densities comes out to the final value of [tex]\Omega[/tex] which if as diazona said is either > 1, or = 1, or < 1, will determine the fate of the universe (essentially).

    As such more than a decade back we had no idea about dark energy and probably hadn't even thought of the term to coin it, so the values for these densities are changing and variants of the densities will surely pop up in the future as we determine more and more about the ingredients of the universe. This in itself is the basis of cosmology.

    As for gravity mixing into this recipe, we are not sure about its overall or future effect on the expansion of the universe. There are a few instruments I believe that will be going up (or are already up) and measuring gravity waves and searching for gravitons and other elusive physical particles. Hell we barely even know how gravity works here on Earth or other bodies. Wtf exactly is causing us to be pulled backwards towards the ground whenever we jump or shoot missiles at other countries?

    For now we just play the waiting game
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