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

Accelerating universe

  1. Mar 14, 2009 #1
    is the rate of expansion of the universe constant? or is accelerating or decelerating?
    can anything such as light ever catch the limits of the universe?
    and is it the space and the matter of the universe thats expanding or is it just the matter thats separating farther and farther apart into an infinite nothing?

    sorry for the multiple questions but they all relate so I thought it might be appropriate, thanks in advance for any answers.
  2. jcsd
  3. Mar 14, 2009 #2
    Based on my limited knowledge...

    It's is thought that since the Big Bang, the expansion of the universe initially decelerated, but have recently (few billion years ago) accelerated for some unknown reason. Hence the theory of Dark Energy.

    No, since the inflation during the early times of the Big Bang is thought to have been actually faster than the speed of light. This seems paradoxical, but apparently, inflation (much like the frame dragging near black holes) can be faster than light.

    It's not just the matter. It's the space-time itself.
  4. Mar 14, 2009 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The only way I can answer is by reference to the standard cosmic model that almost all astronomers use. I don't have my own private model.

    As normally modeled the uni has no limits. Space has no boundary.
    (The chunk of it that we have so far seen is bounded but that's a different issue. Stuff that's currently more than 46 billion LY away, it's light hasn't reached us yet. There is no physical limit or boundary.)

    Have a look at the balloon model sticky thread. It may help.

    The model does not have an "infinite nothing" surrounding occupied space. Space is approximately uniformly filled with matter. Just a bit clumpy and wispy but on average uniform.
    Distances increase. That's it.
    Our theory of gravity is a theory of dynamic (changing) geometry. Especially at large scale (beyond galaxy scale) distances change. As long as we can't find a better theory of the dynamics of gravity/geometry, we have no right to expect that distances won't change. So it's not so surprising to find that they do.
    However there is little or no expansion in smallscale local distances, like the diameter of our galaxy or the distance to the nearest others.

    You need to be definite about what you mean by "rate of expansion."
    Do you mean the hubble rate H(t) or do you mean the time-derivative of the scalefactor a(t)?
    The hubble rate is decreasing and will continue to decrease. The current hubble rate says that largescale distances (like between widely separated galaxies) increase 1/140 of a percent every million years. This percentage is expected to decline towards around 1/160 of a percent.

    However there is this number called the scale factor, denoted a(t). Intuitively it gives us a handle on the average distance between galaxies, but the mathematical definition of it is technical. If you know calculus, the time derivative of a(t) can be written a'(t) and is the slope of the a(t) curve.

    That slope is positive, in other words a'(t) > 0, and that means the scalefactor is increasing. And moreover a'(t) is itself increasing. That's actually what people mean when they say "expansion is accelerating".
  5. Mar 14, 2009 #4
    well I take my first calculus course next year so I didnt really understand the ending bit.
    So is there any theory that tries to state that the universe is infinite and always has been with sections of matter moving outwards on different angles showing that over time the distances between these sections will be greater. Everytime I think of this from what I have read I picture the balloon skin being the outer limit of the universe and the space inside the baloon being the fabric of spacetime in which mass and energy exist. Is this wrong? and if the universe is expanding than it has a finite value for things like diameter right?
  6. Mar 14, 2009 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Yes, that is wrong. The balloon surface is a 2D metaphor for the real 3D space.
    If you picture the universe with the balloon analogy then you have to concentrate and make believe that all existence is on that surface. There is no "inside" the balloon, and no "outside".

    All existence is in what you call the skin.

    There are points painted on the balloon surface (galaxies) and they are getting farther apart.

    Analogous to how galaxies in our 3D space are getting farther apart.

    Any beings in that kind of universe would have to be flattish 2D creatures.
    There world would have a circumference---the longest distance you could in a given direction before getting back to where you started.
    They would have no concept of the center of their universe since no point in it is the center.

    One of those beings could not point his finger in the direction of inside or outside or towards the center, because those directions do not exist in his 2D world.

    Just like in our 3D world you cannot point your finger towards the center, towards the "outside". Any direction you point is at some point in 3D space.

    You ask what about if space is infinite volume? Sure. It could be. The standard model has several versions, a finite volume and an infinite volume.

    If it is infinite volume, then there is an infinite amount of matter distributed more or less evenly throughout.

    Maybe you should have a look at the princeton.edu link in my signature. It is a SciAm article, very easy reading, intended to help get people out of their misconceptions. One of the most common misconceptions is thinking of the big bang as an explosion, with stuff flying out in all directions into empty space :biggrin:. Check it out. It is a good readable introduction to standard cosmology. You may find several things that surprise you.
    Last edited: Mar 14, 2009
  7. Mar 14, 2009 #6


    User Avatar
    Science Advisor

    I know this has been claimed, even by some cosmologists, but it really is an inaccurate use of language.

    It makes no sense whatsoever to call any expansion faster than the speed of light. An expansion rate is given in inverse time (sometimes quoted in speed per distance, which if you cancel out distance, makes for inverse time). You can't state that a number that is given in inverse time is greater or less than any speed. It makes no sense. You might as well try to say that 70mph is faster than 3000rpm.

    To get a speed from the expansion rate of the universe, you have to input a distance. It should be easy to see that at any expansion rate at all, multiplying it by some distance leads to a speed greater than that at light. During inflation, this distance was small (smaller than the nucleus of an atom). Today, the distance is large (around 14 billion light years). There's no reason to pick out one particular sort of expansion an call it "superluminal" while stating that other sorts of expansion are not.

    Heck, in a sense, the current expansion is even of the same character as the expansion during inflation: during inflation, as with right now, the expansion of the universe was accelerating. The difference is only one of magnitude, and since it's impossible to compare the magnitude of expansion against the speed of light (as the units are wrong), it's impossible to say one rate of expansion is superluminal while another is not.

    Finally, I thought I'd just touch on what General Relativity says about the speed of light limitation: what it actually says is that at any given point in space-time, the relative speed between two objects cannot be greater than the speed of light. I bolded the important distinction: this is only at a singular point. In General Relativity, relative speeds between objects at different locations are arbitrary: you can basically make the relative speed anything you like just by using a different coordinate system. Relative speeds are only well defined when objects pass one another. This has the effect, for instance, of stating that you can't ever outrun a light ray (at least, not by taking the same path...in principle it might be possible to take a shorter path).

    What this means is that at any given place in the universe, massive objects are always moving more slowly than the photons whizzing around them. But when you compare velocities of far-away objects, it is possible, depending upon the coordinates you use, to get numbers greater than the speed of light. But this is really meaningless in GR because that comparison is meaningless.
  8. Mar 15, 2009 #7
    Chalnoth, I think you're correct.
    Physics can be so confusing at times.

    Following is an interesting link that answers this and many of OP's questions:

    http://curious.astro.cornell.edu/question.php?number=575 [Broken]
    Last edited by a moderator: May 4, 2017
  9. Mar 15, 2009 #8
    alright thanks for all the insight, and marcus that princeton link definitely helped, its well written. Thanks.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook