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Featured A No metastable type IIB de Sitter vacua

  1. Jun 29, 2018 #21

    Ken G

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    Reading through all this with little background expertise, but accepting the arguments at face value, I am led to wonder if the most natural conclusion isn't, as D-VR 18 say, that "string theory conspires against the existence of dS space," but rather, that the existence of dS space conspires against the usefulness of string theory. And similarly, rather than concluding "string theory has not made much progress on the problem of the cosmological constant during the last 15 years," it is perhaps more tempting to conclude that string theory is not the right course at all, when deSitter space seems to have much more going for it. I realize D-VR 18 are not trying to conclude anything beyond what they can show, but it still seems to me that the language used is placing string theory on a kind of pedastol, such that we should blame KKLT 03 for being careless and wasting 15 years, rather than blaming string theory itself, and using the KKLT 03 affair as a very productive means of helping us decide to try something other than string theory (or at least type IIb string theory, I don't really know the alternatives there). I'm saying that if it took the analysis of the last 15 years to understand that string theory is a red herring for understanding inflation and/or dark energy, then that may be the most significant discovery in theoretical physics in that timeframe. When everyone is doing something, the most constructive possible thing is to find out as quickly as possible if they should be doing something else!
     
  2. Jun 29, 2018 #22

    Urs Schreiber

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    It didn't take 15 years to see that KKLT had problems, that took at most a few months, e.g. here. But if a field does not proceed by rigorous argument, then some handwaving may survive a long long time before being abandoned.

    It's hard to draw any conclusions from this regarding the actual theory, since the point is that the poor theory has been neglected all along, in favor of fantasies.
     
  3. Jun 30, 2018 #23

    Ken G

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    But that conclusion, that only the KKLT 03 flawed version of string theory faces difficulties, is what I don't see here. I see that you are saying KKLT 03 was nonrigorous and is likely deeply flawed, and I am accepting this as fact. But the flaw was not that KKLT 03 didn't work well with dS space, whereas string theory might, the flaw was that KKLT 03 did work well with dS space, and string theory might not. The fervent hope for string theory, and the reason KKLT 03 had such impact, is that it would all play well with the dS space, as dS space seems to be our best means of understanding a plethora of befuddling astronomical observations. So critiquing KKLT 03 on the grounds that a more rigorous argument should not play well with dS is a much deeper issue than simply rejecting KKLT 03 and all the work done following it. It seems to me it must shake string theory to its core if it does not work with dS space, so if KKLT 03 stimulated arguments that demonstrate string theory (or some popular form of it) does not do inflation, then it was an error that was of tremendous value. You could certainly argue that some might have already known that string theory doesn't like to do inflation even without KKLT 03, but would it have the same widespread impact as getting everyone to believe something, and then show it isn't true? The rejection of KKLT 03, as framed here, sounds awful close to a proof by contradiction that string theory won't explain the need for inflation to understand astronomical observations.
     
  4. Jun 30, 2018 #24

    haushofer

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    That's what I also think, but somehow not a lot of people seem to worry about that. Maybe I'm missing the relevant papers on this.
     
  5. Jul 1, 2018 #25
    Vacua with unbroken supersymmetry have zero vacuum energy and are therefore flat. Until 1998 (discovery of dark energy), string phenomenologists hoped that some unknown mechanism could still cancel or absorb vacuum energy contributions in the realistic vacua with broken supersymmetry. But after 1998, people took the problem more seriously, and after KKLT in 2003, opinion moved to the idea that there is no unknown mechanism that damps the gravitation of the vacuum energy, there's just a coincidental-looking, but anthropically necessitated, near-cancellation between negative and positive contributions to the vacuum energy.

    Now apparently we are going to think about whether e.g. an undiscovered generic instability of quantum de Sitter space would decay all those would-be dS vacua after all. In that case it's back to the pre-1998 situation, but with the post-LHC knowledge that there's something wrong with the old ideas of supersymmetric phenomenology too.

    If dark energy isn't vacuum energy, the next candidate is quintessence, and Vafa has his idea for how it fits into string theory (a modulus, e.g. size of a Calabi-Yau "handle", that only interacts with dark matter or hidden sector fields). But the problem with supersymmetry-breaking creating a vacuum energy still exists. Vafa's supervisor Witten wrote a few papers speculating as to how supersymmetry might somehow be unbroken after all (and we have a long-running thread here in "Beyond the Standard Model" where that is also a theme). Then there are string models which are non-supersymmetric from the beginning (but which have a kind of echo of supersymmetric properties).

    String Pheno 2018 opens in Poland today...
     
  6. Jul 2, 2018 #26

    Urs Schreiber

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    Thanks for the alert. From the webpage there seems to be one contributed talk on the subject:

    Timm Wrase:
    "de Sitter extrema and the swampland"
    Abstract: Following the recent claim in 1806.08362 that dS extrema are in the swampland, I will review and scrutinize existing constructions of type II flux compactifications that use only classical ingredients. I will review some no-go results against dS extrema but also show that explicit constructions of (unstable) dS extrema exist that are inconsistent with the conjecture in 1806.08362. I will scrutinize all potential assumptions that go into these existing constructions.

    (Interesting how the attribution goes, now the claim is attributed to Ooguri-Vafa et. al...)

    Probably Wrase will be comparing to

    Diego Gallego, M.C. David Marsh, Bert Vercnocke, Timm Wrase.,
    "A New Class of de Sitter Vacua in Type IIB Large Volume Compactifications"
    (arXiv:1707.01095)

     
  7. Jul 5, 2018 #27

    Urs Schreiber

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    Danielsson now exploring possible alternative constructions:

    Souvik Banerjee, Ulf Danielsson, Giuseppe Dibitetto, Suvendu Giri, Marjorie Schillo,
    "Emergent de Sitter cosmology from decaying AdS"
    (arXiv:1807.01570)

    "Since the discovery of dark energy two decades ago, string
    theory has been faced with the challenge of reproducing
    a small positive vacuum energy. The dominant approach
    has been the reliance on a landscape of different vacua [1]
    equipped with a transition mechanism such that the an-
    thropic principle selects our vacuum [2]. This approach
    became calculable in string theory with the construc-
    tion of KKLT [3], in which one can achieve a landscape
    of scale-separated vacua, with both positive and nega-
    tive cosmological constant (CC), by tuning flux numbers.
    However, issues have recently been raised which indicate
    that the na ̈ıve application of supersymmetry-breaking
    and non-perturbative effects necessary in the construc-
    tion of the landscape is insufficient [4–8]. These concerns
    suggest that not even a single rigorous string vacuum has
    actually been constructed and further hints that string
    theory abhors de Sitter space and any solution with a
    positive vacuum energy will suffer from instabilities.

    "Given these difficulties, it is reasonable to consider the
    possibility that neither metastability nor scale separa-
    tion can be achieved in string theory in the way envi-
    sioned. It seems, therefore, that something completely
    different is needed. In order to construct an alternative,
    we will take motivation from work that received consid-
    erable attention around the turn of the millennium just
    before the idea of the string landscape started to flour-
    ish: braneworlds. In this context, the cosmology we see
    as 4D observers is not due to vacuum energy, but rather
    arises as an effective description on a dynamical object
    embedded in a higher dimensional space.

    "In the scenario developed by Randall and Sundrum
    (RS)[9, 10], two identical AdS5 vacua are glued together
    across a three-brane. The 5D graviton has a zero mode
    confined on the brane that gives rise to an effective 4D
    gravity despite the existence of large extra dimensions;
    this solves the issue of finding scale-separated vacua. We
    will consider a variation of this scenario that starts with
    a metastable false AdS5 vacuum that non-perturbatively
    decays to a supersymmetric true AdS5 vacuum through
    bubble nucleation. Here, a spherical brane separates
    the two phases with an inside and an outside, and 4D
    observers confined to the brane experience an effective
    dS4. This scenario is further motivated by recent argu-
    ments that all non-supersymmetric AdS vacua must pos-
    sess such an instability in a consistent theory of quantum
    gravity [11, 12]

    "In section 2, we discuss the physics experienced by an
    observer riding on an expanding bubble that nucleated in
    a first-order phase transition. In section 3 we discuss the
    restriction of gravity to the brane, performing a consis-
    tency check that the 4D Newton’s constant matches the
    expected value. In section 4 we sketch a concrete con-
    struction of a de Sitter braneworld using explicit type
    IIB string vacua; a metastable vacuum breaking all su-
    persymmetries first obtained by Romans [13] is shown to
    have a decay channel to an orbifold of the maximally su-
    persymmetric vacuum. We conclude by commenting on
    some future developments of this scenario."
     
  8. Jul 5, 2018 #28

    Ken G

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    Interesting, so they are trying to save string theory in cosmology by eliminating elements that string theory is intolerant to, and replacing them with higher-order structures that string theory is more tolerant to. I would call that a rather striking break from the concept of Occam's Razor, where if the simplest description of some phenomenon is a positive vacuum energy, then we should seek theories that are compatible with that simplest description. But I realize that "what is simplest" is a somewhat subjective determination, and for those already married to string theory, maintaining what advances they already perceive is always going to seem like the simplest path. Ultimately, any scientific theory must be judged not on what we see that it can explain, but rather what we have not yet seen that we would only expect to see if the theory is good. We must await that type of confirmation of all these notions. I don't know if any of us will see that in our lifetimes, however, but perhaps.
     
  9. Jul 5, 2018 #29

    That’s an interesting comment...I wonder if Occam’s is a universal, foolproof principle...general relativity is very complex...but correct...one needs just enough complication to get it right...and no more...

    But then we are far from knowing if ST in any of its guises is correct or not...


    IH
     
  10. Jul 5, 2018 #30

    Ken G

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    I agree, the relation between Occam and string theory is still unclear, and Occam is more useful in hindsight than foresight. All I'm saying is, there is a danger when a community builds an expertise that has as yet not proved to be a good theory, there can be the danger of square pegs in round holes. But they are the best ones to figure out which it is at this point-- if they can be objective.
     
  11. Jul 5, 2018 #31

    Agreed, except perhaps that objectivity can sometimes be a wobbly issue...from what I gather mathematical objectivity is perhaps the major focus of ST today...with the hope that this will one day yield a testable physical one...in fine, objectivity is also defined in hindsight...


    IH
     
  12. Jul 6, 2018 #32

    Urs Schreiber

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    Ken G, some information:

    The KKLT 03-constructions that are being abandoned (for being wrong) were so complicated that Susskind 03 famously referred to them as "not at all simple. They are jury-rigged Rube Goldberg contraptions" (p. 5). Hence abandoning KKLT likely means to reduce the perceived complexity, not to increase it.

    On the other hand, beware that there are general arguments in quantum gravity, independent of string theory, that global de Sitter spacetime is inconsistent, see e.g. Rajaraman 16 and references given there. If true, this means that no quantum consistent model for observed cosmology will be totally straightforward, all of them will have to realize a de Sitter cosmology as an effective phenomenon on the backdrop of non-de Sitter cosmology.

    Hence if you are intent on doing fundamental physics by way of slogans, remember the Occam's slogan has an addendum: "as simple as possible, but no simpler".
     
  13. Jul 8, 2018 #33

    Ken G

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    That's well and good, and I don't doubt you are correct, but that's not the reduction in complexity I'm talking about-- I'm talking about the cosmological constant. It sounds like you are saying that the reason KKLT existed in the first place was because it looked like it supported a positive vacuum energy. You are saying that was never a good enough reason to adopt what is essentially wrong physics, but in arguing it is wrong, you are pointing to studies that conclude type IIB string theory is not terribly compatible with positive vacuum energy. So that's the irony here-- the killing blow to KKLT is that it concludes something that string theory prefers to violate, yet it is the thing that KKLT is trying to conclude that is the simplest explanation for astronomical observations. This in turn suggests that string theory may not be terribly compatible with Occam's Razor when applied astronomically.
    That does sound significant, though of course it will also present the choice: accept awkward modifications to quantum gravity to get de Sitter, or just go with a very different approach to gravity that is built to play well with de Sitter from the get go. How much of current quantum gravity is regarded as successful, in the sense of producing expectations of things we actually see? Any of it?
    Calling Occam a "slogan" hardly rescues you from the problem here: physics must be done so as to give the simplest possible explanations to what we actually see. That isn't a slogan, it's the definition of scientific progress.
     
  14. Jul 8, 2018 #34

    Urs Schreiber

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    What I meant to indicate regarding "slogans" is -- given that the field has suffered from people being prejudiced about what the theory should yield, without being careful about doing the actual math -- that it is wrong to react to this by just changing the prejudice about what ought to be the case. It's time to do careful analysis and let the prejudices rest for a while.
     
  15. Jul 11, 2018 #35

    Urs Schreiber

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  16. Jul 13, 2018 #36

    Urs Schreiber

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    I am not an expert on the matter of inhomogeneous cosmology, but in view of the above discussion it is interesting to check the latest developments here:

    What is called inhomogeneous cosmology is the study of cosmology via cosmological solutions to Einstein's equations, whithout assuming or constraining these solutions to be spatially homogeneous (in the technical sense).

    This is in contrast to the standard model of cosmology, based on FRW model-type solutions to Einstein's equations, where spacetime is assumed to be spatially homogeneous.

    Of course the observable universe is clearly not exactly homogeneous, but the question is whether on cosmic scales the deviation from homogeneity is small enough that it may be neglected, to first approximation, for the purpose of modelling cosmological evolution.

    The standard model of cosmology assumes that this is the case, and studies structure formation as a perturbation about a homogeneous background spacetime.

    Given that the standard model of cosmology faces some issues related to dark energy/cosmological constant (and possibly related issues such as cosmic inflation or dark matter), it has been suggested that these may be but an artifact of the overly idealistic approximation of cosmic homogeneity, and that a more accurate inhomogeneous cosmology would not need to assume any dark energy (e.g. Buchert 07, Buchert 11, Buchert-Rasanen 11, also Scharf 13).

    A seminal argument that it is consistent to neglect cosmic inhomogeneity due to (Green-Wald 10, Green-Wald 13), has been called into question in Buchert et al. 15, where it is concluded that the question is more subtle and remains open. Recent review is in Belejko-Korzyński 16.
     
  17. Jul 13, 2018 #37

    Ken G

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    An interesting article to be sure, he raises the interesting point that the death of de Sitter vacuums in string theory might come with a rather nice side benefit: the death of the landscape of potential solutions in favor of a special type of solution that actually does permit a de Sitter vacuum in some unseen corner of string theory that would now have a reason to be favored over all the rest (including a violation of the cosmological principle, like perching our universe on the edge of a bubble). This new way of thinking about alternatives to the "landscape" might seem optimistic, but maybe it's just natural hopefulness. But I did take issue with this part: "It is not at all true that string theory cannot be tested. Its first test, which we thought was easy to pass, is to accommodate the accelerated expansion of the universe. "
    I actually think Danielsson is misapplying the concept of a "test of a theory," in a way that is becoming pretty common among theorists. A test of a theory must work like this: you first suggest the theory, then you suggest the observation which should only come out a certain way if the theory is good, and then you do the observation, which then either falsifies or favors the theory. That's a test! It is not a test to already know the observation, and sift through a mass of potential theories until you find one that fits the observation. This is certainly a valid way to come up with new theories, but it's no kind of test and cannot be used to claim that a theory is testable. If one cannot suggest a test that actually fits the definition of a test, then the theory is indeed untestable.
     
  18. Jul 14, 2018 #38

    Urs Schreiber

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    Not sure how this is on-topic, but just to reply:

    Not potential theories, but parameters of a theory. See the FAQ How do physical theories generally make predictions anyway?
     
  19. Jul 14, 2018 #39

    Ken G

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    Wow, there's a lot of confusion about what a prediction is, and what a parameter is! From that link: "But the point is that once this model has been postulated, then one can use the theory to see what it predicts about the remaining parameters, such as here the fluctuations of the cosmic microwave background radiation in a universe described by this model." I'm afraid you just can't have it both ways-- either you have a prediction, or you have a parameter you are adjusting in a model. Never both at the same time, that's my whole point.

    It's on topic because we are talking about to what extent can what string theory likes, or doesn't like, to do is or is not a "prediction" about how the universe works. If string theory is flexible enough to do anything we might later observe, then we can hardly call any of that a "prediction" of string theory, even though we do see language like that surprisingly often (some people even think string theory "predicts" the landscape!). Danielsson was saying that if string theory turns out to not be so flexible, in that it has a hard time with dark energy, that might actually be a good thing-- because now we have leverage to distinguish between types of string theory, perhaps looking for something non-homogeneous or some other selective agent. That's all fine-- until there is confusion about what is a prediction of a theory, versus what is a constraint we are using to hone the version of the theory we will settle on. Those are two completely different things. What is claimed now is that string theory predicts that whatever it is capable of doing should be happening somewhere, yet anything that string theory finds hard to do must be a useful way to constrain string theories. So which is it, are we looking for ways to falsify string theory, or are we married to always interpreting everything that happens in terms of some sort of string theory? We can do either, but we cannot pretend we are doing both-- the former is about testing theories, the latter is about creating theories from the possibilities.

    When creationists are asked, "what observational outcome could cause you to reject creationism?", they never have a good answer, which tells us that creationism isn't science. So it is clear that string theorists must also have a ready answer to that question. If a string theorist didn't know about the accelerated expansion, but did know that string theory doesn't do de Sitter well, why wouldn't they say "if the universe if found to be accelerating, that would lead me to reject string theory"? Indeed, Danielsson actually said that it would "test" string theory if it could accomodate acceleration, simply because we already see the acceleration. But tests are never a posteriori like that, a prediction is by definition a priori.
     
    Last edited: Jul 14, 2018
  20. Jul 26, 2018 #40

    Urs Schreiber

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    Today Wrase has the article out, which seems to be about research reported on at StringPheno18 (#26):


    In conclusion (section 5), they review the problem...

    "...we have spelled out the problematic details of these [type II dS vacua]. Here two points are particularly important: 1) All dS critical points that have been found up to date involve intersecting O-planes and therefore one cannot solve the ten dimensionally equations of motion pointwise but only after integration over the internal space. 2) The existing critical points were all found numerically and they have not been shown to exist in a large volume and weak coupling regime, for properly quantized fluxes. Here we have taken first steps towards checking point 2)."

    But then they find it's hard:

    "Unfortunately, these symmetries and the flux quantization conditions are model dependent and need to be worked out carefullyfor each model, which we leave to the future. We restricted ourselves to identifying oneuniversal rescaling that allowed us to rescale the tadpole condition..."

    and all they have in the end is this:

    "Based on these results we believe that these dS critical points deserve further study because they might provide the simplest explicit string theory setups that carry the potential of falsifying the dS swampland conjecture."

    and then a curious final sentence:

    "Another, potentially more complicated goal, would be to reach an agreement within our community on the consistency or potential shortcomings of the existing quantum dS vacua scenarios like KKLT [2] and LVS [3]."
     
    Last edited: Jul 26, 2018
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