arXiv subjects

Olaf Hohm

Publications and source records attributed to Olaf Hohm.

At least 37 records · Page 2Linked to original sources

arXiv2023

2D Black Holes, Bianchi I Cosmologies, and $α'$

We report two surprising results on $α'$ corrections in string theory restricted to massless fields. First, for critical dimension Bianchi type I cosmologies with $q$ scale factors only $q-1$ of them have non-trivial $α'$ corrections. In particular, for FRW backgrounds all $α'$ corrections are trivial. Second, in non-critical dimensions, all terms in the spacetime action other than the cosmological term are field redefinition equivalent to terms with arbitrarily many derivatives, with the latter generally of the same order. Assuming an $α'$ expansion with coefficients that fall off sufficiently fast, we consider field redefinitions consistent with this fall-off and classify the higher derivative terms for two-dimensional string theory with one timelike isometry. This most general duality-invariant theory permits black-hole solutions, and we provide perturbative and non-perturbative tools to explore them.

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arXiv2022

U-duality and $α'$ corrections in three dimensions

We consider the target space theory of bosonic and heterotic string theory to first order in $α'$ compactified to three dimensions, using a formulation that is manifestly T-duality invariant under ${\rm O}(d,d,\mathbb{R})$ with $d=23$ and $d=7$, respectively. While the two-derivative supergravity exhibits a symmetry enhancement to the U-duality group ${\rm O}(d+1,d+1)$, the continuous group is known to be broken to ${\rm O}(d,d,\mathbb{R})$ by the first $α'$ correction. We revisit this observation by computing the full effective actions in three dimensions to first order in $α'$ by dualizing the vector gauge fields. We give a formally ${\rm O}(d+1,d+1)$ invariant formulation by invoking a vector compensator, and we observe a chiral pattern that allows one to reconstruct the bosonic action from the heterotic action. Furthermore, we obtain a particular massive deformation by integrating out the external $B$ field. This induces a novel Chern-Simons term based on composite connections that, remarkably, is ${\rm O}(d+1,d+1)$ invariant to leading order in the deformation parameter.

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arXiv2022

An $α'$-complete theory of cosmology and its tensionless limit

We explore the exactly duality invariant higher-derivative extension of double field theory due to Hohm, Siegel and Zwiebach (HSZ) specialized to cosmological backgrounds. Despite featuring a finite number of derivatives in its original formulation, this theory encodes infinitely many $α'$ corrections for metric, B-field and dilaton, which are obtained upon integrating out certain extra fields. We perform a cosmological reduction with fields depending only on time and show consistency of this truncation. We compute the $α'^4$ coefficients of the general cosmological classification. As a possible model for how to deal with all $α'$ corrections in string theory we give a two-derivative reformulation in which the extra fields are kept. The corresponding Friedmann equations are then ordinary second order differential equations that capture all $α'$ corrections. We explore the tensionless limit $α'\rightarrow \infty$, which features string frame de Sitter vacua, and we set up perturbation theory in $\frac{1}{α'}$.

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arXiv2022

Supersymmetric action for 6D $(4,0)$ supergravity

We give a linearized but otherwise complete supersymmetric action for ${\cal N}=(4,0)$ supergravity in six dimensions, using a Kaluza-Klein-type $5+1$ split of coordinates and fields. We provide in particular a significantly simplified version of the bosonic action derived by us recently. This formulation employs fields that are no longer irreducible, subject to a local Lorentz invariance, which in turn simplifies the supersymmetry transformations including the exotic gravitino.

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arXiv2022

Cosmological Perturbations in Double Field Theory

We explore perturbative double field theory about time-dependent (cosmological) backgrounds to cubic order. To this order the theory is consistent in a weakly constrained sense, so that for a toroidal geometry it encodes both momentum and genuine winding modes. We give a self-contained discussion of the consistency problems and their resolution, including the role of cocycle factors and the $O(d,d,\mathbb{Z})$ duality. Finally, as a first step toward the computation of cosmological correlation functions, we propose a generalized scalar-vector-tensor decomposition and use it to construct gauge invariant generalized Bardeen variables. Compared to standard cosmology there are fewer tensor modes but more vector and scalar modes.

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arXiv2022

The Gauge Structure of Double Field Theory follows from Yang-Mills Theory

We show that to cubic order double field theory is encoded in Yang-Mills theory. To this end we use algebraic structures from string field theory as follows: The $L_{\infty}$-algebra of Yang-Mills theory is the tensor product ${\cal K}\otimes \mathfrak{g}$ of the Lie algebra $\mathfrak{g}$ of the gauge group and a `kinematic algebra' ${\cal K}$ that is a $C_{\infty}$-algebra. This structure induces a cubic truncation of an $L_{\infty}$-algebra on the subspace of level-matched states of the tensor product ${\cal K}\otimes \bar{\cal K}$ of two copies of the kinematic algebra. This $L_{\infty}$-algebra encodes double field theory. More precisely, this construction relies on a particular form of the Yang-Mills $L_{\infty}$-algebra following from string field theory or from the quantization of a suitable worldline theory.

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arXiv2022

Double Field Theory as the Double Copy of Yang-Mills

We show that double field theory arises from the color-kinematic double copy of Yang-Mills theory. A precise double copy prescription for the Yang-Mills action at quadratic and cubic order is provided that yields the double field theory action in which the duality invariant dilaton has been integrated out. More precisely, at quadratic order this yields the gauge invariant double field theory, while at cubic order it yields the cubic double field theory action subject to a gauge condition that originates from Siegel gauge in string field theory.

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arXiv2022

Duality invariant string beta functions at two loops

We compute, for cosmological backgrounds, the $O(d,d;\mathbb{R})$ invariant beta functions for the sigma model of the bosonic string at two loops. This yields an independent first-principle derivation of the order $α'$ corrections to the cosmological target-space equations. To this end we revisit the quantum consistency of Tseytlin's duality invariant formulation of the worldsheet theory. While we confirm the absence of gravitational (and hence Lorentz) anomalies, our results show that the minimal subtraction scheme is not applicable, implying significant technical complications at higher loops. To circumvent these we then change gears and use the Polyakov action for cosmological backgrounds, applying a suitable perturbation scheme that, although not $O(d,d;\mathbb{R})$ invariant, allows one to efficiently determine the $O(d,d;\mathbb{R})$ invariant beta functions.

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arXiv2021

Beta functions for the duality-invariant sigma model

The $O(d,d)$ invariant worldsheet theory for bosonic string theory with $d$ abelian isometries is employed to compute the beta functions and Weyl anomaly at one-loop. We show that vanishing of the Weyl anomaly coefficients implies the equations of motion of the Maharana-Schwarz action. We give a self-contained introduction into the required techniques, including beta functions, the Weyl anomaly for two-dimensional sigma models and the background field method. This sets the stage for a sequel to this paper on generalizations to higher loops and $α'$ corrections.

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arXiv2021

General String Cosmologies at Order $α'{}^{\,3}$

We compute the cosmological reduction of general string theories, including bosonic, heterotic and type II string theory to order $α'^{3}$, i.e., with up to eight derivatives. To this end we refine recently introduced methods that allow one to bring the reduced theory in one dimension to a canonical form with only first-order time derivatives. The resulting theories are compatible with a continuous $O(d,d,\mathbb{R})$ invariance, which in turn fixes the B-field couplings.

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arXiv2021

Homotopy Transfer and Effective Field Theory II: Strings and Double Field Theory

We continue our study of effective field theory via homotopy transfer of $L_\infty$-algebras, and apply it to tree-level non-Wilsonian effective actions of the kind discussed by Sen in which the modes integrated out are comparable in mass to the modes that are kept. We focus on the construction of effective actions for string states at fixed levels and in particular on the construction of weakly constrained double field theory. With these examples in mind, we discuss closed string theory on toroidal backgrounds and resolve some subtle issues involving vertex operators, including the proper form of cocycle factors and of the reflector state. This resolves outstanding issues concerning the construction of covariant closed string field theory on toroidal backgrounds. The weakly constrained double field theory is formally obtained from closed string field theory on a toroidal background by integrating out all but the doubly massless' states and homotopy transfer then gives a prescription for determining the theory's vertices and symmetries. We also discuss consistent truncation in the context of homotopy transfer.

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arXiv2021

Gauge Invariant Perturbation Theory via Homotopy Transfer

We show that the perturbative expansion of general gauge theories can be expressed in terms of gauge invariant variables to all orders in perturbations. In this we generalize techniques developed in gauge invariant cosmological perturbation theory, using Bardeen variables, by interpreting the passing over to gauge invariant fields as a homotopy transfer of the strongly homotopy Lie algebras encoding the gauge theory. This is illustrated for Yang-Mills theory, gravity on flat and cosmological backgrounds and for the massless sector of closed string theory. The perturbation lemma yields an algorithmic procedure to determine the higher corrections of the gauge invariant variables and the action in terms of these.

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arXiv2021

String Dualities at Order $α'^{\,3}$

We compute the cosmological reduction of the fourth powers of the Riemann tensor claimed to arise in string theory at order $α'^{\,3}$, with overall coefficient proportional to $ζ(3)$, and show that it is compatible with an $O(9,9)$ symmetry. This confirms the general result in string theory, due to Sen, that classical string theory with $d$-dimensional translation invariance admits an $O(d,d)$ symmetry to all orders in $α'$.

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arXiv2020

Old Dualities and New Anomalies

We revisit the question whether the worldsheet theory of a string admits a global O(d,d) symmetry. We consider the truncation of the target space theory in which fields are independent of d coordinates, which is O(d,d,R) invariant. The worldsheet theory is not O(d,d,R) invariant, unless it is truncated by setting winding and center-of-mass momenta to zero. We prove consistency of this truncation and give a manifestly O(d,d,R) invariant action, generalizing a formulation due to Tseytlin by including all external and internal target space fields. It is shown that, due to chiral bosons, this symmetry is anomalous. The anomaly is cancelled by a Green-Schwarz mechanism that utilizes the external B-field.

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arXiv2020

Homotopy Transfer and Effective Field Theory I: Tree-level

We use the dictionary between general field theories and strongly homotopy algebras to provide an algebraic formulation of the procedure of integrating out of degrees of freedom in terms of homotopy transfer. This includes more general effective theories in which some massive modes are kept while other modes of a comparable mass scale are integrated out, as first explored by Sen in the context of closed string field theory. We treat $L_\infty$-algebras both in terms of a nilpotent coderivation and, on the dual space, in terms of a nilpotent derivation (corresponding to the BRST charge of the field theory) and provide explicit formulas for homotopy transfer. These are then shown to govern the integrating out of degrees of freedom at tree level, while the generalization to loop level will be explored in a sequel to this paper.

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arXiv2020

Toward Exotic 6D Supergravities

We investigate exotic supergravity theories in 6D with maximal (4,0) and (3,1) supersymmetry, which were conjectured by C. Hull to exist and to describe strong coupling limits of ${\cal N}=8$ theories in 5D. These theories involve exotic gauge fields with non-standard Young tableaux representations, subject to (self-)duality constraints. We give novel actions in a 5+1 split of coordinates whose field equations reproduce those of the free bosonic (4,0) and (3,1) theory, respectively, including the (self-)duality relations. Evidence is presented for a master exceptional field theory formulation with an extended section constraint that, depending on the solution, produces the (4,0), (3,1) or the conventional (2,2) theory. We comment on the possible construction of a fully non-linear master exceptional field theory.

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arXiv2020

Leibniz Gauge Theories and Infinity Structures

We formulate gauge theories based on Leibniz(-Loday) algebras and uncover their underlying mathematical structure. Various special cases have been developed in the context of gauged supergravity and exceptional field theory. These are based on `tensor hierarchies', which describe towers of $p$-form gauge fields transforming under non-abelian gauge symmetries and which have been constructed up to low levels. Here we define `infinity-enhanced Leibniz algebras' that guarantee the existence of consistent tensor hierarchies to arbitrary level. We contrast these algebras with strongly homotopy Lie algebras ($L_{\infty}$ algebras), which can be used to define topological field theories for which all curvatures vanish. Any infinity-enhanced Leibniz algebra carries an associated $L_{\infty}$ algebra, which we discuss.

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arXiv2020

Duality Invariance and Higher Derivatives

We dimensionally reduce the spacetime action of bosonic string theory, and that of the bosonic sector of heterotic string theory after truncating the Yang-Mills gauge fields, on a $d$-dimensional torus including all higher-derivative corrections to first order in $α'$. A systematic procedure is developed that brings this action into a minimal form in which all fields except the metric carry only first order derivatives. This action is shown to be invariant under ${\rm O}(d,d,\mathbb{R})$ transformations that acquire $α'$-corrections through a Green-Schwarz type mechanism. We prove that, up to a global pre-factor, the first order $α'$-corrections are uniquely determined by ${\rm O}(d,d,\mathbb{R})$ invariance.

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