Thomas Van Riet

A bstract: We initiate a systematic study of compactifications of type II string theory to two dimensions on Ricci-flat spaces with fluxes and sources. We derive universal constraints on such compactifications, and then develop type IIA compactifications on Spin(7)-holonomy spaces with bulk fluxes whose tadpole is cancelled by OF1-planes. We derive the resulting two-dimensional 𝒩 = (1, 1) supergravity for a toroidal Spin(7) orbifold, and we extend the metric and universal sectors geometrically to general compact Spin(7) manifolds. In candidate supersymmetric Minkowski vacua, all untwisted shape modes appear in the flux scalar potential and can in principle be classically stabilised. However, in an explicit toroidal orbifold example flux quantisation together with the tadpole bound may obstruct the existence of candidate vacua supported entirely within the untwisted sector. Within the approximation retaining the topological curvature contribution and its associated energy while neglecting the remaining higher-derivative corrections, the string-frame volume in string units is bounded by 7 χ /192, so suppressing α ^′ corrections requires Spin(7) manifolds with large Euler characteristic.

We revisit scale separation for compactifications of ten- and eleven-dimensional supergravity. For cosmological solutions rolling down flux-generated potentials, we observe that scale separation is achieved as time flows, and is fairly generic. This is realized without the need of orientifolds nor corrections to the classical supergravity approximation. We then confront scale separation with the covariant entropy bound (CEB) and the Cohen-Kaplan-Nelson bound. We show that a naive application of these bounds to vacua hints at the existence of at least two extra dimensions. For rolling solutions, we observe that the CEB is not always respected, but since these examples lack a cosmic horizon, the application of entropy bounds remains delicate.

Flux compactifications of IIA supergravity on orientifolded G$_2$-manifolds have been argued to allow for classical Minkowski$_3$ vacua with moduli and scale-separated AdS$_3$ vacua with full moduli stabilisation. To further uplift these vacua to meta-stable dS$_3$ vacua using anti-D2 branes, warped throats are desirable. We study the flux-stabilisation of local "CGLP-type" throats in compact G$_2$ spaces, and discuss consistency constraints on anti-brane uplifting. Despite the classical AdS$_3$ vacua to be free of tachyons, we find that uplifting from anti-branes down warped throats is forbidden. If instead we rely on hypothetical AdS$_3$ vacua that arise from quantum corrections to the classical Minkowski$_3$ vacua, we find that (similarly to the 4d analogues) consistency constraints point in opposite directions. However, there is potentially an advantage over 4d when it comes to concrete fine-tuning freedom of numbers, such as tadpole constraints.

Near horizon geometries of Dp-branes with p ≠ 3 are singular with a running dilaton. Bound states of Dp branes with their magnetic cousins, D(6 − p) branes, can stabilise the dilaton such that an AdS factor might appear in the near horizon region, potentially leading to a chain of AdS vacua of the form AdS _p+2 × S ^p+2 × 𝕋^6−2p . The solutions with p = −1, 1, 3 are supersymmetric with the cases p = 1, 3 being well-known examples already. We construct explicit (partially smeared) brane bound state solutions for all such configurations. The D2-D4 and D(−1)-D7 cases are entirely novel, but they do not have a near-horizon AdS geometry. The two novel classes of solutions feature ghost branes (negative tension branes), and we suggest they are physical for the D(−1)-D7 solutions but unphysical for the D2-D4 solutions. The bound state of a D(−1) and a D7 brane in supergravity was only hinted upon recently in [1]. We correct the solution here in order to preserve supersymmetry, and find that the dilaton can indeed be stabilized. This points to a possible dual matrix theory, generalizing the IKKT matrix model to allow for conformal invariance.

The 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.

We reconsider the perturbative stability of Euclidean axion wormholes. The quadratic action that governs linear perturbations is derived directly in Euclidean gravity. We demonstrate explicitly that a stability analysis in which one treats the axion as a normal two-form gauge field is equivalent to one performed in the Hodge-dual formulation, where one considers the axion as a scalar with a wrong-sign kinetic term. Both analyses indicate that axion wormholes are perturbatively stable, even in the presence of a massless dilaton, or saxion, field that couples to the axion.

This paper analyzes the effective field theory perspective on modern physics through the lens of the quantum theory of gravitational interaction. The historical part argues that the search for a theory of quantum gravity stimulated the change in outlook that characterizes the modern approach to the Standard Model of particle physics and General Relativity. We present some landmarks covering a long period, i.e., from the beginning of the 1930s until 1994, when, according to Steven Weinberg, the modern bottom-up approach to General Relativity began. Starting from the first attempt to apply the quantum field theory techniques to perturbatively quantize Einstein's theory, we explore its developments and interaction with the top-down approach encoded by String Theory. In the last part of the paper, we focus on this last approach to describe the relationship between our modern understanding of String Theory and Effective Field Theory in today's panorama. To this end, the non-historical part briefly explains the modern concepts of moduli stabilization and Swampland to understand another change in focus that explains the present framework where some string theorists move.