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Clarification on dark energy please

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  1. Mar 22, 2014 #1
    Since first hearing about dark energy in the popular press I’ve always had the impression that we can’t detect it because it’s some exotic kind of energy that doesn’t interact with matter. Is it true that it doesn’t interact with matter or is that just a popular misinterpretation?

    I mean I know we can’t detect it, but I think we also know that expansion is halted in the presence of matter within galaxies. So isn’t that an interaction of sorts between matter and DE; the fact that expansion is stopped. Is it more the case that DE exerts a uniform force everywhere and so gives no variations against which to test?
     
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  3. Mar 22, 2014 #2

    phinds

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    "Dark Energy" is a stand-in name. What it stands for is "what ever the hell is making the universal expansion accelerate". It may be energy or it may be something else. Since we don't know what it is, we don't know what it is :smile:

    Dark "energy" is a very unfortunate name.

    It has SUCH a tiny effect on local scales that .... well, see the link in my signature for my favorite view on Dark Energy.

    At galactic scales it has no effect because it can't overcome gravity (and other forces on smaller scales). It's like an ant pushing on a house. It isn't that the any has a really tiny effect, it is that it has no effect at all because it can't overcome the forces that hold the house on its foundation.
     
  4. Mar 22, 2014 #3
    And I presume there are no indications that this ‘expansion energy’ could build up within a galaxy over a long enough time and eventually start to have an effect on gravity?
     
  5. Mar 22, 2014 #4
    The most favoured prediction is that this is exactly what will happen. Dark energy is predicted to build up in the universe, accelerating the expansion of the universe indefinitely, eventually overcoming all forces at all scales, ripping apart galaxies and eventually atoms, such that all elementary particles lose causal contact again, as in the case of the very early universe.

    There's something very neat about the argument that recursively, a similar process will take place, to the one shortly after our Big Bang giving temporary causality in subsequent universes. I haven't been able to ascertain how widespread the belief in this latter argument is amongt cosmologists, but if we add in variation on the constants of nature in the subsequent universes, it certainly offers answers to a number of open problems in cosmology.
     
    Last edited: Mar 22, 2014
  6. Mar 22, 2014 #5
    Dark Energy DOES interact with matter. It is what makes galaxies accelerate on a large scale. We can't feel its effects here on Earth because it is way too small compared to the vastness of the cosmos.
     
  7. Mar 22, 2014 #6
    I meant more specifically, within a galaxy, where the normal expansion of ‘space’ has been halted. I guess I’m thinking of it like an extremely slow build up of pressure. Just wondered what the time scale of such a (hypothetical) build up would need to be before it could compete with gravity, compared to the average life-span of a galaxy.
     
    Last edited: Mar 22, 2014
  8. Mar 22, 2014 #7
    Since galaxies have been around since the early universe and expansion is expected to end the life-cycle of galaxies then it's no suprise that the timescales match up. What's the alternative fate of a galaxy? Merging with other galaxies. We expect our local cluster to merge before being ripped apart, so that's one indication you could use for an average, but how many galaxies already merged to form our local cluster? I think that depends on where we draw the distinction between galaxies. Andromeda, for instance, has recently been observed to actually be surrounded by many much smaller galaxies. So it's difficult to talk of an average life time of galaxy.

    One way to compare timescales for the life time of galaxies, would be to consider different models, under the Cosmic Inflation model, with dark energy driving accelerated expansion, the universe has been predicted to be ripped apart 22 billion years from now. In older models of the universe the rate of expansion of the universe could achieve a steady state and in this model the rate of merging of galaxies slows down and galaxies get dimmer, becoming nothing more than supermassive blackholes, over a time scale of 10^40 years. These blackholes would evaporate over a timescale around 10^100 years leaving nothing but radiation. From this perspective the end of the universe as we know it, it very soon, but not worth changing your immediate plans for. There are certainly far more pressing existential threats.
     
    Last edited: Mar 22, 2014
  9. Mar 22, 2014 #8

    phinds

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    No, it does not build up over time. Despite craigi's belief in the "big rip" scenario, it is not at all agreed upon as something that will happen. Another theory is that the stars will gradually all "go out" as their fuel burns up and there becomes no fuel to make new ones and the universe will go cold and dark but NOT as the "big rip" scenario calls for, made up only of elementary particles. The black holes will, as he said, evaporate over time (assuming the currently accepted theory of Hawking Radiation holds true).
     
  10. Mar 22, 2014 #9
    good works guys.
    thanks for the quick responses all round.
    (like the balloonanalogy page too phinds)
     
  11. Mar 23, 2014 #10

    Chalnoth

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    That's not the most favored prediction, not by a long shot. In fact, there are very good reasons to believe it's impossible.
     
  12. Mar 23, 2014 #11
    Well, that is just a theory. Chances are such thing is never gonna happen. I am pretty sure DE will gradually get less intense over time or maybe the astronomical constants will change due to the bigger, faster Universe.
     
  13. Mar 23, 2014 #12

    Drakkith

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    What reason do you have for believing that?
     
  14. Mar 23, 2014 #13

    phinds

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    You are totally missing the point. The reason that DE is not going to have an effect inside galactic clusters is not because it is going to get weaker or stronger, but because it is likely to remain exactly the same per unit volume which means it will never have any effect inside galactic clusters.

    Also, I agree w/ Drakkith's questioning why you believe it will get less intense. This sounds like unsupportable personal speculation which is against the forum rules.
     
  15. Mar 23, 2014 #14
    I had in mind the Inflation theory, which says that the universe inflated exponentially and then slowed down considerably. I know that science is supported by facts, not beliefs, but considering we don't know much about the nature of DE, I assume we can be free for some theories or speculations. However, if there is proof that I am wrong, I will apologize. We are all learning new stuff in this forum after all, right? :D

    cb
     
  16. Mar 23, 2014 #15

    Drakkith

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    You are free to believe what you wish, however the rules of the forum do not allow for speculation on any topic. While this may seem a bit harsh, PF exists to help people understand what existing, mainstream science knows and does not know. We don't know for sure how the universe will look in 100 billion years, but we can extrapolate to that point using known laws of physics. Whether these laws are correct and will still apply that far into the future is unknown. That is not reason enough to start saying "what if" or "I believe", since these are inherently unanswerable and get us nowhere.

    This is not to say that we can't ask about something like that here on PF, you just need to go about it differently. For example, asking, "According to current science, what will the universe be like in 100 billion years" is fine. So is, "Do we have reason to believe that known laws will apply in 100 billion years". These questions, while they may not always be easy to answer, at least have answers, even if it's sometimes, "We don't know".
     
  17. Mar 23, 2014 #16

    phinds

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    OK, I see. You are conflating the inflation that happened in the first tiny fraction of time of the universe's existence
    with that which is going on now. As far as is known, they have nothing to do with each other but it IS an open question whether or not they might possibly have something to do with each other. Since that is totally unknown, hypothesizing on the basis that they ARE somehow related, which is what I now understand you were doing, IS unsupportable personal speculation.
     
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