A No metastable type IIB de Sitter vacua

Urs Schreiber

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what exactly makes it more plausible that they correspond to different "universes" which are actually realized?
I think what is important, since it's a solid fact, is that it makes it possible. From here on, people tell campfire stories (campfire stories with arXiv numbers! :-).

The currently popular campfire story goes like this: If it is possible, it will happen once there is a mechanism that explores all possibilities. One such mechanism might be cosmic inflation, if thought of in the naive way as a small "bubble" (as they like to say these days) of spacetime within a larger ambient spacetime suddenly expanding rapidly. The campfire crowd imagines that each such bubble has a chance to go off with different values of those fields that fix the would-be "constants of nature".

That's fun as a campfire story, but it's not more than that. I wish people would focus more on actually figuring out stuff.

It is ridiculous that we don't even know for sure how much of the apparent cosmological constant deduced from supernova data is actually due to the observable universe not really being completely homogeneous, as assumed in the concordance model. The "backreaction debate" remains inconclusive. This is amazing, it means that potentially we are in for a much larger drama than the disappearance of the KKLT fatansy. Somebody should figure this out. But of course that's much harder than telling campfire stories.
 

atyy

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It is ridiculous that we don't even know for sure how much of the apparent cosmological constant deduced from supernova data is actually due to the observable universe not really being completely homogeneous, as assumed in the concordance model. The "backreaction debate" remains inconclusive. This is amazing, it means that potentially we are in for a much larger drama than the disappearance of the KKLT fatansy. Somebody should figure this out. But of course that's much harder than telling campfire stories.
That is interesting! I knew about Green-Wald, but didn't know their conclusion was not universally accepted.
 

Urs Schreiber

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That is interesting! I knew about Green-Wald, but didn't know their conclusion was not universally accepted.
I am not an expert on this, would love to hear your opinion if you know more.

My understanding is that nobody doubts their mathematical conclusion, but that people feel at a loss understanding how their assumptions/axioms are related to the actual problem of inhomogeneity.

In addition, there seem to be numerical simulations that prove that inhomoheneity may have anything between small and large effects (here). Of course there will be assumptions going into that, too...
 

Ken G

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Ken G, the explanation you are looking for has been explained many times over the years, but you seem intent on turning this into ridiculous philosophy rhetoric instead of actually learning what people are doing (which is perfectly in the scientific tradition):
What is "ridiculous" is to suggest that I am the one bringing basic issues in scientific pedagogy into this thread. Much of the thread, dating back to the OP, has been about the role of mathematical rigor in science ("If at the next social gathering smalltalk with your colleagues you need a good argument that physics does need mathematically airtight proof after all, you’ll have a monumental example here." ) That is quite clearly an issue for scientific pedagogy-- i.e., philosophy. Also, the articles cited in the thread are rife with philosophical statements (a random example from Danielsson: "We therefore think that the most natural assumption, at this point in time, is that string theory conspires against the existence of dS space." Is it not perfectly obvious that talking about what people "think is the most natural assuption" is an example of philosophical thinking?) So no, I did not bring philosophy into the thread, it was already there. I just want it to reasonably reflect proven scientific pedagogy, and to reign in the more glaring variances from that.

It seems to me this thread is about two things: a technical question about whether or not string theory supports dS vacua, and a more widely accessible scientific issue about what are the ramifications if it is or is not supportive of dS vacua. The quote by Danielsson above actually goes so far as to suggest, quite clearly by his choice of language, that if string theory disallows dS vacua, that we should actually look for cosmologies that don't involve dS vacua-- not that we should look for a different physical theory. That's philosophy, and demonstrating a rather complete marriage to string theory thinking.
The state of string theory is very analogous to what the state of GR was, except that we don’t understand which particular stress energy configuration pinpoints the real world. So go back to the state of things in 1918, long before the FRW solutions had been found.
This more or less proves my point actually, the very fact that you think string theory today is at all analogous to GR in 1918. That comparison reflects a departure from sound scientific pedagogy, because although we presently cannot rule out the possibility that string theory might someday explain pre-existing observations that no other theory can explain (like GR could do in 1918 in reference to Mercury's orbit), proper scientific pedagogy calls on us to notice that it has not yet done so. Also, it calls on us to notice the difference between making a prediction that was only later observed (as GR did with the eclipse of 1918), and what people call "predictions" of string theory (like the dubious "landscape", which is no kind of prediction at all, regardless of whether you think it is mathematically sound or unsound). There is no question that string theory is a remarkable theory with great promise, but there is no need to forget what science is in order to praise string theory.
The parameters here are analogous to the parameter K in FRW. Namely you can’t get anything you want, they’re fixed but can take discrete values that have to be matched to the real world (configurations that sometimes correspond to experiments at very high energies which can’t be currently performed).
Yes, that is the natural status of theories, we use observations to fix the free parameters, and then we try to extrapolate beyond what we already know by expecting predictions to come from it-- like how you can use Newton's laws to predict the return of Halley's comet.
As for the multiverse, that question can already be answered at the classical level, no need to bring in string theory. In other words, if you have a philosophy objection, first square it against the classical scenario before complicating it with a currently incomplete quantum gravity program.
I have no problem with people applying philosophy to string theory. My issue is when they pretend they aren't.
 
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haushofer

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I find the introductorion on Danielsson's site https://ulfdanielsson.com/is-string-theory-in-crisis/ ,

"String theory is not in crisis – on the contrary – but string theorists are. The theory refuses to adopt to human preconceptions, and the ruling paradigm since the turn of the millennium is questioned."

curious. The problem of imo is the fact that string theory is a paradigm, and paradigms can't be falsified directly, only certain models/theories in a paradigm. And in string theory we still don't have such theories which reproduce all the phenomena we now describe with the standard model and GR. What I read in the quote above, is a strong believe that the theory will guide us. But to what?
 

Urs Schreiber

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The problem of imo is the fact that string theory is a paradigm
No. Perturbative string theory is a well defined theory. Non-perturbative string theory is an open problem.

The precise rules of the game are this:


While well defined, this is extremely hard, since the space of all 2d SCFTs is so very little understood. Hence people resort to 1001 approximations, and that's the source of all the uncertainty and debate.

One of the approximate ideas is that it should be possible to build 2d SCFTs by a "sigma-model"-prescription (these are the "geometric vacua"). For these it may be argued that the effective target space background is a solution to some supergravity theory. Very few sigma-models in curved spacetime are fully understood (essentially only the WZW models), but, still, what is known about sigma-model 2d CFTs is much more than about about "non-geometric vacua" such as Gepner models.

But, unfortunately, general theorems about supergravity seem to rule out de Sitter vacua for these. This means that if there is a perturbative string de Sitter vacuum, then it is not a "geometric" one, coming from a plain sigma-model.

Indeed, all candidate string de Sitter vacua that have been proposed involve "stringy effects" such as O-planes and non-BPS D-branes, which have, ultimately, only an algebraic definition on the level of 2d SCFTs.

In principle it is a well-defined mathematical problem to ask if the space of 2d SCFTs of central charge 15 contains any elements whose low energy effective S-matrix describes scattering on asymptotic de Sitter backgrounds.

The trouble is that actually solving this mathematical problem is technically so utterly out of reach for the time being, that instead of this actual technical problem a huge story of interlocking plausibility arguments is being discussed.

Eventually. the only way to make real progress, will be in developing better mathematical tools and devices to actually study perturbative string theory.

Once that it accomplished (maybe in a century from now?!) we can really check whether there are any perturbative de Sitter vacua.

Then the next question is if this result, either way, will receive corrections from non-perturbative string theory, also known as M-theory. A theory still to be developed. (Incidentally, the titles and abstracts for the contributed talks at the upcoming conference "Higher Structures in M-theory" are now available here).
 

Ken G

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In principle it is a well-defined mathematical problem to ask if the space of 2d SCFTs of central charge 15 contains any elements whose low energy effective S-matrix describes scattering on asymptotic de Sitter backgrounds.
Now that sounds an awful lot like a paradigm to me, in the sense haushofer seems to mean, and not a "single theory" that could be falsified by observation. For example, you have not claimed to know that any such theory even exists, so you can hardly call it a "well defined theory" if it turns out that no such theory exists. Worse, if it turns out that a vast number of such theories could exist, then you can also not call it a well defined theory, as it might still have enough power to describe anything. That has always been the problem, has it not? The problem is, it's a paradigm, not a falsifiable theory, despite the claims to the contrary.
Eventually. the only way to make real progress, will be in developing better mathematical tools and devices to actually study perturbative string theory.
And this is the main point you are making, which seems perfectly valid, but does not require embuing string theory with pedagogical attributes that it simply does not, as yet, exhibit.
Then the next question is if this result, either way, will receive corrections from non-perturbative string theory, also known as M-theory. A theory still to be developed. (Incidentally, the titles and abstracts for the contributed talks at the upcoming conference "Higher Structures in M-theory" are now available here).
So what we actually have is, a paradigm suggested to make corrections to another paradigm. That's what we've got, let's not pretend the situation is otherwise. I'm not saying it isn't promising, I'm saying it is being misrepresented by a lot of the language we see. Indeed, if you think that the main problem with the landscape is that it is not a mathematically rigorous (or even a mathematically plausible) application of string theory, others could equally point out that it might suffer from the even more serious problem of not being science at all, but instead, a form of mathematical philosophy (and bad mathematics at that, as you point out). Where your comments are most expert are around the actual mathematical questions, so I laud the efforts you are making to make these more accessible to others. This is a great service to everyone, and a fascinating thread that I have learned a lot from. It seems to me that one way to frame this entire thread is with this question:

If we are to say that string theory is a predictive way of thinking about the nature of things, then is it fair to say that the prediction it makes is there cannot be dark energy? If it does not make that prediction, it sure sounds like a paradigm that can be hammered into any shape we need. And if it does make that prediction, then are we not already on a path that could falsify it before it even gets going? That's the double-edged sword of science: to be good, it has to be capable of being wrong.
 
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Urs Schreiber

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Ken G, that's all right, I know your attitude and I have seen what you can provide, you don't have to keep repeating. I suggest you start a fresh thread on the points that are of concern to you, so that we can keep this thread here focused.
 

Ken G

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But focused on which part of what you have already said, the mathematics part, or the philosophy parts, that you put into your OP?
 

haushofer

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No. Perturbative string theory is a well defined theory. Non-perturbative string theory is an open problem.

The precise rules of the game are this:


While well defined, this is extremely hard, since the space of all 2d SCFTs is so very little understood. Hence people resort to 1001 approximations, and that's the source of all the uncertainty and debate.
Maybe this is semantics, but it depends on what you call a theory and a paradigm. I don't consider perturbative string theory as a different theory from string theory; in my view, perturbative string theory is merely a calculational limitation of string theory. From that point of view I say we don't have the precise rules.
 

Fra

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demonstrating a rather complete marriage to string theory thinking.
...
There is no question that string theory is a remarkable theory with great promise, but there is no need to forget what science is in order to praise string theory.
...
I have no problem with people applying philosophy to string theory. My issue is when they pretend they aren't.
I agree.

As we know already I think what's lost is more than 15 years, it may unfortunately be the ideas and contribution of a whole generation of physicists.

I remeber well 20 years ago in Danielssons office how he adviced me against pursuing any future in theoretical physics research unless it was in string theory! and he also didnt seem open to even reflect upon different ideas. So it was clear that either you had to fight research political battles or join the mentality.

For me the integrity of the intellectual quest was far more important than academic route so the choice was easy.

/Fredrik
 

Ken G

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This is why it is so important to maintain a clear view of what differentiates science from other things, including mathematics. This thread makes the key point that in mathematical physics, as anywhere in formal mathematics, following non-rigorous ideas too far can lead to false turns and wasted time. Obviously mathematics is important in science. But it does raise the question, what is different about mathematics and science? In mathematics, it is important to know what is proven, what must be true. In science, it is also important to know that one does not prove, one explores. Are we seeing a kind of collision of these mindsets when people think a theory is so beautiful it doesn't need testing, and doesn't need an observation to be obtained that no other theory can explain? That mindset is pretty close to forgetting what differentiates science from just about everything else, because science is the only place where the gold standard is "look for such-and-such, and if this theory is good, you will see it."
 

Fra

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I largely agree.

But I think one should distinguish between standards in scientific community and the motivation of individual researchers.

I have absolutely no problem with wether some potential genius follows guidelines and intuition in the search for something that could later be either "proved in math" or "corroboorated in physics". It is actually not surprising that even the most briliant idea, can initially be met by scepsism from the scientific community as there are yet not hard proof and only fuzzy subjective motivations. If these nutcases (or geniuses if you prefer) arent "allowed" then the creativity required for the evolution of science is severely crippled. One essential mechanism of evolution is variation, but the wildness must be "tamed".

So in this sense, i have no problems with that we "evaluate" the paradigm of string theory, or that some passionate brave theorists commit to exploring its implications. Because who knows if they find something noone expected.

But I have an objection with how it becomes a self-inforcing mechanism that risk intellectual inbreeding, because the same persons that are supposed to guide students and future physicists. This is where the problem for scientific community and wasting resources comes in.

But Danielsson himself has said several times (I recall one panel discussion where he argues for a popular audience against Max Tegmark) that if it is found that string theory ends up beeing "only" mathematics he will be dissapointed. So I do not think he thinkhe is doing mathematics, he is doing physics. As far as I understand, his perspective is that string theory in his perspective is the most promising candidate to unify GR and QFT, in despited of all its issues and problems. This is the rational reason why you keep developing it.

If I agreed with this, i would also be looking at string theory. But my analysis has found too many problems, in the premises of QFT and GR that are carried overy completely to string theory, which i think are the where the problems are hidden. This is why i find string theory to actually in a sense be too conservative (Danielsson has himself used this word, he thinks string theory is a "natural extrapolation" in some way from QFT, that he calls "conservative"). And in a way i agree its conservative in one way(the wrong way), but crazy in other way (also the wrong way).

And in a way i think it is the most promising, DEVELOPED idea, but the problem is that that is almost like a self-fulfilling prophecy, as you discourage variation.

/Fredrik
 

Urs Schreiber

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This shall be my last message here, as it seems from this point on you can follow this social event from the mass media.

The talk
  • Thomas van Riet, "Status of KKLT", talk at Simons summer workshop 2018 (recording)
is worth watching; not just for the social curiosity after the talk in 1:30-1:40, but since it's a good talk and interesting to see the many questions being raised by the audience, and the replies by the speaker.
 

Urs Schreiber

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Maybe I should keep logging the discussion after all, might be a useful compendium later. But I will be brief.

Today a whole bunch of articles from KKLT-related groups claiming that they didn't make a mistake after all. But it looks like they don't quite address the issues that have been raised. See Motl's summary here.
 

Urs Schreiber

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Van Riet gives a decent analysis of today's flood of papers (here):

--BEGIN QUOTE --

My 2 cents about these papers in case anybody would care. But before so, I must admit I only had a very superficial reading of all of them.

Paper 1 is interesting and a complementary review to the one I wrote with Danielsson. They seem to cover very similar topics (we did not cover quintessence though). But were we concluded trouble, they conclude the opposite. That is ok and makes this review a good read for folks interested in the topic, but I highly suggest to read my review with Danielsson and the references therein, to have a balanced view. But I do not consider this review as a direct attack on the dS Swampland since no new computations were done that refute existing criticism it seems. Also the issue they point out about double fine-tuning in quintessence was already addressed by Vafa et al when they published their papers.

Then the paper 2 by Kachru and Trivedi. It is good that their arguments are in print now. But I think it contains what they have been saying already for many years in defense of KKLT. Their EFT reasoning can make sense for sure. But it is not enough and subtleties are around the corner almost everywhere. But like the previous paper, this is good to have and especially this one is useful to pick apart for anyone that cares about criticism on the flux program. But, again, nothing too new.

Paper 3 by Wrase-Kallosh is really nice for me personally. Contains a very interesting idea. But I doubt it survives a thorough study of the open-string sector as you mention. For instance brane-flux decay here could be perturbative. I will check that. But as an idea paper, it is great! Both these authors are indeed great experts on the supergravity side of the story and they definitely came with a nice suggestion that needs to be studied. Also here, I doubt it counts as any evidence against dS Swampland ideas. But they do not push that either in their paper.

Paper 4 does not really address the issue from the start. As a 4D supergravity paper there might be something interesting, but it says little about the paper by Moritz et al they want to refute . Why? The latter paper is really about a 10D computation. Moritz et al then tried to reformulate their results in 4D sugra and are so nice to put it into the language of constrained superfields. That language is not guaranteed to make sense always. Anyhow, today's paper notices there is small issue with the constrained superfield description in a different position of moduli space. But I do not think that Moritz et al intented their results to apply that far away in moduli space in the first place (in line with Swampland distance conjectures).

Paper 5 could indeed fall into the problem you mention. But observational cosmology papers seem very shaky and there have been various tensions among complementary observations. As far as I can tell the current data are still easily fitted with time-dependent dark energy.

The possible existence of a dS landscape has huge implications. So it better rests on serious solid foundations. Foundations of the kind that makes theoretical physics such a great science. The papers today do not really help in gathering enough people to look into the details in order to find more mathematical support for or against the dS landscape. Vafa and friends created the academic, intellectual space to investigate this and that is great! We should be grateful for that. His move was necessary since too many papers are written about "stringy inspired EFT's". Whereas really listening as to how string theory constrains EFT's seems less popular, although much more relevant. It is less popular since it is simply more complicated. But, in the long run, will be more rewarding. Many things that may seem to have crawled out of the Swamp today, could very well fall back in later.

--END QUOTE--
 
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Urs Schreiber

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Today Moritz-Retolaza-Westphal arxiv.org/1809.06618 still aren't convinced of Kallosh-Linde's arguments for saving KKLT arxiv.org/1808.09428. Independent of how this debates turns out, the key point of Moritz-Retolaza-Westphal is simple: You gotta stick to precise arguments.

(possibly more discussion here)
 

Fra

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Thanks for monitoring this frontier and highlighting it here! It is fascinating to see this happening. I cant wait to see what the dual to this twist will be.

/Fredrik
 

Urs Schreiber

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Today the reply from Kallosh and Linde: http://arxiv.org/abs/1809.09018 Am unsure about their logic. It sounds like saying it's not their problem if the embeddings into 10d of their model doesn't work. Instead they feel free to further modify the 4d model.

Thomas von Riet comments (here): "it is distraction from the essence. Their original paper needs to be addressed and not some detail about a 4d SUGRA description failing at some far point in moduli space"
 
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I have avoided this topic since it is a little distant from my interests, but some string doubters excited that "Vafa's conjecture" might falsify string theory, provoked me to look at it again. The reason for the excitement is that the conjecture creates new constraints regarding dark energy, inflation, and so on. In particular, it is being reported that "the Higgs field also has properties that should actually be forbidden by Vafa's conjecture". The people who did the research surely consider this a reason to amend the conjecture, but for the string skeptics, the excitement is that maybe the Higgs boson already falsifies string theory.

What does the paper say is actually inconsistent with the "dS swampland conjecture" as currently formulated? The standard model Higgs... plus quintessence, and with only a minimal coupling between them. In the discussion, they link to some papers which already propose milder versions of the conjecture... But what interests me more is a related paper from Korea, which points out that a similar dS extremum exists for the pion scalar potential too.

To me this seems ripe for producing an outright counterexample to the conjecture as it stands. The Koreans consider (their eqn 26) a scalar potential involving the pion field and the quintessence field. Well, you should be able to get pions from a brane stack, and Vafa proposed that quintessence could just be a modulus associated with a Calabi-Yau handle. It doesn't sound that complicated (for a string theory vacuum): maybe strings compactified on a torus with one or two holes, with a brane or two wrapped around one of the them. So one hole has the pion branes wrapped around it, and the other hole provides the quintessence modulus...
 

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