arXiv subjects

Giuseppe Dibitetto

Publications and source records attributed to Giuseppe Dibitetto.

At least 37 records · Page 2Linked to original sources

arXiv2020

New accelerating solutions in late-time cosmology

Dark energy models can be seen as dynamical systems. In this paper we show that multi-field models with a curved field space give rise to new critical points and we analyse their stability. These are new accelerating solutions in late-time cosmology which exist even for steep potentials. This opens up the possibility to realise quintessence models even when quantum corrections spoil the flatness of the underlying potential. These non-linear sigma models arise naturally in supergravity and string models where their multi-field dynamics can help to avoid swampland bounds.

gr-qc↗

arXiv2020

Nothing really matters

We study non-perturbative instabilities of AdS spacetime in General Relativity with a cosmological constant in arbitrary dimensions. In this simple setup we explicitly construct a class of gravitational instantons generalizing Witten's bubble of nothing. We calculate the corresponding Euclidean action and show that its change is finite. The expansion of these bubbles is described by a lower-dimensional de Sitter geometry within a non-compact foliation of the background spacetime. Moreover we discuss the existence of covariantly constant spinors as a possible topological obstruction for such decays to occur. This mechanism is further connected to the stability of supersymmetric vacua in string theory.

hep-th↗

arXiv2020

Holographic Description of M-branes via AdS$_2$

We study $\textrm{AdS}_2\times S^4 \times S^2 \times Σ_2$ solutions in type IIB string theory arising from D1 -- D3 -- NS5 brane intersections. These backgrounds enjoy sixteen supercharges and the corresponding internal geometry is non-compact due to the specific form of the warping w.r.t. the Riemann surface $Σ_2$. Even though a direct computation of the holographic free energy of the would-be dual CFT$_1$ yields a divergent behaviour, it reveals the typical $N^3$ scaling of a 6d theory upon introducing a hard cut-off. We claim that such geometries may be interpreted as the gravity duals of M(atrix) models describing an IR phase of the $(2,0)$ theory of M5 branes, in presence of momentum and NUT charges. We discuss parallel $\textrm{AdS}_2$ geometries describing longitudinal M2 branes in the UV, where the counting of the number of degrees of freedom correctly reproduces the expected $N^{3/2}$ behaviour of the dual field theory. These geometries provide explicit examples where deconstructed extra dimensions yield well-defined UV descriptions in terms of higher-dimensional CFTs.

hep-th↗

arXiv2019

6D (1,1) Gauged Supergravities from Orientifold Compactifications

We study dimensional reductions of M-theory/type II strings down to 6D in the presence of fluxes and spacetime filling branes and orientifold planes of different types. We classify all inequivalent orientifold projections giving rise to $\mathcal{N}=(1,1)$ supergravities in 6D and work out the embedding tensor/fluxes dictionary for each of those. Finally we analyze the set of vacua for the different classes of reductions and find an abundance of "no-scale" type Minkowski vacua, as well as a few novel examples of (A)dS extrema.

hep-th↗

arXiv2019

de Sitter Cosmology on an expanding bubble

Constructing an explicit compactification yielding a metastable de Sitter (dS) vacuum in a UV consistent string theory is an incredibly difficult open problem. Motivated by this issue, as well as the conjecture that all non-supersymmetric AdS vacua must decay, we discuss the alternative possibility of realizing an effective four-dimensional dS cosmology on a codimension-one bubble wall separating two AdS$_5$ vacua. The construction further elaborates on the scenario of arXiv:1807.01570, where the aforementioned cosmology arises due to a non-perturbative decay and is embedded in a five-dimensional bulk in a time-dependent way. In this paper we discuss the relation between this scenario and the weak gravity conjecture and further develop the details of the four-dimensional cosmology. We provide a bulk interpretation for the dS temperature as the Unruh temperature experienced by an accelerated observer riding the bubble. A source of four-dimensional matter arises from a string cloud in the bulk, and we examine the consequences for the particle mass spectrum. Furthermore, we show how effective four-dimensional Einstein gravity on the bubble is obtained from the five-dimensional Gauss equation. We conclude by outlining some implications that this paradigm will have for holography, inflation, the standard model, and black holes.

hep-th↗

arXiv2019

Constructing stable de Sitter in M-theory from higher curvature corrections

We consider dimensional reductions of M-theory on $\mathbb{T}^{7}/\mathbb{Z}_{2}^{3}$ with the inclusion of arbitrary metric flux and spacetime filling KK monopoles. With these ingredients at hand, we are able to construct a novel family of non-supersymmetric yet tachyon free Minkowski extrema. These solutions are supported by pure geometry with no extra need for gauge fluxes and possess a fully stable perturbative mass spectrum, up to a single flat direction. Such a direction corresponds to the overall internal volume, with respect to which the scalar potential exhibits a no-scale behavior. We then provide a mechanism that lifts the flat direction to give it a positive squared mass while turning Mkw$_{4}$ into dS$_{4}$. The construction makes use of the combined effect of $G_{7}$ flux and higher curvature corrections. Our solution is scale separated and the quantum corrections are small. Finally we speculate on novel possibilities when it comes to scale hierarchies within a given construction of this type, and possible issues with the choice of quantum vacuum.

hep-th↗

arXiv2019

AdS$_3$ solutions with exceptional supersymmetry

Among the possible superalgebras that contain the AdS$_3$ isometries, two interesting possibilities are the exceptional $F(4)$ and $G(3)$. Their R-symmetry is respectively SO(7) and $G_2$, and the amount of supersymmetry ${\cal N}=8$ and ${\cal N}=7$. We find that there exist two (locally) unique solutions in type IIA supergravity that realize these superalgebras, and we provide their analytic expressions. In both cases, the internal space is obtained by a round six-sphere fibred over an interval, with an O8-plane at one end. The R-symmetry is the symmetry group of the sphere; in the $G(3)$ case, it is broken to $G_2$ by fluxes. We also find several numerical ${\cal N}=1$ solutions with $G_2$ flavor symmetry, with various localized sources, including O2-planes and O8-planes.

hep-th↗

arXiv2019

$\textbf{AdS}_3$ vacua and surface defects in massive IIA

We summarize the results and the ideas in 1707.06152, 1707.06154, 1807.07768 where new warped $\mathrm{AdS}_3$ backgrounds are derived in massive IIA string theory by uplifting exact solutions in $\mathcal N=1$, $d=7$ and $\mathcal N=(1,1)$, $d=6$ gauged supergravities. These solutions are respectively asymptotically $\mathrm{AdS}_7$ and $\mathrm{AdS}_6$ and they are related to the D2-D4-NS5-D6-D8 brane intersection. We provide a particular supergravity solution in 10d describing this bound state and we discuss the relations between its near-horizon geometry and the uplifts from 6d and 7d. Then we give the holographic interpretation of these $\mathrm{AdS}_3$ warped backgrounds in terms of $\mathcal N=(0,4)$ defect $\mathrm{SCFT}_2$ within the $\mathcal N=(1,0)$ $\mathrm{SCFT}_6$ and the $\mathcal N=2$ $\mathrm{SCFT}_5$.

hep-th↗

arXiv2019

Emergent de Sitter Cosmology from Decaying AdS

Recent developments in string compactifications demonstrate obstructions to the simplest constructions of low energy cosmologies with positive vacuum energy. The existence of obstacles to creating scale-separated de Sitter solutions indicates a UV/IR puzzle for embedding cosmological vacua in a unitary theory of quantum gravity. Motivated by this puzzle, we propose an embedding of positive energy Friedmann-Lemaitre-Robertson-Walker cosmology within string theory. Our proposal involves confining 4D gravity on a brane which mediates the decay from a non-supersymmetric false AdS5 vacuum to a true vacuum. In this way, it is natural for a 4D observer to experience an effective positive cosmological constant coupled to matter and radiation, avoiding the need for scale separation or a fundamental de Sitter vacuum.

hep-th↗

arXiv2018

AdS$_{\textbf{2}}$ solutions and their massive IIA origin

We consider warped $\mathrm{AdS}_2\times \mathcal M_4$ backgrounds within $F(4)$ gauged supergravity in six dimensions. In particular, we are able to find supersymmetric solutions of the aforementioned type characterized by $\textrm{AdS}_{6}$ asymptotics and an $\mathcal M_4$ given by a three-sphere warped over a segment. Subsequently, we provide the 10D uplift of the solutions to massive type IIA supergravity, where the geometry is $\mathrm{AdS}_2\times S^{3}\times\tilde{S}^3$ warped over a strip. Finally we construct the brane intersection underlying one of these supergravity backgrounds. The explicit setup involves a D0-F1-D4 bound state intersecting a D4-D8 system.

hep-th↗

arXiv2018

$\textrm{AdS}_{2}\times S^7$ solutions from D0 $-$ F1 $-$ D8 intersections

We study an exhaustive analytic class of massive type IIA backgrounds preserving sixteen real supercharges and enjoying $\textrm{SL}(2,\mathbb{R})\times\textrm{SO}(8)$ bosonic symmetry. The corresponding geometry is described by $\textrm{AdS}_{2}\times S^{7}$ warped over a line, which turns out to emerge from taking the near-horizon limit of D0 $-$ F1 $-$ D8 intersections. By studying the singularity structure of these solutions we find the possible presence of localized O8/D8 sources, as well as of fundamental strings smeared over the $S^{7}$. Finally we discuss the relation between the aforementioned solutions and the known $\textrm{AdS}_{7}\times S^{2}$ class through double analytic continuation.

hep-th↗

arXiv2018

A new light on the darkest corner of the landscape

Motivated by the recently proposed bounds on the slow-roll parameters for scalar potentials arising from string/M-theory compactifications, a.k.a. the Refined de Sitter Swampland conjecture, we explore the sharpness of such constraints within 4D supergravities coming from compactifications of massive type IIA string theory on $\mathbb{T}^{6}/(\mathbb{Z}_{2}\,\times\,\mathbb{Z}_{2})$. With the aid of a numerical technique, known as differential evolution, we are able to find a de Sitter extremum which is fully metastable up to one single flat direction. This solution is supported by spacetime filling sources such as O6 planes and KK monopoles. Our example violates the bound imposed by this conjecture.

hep-th↗

arXiv2018

6d surface defects from massive type IIA

We present a new BPS flow within minimal $\mathcal{N}=1$ supergravity in seven dimensions describing a warped $\textrm{AdS}_{3}$ background supported by a "dyonic" profile of the three-form. Furthermore, we discuss the holographic interpretation of the above solution in terms of a defect $\textrm{SCFT}_{2}$ inside the 6d $(1,0)$ theory dual to the AdS in the asymptotic region. Finally we provide the brane picture of the aforementioned defect CFT as D2- and wrapped D4-branes ending on a D6 - NS5 - D8 funnel in massive type IIA string theory.

hep-th↗

arXiv2018

Surface defects in the D4 $-$ D8 brane system

A new class of exact supersymmetric solutions is derived within minimal $d = 6$ $F(4)$ gauged supergravity. These flows are all characterized by a non-trivial radial profile for the 2-form gauge potential included into the supergravity multiplet. In particular three solutions within this class are featured by an $\mathrm{AdS}_3$ foliation of the 6d background and by an $\mathrm{AdS}_6$ asymptotic geometry. Secondly, considering the simplest example of these, we give its massive IIA uplift describing a warped solution of the type $\mathrm{AdS}_3\times S^2\times S^3$ fibered over two intervals $I_r \times I_ξ$ . We interpret this background as the near-horizon of a D4 $-$ D8 system on which a bound state D2 $-$ NS5 $-$ D6 ends producing a surface defect. Finally we discuss its holographic dual interpretation in terms of a $\mathcal{N} = (0, 4)$ SCFT$_2$ defect theory within the $\mathcal{N} = 2$ SCFT$_5$ dual to the $\mathrm{AdS}_6\times S^4$ massive IIA warped vacuum.

hep-th↗

arXiv2018

Space-filling branes & gaugings

We consider in any dimension the supersymmetric $\mathbb{Z}_2$ truncations of the maximal supergravity theories. In each dimension and for each truncation we determine all the sets of 1/2-BPS space-filling branes that preserve the supersymmetry of the truncated theory and the representations of the symmetry of such theory to which they belong. We show that in any dimension below eight these sets always contain exotic branes, that are objects that do not have a ten-dimensional origin. We repeat the same analysis for half-maximal theories and for the quarter-maximal theories in four and three dimensions. We then discuss all the possible gaugings of these theories as described in terms of the embedding tensor. In general, the truncation acts on the quadratic constraints of the embedding tensor in such a way that some representations survive the truncation although they are not required by the supersymmetry of the truncated theory. We show that for any theory, among these representations, the highest-dimensional ones are precisely those of the 1/2-BPS space-filling branes that preserve the same supersymmetry of the truncated theory, and we interpret this result as the fact that these quadratic constraints after the truncation become tadpole conditions for such branes.

hep-th↗

arXiv2017

BPS objects in D=7 supergravity and their M-theory origin

We study several different types of BPS flows within minimal $\mathcal{N}=1$, $D=7$ supergravity with $\textrm{SU}(2)$ gauge group and non-vanishing topological mass. After reviewing some known domain wall solutions involving only the metric and the $\mathbb{R}^{+}$ scalar field, we move to considering more general flows involving a "dyonic" profile for the 3-form gauge potential. In this context, we consider flows featuring a $\textrm{Mkw}_{3}$ as well as an $\textrm{AdS}_{3}$ slicing, write down the corresponding flow equations, and integrate them analytically to obtain many examples of asymptotically $\mathrm{AdS}_7$ solutions in presence of a running 3-form. Furthermore, we move to adding the possibility of non-vanishing vector fields, find the new corresponding flows and integrate them numerically. Finally, we discuss the eleven-dimensional interpretation of the aforementioned solutions as effective descriptions of $\mathrm{M2}-\mathrm{M5}$ bound states.

hep-th↗

arXiv2017

Holography, Brane Intersections and Six-dimensional SCFTs

We study supersymmetric intersections of NS5-, D6- and D8-branes in type IIA string theory. We focus on the supergravity description of this system and identify a "near horizon" limit in which we recover the recently classified supersymmetric seven-dimensional AdS solutions of massive type IIA supergravity. Using a consistent truncation to seven-dimensional gauged supergravity we construct a universal supersymmetric deformation of these AdS vacua. In the holographic dual six-dimensional (1,0) superconformal field theory this deformation describes a universal RG flow on the tensor branch of the vacuum moduli space triggered by a vacuum expectation value for a protected scalar operator of dimension four.

hep-th↗

arXiv2016

Double Field Theory at SL(2) angles

An extended field theory is presented that captures the full SL(2) x O(6,6+n) duality group of four-dimensional half-maximal supergravities. The theory has section constraints whose two inequivalent solutions correspond to minimal D=10 supergravity and chiral half-maximal D=6 supergravity, respectively coupled to vector and tensor multiplets. The relation with O(6,6+n) (heterotic) double field theory is thoroughly discussed. Non-Abelian interactions as well as background fluxes are captured by a deformation of the generalised diffeomorphisms. Finally, making use of the SL(2) duality structure, it is shown how to generate gaugings with non-trivial de Roo-Wagemans angles via generalised Scherk-Schwarz ansaetze. Such gaugings allow for moduli stabilisation including the SL(2) dilaton.

hep-th↗