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

Why would an infinite universe tend to contract?

  1. Apr 24, 2009 #1
    This is a rather long post, but I hope it will get interesting enough along the way to make you more and more interested to keep reading. Otherwise, never mind ;-)

    I have a seemingly very simple question: why should an infinite universe tend to contract in the absence of a cosmological constant or dark energy? I've read some answers in various places, but they haven't really convinced me so far and I'd like to find out where my own reasoning is going off track (or, more unlikely, I might be right and hundreds of well known scientists might be wrong). I personally don't see any reason for a contraction at all.

    In "A Brief History of Time", Stephen Hawking says that at one point astronomers thought an infinite, homogenously filled universe would not contract because matter has no central point to fall to. He then explains that this is "one of the pitfalls that you can encounter in talking about infinity", and that you should start out with a finite universe (which always contracts) and then add more mass uniformly around the initial part. Since a hollow sphere does not contribute to the field of gravity inside of it, the net effect of the added matter is zero. Since you can keep doing this up to infinity, the infinite universe should contract as well.

    However (with all due respect, and once again, I may be completely wrong), I think he fell into a different pitfall of infinities: integrating a function that is not absolutely convergent over an infinite domain by choosing a convenient sequence of sets (concentric spheres) to approach infinity, while a different sequence may yield a different result.

    If someone were to ask you what is the integral from -infinity to infinity of f(x) = x, the answer would be, at best, nuanced. If you consider this to be the limit of a sequence of integrals from -r ro r with r approaching infinity, the answer is clearly zero. However, if you consider it to be the integral from -r to r+1, the anser is suddenly infinity. You can even choose sequences of integration intervals, for example [-r, r+1/r], to get any value you like. This is an integral that is not absolutely convergent, and therefore has no definite value unless you agree on a specific method of integration for some particular purpose.

    The same kind of flaw is present in Hawking's argument: he keeps adding mass uniformly in hollow spheres around the initial bunch of points, so that it all cancels itself out. But why should you? If you use non-spherical ovoids instead of spheres, you'll find different speeds of contraction in different directions, with less contraction in the direction of the longest axis of the ovoids. The universe might even expand in a particular direction if you use a seqence of coaxial cones with both the top and the base moving away as the cones grow bigger. Yet, these, too, will eventually include any part of the universe so they should be perfectly valid for integration to infinity if the result is to be well-defined.

    I admit these approaches are all much less likely than Hawking's result, because of their asymmetry, but they do show that integration to infinity by using a specific sequence of sets is not guaranteed to give a correct result. Anyway, there's a more important flaw:

    Why should you consider a sequence of sets of spheres around a single point to calculate the difference in gravitational acceleration between two different points? Since gravity travels at the speed of light, a more correct approach would be to compare the gravity acting on a point from a sequence of spheres around that point, with the gravity acting on a second point from a sequence of spheres around that other point. Clearly, then, the net effect should be zero. The last sphere to be considered is the one with a radius equal to the age of the universe times the speed of light. Even if you are wondering about the possibility of an infinitely old universe, I could argue that the result should remain valid since it is valid for any universe with a finite age, using the same kind of argument Hawking used for extrapolating from finite to infinite universes.

    The only reason that finite universes contract is that, as my spheres of integration get bigger (centered around each point respectively), one of them will run into the edge of the universe on one side. The lack of gravity from that far side will completely explain the gravitational "attraction" between the two points. It has nothing to do with local attraction, but rather with the asymmetrical distribution of matter at a distance. In an infinite universe, no such asymmetry exists.

    Since my argument might still not be totally convincing, I will now give a counterexample that (I think) disproves the soundness of extrapolation from finite to infinite universes. Consider this simple infinite "toy" universe, which does not resemble ours, but which I will only use to demonstrate the logical flaw in the argumentation.

    Imagine a cartesian, Newtonian, non-relativistic universe called R^3. It goes on up to infinity in all directions, and you can consider any point to be its center. However, unlike our universe, it does have absolute speeds and accelerations. This pretty much corresponds to the Newtonian view of the universe. Now imagine that, for a convenient choice of coordinates, this toy universe has a stationary object of mass 1 on every integer lattice point.

    Will this universe contract? In that case, you should be able to show at least one object that will start moving in some direction. But in what direction should it accelerate? There can be no preference for any particular direction since you can consider any point to be the center of the universe. Therefore, the only result can be that this toy universe remains static.

    I can even do one better: remove any single object from the universe, and it will start expanding!

    Of course I know that our universe does not resemble my toy universe (for one thing, the real universe does not have absolute speeds or even accelerations), but it only serves as a counterexample to make the point that "finite universes contract" does not necessarily imply "infinite universes contract". In the case of my toy universe, finite universes do contract (even with the same speed for any spherical universe) while the infinite version clearly does not. Yet, you might apply Hawking's argument to my toy universe and conclude that it must contract, which shows the logical unsoundness of the extrapolation from finite to infinite universes.

    I know the real universe has a lot of properties that are different from my toy universe, and a few of them would invalidate the objections against contraction that I used in my example, but I can't see any reason why this should MAKE the real universe contract. Removing objections against contraction is not enough to show a cause for contraction.

    Just to be clear, I'm not saying the universe without dark energy should definitely not contract, I'm just saying that extrapolation from finite universes to the infinite universe cannot, in itself, be used as an argument for contraction. There might be a different reason, maybe I just haven't heard of it yet.


    Anyway, since our universe is relativistic and not Newtonian. let's look at this from a relativistic point of view. My knowledge on the subject is a bit limited (that's an understatement), but I'll give it a shot anyway:

    If I understood correctly, gravity is really the curvature of space-time, which is caused by the presence of mass and can be calculated using differences in potential energy. If the gravitational potential decreases (curvature increases) from one point to another, you will be accelerated towards the latter (as measured by an observer at a distance in his frame of reference).

    So let's look at the change in gravitational potential when we move from one place in the universe to another. As we start moving, we'll distance ourselves from certain objects while getting closer to others. On average, as long as we're not close to any particular object, the net change in gravitational potential seems to be zero since there's always roughly the same amount of matter in any direction from any vantage point. So why should there be any gravtiational pull between two points? I really don't see a cause for contraction, but I may be missing someting.

    I could even go one step further (but now I'm really going out on a limb):

    Distant objects are moving away from us at very high, "relativistic" speeds, and therefore have a higher mass. This could imply an increasing curvature of the universe at further distances, in our frame of reference. That in turn might explain the accelerating expansion of the universe! Of course this would be a subjective point of view, the aliens in that faraway galaxy would say that we are the ones being pulled away from them because of the higher curvature on our far side, but this would be one of those typical relativistic paradoxes in which two seemingly different perceptions of reality result in the non-contradictory result that we are simply being pulled apart. We'd disagree on a lot more things, like whose clock is faster, who's taller, etc... but it all magically works out just fine anyway.

    So, did I just solve the mystery of the accelerating expansion of the universe? Or am I just a rambling, ignorant newbie? I assume the latter, but this is the best way to learn ;-)

    Thanks,

    Michel Colman
     
  2. jcsd
  3. Apr 25, 2009 #2

    Mentz114

    User Avatar
    Gold Member

    It's a long time since I read the book in question, but I agree with you, in re the argument as you quote it. But S. Hawking is smarter^n than me, so maybe he has a better argument somewhere else.

    The Newtonian toy model you quote is fraught with contradictions. You must have a space-time described by a metric in order to define expansion. What you've got is a lot of matter moving in a fixed space. The infinite lattice is unstable and collapses if any point is perturbed even by an infinitesmal amount. It does not expand if a mass is removed. This conclusion is based on assuming incorrectly that some divergent infinite series is convergent.

    In order to get an infinite universe that is contracting in GR, you only need to set up a metric so that dR/dt is negative. Whether or not this has physical meaning is not obvious.
     
  4. Apr 25, 2009 #3
    I did assume that, of course :smile:
    I don't understand. If you assume the initial lattice is statically stable, and you remove one mass, all other masses will have less attraction from the direction of that point. So they will move away from it. The lattice may be unstable if gravity moves infinitely fast, but not anymore if it travels at a finite speed.

    Anyway, my toy universe only served the purpose of showing that, just because finite universes contract, infinite universes don't necessarily. But I guess you agree with me there, so we can forget about the toy universe and talk about the real one.
    But what makes everyone think (in fact, Hawking used the word "know") that our infinite universe should contract and therefore needs dark energy or some other strange effect to keep it from doing so? Why does dR/dt have to be negative?

    If I understood correctly (I may be completely wrong), people are using the Schwarzschild solution (for a finite distribution of mass) on the infinite universe. Why?
     
  5. Apr 26, 2009 #4

    Mentz114

    User Avatar
    Gold Member

    IN GR cosmological models ( FLRW) the expansion/contraction can be linked to the energy density. Observations suggest the universe is expanding at an accelerating rate and that, I believe, is the current thinking in cosmology. One way to model this in GR is to have an energy field that forces the expansion. Look up the "Friedmann equations" on the link below.

    It's not used a cosmological model, since it describes the field of a single spherically symmetric source. The simpler cosmological models assume that the universe is filled with non-interacting 'dust'. It is possible to embed a Schwarzschild field into this to make a 'vacuole', or to include both fields in a single ST (an example of this is the Kottler spacetime , extended by the Meissner ST).

    Link : http://en.wikipedia.org/wiki/Friedmann_equations
     
  6. Apr 26, 2009 #5
    I know the universe is expanding at an accelerating rate, I was just wondering why they found a contracting universe theoretically (or at least one slowing down its contraction) and then had to go hunting for some reason why it's accelerating its expansion after all.
    OK, now it's starting to make sense. If the universe is considered to be homogenous and isotropic, its metric must be reasonably simple, which leaves only a few parameters to be adjusted. Then, by applying Einstein's field equations, the conclusion is that the universe should slow down its contraction, except if a cosmological constant is added or dark energy is invented.

    Did I understand everything correctly now?

    I'm going to read up on those field equations, I think it's time I finally figure out how they work.

    Thanks,

    Michel
     
  7. Apr 26, 2009 #6

    Mentz114

    User Avatar
    Gold Member

    Yes, that's correct. The Einstein universe is closed and in order to get a time independent solution he invented the field to adjust it. Then it was removed, but after the FLRW, it's being used again.
     
  8. Apr 27, 2009 #7
    I don't see why a homogenous universe can't collapse. Using Newtonian gravity there would be no net force in any direction to cause a collapse. The only thing that could cause a collapse is an inhomogenity(?) which makes it not homogenous...(absent of all other forces)

    Sorry if you went over that but I just skimmed the last 2 or 3 posts.
     
  9. Apr 27, 2009 #8

    Mentz114

    User Avatar
    Gold Member

    Hi jefswat,
    the kind of spatial expansion or contraction described in GR has no analogue in Newtonian cosmology. If one of the cited Newtonian universes collapses, it's only the matter which collapses, leaving behind empty space. In GR, the volume of space can change.

    The case of the Newtonian infinite lattice has problems. If a mass element is removed, the force acting on every mass becomes infinite ( try summing the force ). This is unphysical.
     
  10. Apr 27, 2009 #9
    Not if there's an infinite amount of matter. In that case, there will never be empty space.
    Would you mind elaborating on that? As far as I can see, the amount of force on any point would be finite, equal and opposite to the attraction lost from the removed mass element.
     
  11. Apr 27, 2009 #10
    That's what I'm trying to say about Newtonian gravity and what Hawkings said. If the universal were to be fully homogeneous where would a net force come from?
     
  12. Apr 27, 2009 #11

    Mentz114

    User Avatar
    Gold Member

    A universe with no space ? I don't want to talk about that. It's daft !

    OK, I've amended my statement. The potential is an infinite series of 1/Rn which does not converge, and subtracting one series like this from another is not a valid thing to do. So at best you could say that the potential at any point is undefined. If you remove an element the potentials are still infinite.

    There's nothing to be gained from discussing Newtonian cosmology and since you are ignoring my twice stated assertion that you cannot have expanding space in Newtonian cosmology I don't see what more I can say.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook