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Thread: Dark Energy might falsify String Theory

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    Exclamation Dark Energy might falsify String Theory



    https://www.quantamagazine.org/dark-...eory-20180809/

    I don't have an opinion yet. May need some digging.
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    Default Re: Dark Energy might falsify String Theory

    And a whole generation of physicists spent their careers on string theory. Oops.

    Whoa. "[...] vacuum energy must be decreasing"? Guess I should read this thing all the way through.
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    Default Re: Dark Energy might falsify String Theory

    Quote Originally Posted by jcj380 View Post
    And a whole generation of physicists spent their careers on string theory. Oops.
    The ability to be shown wrong and bounce back it a necessary requirement for working in science. So it's really no biggie.

    Whoa. "[...] vacuum energy must be decreasing"? Guess I should read this thing all the way through.
    There is no evidence that vacuum energy is decreasing, though. That "vacuum energy must be decreasing" is just a cry of despair by a disappointed theorist and member of the string theory believer group. "Damn the data it must be otherwise!" is the motto.
    Last edited by not_Fritz_Argelander; 08-09-2018 at 05:51 PM.
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    Default Re: Dark Energy might falsify String Theory

    OK... some initial reaction.

    There is this analogy that so far has led to the only positive results for string theory: the Anti de Sitter - Conformal Field Theory correspondence. It looked like a promising way forward for quantum gravity and helps to make sense of experiments at the Relativistic Heavy Ion Collider.

    https://en.wikipedia.org/wiki/AdS/CFT_correspondence

    But it always stuck a little bit. The is it possible to make this correspondence for Anti de Sitter Spaces but not for de Sitter Spaces?

    If the cosmological constant is constant and we live in a de Sitter Space then this seeming favoritism of String Theory for Anti de Sitter Spaces would be a piece of evidence that things are going to get worse for strings before they get better, if ever.

    I've been waiting for 20 years for some theorist to come up with a Conformal Field Theory correspondence to a de Sitter Space and there's zero, zilch, nada.

    So Vafa's idea that Dark Energy may falsify String Theory has some support from that area.

    That's all I have for the moment.
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    Default Re: Dark Energy might falsify String Theory

    OK I've gone ahead and collected some reading material. The paper at the heart of these interesting claims is at

    https://arxiv.org/abs/1806.08362

    It has been notoriously difficult to construct a meta-stable de Sitter (dS) vacuum in string theory in a controlled approximation. This suggests the possibility that meta-stable dS belongs to the swampland. In this paper, we propose a swampland criterion in the form of |∇V|≥ c⋅V for a scalar potential V of any consistent theory of quantum gravity, for a positive constant c. In particular, this bound forbids dS vacua. The existence of this bound is motivated by the abundance of string theory constructions and no-go theorems which exhibit this behavior. We also extend some of the well-known no-go theorems for the existence of dS vacua in string theory to more general accelerating universes and reinterpret the results in terms of restrictions on allowed scalar potentials.
    Translating: "All spaces with positive cosmological constant are de Sitter Spaces and de Sitter spaces cannot be constructed in String Theory." I've skimmed the full paper and it looks good and reasonable. I would say that the only hope for String Theory is an observational result that shows that the cosmological constant isn't constant.

    In skimming the above paper I also found other references to results on the string landscape.

    https://arxiv.org/abs/1711.00864

    We give a brief overview of the string landscape and techniques used to construct string compactifications. We then explain how this motivates the notion of the swampland and review a number of conjectures that attempt to characterize theories in the swampland. We also compare holography in the context of superstrings with the similar, but much simpler case of topological string theory. For topological strings, there is a direct definition of topological gravity based on a sum over a "quantum gravitational foam." In this context, holography is the statement of an identification between a gravity and gauge theory, both of which are defined independently of one another. This points to a missing corner in string dualities which suggests the search for a direct definition of quantum theory of gravity rather than relying on its strongly coupled holographic dual as an adequate substitute (Based on TASI 2017 lectures given by C. Vafa).
    Translation: It is useless to continue hoping that the AdS-CFT Correspondence can be used to construct a quantum gravity. It is an inadequate approximation and a quantum gravity needs to be constructed directly rather than edged into gradually.

    Lastly, string theory really needs supersymmetry to work smoothly. Supersymmetry is under pressure because of the failure of the LHC to find any supersymmetric partners for standard model particles. Almost all string theories are supersymmetric string theories and the above papers assume that is the case. Nevertheless there are a few string theories that work without supersymmetry. The following paper rules some of these out too just in case:

    https://arxiv.org/abs/1610.01533

    We propose to sharpen the weak gravity conjecture by the statement that, except for BPS states in a supersymmetric theory, the gravitational force is strictly weaker than any electric force and provide a number of evidences for this statement. Our conjecture implies that any non-supersymmetric anti-de Sitter vacuum supported by fluxes must be unstable, as is the case for all known attempts at such holographic constructions.
    Translation: You can't make a stable Anti de Sitter Space where gravity is weaker than electromagnetism even if you give up on supersymmetry.

    Conclusion: This is the most serious challenge yet to String Theory and M Theory. The only way out is for future observations to show that the cosmological constant is not a constant.
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    Default Re: Dark Energy might falsify String Theory

    thank you nFA.
    utterly brilliant posts.
    so happy that you post on this forum.
    it is a joy to read your posts.
    sincerely thank you,
    Charlie

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    Default Re: Dark Energy might falsify String Theory

    I'm missing something here (what else is new? heheheheh) If the crux of the matter revolves around the Cosmological Constant needing to be constant, then well, it is clearly known to NOT be constant. It was different in the distant past, it has a present value, and is expected to be larger in the future.

    If a cubic volume has a standard amount of Dark Energy, and with time expansion gives rise to more cubic volume but the amount of DE stays constant per individual cubic volumes then the Universe must accelerate in it's expansion.(i.e. the Cosmological Constant isn't constant.)

    The empirical measurements of Perlmutter etal in 1999 confirm this picture demonstrating an accelerated expansion starting ~5 billion years ago. The "Cosmological Constant" isn't CONSTANT.

    If this is the case the Cosmological Constant/Hubble Constant/Hubble parameter/Dark Energy Value/Accelerated Expansion Rate or whatever euphemism is currently in vogue is unique to the era in which it is measured. It may appear to be constant in the short observation times that humans are personally limited to but in long time frames it is demonstrated to change and have different values unique to the epoch being studied.

    It boils down to this...In the past it was smaller and in the future it will be larger.

    From what I could gather from the above posts the "fact" that the Cosmological Constant" is constant, kills String theory.

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    Default Re: Dark Energy might falsify String Theory

    Quote Originally Posted by Seeker725 View Post
    I'm missing something here (what else is new? heheheheh) If the crux of the matter revolves around the Cosmological Constant needing to be constant, then well, it is clearly known to NOT be constant. It was different in the distant past, it has a present value, and is expected to be larger in the future.
    One needs to be careful to not confuse that acceleration of the expansion of the universe due to inflation and the cosmological constant. Everything you just said is true about the acceleration of the expansion of the universe but it does not apply to the cosmological constant as simply as this. The cosmological constant IF constant gives future greater acceleration. In the past inflation can be a cause of acceleration over and beyond the cosmological constant term.

    If a cubic volume has a standard amount of Dark Energy, and with time expansion gives rise to more cubic volume but the amount of DE stays constant per individual cubic volumes then the Universe must accelerate in it's expansion.(i.e. the Cosmological Constant isn't constant.)
    This is confused. If the universe has a standard amount of DE density always, then the cosmological constant IS a CONSTANT. That STILL implies that the acceleration of the universe increases. It is easy to see this. At present DM and baryonic matter contributes a certain amount of deceleration via gravity and the cosmological constant contributes a certain amount of acceleration of expansion. The present deceleration is overbalanced by the acceleration giving a net acceleration. In the future the amount of deceleration due to gravity is less because the expansion is decreasing the density of the gravitationally attractive stuff. Due to the cosmological constant the acceleration of expansion is the same. So in the future there is LESS deceleration to balance the acceleration due to the cosmological constant so the net future acceleration appears to increase.

    That's where you are making an error. The contribution of the cosmological constant stays constant, but the contribution of gravity to resist acceleration decreases over time.

    The empirical measurements of Perlmutter etal in 1999 confirm this picture demonstrating an accelerated expansion starting ~5 billion years ago. The "Cosmological Constant" isn't CONSTANT.
    Yes, but the measurements of Perlmutter and all are consistent with the cosmological constant being constant and furthermore the Planck measurements of the CMB are constant with the cosmological constant being constant.

    https://en.wikipedia.org/wiki/Planck...craft)#Results

    If this is the case the Cosmological Constant/Hubble Constant/Hubble parameter/Dark Energy Value/Accelerated Expansion Rate or whatever euphemism is currently in vogue is unique to the era in which it is measured. It may appear to be constant in the short observation times that humans are personally limited to but in long time frames it is demonstrated to change and have different values unique to the epoch being studied.

    It boils down to this...In the past it was smaller and in the future it will be larger.
    What you have said applies correctly to the acceleration of the universe's expansion. It is incorrect to apply it to the cosmological constant. The acceleration decreases because the contribution to acceleration of the cosmological constant IS CONSTANT while the opposing deceleration due to gravity DECREASES leaving a net increase of acceleration.

    From what I could gather from the above posts the "fact" that the Cosmological Constant" is constant, kills String theory.
    And since the Planck CMB results are a strong clue that the cosmological constant is constant string theory has problems. Not only is the measured constancy of the cosmological constant a problem but the sign of the cosmological constant is a problem. Anti de Sitter Spaces that String Theory likes have a NEGATIVE cosmological constant. Planck CMB measurements show a POSITIVE one.

    The only way for String Theory to persist is for observation to show that the present positive value is decreasing towards a negative value.

    The article points out observational campaigns to try to find evidence of a decreasing cosmological constant. So string theorists need to hold their breath and consider a possible change in research topic.
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    Default Re: Dark Energy might falsify String Theory

    Thanks, Seeker725, for your questioning of the results. I'm sure that of there was a confusion about acceleration vis a vis the cosmological constant that many shared it. So it is important that you brought it forward to be aired out. Thanks again.
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    Default Re: Dark Energy might falsify String Theory

    Thank you nFA.

    There still seems to be a logical contradiction. If the U's total amount of DE is fixed, with expansion (accelerated or not), DE must dilute per unit volume with time. If DE remains constant and new space is created due to expansion, the new space's DE must come from somewhere or conservation of energy does not apply.

    That leads to 3 possible conclusions...

    1. Conservation laws do not apply to the Universe as a whole. (distasteful and somewhat metaphysical)
    2. Observations and experiments among several different lines of reasoning are wrong and all conspiring to drive us to false conclusions. (almost as distasteful)
    3. The energy of empty space, DE, is acquired from OUTSIDE the universe. (distasteful to some, obvious to others, proveable to none.)

    Is there an option I don't see?

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