Stringy Things

Notes on the Swampland (4): The Distance Conjecture for Arbitrary Calabi-Yau Manifolds, the Emergence Proposal, and the de Sitter Conjecture

The following collection of notes is based on a series of lectures that I attended by Eran Palti at SiftS 2019 at Universidad Autonoma de Madrid. The theme of the lecture series was ‘String Theory and the Swampland’. Palti’s five lectures were supported by his most recent and impressive 200 page review paper on the Swampland, which includes over 600 references [arXiv: 1903.06239 [hep-th]]. The reader is directed to this paper in addition to supplementary references that I also provide at the end of each set of notes.

In the following entry, the notes presented follow the fourth and fifth lectures of Palti’s series.

1. Introduction

In this final entry, we approach the conclusion of this collection of notes by focusing on the fourth and fifth of Palti’s lectures. Due to lack of space, we will not cover every topic in lectures four and five. Instead, we shall focus our energy on paying particular attention and detail to one of the most important and interesting subjects of study presented by Palti (from lecture four): namely, the study of the Distance Conjecture in the context of arbitrary Calabi-Yau (CY) manifolds. Then we will conclude these notes by briefly thinking about a few choice cosmological implications of the Swampland (the topic of Palti’s fifth lecture), particularly the de Sitter conjecture in the context of Type IIA string theory on CY with flux and in the context of 11-dimensional supergravity.

But before all this, we spend a short amount of time reflecting on the ‘Emergence Proposal’ (a concept introduced at the end of lecture four) and on some timely issues facing the Swampland programme.

2. A House of Cards? Emergence and the Swampland

As a summary review, let us quickly recall what we have so far emphasised in this series of notes. One of the featured viewpoints to be highlighted in Palti’s lectures is the observation that, among the growing list of Swampland Conjectures, there is ample reason to suggest that the Distance Conjecture and the Weak Gravity Conjecture are among two of the most established in terms of evidence. That is to say, short of complete and formal proof, the amount of evidence supporting these two conjectures in particular is very solid. We have already spent quite a bit of time exploring a number of tests and we have already begun to develop a deeper understanding for why the DC and the WGC are supported by significant evidence.

This emphasis on mathematical testability is important. In general, it does not seem too egregious to admit that the Swampland programme as a while has being experiencing both internal and external controversy. This controversy would seem as much scientific as philosophical. For example, consider the most recent Swampland conference (also held at IFT) that followed a couple months after the summer school from which these notes were originally written. One description of the situation at the September conference is this: the list of conjectures has experienced unrelenting growth but at the result of questionable rigour. In a moment of hyperbole issued for exaggerating effect, we might say that the programme itself is developing analogously as an infinite tower of conjectures. It is a well-established concern among portions of the Swampland research community that we are not proving/disproving conjectures faster than the rate in which new conjectures are being introduced. And from the perspective of this humble student, the situation has reached a point where proof and disproof are imperative.

In listening to and monitoring debates about the programme, I have come to be of the mind that we should proceed with particular caution. The caution is this: there is a genuine concern growing about a lack of mathematical rigour, which would seem verified by the observation that the list of conjectures is growing at a much faster rate than formal proof/disproof. This concern gains further urgency when considering the surrounding sociology, where calls for systematic evidence have been matched with what seems a more generally developing narrative against String Theory / M-theory writ large. It is understandable that the Swampland programme is compelled to react with a mission to make predictions and provide such evidence. But these sorts of commitments may still be premature. Predictions are key, but if we do not even know the theory to be accurate, any predictive claims or evidence would seem to put not just the Swampland but the entire reputation of String Theory at risk. In other words, it would seem reckless to begin contemplating demands for evidence without exhaustive rigour and confirmation of the theory at hand. In the very least, and in the best possible scenario, we have a theory not completely understood. But in either case, to then make predictions on these grounds – on a tower of conjectures, which, at the end of the day could very well be a house of cards – is risky.

But perhaps it is in this context that the moral tone of Palti’s lecture series earlier in the summer (prior to the Swampland conference in September) might be seen to be profoundly insightful and of timely inclination. In Palti’s fourth lecture, for example, the message becomes much more pronounced – that we may take the view that the DC and the WGC are in fact two fundamental pillars of the Swampland. Let me state this slightly differently. In reflecting on Palti’s lectures, to take the view that the entire programme depends on the DC and the WGC, and to map the relation of all the other conjectures from this foundation, it provides clarified view on a programme of proof/disproof.

Furthermore, if what we have done so far is focused on studying and reviewing examples of why the DC and the WGC can be trusted – why, short of complete formal proof – the evidence for these two conjectures is both substantial and inescapable, what we are coming to learn is precisely why both the DC and the WGC are an example of two first-class constraints. Taking this view has consequences. If the DC and the WGC are first-class constraints, it follows that if one understands these two conjectures they may then go on to understand all of the other conjectures. If we can disprove any number of the second-class conjectures, the Swampland programme would not collapse. If, on other hand, we should disprove the DC or the WGC, it is likely the entire programme collapses in on itself. The picture is illustrated quite explicitly in the above image.

The above picture describes what is called the ‘Emergence Proposal’ [1], based, in a sense, on the idea that Swampland Conjectures are consequences of the emergent nature of dynamic fields in quantum gravity. In lecture 4, we learned that if a coherent picture is emerging that outlines the relations between the growing conjectural assertions of the Swampland, a related internal programme of proof/disproof may also most effectively work from the bottom-up. But with the Emergence Proposal (as I currently understand it), not only is there the idea of first and second-class constraints – an idea for how we may perhaps pursue a foundational line of enquiry – another deep idea also comes to the fore: namely, the Swampland constraints are rooted in some underlying microscopic physics to be discovered. We don’t know what defines this microscopic physics, if it exists at all. That is a subject for another time. But we know, currently placed just above it in an overall web of constraints, the DC and the WGC may still offer some direct insight.

On that end, we now turn our attention to one of the deepest tests yet of the DC, beginning with a brief discussion of the refined version of the conjecture.

3. The Distance Conjecture (Refined)

We begin with the following message in mind: already we have seen several tests of the WGC and the DC. Each time, we have focused on increasing the complexity of the test and each time we have found strong evidence that both the WGC and the DC are deeply general. What we want to do now is proceed to review more tests of the DC, this time for even more complex geometry: namely, arbitrary Calabi-Yau manifolds.

Formally, the DC can be understood as follows [2]. As Palti put it in lecture four, consider how if we have a scalar field that is canonically normalised then we have already come to expect that there should be an infinite tower of states that goes something like,

\displaystyle  (\partial \phi)^{2}, \ m \sim e^{- \alpha \phi} \ \ (1)

Indeed, we are starting to understand that the behaviour in (1) would seem a general property of string theory. But we might ask, following Palti, what if the scalar is not canonically normalised? Consider, for instance, the scenario where we have some complicated function {f (\phi)} in front of the kinetic term,

\displaystyle  f(\phi) (\partial \phi)^{2} \ \ (2)

Moreover, let us consider for a moment a theory with a moduli space, {\mathcal{M}} (remember: a moduli space is a space parameterised by the value of some scalar fields). We will make it so {\mathcal{M}} is parameterised by {\phi^{i}}, and we should note that {\phi^{i}} has no potential (typically, this implies that there should be some supersymmetry in the theory). Now, take any point {P \in \mathcal{M}}, where a point in Moduli space is given by the expectation value for the scalar fields {\phi^{i}}. We define another point {Q \in \mathcal{M}} such that, in this set-up, the geodesic proper distance (i.e., the distance is equivalent to the vacuum expectation value in field space) between {P} and {Q} may be denoted as {d(P, D)} (note: we measure the distance using the field space metric in front of the kinetic terms). Crucially, the first statement of the DC says this geodesic distance is infinite, which is to say the scalar field obtains an infinite vacuum expectation value. The second statement describes the behaviour at this infinity. That is to say, the second state describes that there exists an infinite tower of states with mass scale m , such that m(Q) \sim m(P)e^{-\alpha d(P,Q) / M_{P}} as d(P,Q) \rightarrow \infty and where \alpha \sim \mathcal{O}(1) .

This is the key idea. Given two points of great distance in field space – at least greater than the Planck scale – we obtain an infinite tower of exponentially light states.

We have of course already started to become familiar with this statement. The point that ought to be highlighted here, however, is that if we have some basic canonically normalised scalar field {(\partial \phi)^{2}}, then all that we get is the familiar {m \sim e^{-\alpha \phi}}. In more complicated situations, such as when the scalar field is not canonically normalised, the refined DC tells us that we can apply it also to such completely general situations.

In these notes, we will not explore any further an example of the trivial canonical case. Instead, having discussed is the refined distance conjecture [1], what we want do is review whether it holds in the case of arbitrary complex extended structures.

4. Type IIB on Calabi-Yau C3-fold

4.1. Supergravity Set-up

In this example, we invoke Type IIB string theory on a Calabi-Yau C3-fold (i.e., we have a 6-dimensional CY space). In the construction we are about to study, the geometry we will be working with is about as complicated as it gets, so we start with some basics.

We should first note that Type IIB string theory on CY gives {\mathcal{N} = 2} supergravity (SUGRA) in 4-dimensions. Due to limited space, we are not going to establish the supergravity formalism in these notes. The reader is instead directed to ref. [1, 3-5] for an introduction, where, for these notes, we are of course following Palti in ref. [1] quite strictly. Another very important paper, which we will cover in some depth is ref. [6] on infinite distances in field space. In fact, majority of what follows is based on this paper.

Meanwhile, to continue establishing the basics, the general supergravity set-up is this: we have {n_{V}} vector multiplets with bosonic content of a complex scalar field. Similar in a sense to past discussion about the presence of scalar fields with regards to the radius of the circle, in the present case the scalar fields we are interested in studying are complex structure moduli, {t^{i}}, where {i = 1, ..., n_{V}}. These complex structure moduli parameterise the geometry of the CY.

We also have gauge fields, {A^{i}}. These gauge fields are quite interesting, as we will elaborate. For now, note that there is a gravity multiplet which contains a (bosonic fields) graviton and graviphoton, {A^{0}}. All of the gauge fields can be combined such that {I = \{0, i \}} for {A^{I}}.

The number of fields, {t^{i}} and {A^{I}}, is counted by the number of 3-cycles in the CY, which, for a typical CY, is {\sim \mathcal{O}(100)}. This means that for the field space in the effective field theory we find a space with {\sim 100} complex dimensions (and so we have a 200 dimensional field space in total).

Based on previous discussions, one might wonder whether there are charged states under the gauge field {A^{I}}. The answer is that there are charged states, they are BPS states which are {D3-branes} wrapping 3-cycles in the CY. Schematically, the moduli describe the size of the 3-cycle and then they describe the mass of the D3-branes that are wrapping the 3-cycles, behaving like particles in the external dimensions.

Generally, in this set-up, we find an action of the form,

\displaystyle  S_{\mathcal{N} = 2} = \int d^{4}x \sqrt{-g} [\frac{R}{2} - g_{ij} \partial_{\mu} t^{i} \partial^{\mu} \bar{t}^{j} -h_{\sigma \lambda} \partial_{\mu} l^{\sigma} \partial^{\mu} l^{\lambda} + \mathcal{I}_{IJ}\mathcal{F}^{I}_{\mu \nu}\mathcal{F}^{J, \mu \nu} + \mathcal{R}_{IJ}\mathcal{F}^{I}_{\mu \nu} (\star \mathcal{F})^{J, \mu \nu}]  \ \ (3)

The structure of which can be read off beginning with metrics, {g_{ij}} and {h_{\sigma \lambda}}. In totality, the moduli space is split into vector multiplets and hypermultiplets, {\mathcal{M} = \mathcal{M}_{V} \times \mathcal{M}_{H}}. And so, as one would expect even notationally, these two metrics describe two separate manifolds. We are going to focus on the vector multiplets which span a special Kähler manifold, from which we can generalise for the hypermultiplets on the quaternionic Kähler manifold. What is important to note is the periodicity {\{X^{I}, F_{I} \}} for the multiplet field space, in which we are dealing with holomorphic functions of {t^{i}}.

Notice also the gauge kinetic functions, {\mathcal{R}} and {\mathcal{I}}. These both contain real and imaginary parts of a complex matrix.

4.2. Charge Vector and Kähler Potential

It was mentioned that we have D3-branes wrapping 3-cycles. When a certain D3-brane wraps the 3-cycles in the CY, this is labelled by a charge vector {q \in \mathbb{Z}} (of {\mathcal{O}(100)}). This charge vector is in fact a 100-dimensional vector, where each entry is some holomorphic function of the 100’s of scalar fields in our theory. The basic idea, to give some more intuition, is that once we know the charge vector we know the mass of the BPS state, which, again, are the charged states under the gauge fields. Study (4) below,

\displaystyle  m(\underline{q}) = \mid z(\underline{q}) \mid = \mid \frac{\underline{q \eta \underline{\prod}}(t)}{[i \underline{\prod}^{T}(t) \eta \bar{\prod}(t)]^{1/2}} \mid \ \ (4)

Where {\prod} is the period vector. Notice that in the denominator we have complex conjugation as the object {\underline{\prod}^{T}(t)} must be real. Furthermore, all of the geometry of the CY is captured in the period vector {\underline{\prod}(t)}. One can see that it is a function of {t^{i}}. This is because it is a 100-dimensional vector that is an arbitrary function of the complex structure moduli. We should also highlight, for pedagogical purposes, that the expressions for {\eta} and {\prod} are a local choice of basis on the moduli space. Without going into all of the details, the period vector {\prod} can be defined on a local coordinate basis such that,

\displaystyle \prod = \begin{bmatrix} X^{0} \\ x^{i} \\ F_{j} \\ F_{0} \\ \end{bmatrix} (5)

So that the electric index increases down the vector and the magnetic index increases from the bottom-up. The {eta} term in (4) is the natural symplectic form of this multiplet vector space, and so we may indeed construct the appropriate symplectic inner products.

It is not difficult to understand that the field space that we are working with is very complicated. In [6], the metric is given by the derivative of the Kähler potential (also note, much of the same notation and general construction is in this paper, which as with other points discussed can also read in ref. [1]),

\displaystyle  g_{t^{i} \bar{t}^{j}} = \partial_{t^{i}} \partial_{\bar{t}^{j}} K \ \ (6)

Where the {K} is the Kähler potential, {K = -log [i \prod^{T} \eta \bar{\prod}]}. In other words, we have the log of the period vector. This potential is actually very interesting, and one can derive it by considering in general a Kähler potential for some CY manifold, {Y_{D}}, of complex dimension {D} where the complex structure moduli is given by a {h^{D-1, 1} (Y_{D})}-dimensional Kähler manifold. The potential is then generally written as K = -log [-i^{D} \int_{Y_{D}} \omega \wedge \bar{\omega}] in which one finds metric components of the form above. Once one finds the appropriate integral basis, the potential above is found.

4.3. Studying the Field Space

The discussion in this section is based almost entirely on [6], as well as parts of Palti’s summary in lecture 4 and his review in [1]. Additionally, we will be working with a number of very powerful mathematical theorems offered by Wilfried Schmid [7] building on Deligne’s work [8] in Hodge theory. (Please note, while we will not explore a detailed study / re-derivation of some of the theorems found in [7], I am very interested in this work and also in [6] which leverages Schmid’s nilpotent orbit theorem, so I will offer a detailed review in a future post).

In a schematic way, what we want to do is consider some point of infinite distance on this field space. Following Palti in his lecture, we shall label this point by the parameter {t} going to {+i \infty}. We now invoke the theorem that tells us that for such a point the period vector has a monodromy around it. In other words, if we send the real part of {t} to infinity, {\text{Re} t \rightarrow \text{Re} t + 1}, which, in a sense, is like encircling the point at infinity, we have a transformation of the period vector. In fact, we see that the period vector transforms by the action of a monodromy matrix, \prod (t) \rightarrow T_{i} \prod(t) . Then, due to properties studied in [6], we see that each {T_{i}} can be decomposed and, with the monodromy matrix massaged in a way that it only gives its infinite order part, we can define the log of this {T_{i}} in the form of a matrix equation,

\displaystyle  N_{i} = \log T^{u}_{i} = \sum_{k = 1}^{\infty} (-1)^{k + 1} \frac{1}{k} (T^{(u)}_{i} - Id)^{k} = \frac{1}{m_{i}} \log T^{mi}_{i} \ \ (7)

From this, we invoke the nilpotent orbit theorem [7]. With space limited the essentially idea may be summarised in the result that {N} is nilpotent. This means that if we take a high enough power we will get zero, {N^{n+1} = 0, \ n \leq 3}. Moreover, remember that we have sent {t} an infinite distance, and as things are currently constructed we need to know what this point looks like. What Schmid’s theorem in ref. [7] tells us is precisely what the period vector looks like around any point at infinite distance. In fact, it says that the period vector must look like,

\displaystyle  \prod (t) = \exp [t N](a_{0} (S) + \mathcal{O}(E^{2\pi i t})) \ \ (8)

What is this telling us exactly? It says that we have a parameter {t}, and as {t \rightarrow i \infty} we get exponentially small corrections. In other words, because {N} is nilpotent we see in (8) that we get some polynomial in {t}. The vector {a_{0}} depends on the other moduli, but not {t}, and as the exponential term may be neglected we see that we can know the form of the period vector around any point.

There is another theorem in [7], as Palti cites in his lecture, which, using again the nilpotent theorem, tells us if this point is indeed an infinite distance then it must be the matrix {[t N]} does not annihilate the vector {a_{0} (S)}. And so what we have, to be terse, is the following,

\displaystyle  \text{Infinite distance} \longleftrightarrow N^{d + 1} a_{0} \neq 0, \ d > 0 \ \ (9)

Now, all we need to do is take the period vector and this form {[t N]} and plug it into the formulae for the mass of the BPS states and for the metric on the moduli space. What we find is that we must have some local expression near any infinite locus in the moduli space. Schematically, from section 3.2 in ref. [6] we may write,

\displaystyle  g_{t \bar{t}} = \partial_{t} \partial_{\bar{t}} K = \frac{1}{4} \frac{d}{\text{Im} t^{2}} \ \ (10)

Where we have dropped the subleading terms. With the universal leading term only depending on degree {d}, quadratic {1 / \text{Im} t} it is found that the proper field distance is logarithmic when we send {t} to infinity,

\displaystyle  d_{\gamma}(P, Q) = \int_{Q}^{P} \sqrt{g_{t \bar{t}}} \mid dt \mid \sim \frac{\sqrt{d}}{2} \log (\text{Im} t) \ \ (11)

From which it is found that, in the case of a CY compactification that preserves {\mathcal{N} = 2} supersymmetry the BPS states become massless at the singularity point. More technically, in the paper these singular points have to do with what the author’s study as infinite quotient monodromy orbits. But for our purposes we note in particular for the mass,

\displaystyle  M_{q} \sim \frac{\sum_{j}\frac{1}{j!}(\text{Im} t)^{j} S_{j}(q, a_{0})}{(2^{d} / d!)^{1/2} (\text{Im} t)^{d/2}} \ \ (12)

In other words, as Palti motivates it, we see that the D3-branes become massless as the imaginary part goes to infinity. The behaviour of the mass is argued to be universal for any massless BPS states. Furthermore, what is observed is the presence of a power law in {t} whilst the proper distance is logarithmic in {t}. If we consider some path, {\gamma}, as implied in (11), the effective theory at two points (P, Q) in the moduli space approach singularity. The mass of the BPS states decreases exponentially fast in the proper distance. And so, in a schematic way in these notes, we may describe this in the form of {\Delta \phi \sim \log t} and M \sim \frac{1}{t^{\alpha}} \sim e^{-\alpha \Delta \phi}, which is just the Distance Conjecture and the Weak Gravity Conjecture at work.

We have of course been crude in our description, and there is a subtlety about the state not necessarily being confirmed in the theory, with the need remaining that one must show the BPS states being in the spectrum. Perhaps a detailed individual post would be beneficial in the future. For now, we can say that in [6] the case is shown for when {d = 3}. For our current purposes, the result is notable it shows that the DC and WGC hold for any CY compactification for Type IIB string theory. And this result should not in any way be understated. Altgough we are dealing with a very complicated 100-dimensional field space, the fact the it can be proven mathematically that both of these first-class Swampland conjectures hold for any CY compactification – and that very powerful mathematical theorems tell us this is necessarily true – we are driven directly toward the suggestion of some deeply general physics.

5. de Sitter Conjecture

5.1. Introduction

To conclude this series of notes, and to celebrate what has been a fairly lengthy and detailed engagement with Palti’s lectures at IFT this past summer, we turn our attention to a brief discussion on some of the cosmological implications of the Swampland. We will not discuss things like tensors modes in inflation or other topics covered in the lectures, which can be easily reviewed in [1]. Instead, we begin with a brief review of the de Sitter Conjecture, which states that the gradient of the potential is bounded,

\displaystyle  \mid \nabla V \mid \geq \frac{c}{M_{P}} V \ \ (13)

In other words, the scalar potential of the theory must satisfy (13) or the refined version below,

\displaystyle  \text{min} (\nabla_{i} \nabla_{j} V) \leq - \frac{c^{\prime}}{M_{P}^{2}}V \ \ (14)

Where this second condition is based on or motivated by entropy arguments. There are a number of connections between the de Sitter conjectures and ongoing experiments, including dark energy constraints and constraints from inflation. Interaction with experimental observation is quite active here, as Palti summarises. What we shall focus on is what motivates the de Sitter conjecture from string theory.

5.2. Evidence of the de Sitter Conjecture – Type IIA on CY with Flux

What follows is based on a simplified version of the more general study in ref. [8], where flux compactifications of Type IIA string theory are considered and the author’s study the classical stabilisation of geometric moduli. The main idea that we consider in general is that we want to switch on the fluxes for the background CY and then we study them from the perspective of the 4-dimensional effective theory. That is to say, we study the potential from the fluxes in the 4-dimensional theory. In the referenced study there are two fields in the low-energy effective theory. More precisely, there are two moduli fields that parameterise the geometry of the CY, \rho = (vol)^{1/3} , which is the volume of the CY and another field, \tau = e^{-\phi} (vol)^{1/2} , which is the string coupling times the volume of the CY. As a result of the flux being switched on, these two fields will have some potential.

Now let us consider the canonically normalised fields,

\displaystyle  \hat{\rho} = \sqrt{\frac{3}{2}} M_{P} \ln e \ \ (15)

\displaystyle  \hat{\tau} = \sqrt{2} M_{P} \ln \tau \ \ (16)

As these fields are canonically normalised, we may write the following Lagrangian in the Einstein frame,

\displaystyle \mathcal{L} = \frac{M_{P}^{2}}{2} R - \frac{1}{2} (\partial \hat{\rho})^{2} - \frac{1}{2} (\partial \hat{\tau}) + ... V(\rho, \tau) \ \ (17)

Now, the featured point here is that the potential is of course quite complicated. We can include any number of things to generate the potential – for example, we can turn off and on certain RR-fluxes or a combination of fluxes. What is interesting is that, in playing with different scenarios, a number of general properties are found. For instance, consider the case of turning on only certain RR-fluxes, where we have an expectation value for the p-form field strength, and also the H-flux which is the field strength of the NS sector,

\displaystyle \text{RR-flux:} \ V_{p} \sim \rho^{3 - p} \tau^{-4} \ \ (18)

\displaystyle \text{H-flux:} \ V_{3} \sim \rho^{-3} \tau^{-2} \ \ (19)

And with these contributions, we can also have in this case D6-branes and 06-branes that contribute to the potential,

\displaystyle V_{D6} \sim \tau^{-3} \sim V_{06} \ \ (20)

It turns out that, completely generally (regardless of the fluxes we switch on or off, their combination, and the branes we choose), the potential always takes the form,

\displaystyle V = \frac{A_{3} (\phi^{i})}{\rho^{3} \tau^{2}} + \sum_{p} \frac{A_{p} (\phi^{i})}{\rho^{3 - p} \tau^{4}} + \frac{A_{}}{\tau^{3}} \ \ (21)

Where in the first two terms in the equality we have in the numerator some function of the other fields included in our theory over the contribution from the H-flux and RR-flux, respectively. In the last term, there is a contribution from localised sources in the numerator over the brane contribution. This is the most general form the potential can take, even when we consider the inclusion of hundreds of other fields.

Inspecting the general form of the potential (21), we may consider the following combination of derivatives,

\displaystyle -\rho \frac{\partial V}{\partial \rho} - 3\tau \frac{\partial V}{\partial \tau} \ \ (22)

It turns out that, in fact,

\displaystyle -\rho \frac{\partial V}{\partial \rho} - 3\tau \frac{\partial V}{\partial \tau} = 9V + \sum_{p} pVp \ \ (23)

Where {pVp} are positive components of the potential and so the following statement is made that, {9V + \sum_{p} pVp \geq 9V}. But what does this mean? Well, if we write this in terms of the canonically normalised fields,

\displaystyle M_{P} \mid \sqrt{\frac{3}{2}} \frac{\partial V}{\partial \hat{p}} + 3\sqrt{2} \frac{\partial V}{\partial \hat{\tau}} \mid \ \geq 9V \ \ (24)

We notice something striking. If, moreover, we consider the gradient of the potential as it also pertains to the statement made in the de Sitter Conjecture, notice that after some work we can go from a completely general statement to the below,

\displaystyle M_{P} \mid \nabla V \mid \geq M_{P} \mid \frac{\partial V}{\partial \hat{p}} +\frac{\partial V}{\partial \hat{\tau}} \mid \geq \frac{27}{13} V, \ \ \nabla V > 0 \ \ (25)

Where we see that the de Sitter Conjecture has been satisfied. As it is a completely general result for any choice of fluxes and any choice of branes for the given compactification, this result is quite striking. In other words, regardless of the complexity of the potential, there is also a lower bound to it.

6. 11-dimensional Supergravity

But what about other scenarios? Let us consider one last example, namely 11-dimensional supergravity and quickly think about what sort of potentials may be generated.

We start by noting the Maldecena-Nunez no-go theorem, which tells us that there is no de Sitter vacua in compactifications of 11-dimensional SUGRA down to any dimension. Moreover, it is shown in [10] that for 11-dimensional SUGRA on a smooth manifold compactified down to d-dimensions there is once again a lower bound which may be written as follows,

\displaystyle \frac{\mid \nabla \mid}{V} \geq \frac{6}{\sqrt{(d-2)(11-d)}} \ \ (26)

This is consistent with the de Sitter conjecture. But there are caveats, such as when orientifolds are present, as once again summarised [1]. The main point, with (13), (14), and (26) in mind, is that it is very difficult, if not somewhat extraordinary, to evade these constraints. The statement here is not that it is impossible, but that it is very difficult. Most notably, one is required to use stringy ingredients. For instance to violate these constraints you can include,

* Orientifolds (without D-branes and so where charges cannot be cancelled locally) – i.e., `naked’.

* Higher derivative corrections

* Type IIA with orientifolds / something not CY

* Quantum corrections – i.e., quantum vacuum (large, like KKLT)

But these all imply a level of great difficulty, pertaining to the use of stringy ingredients of which we do not yet have a great understanding. So this is one problem, which already requires great consideration. But there is another, which refers to the Dine-Seiberg problem [11], and when combined with the first means one has to work doubly hard. The basic idea with the latter is that the source of the potential vanishes when {g_{s} \rightarrow 0}. Moreover, it says in the weakly coupled regime there is a non-interacting theory, and so any fluxes etc. vanish. This is a very generic statement; it applies to any point in the Hilbert space where many possible light tower of states may dominate. Consider, for example, a potential subject to the above statement regarding the string coupling in some expansion,

\displaystyle V \sim g_{s}^{n} + \sum_{k=1}^{\infty} g_{s}^{n+k}C_{k}

Now, imagine the expansion is controlled. To leading order,

\displaystyle V \sim g_{s}^{n} \sim e^{-n\phi} + \text{small corrections} \ \ (27)

With only small corrections in the well controlled limit such that {g_{s} << 1}. If the potential looks like {e^{-n\phi}} then one can quickly work out,

\displaystyle  \mid \partial_{\phi} V \mid \sim nV \ \ (28)

Which satisfies the conjecture. But as Palti points out, one can always fight this with coefficients, say, for instance, with some potential,

\displaystyle  V = Ag_{s} + B g_{s}^{2} + cg_{s}^{2} + ..., \ \ g_{s} << 1 \ \ (29)

Which is what people do when performing flux compactifications. As we know, we can always play with the fluxes and other things which corresponds in the above expansion to playing with the coefficients. So we can consider A and B and chose that {\frac{B}{A} > \frac{1}{g_{s}}} for which it is possible to then have these fields in minimum balance against each other. But then what of the C coefficient? One must ensure that this doesn’t takeover, so we could say {c \sim B}. But what the Dine-Seiberg argument says that if {A \sim B \sim C \sim O(1)} then we will never find the minimum to the potential, because {Ag_{s}} must be the leading term and we end up with a runaway direction in the field space. That is why for flux compactifications a general approach is to balance the coefficients by playing with the fluxes so that we can get a minimum for the potential.

We can see clearly that the situation is one where we have to overcome both problems, the no-go and the Dine-Seiberg problem, in order to show a de Sitter vacuum in string theory. One interpretation is that both the Maldecena-Nunez no-go theorem and the Dine-Sieberg problem motivates the de Sitter conjecture: i.e., string theory does not foster or does not like de Sitter vacua. But another, perfectly legitimate interpretation is that all that these two accounts are saying is that we just have to work very hard to obtain a de Sitter vacuum in string theory. For the no-go theorem, for example, to evade it requires working with stringy ingredients that we do not yet have much understanding of – such as working with naked orientifolds or in the case of higher derivative corrections. And so maybe the reality of the situation is not best described by the de Sitter Conjecture but instead motivates the need for even deeper thinking in string theory. In time, which of these interpretations is correct will likely clarify.

References

[1] E. Palti, `The Swampland: Introduction and Review’, [arXiv:1903.06239v3[hep-th]].

[2] H. Ooguri and C. Vafa, On the Geometry of the String Landscape and the Swampland, Nucl. Phys. B766 (2007) 21–33, [hep-th/0605264 [hep-th]].

[3] A. Ceresole, R. D’Auria, and S. Ferrara, The Symplectic structure of N=2 supergravity and its central extension, Nucl. Phys. Proc. Suppl. 46 (1996) 67–74, [hep-th/9509160 [hep-th]].

[4] L. Andrianopoli, M. Bertolini, A. Ceresole, R. D’Auria, S. Ferrara, P. Fre, and T. Magri, N=2 supergravity and N=2 superYang-Mills theory on general scalar manifolds: Symplectic covariance, gaugings and the momentum map, J. Geom. Phys. 23 (1997) 111–189, [hep-th/9605032 [hep-th]].

[5] J. Polchinski, ‘String Theory: Superstring Theory and Beyond’, Vol. 2. (2005).

[6] T. W. Grimm, E. Palti, and I. Valenzuela, ‘Infinite Distances in Field Space and Massless Towers of States’, JHEP 08 (2018) 143, [arXiv:1802.08264 [hep-th]].

[7] W. Schmid, ‘Variation of Hodge structure: the singularities of the period mapping’, Invent.
Math. , 22:211–319, 1973.

[8] P. Deligne, Theorie de Hodge: III, Publications Mathematiques de l’IHES´ 44 (1974) 5–77.

[9] O. DeWolfe, A. Giryavets, S. Kachru, and W. Taylor, ‘Type IIA moduli stabilization’, JHEP 07 (2005) 066, [hep-th/0505160 [hep-th]].

[10] J. M. Maldacena and C. Nunez, ‘Supergravity description of field theories on curved manifolds and a no go theorem’, Int. J. Mod. Phys. A16 (2001) 822–855, [hep-th/0007018 [hep-th]].

[11] M. Dine and N. Seiberg, ‘Is the Superstring Weakly Coupled?’, Phys. Lett. 162B (1985) 299–302.

Standard
Swampland Conjectures
Stringy Things

Notes on the Swampland (3): Testing the Weak Gravity Conjecture – Gauge Fields, Dp-branes, Type II Strings, and F-Theory-Heterotic Duality

The following collection of notes is based on a series of lectures that I attended by Eran Palti at SiftS 2019 at Universidad Autonoma de Madrid. The theme of the lecture series was ‘String Theory and the Swampland’. Palti’s five lectures were supported by his most recent and impressive 200 page review paper on the Swampland, which includes over 600 references [arXiv: 1903.06239 [hep-th]]. The reader is directed to this paper in addition to supplementary references that I also provide at the end of each set of notes.

In the following entry, the notes presented follow the third lecture of Palti’s series.

1. Introduction

In this collection of notes, we look to review some more basic tests of the Weak Gravity Conjecture. In the last entry, recall that we reviewed a basic relation between the WGC and the Distance Conjecture. We then considered a first test of the Distance Conjecture having compactified our theory on a circle. Additionally, we reviewed evidence for the DC where we found that if we have large expectation values for the scalar fields in string theory, we obtain an infinite tower of exponentially light states. In this sense, we also reviewed the extreme parameter regime for weak and strong coupling. Finally, we reviewed a number of lessons about the DC and T-duality, concluding with a brief review of the parameter space of M-theory.

In the present entry – the third in this series of notes – we continue to expand on past discussions, turning particular attention to another basic test of the WGC. In further testing of the WGC we will also focus on a number of related topics ranging from gauge fields to Dp-branes and Type II strings, ending with a few brief comments on F-theory {\longleftrightarrow} Heterotic duality. This will then lead us directly into the fourth and second-last entry of the series, where we will begin to review more advanced tests of the DC and WGC, using for instance arbitrary Calabi-Yau manifolds.

2. Weak Gravity Conjecture

In this section we return to the WGC, which we have already grown to understand as being closely related to the DC. Following Palti’s lecture series, although the WGC is studied quite extensively from the infrared point of view, we shall instead be studying it from the ultraviolet and maximally stringy perspective.

Proceeding directly from the last entry we return to the simple example of string compactification on a circle and consider some of the physics in [3] as discussed in [1]. This time, in compactifying on {S^{1}}, we are going to instead consider a more general solution for the metric. The reason for this is because we want to study in particular the case of compactification with gauge fields. The metric may be written as follows,

\displaystyle  ds^{2} = e^{2 \alpha \phi}g_{\mu \nu}dX^{\mu}dX^{\nu} + e^{2 \beta \phi}(dX^{d} + A_{\mu}dX^{\mu})^{2} \ \ (1)

A few comments are necessary before proceeding. First, remember that we are working in perturbative superstring theory, so this metric is very similar to the one before, where the first term in the equality is a 9-dimensional object. Second, also remember from the last entry that our original metric encoded the parameter {\phi} such that it became a dynamical field in the lower d-dimensional theory. But, as Palti notes, there is also an additional degree of freedom in the metric. What does this mean? This additional degree of freedom becomes a U(1) gauge field {A_{\mu}} in the d-dimensional theory, as opposed to a scalar field, which will also have a coupling {g}. Furthermore, in that we have added another component to the metric, namely the 9-dimensional {A_{\mu}} term on the right-hand side, this is in fact the graviproton. Altogether, it follows that this is the most general solution for stringy compactification on a circle.

Now, what is of present interest is the Ricci scalar. So let’s look at what dimensional reduction now gives for the Ricci scalar,

\displaystyle  \int d^{D}X \sqrt{-G}R^{D} = \int d^{d}X\sqrt{-g} [R^{d} - \frac{1}{2}(\partial \phi)^{2} - \frac{1}{4}e^{-2(d - 1)\alpha \phi}F_{(A), \mu \nu} F^{\\mu \nu}_{(A)}] \ \ (2)

Where {F_{(A), \mu \nu} =\frac{1}{2} \partial_{[\mu}A_{\nu]}} is the gauge field kinetic term or, in other words, the field strength of the gauge field. Recall, also, from before that the {\phi} in the exponential is related to the radius in the extra dimensions. So from (2) we can read off the gauge coupling for the U(1) gauge field as follows,

\displaystyle  g_{(A)} = e^{d - 1}\alpha \phi = \frac{1}{2 \pi R} (\frac{1}{2 \pi R})^{\frac{1}{d - 2}} \ \ (3)

Which is telling us, similar to the last entry, that if we make the circle very large the theory becomes weakly coupled. But what is the symmetry of the U(1) gauge field? How do we know that symmetry of the gauge field? Consider a general U(1) gauge symmetry transformation of the form (i.e., the circle isometry),

\displaystyle  A_{\mu} \rightarrow A_{\mu} - \partial_{\mu} \lambda (X^{\nu}), \ \ X^{d} \rightarrow X^{d} + \lambda (X^{\nu}) \ \ (4)

Where {\lambda (X^{\nu})} is a local gauge parameter. Notice that the metric remains invariant, and from this we can indeed see how lower d-dimensional theory has a U(1) gauge field with the above gauge coupling.

Now, just like in the past entry, we want to look at the Kaluza-Klein expansion. Moreover, recalling the KK expansion for the higher D-dimensional field {\Psi (X^{\mu}) = \sum_{n = -\infty}^{\infty} \psi_{n} (X^{\mu})e^{2\pi i n X^{d}}}, notice that the gauge transformation (4) reveals that the KK modes {\psi_{n}} obtain a charge under the U(1) gauge field. This charge is quantised, as anticipated, and for the nth KK mode it may be given as,

\displaystyle  q_{n}^{A} = 2\pi n \ \ (5)

But what is the relation between the charge and the KK modes? Note, firstly, that the charge of {\psi_{n}} are just the phases of these objects. Secondly, the emphasis at this point in Palti’s talk is to remember that the mass of the KK states calculated in a past entry in the Einstein frame, {M^{2}_{\text{n kk mode}} = (\frac{n}{R})^{2} (\frac{1}{2 \pi R})^{2 \ d - 2}}, is related to the charge. More pointedly, we are already familiar with how, for the KK modes, there is an infinite tower of states. We see that the mass increases along this tower, and so too does the charge. In other words, it is argued that we have a charge-mass relation for the infinite tower of states. Here it is for arbitrary {n},

\displaystyle  g_{(A)} q_{(n)}^{(A)} = M_{n, 0} \ \ (6)

This relation between the charge, mass, and couping may have already been anticipated. Since all we’ve considered here is really just a reduction of Einstein gravity, let us consider the effective string action from a past set of notes, written below for convenience,

\displaystyle  S_{D} = 2\pi M_{s}^{D - 2} \int d^{D} X \sqrt{-G}e^{-2 \phi} (R - \frac{1}{12} H_{\mu \nu \rho} H^{\mu \nu \rho} + 4\partial_{\mu} \Phi \partial^{\mu} \Phi) \ \ (7)

If we compactify this action on a circle, as we are so inclined, there is a gauge field obtained from the gravitational sector. This is similar to before, and is nothing new. What is new is that we also now obtain a second gauge field, {V_{\mu}}, which comes from the Kalb-Ramond B-field with a single index in the {X^{d}} direction. For this Kalb-Ramond field we may write,

\displaystyle  V_{\mu} \equiv B_{[\mu d]} \ \ (8)

Where we note that, generally, {B_{[mn]}} is an antisymmetric 2-form. If we also reduce {B_{[mn]}}, this also leads to a gauge field. Additionally, look at {V_{\mu}} in (8). The kinetic terms for this additional gauge field are produced by the dimensional reduction of the kinetic terms from the Kalb-Ramond field. In other words, we can compute the kinetic term for the gauge field, {V_{\mu}}, as it comes from the strength of the 2-form in 10-dimensions,

\displaystyle  \int d^{d}X \sqrt{-g} [R^{d} - \frac{1}{4}e^{-2(\alpha + \beta)\phi}F_{(V), \mu \nu}F^{\mu \nu}_{(V)}] \ \ (9)

The factor in front of the kinetic terms is produced when we reduce {\sqrt{-G}H_{\mu \nu \rho}H^{\mu \nu \rho}}. From (9) one can again read off the gauge coupling,

\displaystyle  g (v) = e^{(\alpha + \beta)\phi} = 2\pi R (\frac{1}{2 \pi R})^{\frac{1}{d - 2}} \ \ (10)

What is different here? Notice, if we now make the circle of radius {R} very large, we obtain a strongly coupled theory. So, in taking from what we reviewed in the last entry, we know that charges under this gauge field are the winding modes of the string. That is, we have stringy or indeed quantum gravity states. Moreover, think about how if we take the basic Polyakov action for a string wrapping in the {X^{d}} direction {w} times in the Einstein frame, which means that we can set {\sigma = \frac{2\pi}{w}X^{d}}, then notice we have

\displaystyle S_{P} = -\frac{T}{2} \int_{\sum} d\tau d\sigma [2i V_{\mu} \partial_{\tau} X^{\mu} \partial_{\sigma} (\frac{w\sigma}{2 \pi})]

\displaystyle  = -i\frac{w}{2 \pi \alpha^{\prime}} \int_{\gamma} d\tau (\partial_{\tau} X^{\mu})V_{\mu} \ \ (11)

Which is the worldline action for a charged particle,

\displaystyle  q_{w}^{(V)} = \frac{w}{2 \pi \alpha^{\prime}} (2\pi R)^{\frac{2}{d - 2}} \ \ (12)

Or we can think of this in another way by remembering that if we have some antisymmetric form of rank {n}, there is going to be some object coupling to it. Hence, we may notice that, if we integrate some Kalb-Ramond 2-form on the string worldsheet, where the 2-form has one leg along the 9th direction and one leg along the extra dimension, and if we consider a string winding around the extra dimension, we find the string worldsheet is just a worldline in the 9th direction times a circle. If we then perform the integral along the extra direction, we obtain the coupling {V_{\mu}}. And so, we may write,

\displaystyle  \int_{\sum = C \times S^{1}} B_{[\mu d]} dX^{\mu} \wedge dX^{d} \sim \int_{C} V_{\mu} \ \ (13)

Where a worldline coupled to a gauge field means that, as in (4.11), we have a particle in the lower d-dimensional theory. What this is telling us is that winding modes in the d-dimensional theory produce charged particles that are gauge fields under the Kalb-Ramond field. Consider again (4.12), we find once again a relation between the coupling, charge, and mass, except this time it is for the winding modes. These are interesting relations,

\displaystyle  g_{(V)}q_{w}^{(V)} = M_{0, w} \ \ (14)

Which are strictly stringy – or quantum gravitational – in nature. Moreover, what we are discovering are what appear to be deeply general relations, where there is always some particle with a relation between its charge and its mass. And if these relations are, in fact, deeply general, then this means they are also intrinsic properties of quantum gravity. We will investigate this idea more deeply in the context of the Swampland in a moment.

In the meantime, also notice something else that is interesting. If we send the gauge coupling to zero (either by making the circle small or large), {g \rightarrow 0}, we obtain an infinite tower of light states. But this is just a special case of the DC, emphasising again the relation between the DC and the WGC. Furthermore, notice that the gauge coupling depends on the scalar field. So should we want to go to weak coupling, we must give the scalar a large expectation value that directly implies an infinite tower of states.

Also notice that, in the context of our wider discussion in these notes, there is a noticeable symmetry in the theory, which until now has been left implicit; because we can exchange the two gauge fields and also the KK and winding modes. This is T-duality.

3. Quick Review: Type IIA String Theory

Let us quickly review another example and think about Type IIA string theory (from the last entry). Remember, Type IIA in the strongly coupled regime is just 11-dimensional supergravity reduced on a circle. Also remember, in thinking of the Type IIA string we have a massive Ramond-Ramond 1-form, {C^{(1)}}, which is just a gauge coupling that is the graviphoton. The gauge group is U(1) and, it follows,

\displaystyle  g_{C^{(1)}} \sim \frac{1}{g_{s}^{3/4}} \ \ (15)

The states charged under this gauge field? A D0-brane, with a D6-brane representing the magnetic dual. Again, we find the following mass-charge relation,

\displaystyle  M_{D0} = g_{c^{(1)}} q_{D0} \ \ (16)

So, as Palti summarises, we have another piece of evidence that the mass-charge-coupling relation is indeed general. And, in fact, the more we search the more we become convinced this relationship is a property of quantum gravity.

4. Weak Gravity Conjecture (d-dimensions)

These considerations bring us to a more formal definition of the WGC than what we have so far previously offered. Consider the following: take a theory coupled to gravity with a U(1) gauge coupling, {g},

\displaystyle  S = \int d^{d}X \sqrt{-g} [] (\frac{M_{p}^{d}}{2})^{d-2}R^{d} - \frac{1}{4g_{s}^{2}} F^{2} + ... ] \ \ (17)

For the Electric WGC, there exists a particle with mass {m} and charge {q} satisfying,

\displaystyle  M \leq \sqrt{\frac{d - 2}{d - 3}} gq (M_{p}^{d})^{\frac{d - 2}{2}} \ \ (18)

And for the Magnetic WGC, the cutoff scale of the effective theory is bounded from above by the gauge coupling, such that we have the general statement,

\displaystyle  \Lambda \lesssim g(M_{p}^{d})^{\frac{d - 2}{2}} \ \ (19)

Where the cutoff, as we understand, should correspond to the mass scale of an infinite tower of charged states. It is argued to be completely general.

5. Testing the WGC: The Heterotic String

Following Palti, let’s now consider testing the WGC even more than what we have done previously. For example, a leading question might be: Is the WGC true for the Heterotic string? The first formal test of the WGC was for the Heterotic string on a {T^{6}} [3]. Again, much of the following discussion also echoes [1], where a summary with additional pedagogical references can be found.

One of the first things we must consider is that we have the non-abelian gauge group {SO(32)}. This is important to note because compactifying on a {T^{6}} yields the following 4-dimensional gauge fields: {U(1)^{28}}. To understand why there are 28 U(1) gauge fields, simply remember that a {T^{6}} may be thought of as a product of 6 circles. In 4-dimensions we obtain 12 gauge fields from the metric and the Kalb-Ramond field. We may break these up into 6 {B_{[mn]}} yielding 6 U(1)’s and 6 graviphotons. Additionally, particular to the Heterotic string is a 10-dimensional gauge group. This gauge group may be broken by Wilson lines on a circle to its Cartan subalgebra. That is to say, if we have a circle and take a gauge field on that circle, this will give us a Wilson line to which we can then give an expectation value. The Wilson line will break the non-abelian group to its Cartan subalgebra. For these reasons, one can see what the Cartan subalgebra gives {U(1)^{16}}.

Let us focus on these last 16 U(1) gauge fields that come from breaking the {SO(32)} gauge group. The states charged under these are string oscillators {\underbar{q} = (q_{1}, ..., q_{16})} from which we once again obtain an infinite tower of states. The first massive excitation is the {SO(32)} spinor with mass,

\displaystyle  m^{2} = \frac{4}{\alpha^{\prime}} \ \ (20)

When we compactify on a {T^{6}} we obtain charged states that correspond to the 16-dimensional charge vectors,

\displaystyle  \textbf{q} = (\pm \frac{1}{2}, ..., \pm \frac{1}{2}) \ \ (21)

The idea now is to consider how, in the Einstein frame, and working in Planck units, we have the following gauge coupling for any of the U(1) gauge fields,

\displaystyle  g^{2} = g_{s}^{2} = \frac{2}{\alpha^{\prime}} \ \ (22)

In which the gauge coupling is equal to the string coupling, and where {\alpha^{\prime}} depends on the expectation value of the dilaton. To put it explicitly, we have a dilatonic coupling. And, so, in terms of the bound set by the WGC for the mass the following inequality is satisfied,

\displaystyle  m^{2} \leq g^{2} \mid \textbf{q} \mid^{2} = \frac{8}{\alpha^{\prime}} \ \ (23)

Which is the limit of the expectation values of the small Wilson lines. As Palti notes, an interesting further test would be for arbitrary Wilson lines, but what he focuses on in his presentation is the way in which the entire analysis may be generalised for the complete {U(1)^{28}} gauge fields in which the U(1)’s from the {T^{6}} are included. So now we consider the mass of the higher oscillator modes,

\displaystyle  m^{2} = \frac{2}{\alpha^{\prime}} (\mid \underbar{q} \mid^{2} - 2) \ \ (24)

For which, in his talk, Palti gives the possible charges,

\displaystyle  \textbf{q} = (q_{1} + \frac{c}{2}, ..., q_{16} + \frac{c}{2}) \ \ (25)

Where {q_{i} \in \mathbb{Z}} and {c = 0,1}. In that the charges should be integer, they must satisfy the lattice condition {\mid \underbar{q} \mid^{2} \in 2N}.

Now, the whole point of the analysis up to the present is to consider the mass-charge relation. And, in fact, what we find is the following mass-to-charge ratio,

\displaystyle  \mid \textbf{z} \mid^{2} = \frac{\mid \textbf{q} \mid^{2}}{\mid \textbf{q} \mid^{2} - 2} \ \ (26)

Or, to put the matter differently, notice in (24) the {\frac{2}{\alpha^{\prime}}} factor is just {g_{s}^{2}}, and {g_{s}^{2} = \frac{m^{2}}{M_{P}^{2}}}. And so,

\displaystyle  \frac{m^{2}}{g^{2} \mid \textbf{q} \mid^{2}} = \frac{\mid \underbar{q} \mid^{2} - 2}{\mid \underbar{q} \mid^{2}} < 1 \ \ (27)

Where we find quite explicitly that the mass is bounded by the charge for all of the states. This again satisfies the WGC, where, for all the U(1)’s, the mass is less than the charge. We also find that there is an infinite tower of states charging at {g}, and as we go further up the tower (so to speak) the bound in (27) becomes saturated but never violated. So all of our results so far are consistent, and the WGC indeed proves true for the Heterotic string.

6. What About Other Gauge Fields?

The following question we might now ask, as Palti motivates it: what other gauge fields might we consider? So far we have consider some fairly straightforward or simple examples. Can we continue to generalise?

6.1. Testing the Electric WGC: Open String U(1)’s

Another U(1) we get in string theory is an open string U(1), which, considering again Dp-branes, it is a U(1) gauge field on the world-volume. D-branes of course live in Type II string theory, so we could in general consider Type IIA/IIB on {\mathbb{R}^{1, (q - n)} \times T^{6}}, where there is equal radius for the torus. The D-brane can be thought of as filling the non-compact spacetime. In considering string theory on this background, take in particular a Type IIB on a {T^{6}} with 6 circles of radius R as an example. We therefore have some 4-dimensional {M_{1,3} \times T^{6}}, and what we want to do is specifically put a D3-brane with its 4-dimensional world-volume completely in the {M_{1,3}} external spacetime. The D3-brane of course carries U(1), so we therefore now have a U(1) gauge symmetry in our 4-dimensional theory.

Now, with the scenario partly constructed, notice we only have one spacetime filling D-brane, which, impliedly, means that we have some fundamental open string with its endpoints ending on this brane. But this is not consistent. Why? The gauge symmetry we have included is an open string gauge symmetry, and so it is a gauge symmetry being carried by the non-perturbative D3-brane. But if we have just the single D3-brane, it will source the charge inside the 6-dimensional torus, and, one way to put it is that this scenario is akin to inserting a charged particle in a confined space in which there is nowhere for the field lines to propagate. In other words, we have a U(1) neutral state; but D-branes also source R-R fields. This is one of the great facts about D-branes, because insofar that they carry R-R charges, this gives string theory its power of being able to have a source for every gauge field [8]. In our current construction, however, the presence of the D-brane means that it will provide a source in the compact {T^{6}} whilst we lack an appropriate sink for the R-R field lines. This is obviously a problem because the field lines must end somewhere. This is why Palti points in another direction in his talk.

One option is that we could add an anti-brane; but means that the branes will then annihilate one another and, as this is an unstable configuration, it doesn’t really remedy the situation. Instead, the solution is based on a well known fact that orientifold planes are sinks for R-R charge. We might therefore instead introduce the needed negative charges by way of invoking orientifold planes. In doing so, this implies that the spectrum now also contains unoriented strings. These unoriented strings have charge 2 under the U(1), as, under orientifold involution, they stretch between the D-brane and its image. With this configuration, we have a consistent construction, which, with the presence of the orientifold, then means we have a second D3-brane as illustrated below.

In considering the scenario we have constructed, the actual states being charged under the U(1) are open strings whose endpoints end on the D3-branes with a charge {+1}.

Now let us think more deeply about the scenario in relation to the WGC. Is it not possible to violate the WGC? For instance, if the state has charge {+1}, what if we pull the D3-branes apart (i.e., moving away from the orientfold)? The string that is already stretched between the D3-branes would stretch even more over some spatial distance. This would make it massive. But what of the charge? Well, the charge would remain constant. On first inspection, this would seem to violate the WGC. Let us quantify these ideas as follows.

In {D=10}, the relation between the string scale and the Planck scale can be found as (from dimensional reduction and re-writing everything in Planck units),

\displaystyle  M_{s}^{2}g_{s}^{-2} (RM_{s})^{6} \sim M_{P}^{2} \ \ (28)

And the gauge coupling on the D3-brane is simply,

\displaystyle  g \equiv \sqrt{g_{s}} \ \ (29)

Now, for the stretched string, the mass is given as

\displaystyle  m^{2} \sim (RM_{s})^{2}M_{s}^{2} \sim \frac{g_{s}^{2}M_{P}^{2}}{(RM_{s})^{4}} \ \ (30)

Rearranging (30) it can be found that,

\displaystyle  \frac{m^{2}}{g^{2}_{s} M_{P}^{2}} \sim \frac{g_{s}}{(RM_{s})^{4}} \ \ (31)

If the main task was to try and violate the WGC by stretching the string to great length, as we pull the D3-branes away from the orientfold, the question is: have we succeeded? More precisely, to violate the WGC (31) would have to be greater than 1. Is this the case? No, it is not! The reason is because, if we’re working in the perturbative string description – i.e., the controlled weak-coupling regime – than the coupling {g_{s}  1}. So, in fact, the WGC is satisfied. That is,

\displaystyle  \frac{m^{2}}{g^{2}_{s} M_{P}^{2}} \sim \frac{g_{s}}{(RM_{s})^{4}} < 1 \ \ (32)

As we stretch the string and make it massive, with the orientfold growing very large, the gauge coupling does not change. What we are doing, in effect, is diluting gravity. What’s more, we are diluting gravity faster than the mass can increase. And, it turns out, when {M_{P} \rightarrow \infty} we obtain a weakly coupled theory.

6.2. In General for different cases of {n}

Notice that, in general, the scenario constructed above may be considered in terms of compactification of Type IIA/B string theory on {4\mathcal{R}^{1, 9-1} \times T^{n}}. We considered the case for {n >2} when we compactified on a {T^{6}}. But other subtleties arise when considering the case of {n = 2} and especially {n < 2}, particularly due to backreaction on the space. In all cases, it can be seen that the Electric WGC holds for open string U(1)s [1].

7. Testing the Magnetic Weak Gravity Conjecture: Type IIB String Theory in 6d F-theory

In the last example we considered a test of the Electric WGC for open string U(1)s. What about the Magnetic WGC? Does the MWGC likewise hold for open string U(1)s? Recall from earlier in our discussion the MWGC is not making a statement about a single charged state but about an infinite tower of charged states. Where is the infinite tower of charged states in our scenario? The answer is rather non-trivial and can be reviewed in a series of incredibly interesting and mathematically rich papers [5, 6, 7], which display some lovely stringy physics.

We will save a detailed review of these papers for a separate entry (following the formal conclusion of this series of notes on Palti’s lectures). In the meantime, looking at [5] in particular, a brief if not altogether terse description may be considered. What the authors find is that, for the infinite tower of states, they turn out to be non-perturbative states of the theory.

To see these non-perturbative states is difficult. The set-up is this: consider Type IIB string theory on a 4-dimensional manifold, meaning compactification down to 6-dimensions. A powerful method to study non-perturbative type IIB string theory is by way of uplifting to F-theory (or, for Type IIA, uplifting to M-theory). So the framework is 6-dimensional F-theory. The 6-dimensional Planck mass is defined by the volume of the F-theory compactification space, which is a complex Kähler surface {B_{2}} at the base of a Calabi-Yau 3-fold. In these notes, we have not yet considered such complex extended objects. But the idea is that we then consider a D7-brane filling the 6 external dimensions and wrapping a holomorphic curves on the Kähler surface in the 4-dimensional space. In the uncompactified 6 dimensions, the D3-brane wrapping the 2-cycle produces a solitonic ring. Associated strings on the curve {C_{0}} contained in {B_{2}} are sourced under the D7-brane gauge group.

From this construction, however roughly described, the idea is to uplift to a strong coupling (using F-theory). From this, if the goal is {g_{D7} \rightarrow 0}, where the tower of states become light according the WGC, then the 2-cycle must become very large. But, if the 2-cycle becomes big, the volume of the 4-dimensional manifold changes and, impliedly, the values of {M_{P}} and the string scale also change. So one approach is to keep the volume fixed. However, fixing the volume while making the 2-cycle big means that another 2-cycle needs to be small!

\displaystyle  volume \ fixed \rightarrow small \ 2-cycle

Now consider the following. If a D3-brane wrapped in internal dimensions gives a string in external dimensions, impliedly, in the above construction, it seems a D3-brane wrapped on the small 2-cycle is found to produce a string in the 6 external dimensions. But this string propagating in the 6-dimensions is tensionless as the volume of the curve {C_{0}} contained in B_{2} goes to zero, \text{vol}_{j}(C_{0}) \rightarrow 0 . Moreover, as the tension of the string is actually the size of the cycle, the string itself asymptotically describes an open Heterotic string. And so we observe,

\displaystyle  F-theory \longleftrightarrow Heterotic \ duality

And, as it is found that the string is charged under U(1), to finalise what is an incredible piece of evidence, the oscillator modes become massless and again what is found is an infinite tower of light states.

This concludes the summary. In a separate future entry we will study the technicalities in detail.

In the next collection of notes from Palti’s lecture series, we will continue our study by considering more complex manifolds – that is, arbitrary Calabi-Yau manifolds – to see if the WGC still holds! We will also looks to some more advanced tests of the DC, particularly in the context of Type IIB string theory.

Reference

[1] E. Palti, `The Swampland: Introduction and Review’, [arXiv:1903.06239v3[hep-th]]

[2] B. Heidenreich, M. Reece, and T. Rudelius, Sharpening the Weak Gravity Conjecture with Dimensional Reduction, JHEP 02 (2016) 140, [arXiv:1509.06374 [hep-th]].

[3] N. Arkani-Hamed, L. Motl, A. Nicolis, and C. Vafa, The String landscape, black holes and gravity as the weakest force, JHEP 06 (2007) 060, [hep-th/0601001].

[4] B. Heidenreich, M. Reece, and T. Rudelius, Evidence for a sublattice weak gravity conjecture, JHEP 08 (2017) 025, [arXiv:1606.08437].

[5] S.-J. Lee, W. Lerche, and T. Weigand, Tensionless Strings and the Weak Gravity Conjecture, JHEP 10 (2018) 164, [arXiv:1808.05958].

[6] S.-J. Lee, W. Lerche, and T. Weigand, Modular Fluxes, Elliptic Genera, and Weak Gravity Conjectures in Four Dimensions, [arXiv:1901.08065].

[7] S.-J. Lee, W. Lerche, and T. Weigand, A Stringy Test of the Scalar Weak Gravity Conjecture, Nucl. Phys. B938 (2019) 321–350, [arXiv:1810.05169].

[8] J. Polchinski, String theory. Vol. 2: Superstring theory and beyond. Cambridge Monographs on Mathematical Physics. Cambridge University Press, 2007.

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