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Expanding universe

  1. Dec 14, 2007 #1
    Empty space contains a surprising amount of matter in the form of gas. I couldn't tell you the number because I can't find it online but it is there. Has anyone thought that the universe might be expanding because this gas in empty space far outnumbers the mass of galaxies and that the gas is far out in the universe past the galaxies. It could account for the fact that galaxies are expanding and simplify everything involved. thanks
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  3. Dec 15, 2007 #2


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    I don't really understand what you are trying to say. So I'm willing to take your word for it there is a lot of gas between galaxies. How does this explain the universe expanding? And why would the gas be farther out than the galaxies? And farther out with respect to what (e.g., how do you measure this without unjustly giving a special meaning to our point of view or defining a "center" of the universe)
  4. Dec 15, 2007 #3


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    It seems there is a fundamental misconception here.

    If there was a intergalactic gas of high pressure then counter-intuitively that pressure would cause the universe to slow down in its expansion and tend to contract. This is because pressure acts like energy in GR and is a source of gravitation, which in the cosmological case causes space to decelerate in its expansion.

    Interestingly it is now thought the universe is accelerating in its expansion and this is attributed to Dark Energy, which has the characteristic of negative pressure that has the reverse effect of ordinary gas pressure.

    The intergalactic gas is of very low density and that extra density could have a small effect on the cosmological expansion rate but its pressure is negligible.

    Last edited: Dec 15, 2007
  5. Dec 15, 2007 #4


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    The density of hydrogen in intergalactic space is somewhere on the order of one molecule per cubic meter. I don't know if that is surprisingly high, but it is a more perfect vacuum than we can create on earth.
  6. Dec 15, 2007 #5
    I was more trying to say that the gas was out there in front of the galaxies pulling them towards it. I'm not really saying it's a high pressure either. If you ask me a hydrogen molecule every cubic meter in the whole of space would add up quick. Since I'm probably wrong, do they count all these molecules when they take a approximation to the mass of the universe?
  7. Dec 15, 2007 #6


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    If I follow what you are trying say, you are suggesting that there is a "outer shell" of gas surrounding an "inner core" of galaxies, and that the gravity of this outer shell pulls outward on the galaxies. Is this right?

    If so, there is a fatal flaw in this scenerio: There would be no net outward gravitational pull on any part of the inner core by the outer shell. Newton showed that a hollow shell has zero net gravitational effect on the area inside of the shell.
  8. Dec 15, 2007 #7
    would this supposed gas eventually be pulled in to the galaxies by gravity
  9. Dec 15, 2007 #8
    is the expansion due to a repulsive effect in gravity
  10. Dec 15, 2007 #9


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    The effect of gravity, attractive or repulsive, is either a deceleration or an acceleration of the expansion rate.

    The initial cause of the expansion itself is quite mysterious and bound up with the initial conditions of the BB.

    All we know is that the cosmological solution to Einstein's Field Equation will admit only expanding or contracting solutions, the Hubble red shift of distant galaxies shows that we happen to live in an expanding one.

    The only static solution, Einstein's initial suggestion, which used a cosmological constant to hold cosmological gravitation at bay, proved to be unstable.

    Last edited: Dec 15, 2007
  11. Dec 31, 2007 #10
    if we go by your predictions we should get the emission or absorbtion spectra for each and every star due to gas which we are not getting. this shows that gas exixts but not in huge quantity.there r no reasons as to why universe is expanding but the graphs of big bang and big crunch can tell us about this.instead of getting an absorbtion spectra we r getting highly redshifted spectrums which say that sources r moving far from us and so r we. according to me the space between galaxies is increasing or the hubbles laws can be true.
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