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Accelerating universe

  1. Jun 30, 2004 #1
    As the universe expands and disperses, there is less of a gravitational well for photons to climb out of before they reach us. Photons from the early universe must climb out of the gravity well of a more densely populated universe than photons emitted from sources today. Therefore early photons would have to appear more redshifted than later photons since they loose more of their energy by climbing out of a deeper universal gravity well, right?

    And the change of the potential that a photon must climb out of would not be linear either? I imagine that there would be less of a change of the potential of the univeral gravity well today than earlier, right? Wouldn't this make it appear as if the emount of redshift is decelerating, or that the universe is accelerating? Thanks. :yuck:
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  3. Jun 30, 2004 #2


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    there is an effect connected with expansion of u
    called integrated SachsWolfe
    which adds energy to photons, as they are cruising along through galaxies and clusters of galaxies on their way to us

    it seems to operate the opposite of what you say. the expansion of the U actually blueshifts the photons slightly

    Mike I have a hard time picturing the "gravity well" you mean because the expansion of the u has no center. Where is the center of the well, the one you are imagining?

    If the density in the early U is uniform and centerless then
    a photon would neither be coasting down in or climbing up out
    he would just be cruising along on a level field
    neither helped nor hindered

    the Sachs Wolfe effect happens where the photons path takes him through some accumulation of mass like a cluster of galaxies (which is in the process of expanding)
    he gains some energy by coasting into the clump
    and then when it is time to continue on out of the clump
    he finds that it has dispersed some
    and the well associated with the clump is not so deep
    so he has picked up a bonus of energy

    it doesnt cost as much to get out
    as he gained by falling in

    in a static situation it costs exactly the same to get out as
    you pick up by coasting in---like a dip in the road---so there is no bonus
    left over.

    the effect of the SW is taken into account when they study the CMB because the CMB photons have had plenty of opportunity to pick up a little bonus here and there

    although it is not a big effect percent-wise

    I havent heard of a gravity well effect like the one you describe (tho it has some resemblance to the SW blueshifting) and think it seems to require that the Universe have a center from which it expands---in order for there to be a center of the well---and therefore it would not happen because the expansion has no center.

    Putting it perhaps somewhat too intuitively and informally, even in a much denser universe, if the density is uniform the photon is not being "pulled" in any direction, so it is not encumbered by gravitational potential with respect to anything and does not have to fight to go anywhere----so no gravitational redshift.
  4. Jun 30, 2004 #3
    OK, let's see. What would happen to photons emitted within the clump if one is emitted while the clump is dense and another while the clump is spars? According to the Sachs Wolfe effect we would see the one emitted while the clump was dense to be more red shifted than the one emitted when the clump was spars. Now consider that we were all members of the same clump at one time and we have all dispersed.

    I don't think it matters whether there is a center or not. The effect would be the same for photons travelling in opposite directions between the same two distant points. By the time the photon reaches its destination (no matter which way it was going), the density of the entire universe has diminished. The gravitational effect of the whole has less of an effect now than at first. We weigh less heavily on space now then at first.

    What you seem to be suggesting is that the gravitational effect of the "whole" cancels itself out always. Do you think that this gravitational cancelation of the whole is still in effect when the universe was just the size of an atom? Or did photons then feel the force of gravity much more forcefully than now? Just as everything else felt more of a gravitational force then than now, so did photons.

    I've not actually done the math; I could be wrong.
  5. Jun 30, 2004 #4


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    I dont think photons felt the force of gravity more back then in a denser U.

    As you know, i dont picture the universe ever being "the size of an atom"
    because it seems consistent with the data to assume that it has always been infinite in spatial extent. maybe there will sometime be some astronomical observation that suggests the u was once the size of an atom. If that ever happens I will be happy to change. but so far nothing implies that, so I prefer not to imagine it that way---the size of an atom, or a pea or whatever.

    but I suppose this does not matter

    whether in a finite peasize or an infinite universe----who cares as long as it is very very dense!

    and if it is very dense then, as long as it is uniform, nobody not even a photon will feel any gravity----because the pull will be equal in all directions

    so nobody has to endure a redshift merely because they happened to be alive back when the world was dense

    the CMB photons, we know what color they were, they came from partially ionized gas at 3000 kelvin. we all know that kind of orange glow!
    and we know that their wavelengths are longer now by a factor of 1100

    and none of that is attributed to a gravitational redshift of having lived when the universe was much denser----a billionfold denser!
    indeed ALL of that is attributed to what had to happen to the light when space stretched out by a factor of 1100.

    if what you say made sense then the fact that matter/ energy was roughly a billion times denser back when those cmb photons were born would show up in some gravitational effect---but it does not show up, not even a sliver.

    so I'm doubtful about your proposed gravitational effect for two reasons----I cant imagine how it would happen (cause its uniform, no well)
    and there doesnt seem to be any evidence of it either

    however enjoyed the question, keep on truckin'!
  6. Jul 1, 2004 #5
    You acknowledge the Sachs Wolfe effect at the local level but deny it at the global level. What gives?
  7. Jul 1, 2004 #6


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    Hi Mike, i sure do acknowledge the SW effect! it is a delightful effect
    and i love it dearly, but it does not exist in a uniform flat potential. It needs a well to operate. So in the "global" situation we were imagining there is no SW effect!
    I have nothing more i can say on the subject, so must take leave on that note. Maybe someone else will take over
  8. Jul 2, 2004 #7
    yes.... and the whole question is whether a denser, smaller universe constitutes a deeper gravitational well then a larger, less dense universe? Spacetime curves under more dense concentrations of mass. And as I understand it, the universe was so dense at the beginning that spacetime curved back in on itself; not even light can escape the universe.

    Maybe someone could describe the math used to calculate the velocity of galaxies from the redshift of its light. Or better yet, mathematically prove why a younger, denser universe is not a deeper gravity well out of which photons must climb to reach us in a less dense universe.

    Thank you.
  9. Jul 3, 2004 #8
    From the link:
    http://www.astronomycafe.net/anthol/expan.html [Broken]

    "It is tempting to refer to cosmological redshifts as Doppler shifts. This choice of interpretation has in the years since Hubble's work led to an unfortunate misunderstanding of big bang cosmology, obscurring one of its most mysterious beauties. As noted with a hint of frustration by cosmologists such as Steven Weinberg and Jaylant Narlikar and John Wheeler, "The frequency of light is also affected by the gravitational field of the universe, and it is neither useful nor strictly correct to interpret the frequency shifts of light...in terms of the special relativistic Doppler effect."."

    My question is how much does the gravitational contribution to the redshift affect the existence of "dark energy" that is supposed to accelerate the expansion of the universe? The more the universe expands, the more gravitational potential energy increases at each point so that each photon is redshifted to conserve energy. If expansion itself causes redshift, then the true rate of expansion is less than if we consider the redshift as due to the Dopler effect alone. Is the gravitational contribution to the redshift enough to offset an acceleration and erase the need for dark energy?
    Last edited by a moderator: May 1, 2017
  10. Jul 5, 2004 #9
    If the redshift due to the universal gravity well has more of an effect in the far past then in the near past, then the true velocities of the far past are less than in the near past. For then you'd have to subtract more from the velocities in the far past. So even if the universal gravity effect has not been taken into account, if they were, then there would be more of an acceleration than otherwise expected, right?
  11. Jul 5, 2004 #10
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