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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> We study supersymmetric scale-separated AdS₃flux vacua of massive IIA on G2 orbifolds with smeared orientifold planes. We consider two types of$$ {T}^7/{Z}_2^3 $$ $T^7/Z_2^3$orbifolds which, with appropriate flux choices, yield integer dual dimensions for the operators corresponding to the closed string scalar fields in the dual CFT. As with all other known examples, the dual conformal dimensions are only parametrically close to integer values. 

A<sc>bstract</sc> Orientifold planes play a crucial role in flux compactifications of string theory, and we demonstrate their deep connection to achieving scale-separated solutions. Specifically, we show that when an orientifold plane contributes at leading order to the non-zero value of the scalar potential, then either the weak coupling limit or the large volume limit implies scale separation, meaning that the Kaluza-Klein tower mass decouples from the inverse length scale of the lower-dimensional theory. Notably, in the supergravity limit such solutions are inherently scale-separated. This result is independent of the spacetime dimension and the dimensionality of the Op-plane as long asp <7. Similarly, we show, extending previous results, that parametric scale separation is not possible for isotropic compactifications with a leading curvature term that generically arise in the AdS/CFT context. We classify all possible flux compactification setups in both type IIA and type IIB string theory for Op-planes with 2 ≤p≤ 6 and present their universal features. While the parametrically controlled scale-separated solutions are all AdS, we also find setups that allow for dS vacua. We prove that flux quantization prevents these dS vacua in isotropic compactifications from arising in a regime of parametric control. 

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> For decades intersecting D-branes and O-planes have been playing a very important role in string phenomenology in the context of particle physics model building and in the context of flux compactifications. The corresponding supergravity equations are hard to solve so generically solutions only exist in a so-called smeared limit where the delta function sources are replaced by constants. We are showing here that supergravity solutions for two perpendicularly intersecting localized sources in flat space do not exist for a generic diagonal metric Ansatz. We show this for two intersecting sources withp= 1, 2, 3, 4, 5, 6 spatial dimensions that preserve 8 supercharges, and we allow for fully generic fluxes. 

A<sc>bstract</sc> We set up a unified framework to efficiently compute the shear and bulk viscosities of strongly coupled gauge theories with gravitational holographic duals involving higher derivative corrections. We consider both Weyl⁴corrections, encoding the finite ’t Hooft coupling corrections of the boundary theory, and Riemann²corrections, responsible for non-equal central chargesc≠aof the theory at the ultraviolet fixed point. Our expressions for the viscosities in higher derivative holographic models are extracted from a radially conserved current and depend only on the horizon data. 

A<sc>bstract</sc> We explore the possibility that our universe’s current accelerated expansion is explained by a quintessence model with an exponential scalar potential,V=V₀e^{−λ ϕ}, keeping an eye towardsλ≥$$ \sqrt{2} $$ $\sqrt{2}$and an open universe, favorable to a string theory realisation and with no cosmological horizon. We work out the full cosmology of the model, including matter, radiation, and optionally negative spatial curvature, for allλ> 0, performing an extensive analysis of the dynamical system and its phase space. The minimal physical requirements of a past epoch of radiation domination and an accelerated expansion today lead to an upper boundλ≲$$ \sqrt{3} $$ $\sqrt{3}$, which is driven slightly up in the presence of observationally allowed spatial curvature. Cosmological solutions start universally in a kination epoch, go through radiation and matter dominated phases and enter an epoch of acceleration, which is only transient forλ>$$ \sqrt{2} $$ $\sqrt{2}$. Field distances traversed between BBN and today are sub-Planckian. We discuss possible string theory origins and phenomenological challenges, such as time variation of fundamental constants. We provide theoretical predictions for the model parameters to be fitted to data, most notably the varying dark energy equation of state parameter, in light of recent results from DES-Y5 and DESI. 

A<sc>bstract</sc> The requirement that particles propagate causally on non-trivial backgrounds implies interesting constraints on higher-derivative operators. This work is part of a systematic study of the positivity bounds derivable from time delays on shockwave backgrounds. First, we discuss shockwaves in field theory, which are infinitely boosted Coulomb-like field configurations. We show how a positive time delay implies positivity of four-derivative operators in scalar field theory and electromagnetism, consistent with the results derived using dispersion relations, and we comment on how additional higher-derivative operators could be included. We then turn to gravitational shockwave backgrounds. We compute the infinite boost limit of Reissner-Nordström black holes to derive charged shockwave backgrounds. We consider photons traveling on these backgrounds and interacting through four-derivative corrections to Einstein-Maxwell theory. The inclusion of gravity introduces a logarithmic term into the time delay that interferes with the straightforward bounds derivable in pure field theory, a fact consistent with CEMZ and with recent results from dispersion relations. We discuss two ways to extract a physically meaningful quantity from the logarithmic time delay — by introducing an IR cutoff, or by considering the derivative of the time delay — and comment on the bounds implied in each case. Finally, we review a number of additional shockwave backgrounds which might be of use in future applications, including spinning shockwaves, those in higher dimensions or with a cosmological constant, and shockwaves from boosted extended objects.