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Tomás Ortín

Publications and source records attributed to Tomás Ortín.

At least 19 recordsLinked to original sources

Local supersymmetry implies momentum map/Gauntlett-Pakis equations and vanishing generalized Komar forms

The first-order differential equations satisfied by Killing spinor bilinears (momentum map equations or Gauntlett--Pakis equations, Killing vector equations and the vanishing of the generalized Komar charge of the supersymmetric Killing vector) can be derived directly from the action of the local supersymmetry algebra on bosonic fields. We show how this comes about in several examples of increasing complexity, one of which is 11-dimensional supergravity.

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On the quantum statistical relation and the covariant phase space Hamiltonian

We show how the covariant phase space Hamiltonian can be constructed in theories in which some of the fields have gauge freedoms. The addition to the action of boundary terms that guarantee the well-posedness of a variational principle and its exact gauge invariance for the prescribed boundary conditions turn out to be essential in this construction. The same ingredients are essential in the use of the action in the framework of Gibbons and Hawking's Euclidean approach to Quantum Gravity and we show how the on-shell action constructed with a Lagrangian with those properties can be expressed in terms of the covariant phase space Hamiltonian. We compute the Hamiltonian of several theories, one of which has Chern--Simons terms, using our prescription and show that, in black-hole backgrounds, the Hamiltonian gives directly the quantum statistical relation including terms related to non-gravitational charges and chemical potentials, as expected.

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The Komar charges of type II supergravity and supersymmetric localization

We compute the on-shell closed, generalized Komar charges associated to an arbitrary Killing vector of the two 10-dimensional $\mathcal{N}=2$ supergravity theories which are the low-energy effective field theories of the type~IIA and IIB superstrings. We study different rewritings of this charge in terms of Page charges and forms with special properties that are used in the derivation of thermodynamical relations in black $p$-brane spacetimes. The, we show that, when the Killing vector is the supersymmetric Killing vector built as a bilinear of a Killing spinor, the momentum maps of the different fields of the theory are

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Derivation of the Smarr formula from the Komar charge in Einstein-nonlinear electrodynamics theories and applications to regular black holes

We construct the generalized Komar charge of generic, non-linear theories of electrodynamics (NLED) in 4 dimensions coupled to Einstein gravity. The contribution of the dimensionful coupling constant present in all these theories is obtained by promoting it to a dynamical field which is forced to be constant on-shell by a Lagrange multiplier. We use this charge to derive a Smarr formula for asymptotically-flat black-hole and soliton solutions of these theories that includes the contribution of the coupling constant. Previously, this contribution had been found using homogeneity arguments. We test our results on a broad class of Einstein--NLED theories and analyze in detail the thermodynamics of the regular Bardeen black hole using the conservation of the generalized Komar charge to understand the regularity of regular black holes inside the event horizon.

gr-qc↗

$α'$ corrections to self-dual gravitational instantons

We study the $α'$ corrections to self-dual gravitational instantons in the context of the four-dimensional Cano--Ruipérez action, which can be obtained by the compactification of the Bergshoeff--de Roo heterotic string effective action on $\mathbb{T}^{6}$ followed by a truncation and a field redefinition. We show that the metric of spaces of self-dual curvature does not receive any corrections, but their (initially trivial) dilaton and axion fields do, owing to their couplings to Gauss--Bonnet and Pontrjagin densities. We find the generic form of the corrections of the dilaton and axion fields for the Gibbons--Hawking multi-instanton solutions and their explicit form for the particular cases of the Euclidean Taub--NUT and Eguchi--Hanson spaces. We construct the boundary terms required to define a well-posed Dirichlet variational principle in the Euclidean Cano--Ruipérez theory, including the contributions associated with the Gauss--Bonnet and Pontrjagin terms. The boundary terms are normalized for asymptotically-locally-Euclidean solutions, and we evaluate with them the Euclidean action of the $α'$-corrected Eguchi--Hanson instanton showing that the total action receives no corrections to first order in $α'$. We also show that, at zeroth order in $α'$, one can construct Euclidean solutions similar to the string theory D-instanton with non-trivial dilaton and axion on the background of a self-dual purely gravitational instanton which remains unmodified. We also compute the $α'$ corrections to these solutions.

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On the generalized Komar charge of supersymmetric solutions

We consider the supersymmetric solutions of several supergravity theories (ungauged $\mathcal{N}=2,d=4$ and $\mathcal{N}=1,d=5$ supergravities coupled to vector supermultiplets and pure $\mathcal{N}=1,d=10$ supergravity) and show that their generalized Komar charges vanish identically for the supersymmetric Killing vector which is constructed as a bilinear of the Killing spinors of those solutions. This property can be used to prove the supersymmetry (``BPS'') bounds satisfied by some of those solutions in a rigorous, coordinate-independent way.

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Higher-form symmetries in supergravity, scalar charges and black-hole thermodynamics

Minimal 5-dimensional supergravity compactified on a circle gives the T$^{3}$ model of $\mathcal{N}=2,d=4$ supergravity, whose duality group is SL$(2,\mathbb{R})$. We study exhaustively the relations between all the local and global symmetries of both theories and between the corresponding conserved currents and charges, including the on-shell closed generalized Komar charges associated to isometries. We find that the 2-dimensional subgroup of SL$(2,\mathbb{R})$ that does not include electric-magnetic transformations is realized as a higher-form symmetry group that acts on the 5-dimensional metric and vector field. Using the generalized Komar charges we compute the Smarr formulas for black holes, showing that they are identical once the relations between all the 5- and 4-dimensional thermodynamical quantities are taken into account, which is only possible if certain constraints on the fields are satisfied. We notice that on-shell closed 5-dimensional 3-form charges give, upon dimensional reduction, on-shell closed 3-form currents and 2-form charges. The dimensional reduction of the 5-dimensional generalized Komar 3-form charge associated to a Killing vector gives a new 4-dimensional on-shell closed 3-form current which must be associated to a new global symmetry of the theory when it admits that Killing vector. Some of the results that we have derived are valid for theories of Einstein--Maxwell-like theories of $(p+1)$-forms with Chern--Simons terms, which includes 11-dimensional supergravity as a particular example.

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On-shell Lagrangians as total derivatives and the generalized Komar charge

Lagrangians which transform homogeneously under a global transformation of the fields (a global rescaling, for instance) can be written on-shell as a total derivative which has a universal, solution-independent expression, using a functional version of the Euler theorem for homogeneous functions. We study the uniqueness of this expression and how this result can be used in the construction of generalized Komar charges.

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The Komar charge in presence of the Holst term and the gravitational Witten effect

In the first-order formalism, the Einstein--Hilbert action can be modified by the addition of a Holst term multiplied by the Barbero parameter $α$. This modification breaks parity although it does not affect the equations of motion. We show that the standard Komar charge is also modified by the addition of a topological term multiplied by the Barbero parameter $α$. For the Killing vector that generates time translations, the value of the Komar integral at infinity is modified by the addition of a term proportional to the NUT charge $N$ and the parity-breaking Barbero parameter. Thus, as in the standard Witten effect, a non-vanishing NUT charge $N$ induces a non-vanishing mass $αN$.

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A note on the calculation of the Komar integral in the Lorentzian Taub-NUT spacetime

It has recently been shown that one can derive consistent thermodynamical expressions in the Lorentzian Taub--NUT spacetime keeping the Misner-string singularities and taking into account their contributions in the Komar integrals. We show how the same results are obtained when the Mister-string singularities are removed by using Misner's procedure because, even though the complete spacetime has no such singularities anymore, they are unavoidable in all spacelike hypersurfaces which are used in the Komar integrals. Different choices of hypersurfaces may contain different strings and lead to different physics, though.

gr-qc↗

Komar charge of N = 2 supergravity and its superspace generalization

We find the defining equations for a Killing vector and its superpartner (called the generalized Killing spinor) and use them to construct the generalized Komar superform of minimal N=2, d=4 supergravity using the superspace formulation. The superspace procedure presented here can be used for the construction of generalized Komar forms in more general supergravity theories. We also present the (more cumbersome) calculation of the generalized Komar 2-form and of the on-shell closed 2-form used to prove the first law in the component formalism, as an independent confirmation of our main result.

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On the thermodynamics of the black holes of the Cano-Ruipérez 4-dimensional string effective action

The Cano-Ruipérez 4-dimensional string effective action is the simplest consistent truncation of the first-order in $α'$ heterotic string effective action compactified on T$^{6}$. This theory contains, on top of the metric, a dilaton and an axion that couple to the Gauss-Bonnet term and to the Pontrjagin density which suggests a very strong relation between the physical and geometrical properties of its solutions. In this paper we study the thermodynamics of the string black-hole solutions of this theory using Wald's formalism. We construct the on-shell closed generalized Komar, dilaton and axion 2-form charges, and we used them to find the Smarr formula (that we test on the Schwarzschild and Kerr solutions) and to prove no-(primary)-hair theorems for the scalar. We find that $α'$ plays the role of a thermodynamical variable with an associated chemical potential. We also notice the occurrence of non-standard gravitational charges in the Wald entropy and scalar charges.

hep-th↗

Noether-Wald and Komar charges in supergravity, fermions, and Killing supervectors in superspace

The supersymmetry properties of Killing vectors and spinors in supergravity theory can be clarified by relating them to Killing supervectors in the supergravity superspace. In the superspace approach it is manifest that supersymmetry 'mixes' a Killing vector with its fermionic spinor 'superpartner' and the Killing equations with the generalization of the Killing spinor equations. The latter reduces to the standard Killing spinor equation, albeit with a fermionic spinor, when the fermionic fields are set to zero. Using these supersymmetry transformations in the spacetime component approach, we construct a Noether-Wald charge of ${\cal N}=1$, $D=4$ supergravity with fermionic contributions which is diff-, Lorentz- and supersymmetry-invariant (up to a total derivative). The Killing supervector formalism for the maximal $D=11$ supergravity and some related issues are also discussed.

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Gravitational higher-form symmetries and the origin of hidden symmetries in Kaluza-Klein compactifications

We show that, in presence of isometries and non-trivial topology, the Einstein--Hilbert action is invariant under certain transformations of the metric which are not diffeomorphisms. These transformations are similar to the higher-form symmetries of field theories with $p$-form fields. In the context of toroidal Kaluza--Klein compactifications, we show that these symmetries give rise to some of the ``hidden symmetries'' (dualities) of the dimensionally-reduced theories.

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Extremal stringy black holes in equilibrium at first order in $α'$

We compute the first-order $α'$ corrections to well-known families of heterotic multi-center black-hole solutions in five and four dimensions. The solutions can be either supersymmetric or non-supersymmetric, depending on the relative sign between two of the black-hole charges. For both cases, we find that the equilibrium of forces persists after including the $α'$ corrections, as the existence of multi-center solutions free of unphysical singularities shows. We analyze the possibility of black-hole fragmentation.

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Singular limits of spacetimes and their isometries

We consider spacetime metrics with a given (but quite generic) dependence on a dimensionful parameter such that in the 0 and infinity limits of that parameter the metric becomes singular. We study the isometry groups of the original spacetime metrics and of the singular metrics that arise in the limits and the corresponding symmetries of the motion of p-branes evolving in them, showing how the Killing vectors and their Lie algebras can be found in general. We illustrate our general results with several examples which include limits of anti-de Sitter spacetime in which the holographic screen is one of the singular metrics and of pp-waves.

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