Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What makes Dark Energy cause Acceleration of the Expanding Universe?

  1. Nov 15, 2011 #1
    According to Misconceptions about the Big Bang in Scientific American by Charles H. Lineweaver and Tamara M. Davis, space-time is expanding and carrying galaxies with it, rather than galaxies being moved ever faster and further into space-time. See:


    A gravity field, such as produced by the mass our sun, warps space-time. However, I have not heard of energy warping space-time, even though E=M(C**2).

    Why is dark energy assumed to be accelerating space-time expansion, rather than some unknown force (for grins call it ytivarg) that accelerates space-time.
  2. jcsd
  3. Nov 16, 2011 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I'm a Ytivarg skeptic edearl. And even more of a "dark energy" skeptic. What has been observed corresponds most simply to a constant λ (lambda, which occurs naturally in the Einstein law of geometric gravity) being a certain intrinsic curvature.

    We can say approximately what this curvature is. There is nothing mysterious about it.
    But you can always multipy a curvature by garbage and put the wrong units on it and dress it up as an energy density. Then it gets mysterious. A lot of "dark energy" hype goes along with that. All the astronomical observations tell you is that lambda is a very small constant intrinsic curvature in spacetime geometry.

    Check this out: http://arxiv.org/abs/1002.3966 . Click on "pdf" and you get the full text

    This article gives the value of λ on page 5

    λ ∼ 10−35 s−2

    I worked it out for myself using the cosmological parameters in Ned Wright's calculator, which are fairly standard, and I recall it came to around

    λ = 1.16 x 10−35 s−2

    You can think of the unit of curvature as reciprocal area, or reciprocal length squared. But setting c = 1 as is commonly done in relativity puts time and length on the same footing, so the unit of curvature is also reciprocal time squared.
    Hence the "per second per second"
    Last edited: Nov 16, 2011
  4. Nov 16, 2011 #3
    marcus, the acceleration of a mass moving through space-time increases its energy. We use the gravitational constant in equations, and observations of gravity allow us to calculate that constant.

    Your calculation of λ is a valid effort, but I would like to know more about why that constant is needed. We know only a little about gravity, but we know nothing about dark energy, ytivarg or whatever it is that λ represents.

    I do not know if expanding space-time implies an increase of energy or not; conservation of mass-energy seems to indicate the net increase of energy due to expanding space-time is zero. Apparently, gravity warps space-time with a net change in mass-energy of zero; thus, dark energy as an explanation for the accelerating expansion of the universe seems misleading, because it implies that space-time absorbs energy to accelerate inflation.

    I fear that I may have inadvertently proposed a personal hypothesis, since I am not a highly trained physicist. Therefore, I prefer to ask questions rather than make statements. On the other hand, I suppose I must expose my ignorance to enable someone to correct my misconceptions and teach me new things. (the moderators must be very perceptive to be fair on this forum).
  5. Nov 16, 2011 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I would say don't worry! I'm not a moderator of course and it's just my opinion but I think you raise quite legitimate questions (given where you are coming from). Obviously ytivarG is just a bit of fun.
    Nice name for it actually. :biggrin:

    One point is that in the largescale pattern of expansion of distances nobody gets anywhere so it is not like ordinary motion. Nobody approaches a destination. Everybody thinks they are sitting still, and just getting farther from everywhere.

    The 1915 Einstein equation is both our law of geometry (how geometry evolves and interacts with matter) and our law of gravity. We have no right to expect that distances won't change or that geometry will always be flat the way Euclid told us. The 1915 equation keeps passing all the tests that have been devised, with exquisite accuracy. So what can we do?

    Certain things that were true in a fixed foursquare Euclid-style geometry turn out to be only very nearly true. Because nature's real geometry is not perfectly flat fixed and non-expanding---only very nearly so.
    Newton's laws of motion and the intuition that goes with them turn out to be useful in our local neighborhood---even within our own group of galaxies---where flat geometry is an excellent approximation, but become less applicable at wider scale.

    So you need to be somewhat cautious about applying a Newtonian intuition to this larger world beyond our local group of a dozen or so galaxies. It will be NEARLY right a lot of the time but there will be slight discrepancies.

    So at very large scale global energy is not welldefined and a good old law like conservation of energy can't be applied, at least without some technical caveats. There are FAQ entries about this kind of thing, I believe. And you ask "if a distant galaxy is receding at an increasing rate, where is it getting the required influx of kinetic energy?" But the galaxy is not going anywhere. The distance to it is simply increasing. Recession is not like ordinary motion. Distances can increase at rates exceeding c. And indeed the distances to most of the galaxies we can see with a telescope like the Hubble ARE increasing at rates >c. I don't know any simple way to associate a kinetic energy with that.

    The nice thing is that these effects are very slight percentagewise. The current rate of expansion of distance is only about 1/140 percent per million years. That is tiny! In a million years the distance between two stationary observers only increases by 1/140 of one percent. The acceleration rate is even more negligible, percentagewise. I forget what it is except that it comes out to be a ridiculously small number. These things only matter when you look at the world at very very large scale.

    So we can safely continue to rely on our good old Newtonian laws for most practical purposes, conservation of energy included!
    Check out the FAQ, they may do a better job with this.

    the really interesting question you raise is about why the 1915 equation naturally has a second constant lambda in it.

    That article by Bianchi and Rovelli has a discussion of this. It turns out that Einstein already in 1916 noticed that a lambda term naturally belonged in the equation because it was allowed by the ruling symmetry of the theory which he called "general covariance". At that point he had not conceived of a need for the lambda term, so he only mentioned it in a footnote.

    Later there was all that blunder-fuss about it. Misusing the lambda term in a blind attempt to force the model to be static. Who says geniuses are always wise? But that was LATER. The first thing he does is point out that the lambda term naturally appears in the theory because it is allowed by symmetry. This is pretty esoteric but I'll venture a comment or two.

    Symmetry is a sweet and subtle feature of modern theory. In the 20th century they discovered that you can derive physics equations from symmetries. It is viewed as the cool way to do it. In the Beginning was the Symmetry, that the system or the socalled "Lagrangian" describing it is supposed to satisfy and you include every term that you can without spoiling the symmetry.

    I think Tom Stoer had a very down-to-earth description of why the lambda term naturally appears in the Einstein equation. It could have been in the "Why all these prejudices against a constant" thread. You might have a look at that. Tom, or someone, remarked that it was analogous to what you do in First Year Calculus when you learn to do integrals. The answer is not right unless you introduce a constant of integration. Because, in the most general form, it belongs.

    Sorry if this is too esoteric but I can't do better at the moment. Given how A.E. conceived the theory, in its most general and elegant form it must contain TWO constants of nature, namely Newton G and this other Einstein one called λ. Rather than merely having G, like the earlier 1687 law of gravity.

    And the amusing bit is that the effect of this constant (just an A.E. footnote in 1916) was only noticed in 1998 by people comparing distances and redshifts of supernovae. Nature keeps reminding us that we are, after all, just a type of monkey. :biggrin:
    We don't know everything yet.

    In Loop gravity the lambda arises from the use of a new kind of symmetry called a "quantum group". Loop is an attempt to make the classic 1915-1916 theory of geometry/gravity emerge from some deeper description. Which will allow geometry to be uncertain, as all good quantum creatures are. But the Loop description, from which the classic one arises, is as yet just a guess (one of several that the monkeys are working on.)

    We don't know the fundamental descriptors (the deeper "degrees of freedom") that describe geometry in conjunction with matter. When we have a better idea we may see how both Newton G and λ emerge from a deeper symmetry.

    In any case I think "dark energy" is just a myth or fairy tale. It may turn out to have been quite misleading to speak of lambda as an energy. So far all we know is that it is simply a constant, like Newton G, which occurs in the contemporary law of gravity.

    If you want the Bianchi Rovelli paper (which is quite readable) just google "constant prejudices".
    Last edited: Nov 16, 2011
  6. Nov 16, 2011 #5


    User Avatar

    this is one of the few times which I agree with marcus. since alpha(FSC) also changes at different distances and in QG G also suppose to run so there is no fundamental reason why it should stay constant at large distances. I also have my own reason but awaits a future proof.
  7. Nov 16, 2011 #6
    I thought that dark energy was called dark because there's no known force or whatever to associate it with.

    As usual, clearly expressed and informative (for me anyway) posts by marcus.

    By way of conjecture, is it even possible that, assuming our universe is expanding in a preexisting medium, the observed/inferred expansion acceleration might have to do with topological anomalies in the preexisting medium? Of course, this isn't an empirical question (or assumption). But is it a possible way to make sense of observations, and inferences from those observations?
  8. Nov 16, 2011 #7


    User Avatar
    Gold Member

    Could be, but my own recollection is of reading that it's called dark energy as a carry-over from dark matter, which was called dark because we can't see it but the name came to mean somewhat "mysterious" or "unexplained", and THAT is the meaning of the "dark" in dark matter, not anything about energy/force.

    Uh ... preexisting medium? That's pure speculation on your part, yes?
  9. Nov 16, 2011 #8
    Yeah. :smile: Let's not dwell on that. I'm just back from getting temporarily banned for my political opinions. Anyway, thanks for the feedback.
  10. Nov 18, 2011 #9
    Thanks for your reply.

    Ytivarg doesn't work quite right, as I understand it. Space-time can inflate faster than c, but gravity waves are limited by c. Thus, as I understand space-time could not inflate faster than c if ([itex]gravity^{-1}[/itex] = ytivarg) were the driving force.
  11. Nov 19, 2011 #10


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    But why is a "driving force" needed? Space is not a material that is being moved.

    Geometry changes according to an equation (the more general 1915 Einstein eqn or the simpler Friedmann eqn derived from it a few years later).

    Widespread expansion of distances is one of the natural solutions and tends to continue once started. Gravity can gradually slow it down but you don't need some ytivarg to keep it going.

    Distances can certainly expand faster than c without a driving force!

    Most of the galaxies we can see, the distances to them are expanding >c, and this is no big deal. Just a consequence of them being far enough away that small percentage expansion that currently prevails (1/140 of a percent per million years) amounts to >c.

    Somewhere between two and three times the speed of light is fairly common. Distances just increase. Not a big deal. No exotic "force" needed for that.

    I do like the name ytivarg :biggrin: we should find a good use for it.
  12. Nov 19, 2011 #11
    I do believe that is the name given to the force that holds open a wormhole, yes? ;)
  13. Nov 19, 2011 #12
    TY :biggrin:

    Either is possible, AFAIK, but I feel compelled to ask why is inflation accelerating--that seems to need a force.
  14. Nov 19, 2011 #13
    There is not such concept of force that can do that in GR.
  15. Nov 20, 2011 #14
    Which explains why dark energy. It is an enigma that is neither energy nor force. Maybe it is a random occurrence with no reason--something that cannot be known.
  16. Nov 20, 2011 #15


    User Avatar

    marcus, would it be more correct if you stated... "Everybody thinks they are sitting still, and everywhere is just getting farther"... or, "expanding"... with respect to everywhere?

    To me, "getting farther from"... seems to imply motion.

    Too picky?...

    Last edited: Nov 20, 2011
  17. Nov 20, 2011 #16


    User Avatar
    Science Advisor

    Let me respond from the view that general relativity (GR) is a spacetime theory, i.e., it provides a 4D spacetime metric (g) consistent with a particular stress-energy tensor (SET, which, of course, cannot be constructed without g itself, so the "consistency" is a "self-consistency" here). In this view, you find a particular SET and g which are self consistent per Einstein's equations and you have a solution. Now, you can in most cases foliate the spacetime manifold of a solution into spatial hypersfcs and tell a time-evolved story about the different configurations of the 3D entities on those spatial hypersfcs. In that context you can ask questions involving "forces," e.g., what makes galaxy A move as it does wrt galaxy B? But, keep in mind that the solution is a 4D manifold built via self consistency between g and SET, so the ability to tell a reasonable dynamical story is not a requirement of the solution. If you can't find a "good dynamical story" for a particular solution, as may end up being the case with Lambda, then you just don't get to tell a dynamical story.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook