1,720,982 research outputs found
A mechanism for freezing moduli into Minkowski spacetime
Full text linkWe discuss FLRW-cosmologies with negatively-curved spatial slices that induce a late-time freezing of all the moduli of an effective field theory through Hubble friction, independently of the moduli-space curvature. This holds for pure moduli, which appear ubiquitously in string compactifications. Crucially, the cosmological solutions approach a Milne universe. Hence, the mechanism we describe in fact corresponds to late-time moduli freezing into Minkowski spacetime
Non-supersymmetric string models from anti-D3-/D7-branes in strongly warped throats
Full text linkThis article discusses model-building scenarios including anti-D3-/D7-branes, in which supersymmetry is broken spontaneously, despite having no scale at which sparticles appear and standard supersymmetry is restored. If the branes are placed on singularities at the tip of warped throats in Calabi-Yau orientifold flux compactifications, they may give rise to realistic particle spectra, closed- and open-string moduli stabilisation with a Minkowski/de Sitter uplift, and a geometrical origin for the scale hierarchies. The paper derives the low-energy effective field theory description for such scenarios, i.e. a non-linear supergravity theory for standard and constrained supermultiplets, including soft supersymmetry-breaking matter couplings. The effect of closed-string moduli stabilisation on the open-string matter sector is worked out, incorporating non-perturbative and perturbative effects, and the mass and coupling hierarchies are computed with a view towards phenomenology
A distance conjecture beyond moduli?
No full textThe distance conjecture states that for theories with moduli coupled to gravity a tower of states becomes exponentially light in the geodesic distance in moduli space. This specifies how effective field theories break down for large field values. However, phenomenological field theories have no moduli, but a scalar potential that deforms dynamical trajectories away from geodesic curves. In this note, we speculate on how one should generalise the distance conjecture, in asymptotic field regimes, to include a scalar potential. We test the generalized distance conjecture in a few cases, demonstrate a link with pseudo-/fake supersymmetry and apply it to the ekpyrotic scenario in cosmology. For the latter we observe that the pre-uplift KKLT potential could provide a stringy embedding of ekpyrosis away from asymptotic regimes in field space
Analytic bounds on late-time axion-scalar cosmologies
No full textThe cosmological dynamics of multiple scalar/pseudoscalar fields are difficult to solve, especially when the field-space metric is curved. This presents a challenge in determining whether a given model can support cosmic acceleration, without solving for the on-shell solution. In this work, we present bounds on late-time FLRW-cosmologies in classes of theories that involve arbitrary numbers of scalar and pseudoscalar fields coupled both kinetically (leading to a curved field space metric) and through scalar potentials. Such bounds are proven analytically, independently of initial conditions, with no approximation in the field equations and without referring to explicit solutions. Besides their broad applications to cosmological model building, our bounds can be applied to studying asymptotic cosmologies of certain classes of string compactifications
Accelerating universe at the end of time
No full textWe investigate whether an accelerating universe can be realized as an asymptotic late-time solution of Friedmann-Lemaître-Robertson-Walker (FLRW)-cosmology with multifield multiexponential potentials. Late-time cosmological solutions exhibit a universal behavior which enables us to bound the rate of time variation of the Hubble parameter. In string-theoretic realizations, if the dilaton remains a rolling field, our bound singles out a tension in achieving asymptotic late-time cosmic acceleration. Our findings go beyond previous no-go theorems in that they apply to arbitrary multiexponential potentials and make no specific reference to vacuum or slow-roll solutions. We also show that if the late-time solution approaches a critical point of the dynamical system governing the cosmological evolution, the criterion for cosmic acceleration can be generally stated in terms of a directional derivative of the potential
AdS scale separation and the distance conjecture
Full text linkIt has been argued that orientifold vacua with fluxes in type IIA string theory can achieve moduli stabilisation and arbitrary decoupling between the AdS and KK scales upon sending certain unconstrained RR-flux quanta to infinity. In this paper, we find a novel scalar field in the open-string sector that allows us to interpolate between such IIA vacua that differ in flux quanta and find that the limit of large fluxes is nicely consistent with the distance conjecture. This shows that the massive IIA vacua pass an important Swampland criterion and suggests that scale-separated AdS vacua might not be in the Swampland. Our analysis also naturally suggests a flux analogue of “Reid’s fantasy” where flux vacua that differ in quantised flux numbers can be connected through trajectories in open-string field space and not just via singular domain walls
Modular invariance, misalignment and finiteness in non-supersymmetric strings
Full text linkIn this article we show that finite perturbative corrections in non-supersymmetric strings can be understood via an interplay between modular invariance and misaligned supersymmetry. While modular invariance is known to be crucial in closed-string models, its presence and role for open strings is more subtle. Nevertheless, we argue that it leads to cancellations in physical quantities such as the one-loop cosmological constant and prevents them from diverging. In particular, we show that if the sector-averaged number of states does not grow exponentially, as predicted by misaligned supersymmetry, all exponential divergences in the one-loop cosmological constant cancel out as well. To account for the absence of power-law divergences, instead, we need to resort to the modular structure of the partition function. We finally comment on the presence of misaligned supersymmetry in the known 10-dimensional tachyon-free non-supersymmetric string theories
Misaligned supersymmetry and open strings
Full text linkThe study of non-supersymmetric string theories is shedding light on an important corner of the string landscape and might ultimately explain why, so far, we did not observe supersymmetry in our universe. We review how misaligned supersymmetry in closed-string theories leads to a cancellation between bosons and fermions even in non-supersymmetric string theories. We then show that the same cancellation takes place for open strings by studying an anti-Dp-brane placed on top of an Op-plane in type II string theory. Misaligned supersymmetry consists in cancellations between bosons and fermions at different energy levels, in such a way that the averaged number of states grows at a rate dominated by a factor eCeffn, with Ceff< Ctot, where Ctot is the inverse Hagedorn temperature. We prove the previously conjectured complete cancellation, i.e. we prove that Ceff = 0, for a vast class of models
Branes, fermions, and superspace dualities
Full text linkWe use the superspace formulation of supergravity in eleven and ten dimensions to compute fermion couplings on the M2-brane and on Dp-branes. In this formulation fermionic couplings arise naturally from the θ-expansion of the superfields from which the brane actions are constructed. The techniques we use and develop can in principle be applied to determine the fermionic couplings to general background fields up to arbitrary order. Starting with the superspace formulation of 11-dimensional supergravity, we use a geometric technique known as the ‘normal coordinate’ method to obtain the θ-expansion of the M2-brane action. We then present a method which allows us to translate the knowledge of fermionic couplings on the M2-brane to knowledge of such couplings on the D2-brane, and then to any Dp-brane. This method is based on superspace generalizations of both the compactification taking 11-dimensional supergravity to type IIA supergravity and the T-duality rules connecting the type IIA and type IIB supergravities
Connecting flux vacua through scalar field excursions
No full textWe show how flux vacua that differ from each other in flux quanta can be seen as different vacua in a single scalar potential of an enlarged field space, which resolves the separation by thin domain walls. This observation, which is motivated by the anti-de Sitter distance conjecture, allows one to compute distances between different vacua using the usual field-space metric. We verify for explicit examples such as scale-separated IIA flux vacua and the IIB Freund-Rubin vacua that the distance conjecture (for scalar fields) is satisfied and that the asymptotic directions in the enlarged field space are indeed hyperbolic. This enlarged field space contains the tachyon fields on the unstable Dp-branes of type II string theory, which can induce the brane charges of the stable D-branes. We suggest that requiring continuous interpolations refines the cobordism conjecture and postdicts the existence of unstable Dp-branes
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