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B Before accelerated expansion

  1. Mar 2, 2017 #1
    Before the period of the the accelerated expansion started about 5 billion years ago.. where was the dark energy and what was it doing. Or could it only got produced 5 billion years ago and non-existent before that?
     
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  3. Mar 2, 2017 #2

    phinds

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    It was there and it was exerting the same force per unit volume that it is now (give or take a very small amount) ... BUT, it was not ENOUGH of a force to overcome gravity. Only about 5 to 6 Billion years ago did the expansion weaken gravitational attractions enough on cosmological scales that dark energy started being stronger than the gravitational attraction.
     
  4. Mar 2, 2017 #3
    This excerpt from Wiki's "Metric Expansion of Space" article seems to sum it up rather well...

    "According to measurements, the universe's expansion rate was decelerating until about 5 billion years ago due to the gravitational attraction of the matter content of the universe, after which time the expansion began accelerating. The source of this acceleration is currently unknown. Physicists have postulated the existence of dark energy, appearing as a cosmological constant in the simplest gravitational models as a way to explain the acceleration. According to the simplest extrapolation of the currently-favored cosmological model (known as "ΛCDM"), this acceleration becomes more dominant into the future."

    Seems like the relationship between 'gravity decelerating the expansion' and 'dark energy (eventually overcoming gravity) accelerating the expansion' might be a parabolic-type function, such as a graph (time plotted against metric expansion speed) with a parent function:
    f(x) = x2
    Once it hits that absolute minimum point, the deceleration turns into an acceleration.
    Any thoughts?
    EDIT: Originally had in -(x2) by mistake. Changed to x2
     
    Last edited: Mar 2, 2017
  5. Mar 2, 2017 #4
    What was the proof the dark energy was there before the expansion? Anything to do with the total mass energy from the Big Bang and the expansion? Or just assuming it was there because it couldn't just appear 5 billion years ago?
     
  6. Mar 2, 2017 #5

    phinds

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    As far as I'm aware, it's the latter.
     
  7. Mar 2, 2017 #6
    It's to do with observation rather than proof.
    Looking at the redshifting of distant galaxies it seems that they would have been redshifting less quickly in the distant past than they are now,
     
  8. Mar 3, 2017 #7

    Chalnoth

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    There may be some evidence from the effect of dark energy on large scale structure at relatively early times, such as the Integrated Sachs-Wolfe effect, but it's necessarily weak because the density is so low.

    In the simplest model of dark energy, it is simply a cosmological constant, and a cosmological constant has a constant value at all points in space and at all times. It just wouldn't have had much effect until a few billion years ago because everything else in the universe was so much more dense until then.
     
  9. Mar 9, 2017 #8
    I suddenly have a mental block over something. Please follow this logic:

    Negative curvature is tidal gravity where things antigravitate (geodesic converge).
    Dark matter expand spacetime so dark matter antigraviate.
    Dark matter causes negative curvature where the metric expand?
    Does it mean whenever there is negative curvature, the metric expand?
    But high above the earth where there is negative curvature.. it doesn't mean there is metric expansion.
    So how do you know whether metric expansion occurs in negative curvature or there is no metric expansion and just tidal gravity??
     
  10. Mar 9, 2017 #9

    PeterDonis

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    No. Negative curvature means geodesics diverge. That is what dark energy (not dark matter--see below) does.

    No. "Expand spacetime" doesn't make sense (neither does "metric expansion"). Dark matter gravitates the same way ordinary matter gravitates. That means, heuristically, that it causes positive curvature in tangential directions (geodesics falling radially inward at different angular coordinates will converge), and negative curvature in the radial direction (geodesics falling radially inward at the same angular coordinates, but starting from different radial coordinates, will diverge).

    Did you mean dark energy?

    The rest of your post just seems confused.
     
  11. Mar 9, 2017 #10
    I'm sorry. Yes. I meant Dark energy and incorrectly written dark matter. After thinking about it for a day. I think negative curvature doesn't automatically mean there is expansion and only in the presence of dark enegy is there expansion.. right.. but then can't we say whenever there is negative curvature, there is very very minimum expansion (say close to distances in the planck length)?

    What's the right word for "metric expansion"? I thought one couldn't use "space expansion" nor "spacetime expansion"? Thank you.
     
  12. Mar 9, 2017 #11

    PeterDonis

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    They are two separate concepts. Whether there is a connection between them depends on the specific spacetime. There are spacetimes that have negative curvature but no expansion; and there are spacetimes that have expansion but no negative curvature. There are also spacetimes that have both.

    If you mean, only in the presence of dark energy is there negative curvature and expansion, that's not correct either, at least not with the usual definition of "dark energy", where it has constant density everywhere. There is a much wider range of possible models which have negative curvature and expansion, but allow the density of whatever-it-is that is causing the expansion and negative curvature to vary.

    No. See above.

    It's better to describe the specific model you are using; general terms like this are too vague.
     
  13. Mar 9, 2017 #12
    In the Scientific American article "Misconceptions of the Big Bang" where it was supposed to correct misconceptions http://coe.kean.edu/~afonarev/Physics/ExtraCredit_files/Big bang/Big Bang-1.htm , there seems to be a misconception within:

    "Renowned physicists, authors of astronomy textbooks and prominent popularizers of science have made incorrect, misleading or easily misinterpreted statements about the expansion of the universe. Because expansion is the basis of the big bang model, these misunderstandings are fundamental. Expansion is a beguilingly simple idea, but what exactly does it mean to say the universe is expanding? What does it expand into? Is Earth expanding, too? To add to the befuddlement, the expansion of the universe now seems to be accelerating, a process with truly mind-stretching consequences."
    <snipped>
    "This balloon analogy should not be stretched too far. From our point of view outside the balloon, the expansion of the curved two-dimensional rubber is possible only because it is embedded in three-dimensional space. Within the third dimension, the balloon has a center, and its surface expands into the surrounding air as it inflates. One might conclude that the expansion of our three-dimensional space requires the presence of a fourth dimension. But in Einstein's general theory of relativity, the foundation of modern cosmology, space is dynamic. It can expand, shrink and curve without being embedded in a higher-dimensional space."

    I thought it was emphasized in physicsforums that space couldn't expand, shrink and curve.. only the metric could do that, right?

    If you were to simply describe the universe expansion. What should you really use when so many phrases can't be used? I thought "metric expansion" was correct.. could "metric expands" be more correct than "metric expansion" again in describing the universe expansion?
     
  14. Mar 9, 2017 #13

    PeterDonis

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    Yes.

    Spacetime can curve, but it can't expand or shrink. What expands or shrinks are families of worldlines.

    The family of worldlines describing "comoving" observers--observers who always see the universe as homogeneous and isotropic--is expanding. That means these observers are moving apart.
     
  15. Mar 9, 2017 #14
    Dark energy is assumed to have a fixed density in the universe, regardless of the scale of the universe. This is in contrast to matter and radiation, which decrease in density as the universe expands. (The matter and energy get spread out over more space, decreasing the density. Moreover, energy is also redshifted as the universe expands.) That's because dark energy isn't a material that can be spread out over space, but rather some kind of property of space itself. Probably. We really are just guessing with this dark energy business.

    So in the early universe, dark energy was still present, but the matter and radiation densities were much larger and the dark energy could be ignored.
     
  16. Mar 9, 2017 #15
    I'm reading all your justifications in PF archives why "space expand" is not correct terms. Then I came across this:
    https://www.physicsforums.com/threads/spacetime-what-is-it.862012/page-4
    in message #65 you wrote regarding a video shared by a poster where a professor described about the concept of space expanding or at a very specific number for the doubling of space every 10^-35/s.

    certain Tionis wrote: Is the professor wrong?
    you wrote: "He isn't "right" or "wrong". He's talking about an interpretation, not about the actual physics. He's not trying to give you an actual physical model that you can use to draw physical conclusions. He's just making an analogy for people who don't know the physical model and don't want one."

    When you used the word "interpretation" above. Did you mean describing space as expanding was a valid interpretation like Bohmian mechanics where particles have trajectories? Is Bohmian mechanics an "actual physical model"? Or can "space expanding" not a valid interpretation at all like BM, MWI, etc.?
     
  17. Mar 9, 2017 #16
    Is it a mainstream view that dark energy can either be on the left or right side of the Einstein Equations? Were you describing in on the left or right side. What happens if it is on the right side? See this for reference: https://www.physicsforums.com/threads/what-does-a-cosmological-constant-mean.798684/
     
  18. Mar 9, 2017 #17

    PeterDonis

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    Yes, if you also understand that I meant that neither is worth spending time thinking about, IMO. Interpretations are not physics. You can't use them to make predictions or analyze experiments. You have to first make predictions or analyze experiments using the actual physical theory--the actual math. Then, if you really feel the need, you can use an "interpretation" to make up a story after the fact about what is going on. But I don't see the point of the last part; it doesn't actually tell you anything you don't already know. But it can mislead you into thinking you know something new, which is not good.

    No. It's an interpretation. See above.

    All of those are interpretations; I don't see the point of asking whether an interpretation is "valid" or "invalid", since all interpretations of a given physical theory (QM for BM/MWI, the FRW spacetime in cosmology for "space expanding") make the same predictions, so as far as physics is concerned they're all the same.
     
  19. Mar 9, 2017 #18

    PeterDonis

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    In the sense that there are mainstream treatments of the subject that put it on either side, yes. But any single mainstream treatment will probably put it on just one side, and might even make arguments that claim to justify that placement over the other placement.

    Mathematically, there is no difference; the equation is just as valid either way.

    Physically, putting dark energy on the left makes it seem like a property of spacetime, whereas putting it on the right makes it seem like a kind of stress-energy. The latter view is probably more common in recent treatments. But both versions make all of the same physical predictions, so they're not different models, they're just different ways of writing down the same model mathematically.

    Everything I've said is consistent with it being on either side. Both versions make all of the same predictions. See above.
     
  20. Mar 10, 2017 #19
    Uhm.. what spacetime have expansion but no negative curvature??

     
  21. Mar 10, 2017 #20

    martinbn

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    There was an example in the other thread. A cone that opens up have horizontal slices circles, which get bigger as you move up. So there is space expansion but the curvature of the cone is zero.

    But you need to specify what you mean by curvature. If that notion is not somewhat clear to you, then you'll only confuse yourself more.

    Also it is probably not a good idea to think of curvature causing geodesics to diverge or converge. That can happen with zero curvature. Two observers moving away from each other with constant velocity will have world lines that diverge, and two observers moving towards each other will have world lines that converge. But there is no gravity nor curvature. You should think of curvature (and gravity) as related to world lines converging or diverging at an accelerated rate as oppose to at a constant rate.
     
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