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A<sc>bstract</sc> Effective field theories are constrained by the requirement that their constituents never move superluminally on non-trivial backgrounds. In this paper, we study time delays experienced by photons propagating on charged shockwave backgrounds in five dimensions. In the absence of gravity — where the shockwaves are electric fields sourced by boosted charges — we derive positivity bounds for the four-derivative corrections to electromagnetism, reproducing previous results derived from scattering amplitudes. By considering the gravitational shockwaves sourced by Reissner-Nordström black holes, we derive new constraints in the presence of gravity. We observe the by-now familiar weakening of positivity bounds in the presence of gravity, but without the logarithmic divergences present in 4d. We find that the strongest bounds appear by examining the time delay near the horizon of the smallest possible black hole, and discuss on the validity of the EFT expansion in this region. We comment on our bounds in the context of the swampland program as well as their relation with the positivity bounds obtained from dispersion relations. 

A<sc>bstract</sc> In this work, we investigate the properties of string effective theories with scalar field(s) and a scalar potential. We first claim that in most examples known, such theories aremultifield, with at least 2 non-compact field directions; the few counter-examples appear to be very specific and isolated. Such a systematic multifield situation has important implications for cosmology. Characterising properties of the scalar potentialVis also more delicate in a multifield setting. We provide several examples of string effective theories withV> 0, where the latter admits an asymptotically flat direction along an off-shell field trajectory: in other words, there exists a limit$$\widehat{\varphi }\to \infty $$for which$$\frac{\left|{\partial }_{\widehat{\varphi }}V\right|}{V}\to 0$$. It is thus meaningless to look for a lower bound to this single field quantity in a multifield setting; the complete gradient ∇Vis then better suited. Restricting to on-shell trajectories, this question remains open, especially when following the steepest descent or more generally a gradient flow evolution. Interestingly, single field statements in multifield theories seem less problematic forV< 0. 

A<sc>bstract</sc> We study moduli stabilization via fluxes in the 2⁶Landau-Ginzburg model. Fluxes not only give masses to scalar fields but can also induce higher order couplings that stabilize massless fields. We investigate this for several different flux choices in the 2⁶model and find two examples that are inconsistent with the Refined Tadpole Conjecture. We also present, to our knowledge, the first 4d$$ \mathcal{N} $$ $N$= 1 Minkowski solution in string theory without any flat direction. 

A<sc>bstract</sc> Recent work on flux compactifications suggests that the tadpole constraint generically allows only a limited number of complex structure moduli to become massive, i.e., be stabilized at quadratic order in the spacetime superpotential. We study the effects of higher-order terms systematically around the Fermat point in the 1⁹Landau-Ginzburg model. This model lives at strong coupling and features no Kähler moduli. We show that indeed massless fields can be stabilized in this fashion. We observe that, depending on the flux, this mechanism is more effective when the number of initially massless fields is large. These findings are compatible with both the tadpole conjecture and the massless Minkowski conjecture. Along the way, we complete the classification of integral flux vectors with small tadpole contribution. Thereby we are closing in on a future complete understanding of all possible flux configurations in the 1⁹Landau-Ginzburg model. 

A<sc>bstract</sc> We examine bounds on accelerated expansion in asymptotic regions of the moduli space in string theory compactifications to four spacetime dimensions. While there are conjectures that forbid or constrain accelerated expansion in such asymptotic regions, potential counter examples have been discussed recently in the literature. We check whether such counter examples can arise in explicit string theory constructions, focusing in particular on non-geometric compactifications of type IIB string theory that have no Kähler moduli. We find no violation of the Strong Asymptotic dS Conjecture and thus provide support for the absence of accelerated expansion in asymptotic regions of a barely explored corner of the string landscape. Moreover, working in a simplified setting, we point out a new mechanism for potentially connecting the Sharpened Distance Conjecture and the Strong Asymptotic dS Conjecture. If this argument could be generalized, it would mean that the Sharpened Distance Conjecture is implied by the Strong Asymptotic dS Conjecture, and that their exponential factors are naturally related by a factor of 2. 

A bstract Classical flux compactifications contribute to a well-controlled corner of the string landscape, therefore providing an important testing ground for a variety of conjectures. We focus here on type II supergravity compactifications on 6d group manifolds towards 4d maximally symmetric spacetimes. We develop a code where the truncation to left-invariant scalars and the dimensional reduction to a 4d theory are automated, for any possible configuration of O p -planes and D p -branes. We then prove that any such truncation is consistent. We further compute the mass spectrum and analyse the stability of many de Sitter, Minkowski or anti-de Sitter solutions, as well as their consistency with swampland conjectures. 

A bstract We revisit flux compactifications of type IIB string theory on ‘spaces’ dual to rigid Calabi-Yau manifolds. This rather unexplored part of the string landscapes harbors many interesting four-dimensional solutions, namely supersymmetric $$ \mathcal{N} $$ N = 1 Minkowski vacua without flat direction and infinite families of AdS vacua, some potentially with unrestricted rank for the gauge group. We also comment on the existence of metastable dS solutions in this setup. We discuss how these solutions fit into the web of swampland conjectures.