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Why the Universe is accelerating as it is expanding

  1. Jun 14, 2014 #1
    Hello all,

    thank you for taking the time to read this post, i appologise if it is in the wrong section, as i am new to this.

    Why did people just invent Dark Energy to explain the accelerating expansion of the universe? I propose another idea that doesn't rely on fudging numbers to explain observation. As i understand things, space/time can be altered by gravity (warping) it. So objects near to a massive object appear to slow down (Time Dilation i believe?). As i understand, the universe is a measurable size, so it only contains a finite amount of matter (unless the Universe exists in a Steady State?). In the same way that a star collapsing becomes more dense, and consequently warps space/time more. Could you not argue that the Universe is like a star collapsing in reverse? Thus the Universe is becoming less dense, so altering how the universe as a whole warps its own space/time. Effectively what we measure as an increase in the speed of expansion, could be the fact that as the universe expands time flows (for want of a better term) faster. What im surgesting is that looking at the Universe from the outside it would be an expanding bubble, increasing at a steady rate. Inside our space/time expands, the universal gravitational constant decreases, allowing time to accelerate.

    I'm sorry if I'm not explaining very well, I'm not a physicist, so would welcome any observation pointing out obvious mistakes.

    Thank you for reading and i look forward to your responses!
     
  2. jcsd
  3. Jun 14, 2014 #2

    PeterDonis

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    Hi, Zero Ryoko, and welcome to PF!

    More precisely, spacetime can be curved by the presence of mass/energy; "gravity" is the term we use to refer to a variety of phenomena that result from this curvature.

    More precisely, objects near an isolated, stationary massive object appear to slow down, and "gravitational time dilation" is a term used to describe this. But the qualifiers are essential: to even define "gravitational time dilation", you need to have mass/energy restricted to a region with finite radius, outside which there is vacuum all the way to spatial infinity (that's the "isolated" part), and the spacetime needs to be stationary, i.e., not changing with time. The universe as a whole does not meet either of these conditions; see below.

    This is only true if the universe is spatially closed; but according to our current best-fit model, the universe is not spatially closed; it is infinite in spatial extent and contains an infinite amount of matter. Our *observable* universe contains a finite amount of matter, but that's not all the matter in the universe, just all the matter that we can currently see (because there has been enough time since the Big Bang for the light from that matter to reach us).

    No, because a star collapsing, while it is not stationary (it changes with time), is still isolated. The universe as a whole is not.

    This is true, but the key point is *how* the changing density of the universe affects the dynamics. Ordinary matter, the kind we can observe (and even "dark matter", the kind we can't observe but can infer the presence of by the orbital motions of stars in galaxies), slows down the universe's expansion; the less dense the matter, the less it slows down the expansion, but it always slows it down by some amount. So there is *no* way to get an accelerating expansion (i.e., an expansion that speeds up) with ordinary matter. The only way to get an accelerating expansion is with some kind of "stuff" that has very different properties from ordinary matter; "dark energy" is just the name for the simplest form that such "stuff" can take, mathematically, and since it's the simplest kind and we don't have any evidence to force us to consider more complicated kinds, our current best-fit model of the universe includes dark energy.

    No, it can't. The concept of "gravitational time dilation" doesn't apply to the universe as a whole. See above.

    There is no "outside" to the universe; it's not expanding into a pre-existing space.

    Now you're changing your hypothesis; first it was "gravitational time dilation", now it's a changing gravitational constant. They're not the same thing. Everything I said above was about "gravitational time dilation". If you want to talk about whether or not the gravitational constant can change, and how that might affect the universe's dynamics, you should start a separate thread.
     
  4. Jun 15, 2014 #3

    timmdeeg

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    Note that the flow of time is frame dependent. A clock close to a black hole ticks slower in the frame of a far away observer, whereas this clock 'ticks just normal' (check up proper time) in her own frame. The same is true regarding a clock in a cosmological distance, but called cosmological time dilation in this case instead of gravitational time dilation. The reason for the cosmological time dilation is not the decreasing matter density but the expansion of the universe itself. The photons emitted at subsequent ticks of the clock (the luminosity profile of a supernova may also serve as a clock) are increasingly separated due to the expansion while travelling towards the observer and therefore the far away clock ticks slower in his view.
    The "speed of expansion" is a misleading term. What happens is that the distances between comoving objects are increasing due to expansion (not to another time flow due to decreasing matter density). If you are interested check up Hubble's law to see the relationship between distance and recession velocity. So, accelerated expansion means that those distances grow accelerated.
     
  5. Jun 15, 2014 #4

    phinds

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    zero ryoko, when you come up against a belief/hypothesis/whatever that flies utterly in the face of established science, it is not a good idea to start off reaching different conclusions but rather to start off with the assumption that you have made a mistake somewhere and try to find out where it is. If you have NOT made a mistake you will find the flaw in the established science, but that is very unlikely to happen. If you start off thinking that you have overturned established science you are likely to just end up embarrassed.
     
  6. Jun 18, 2014 #5
    We are inside the universe and so we won't feel any time dilation. Someone outside the universe all feel that all events in the universe are hai happening to slowly.

    And can you please explain what do you mean by the universal gravitational constant changes? I don't think there is any experimental evidence supporting it.
     
  7. Jun 18, 2014 #6

    phinds

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    Since there is no such thing as "outside the universe", your statement makes no sense.
     
  8. Jun 18, 2014 #7

    OCR

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    There is no outside of, or inside of, the universe... as far as I know...

    http://en.wikipedia.org/wiki/Lee_Smolin#Views

    http://en.wikipedia.org/wiki/Lee_Smolin
     
    Last edited: Jun 18, 2014
  9. Jun 19, 2014 #8
    I understand that. But I was just trying to show even if we consider it right, there is no way the idea could work.
     
  10. Jun 19, 2014 #9

    phinds

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    That's equivalent to saying "if the laws of physics don't apply then the laws of physics say <fill in any nonsense you like>.
     
  11. Jun 20, 2014 #10
    I din't mean that. It's my fault. I shouldn't have commented on something on which I have no expertise.
     
  12. Jun 24, 2014 #11
    Thank you all, particularly PeterDonis for stepping through my original post. I see that i have a lot to think about. I assumed that when they mentioned an age for the universe that implied a finite size. The fact that it does not is interesting, and ill have to think about it some more, I wonder how something can be infinite and have an age.
     
  13. Jun 24, 2014 #12

    PeterDonis

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    It's infinite in spatial extent; but spacetime includes time as a dimension, and according to our current best-fit model, the universe is not infinite in the time dimension (at least, not into the past).
     
  14. Jun 25, 2014 #13
    How could have something which is infinite in spatial dimensions originated from a point?
     
  15. Jun 25, 2014 #14

    phinds

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    It could not, but that's not a problem because it didn't. If you think the universe began as a point, you've been watching too much pop-sci on TV

    All we know for sure is that the it wasn't a point and it was a hot, dense plasma. It MIGHT have been infinite in extent or it might have been finite but unbounded.
     
  16. Jun 25, 2014 #15
    No. Now I remember reading that the big bang did not happen at a point. It was an overall expansion. And it was not infinitely dense but a lot denser compared to the current state of the universe.
     
  17. Jun 25, 2014 #16

    phinds

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    Right.

    Keep in mind also that the "big bang" really has two meanings. First, the big bang singularity, which we do not understand and second the big bang theory which describes what happened starting one Plank time (more or less) after the singularity. The BB theory does not explain everything completely but it is very solid in most ways.
     
  18. Jun 25, 2014 #17
    What do we mean exactly by a singularity? An extremely dense point or the point at which our theories break down?
     
  19. Jun 25, 2014 #18

    phinds

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    No, just an extrapolation of the math that doesn't make sense ("theory breaks down"). I don't think it would be correct to say it is dense or a point or anything else other than that it is a place where the math of the theory gives a ridiculous result (that is, a result that we do not think could possibly represent physical reality).
     
  20. Jun 25, 2014 #19

    OCR

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    ... :thumbs:
     
  21. Jun 26, 2014 #20
    Ok. Thanks for explaining.
     
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