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Can Dark Energy be eventually infinite?

  1. Jun 16, 2015 #1
    Assuming the most common Dark Energy models, it's density remains constant even with the universe expansion. As new space volume is created, it contains the same amount of dark energy as the previously existing space for the same volume unit.

    If we assume that at a certain epoch of cosmic time, say 13 billion years elapsed from the Big Bang, the universe is finite in volume and time (we opt to dismiss the block-time view), this means that the amount of dark energy contained in the universe at that epoch is finite.

    But if the universe is expanding and accelerating as it is currently mostly believed, it should mean that the expansion will continue forever towards infinity, therefore also Dark Energy will need to be created in infinite quantity.

    Where does the additional dark energy contained in newly created space come from? Doesn't this violate the principle of conservation of energy?

    And eventually in case of infinite expansion, how can infinite energy become available to fill up all that space?
     
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  3. Jun 16, 2015 #2

    marcus

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    So far we have no evidence that what is called "dark energy" corresponds to an actual energy.
    The simplest explanation is that there is a cosmological curvature constant Λ, which is a constant intrinsic curvature in spacetime (which could have various explanations or simply be a constant of nature).

    this curvature Λ does not have to be caused by some mysterious "energy", it is reflected in the observations of how the hubble expansion rate H(t) evolves over time. H(t) seems to be declining but not to zero, it seems to be leveling out at a longterm asymptotic rate we can call H ≈ 1/17.3 percent per million years.
    The current expansion rate is H0 = H(now) = 1/14.3 percent per million years. But this has been declining and best fit to the accumulated data indicates H→H.

    that is essentially what they mean by "acceleration". the rate is going to a positive rate as limit instead of zero.
    growth at a constant positive rate is, of course, exponential, even if the rate is rather slow in percentage terms.

    this does not require that space be full of some mysterious "energy" :oldbiggrin:
    energy is not required for distances to increase, geometry is dynamic, that has been known for generations.
    there is a lot of excited hype surrounding the discovery that the limiting longterm rate is not zero.
     
    Last edited: Jun 16, 2015
  4. Jun 16, 2015 #3

    Chalnoth

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    The rate of expansion doesn't increase. That rate is decreasing and approaching a constant value. This is described as accelerated expansion because if you have a constant rate of expansion (speed per distance), then the distance between objects increases at an accelerating rate.

    There is a (rather unphysical) model where precisely what you describe does happen. It's known as the big rip. This isn't normal dark energy, though: it's dark energy where the energy density grows over time. In such a universe, the rate of expansion increases, and yes, it becomes singular in a finite amount of time. This model is almost certainly impossible, however, as it violates all of General Relativity's energy conditions (if you're curious what that means, see the Wikipedia page here.

    Energy is not a conserved quantity in General Relativity. Here is Sean Carroll's description of why this is:
    http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
     
  5. Jul 26, 2016 #4
    Chalnoth, why would you call phantom energy an "unphysical model"? It's perfectly physical.
     
  6. Jul 26, 2016 #5

    Chalnoth

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    It violates all of the energy conditions of General Relativity. For example, it violates the weak energy condition which comes from the statement that matter density is non-negative for any observer. It also violates the more stringent dominant energy condition which states that mass-energy can never flow faster than light.

    You can see more here:
    https://en.wikipedia.org/wiki/Energy_condition

    Note that dark energy in general violates the strong energy condition, but dark energy which grows in density over time violates them all.
     
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