arXiv subjects

Samuele Chimento

Publications and source records attributed to Samuele Chimento.

13 recordsLinked to original sources

arXiv2019

$α'$ corrections of Reissner-Nordström black holes

We study the first-order in $α'$ corrections to non-extremal 4-dimensional dyonic Reissner-Nordström (RN) black holes with equal electric and magnetic charges in the context of Heterotic Superstring effective field theory (HST) compactified on a $T^{6}$. The particular embedding of the dyonic RN black hole in HST considered here is not supersymmetric in the extremal limit. We show that, at first order in $α'$, consistency with the equations of motion of the HST demands additional scalar and vector fields become active, and we provide explicit expressions for all of them. We determine analytically the position of the event horizon of the black hole, as well as the corrections to the extremality bound, to the temperature and to the entropy, checking that they are related by the first law of black-hole thermodynamics, so that $\partial S/\partial M=1/T$. We discuss the implications of our results in the context of the Weak Gravity Conjecture, clarifying that entropy corrections for fixed mass and charge at extremality do not necessarily imply corrections to the extremal charge-to-mass ratio.

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arXiv2018

Beyond the near-horizon limit: Stringy corrections to Heterotic Black Holes

We study the first-order in $α'$ corrections to 4-charge black holes (with the Reissner-Nordström black hole as a particular example) beyond the near-horizon limit in the Heterotic Superstring effective action framework. The higher-curvature terms behave as delocalized sources in the equations of motion and in the Bianchi identity of the 3-form. For some charges, this introduces a shift between their values measured at the horizon and asymptotically. Some of these corrections and their associated charge shifts, but not all of them, can be canceled using appropriate SU$(2)$ instantons for the heterotic gauge fields. The entropy, computed using Wald's formula, is in agreement with the result obtained via microstate counting when the delocalized sources are properly taken into account.

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arXiv2018

Regular Stringy Black Holes?

We study the first-order $α'$ corrections to the singular 4-dimensional massless stringy black holes studied in the nineties in the context of the Heterotic Superstring. We show that the $α'$ corrections not only induce a non-vanishing mass and give rise to an event horizon, but also eliminate the singularity giving rise to a regular spacetime whose global structure includes further asymptotically flat regions in which the spacetime's mass is positive or negative. We study the timelike and null geodesics and their effective potential, showing that the spacetime is geodesically complete. We discuss the validity of this solution, arguing that the very interesting and peculiar properties of the solution are associated to the negative energy contributions coming from the terms quadratic in the curvature. As a matter of fact, the 10-dimensional configuration is singular. We extract some general lessons on attempts to eliminate black-hole singularities by introducing terms of higher order in the curvature.

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arXiv2018

On a family of $α'$-corrected solutions of the Heterotic Superstring effective action

We compute explicitly the first-order in $α'$ corrections to a family of solutions of the Heterotic Superstring effective action that describes fundamental strings with momentum along themselves, parallel to solitonic 5-branes with Kaluza-Klein monopoles (Gibbons-Hawking metrics) in their transverse space. These solutions correspond to 4-charge extremal black holes in 4 dimensions upon dimensional reduction on $\mathrm{T}^{6}$. We show that some of the $α'$ corrections can be cancelled by introducing solitonic $\mathrm{SU}(2)\times \mathrm{SU}(2)$ Yang-Mills fields, and that this family of $α'$-corrected solutions is invariant under $α'$-corrected T-duality transformations. We study in detail the mechanism that allows us to compute explicitly these $α'$ corrections for the ansatz considered here, based on a generalization of the 't Hooft ansatz to hyperKähler spaces.

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arXiv2018

Supersymmetric solutions of the cosmological, gauged, C magic model

We construct supersymmetric solutions of theories of gauged $\mathcal{N}=1,d=5$ supergravity coupled to vector multiplets with a $\mathrm{U}(1)_{\rm R}$ Abelian (Fayet-Iliopoulos) gauging and an independent SU$(2)$ gauging associated to an $\mathrm{SU}(2)$ isometry group of the Real Special scalar manifold. These theories provide minimal supersymmetrizations of 5-dimensional SU$(2)$ Einstein-Yang-Mills theories with negative cosmological constant. We consider a minimal model with these gauge groups and the "magic model" based on the Jordan algebra $\bf{J}_{3}^\mathbb{C}$ with gauge group $\mathrm{SU}(3)\times\mathrm{U}(1)_{\rm R}$, which is a consistent truncation of maximal $\mathrm{SO}(6)$-gauged supergravity in $d=5$ and whose solutions can be embedded in Type IIB Superstring Theory. We find several solutions containing selfdual $\mathrm{SU}(2)$ instantons, some of which asymptote to AdS$_{5}$ and some of which are very small, supersymmetric, deformations of AdS$_{5}$. We also show how some of those solutions can be embedded in Romans' $\mathrm{SU}(2)\times\mathrm{U}(1)$-gauged half-maximal supergravity, which was obtained by Lu, Pope and Tran by compactification of the Type IIB Superstring effective action. This provides another way of uplifting those solutions to 10 dimensions.

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arXiv2017

Yang-Mills instantons in Kaehler spaces with one holomorphic isometry

We consider self-dual Yang-Mills instantons in 4-dimensional Kaehler spaces with one holomorphic isometry and show that they satisfy a generalization of the Bogomol'nyi equation for magnetic monopoles on certain 3-dimensional metrics. We then search for solutions of this equation in 3-dimensional metrics foliated by 2-dimensional spheres, hyperboloids or planes in the case in which the gauge group coincides with the isometry group of the metric (SO(3), SO(1,2) and ISO(2), respectively). Using a generalized hedgehog ansatz the Bogomol'nyi quations reduce to a simple differential equation in the radial variable which admits a universal solution and, in some cases, a particular one, from which one finally recovers instanton solutions in the original Kaehler space. We work out completely a few explicit examples for some Kaehler spaces of interest.

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arXiv2017

On timelike supersymmetric solutions of Abelian gauged 5-dimensional supergravity

We consider 5-dimensional gauged supergravity coupled to Abelian vector multiplets, and we look for supersymmetric solutions for which the 4-dimensional Kähler base space admits a holomorphic isometry. Taking advantage of this isometry, we are able to find several supersymmetric solutions for the ST$[2,n_v+1]$ special geometric model with arbitrarily many vector multiplets. Among these there are three families of solutions with $n_v+2$ independent parameters, which for one of the families can be seen to correspond to $n_v+1$ electric charges and one angular momentum. These solutions generalize the ones recently found for minimal gauged supergravity in JHEP 1704 (2017) 017 and include in particular the general supersymmetric asymptotically-AdS$_5$ black holes of Gutowski and Reall, analogous black hole solutions with non-compact horizon, the three near horizon geometries themselves, and the singular static solutions of Behrndt, Chamseddine and Sabra.

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arXiv2017

On timelike supersymmetric solutions of gauged minimal 5-dimensional supergravity

We analyze the timelike supersymmetric solutions of minimal gauged 5-dimensional supergravity for the case in which the Kähler base manifold admits a holomorphic isometry and depends on two real functions satisfying a simple second-order differential equation. Using this general form of the base space, the equations satisfied by the building blocks of the solutions become of, at most, fourth degree and can be solved by simple polynomic ansatzs. In this way we construct two 3-parameter families of solutions that contain almost all the timelike supersymmetric solutions of this theory with one angular momentum known so far and a few more: the (singular) supersymmetric Reissner-Nordström-AdS solutions, the three exact supersymmetric solutions describing the three near-horizon geometries found by Gutowski and Reall, three 1-parameter asymptotically-AdS$_{5}$ black-hole solutions with those three near-horizon geometries (Gutowski and Reall's black hole being one of them), three generalizations of the Gödel universe and a few potentially homogenous solutions. A key rôle in finding these solutions is played by our ability to write AdS$_{5}$'s Kähler base space ($\overline{\mathbb{CP}}^{2}$ or SU$(1,2)/$U$(2)$) is three different, yet simple, forms associated to three different isometries. Furthermore, our ansatz for the Kähler metric also allows us to study the dimensional compactification of the theory and its solutions in a systematic way.

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arXiv2016

On 2-dimensional Kaehler metrics with one holomorphic isometry

We show how to write any Kaehler metric of complex dimension 2 admitting a holomorphic isometry as a simple 1-real-function deformation of a Gibbons-Hawking metric. Hyper-Kaehler metrics with a tri-holomorphic isometry (Gibbons-Hawking metrics) or with a mono-holomorphic isometry are recovered for particular values of the additional function. The new general metric can be used as an Ansatz in several interesting physical problems.

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arXiv2015

Supersymmetric black holes and attractors in gauged supergravity with hypermultiplets

We consider four-dimensional $N=2$ supergravity coupled to vector- and hypermultiplets, where abelian isometries of the quaternionic Kähler hypermultiplet scalar manifold are gauged. Using the recipe given by Meessen and Ort\'ın in arXiv:1204.0493, we analytically construct a supersymmetric black hole solution for the case of just one vector multiplet with prepotential ${\cal F}=-iχ^0χ^1$, and the universal hypermultiplet. This solution has a running dilaton, and it interpolates between $\text{AdS}_2\times\text{H}^2$ at the horizon and a hyperscaling-violating type geometry at infinity, conformal to $\text{AdS}_2\times\text{H}^2$. It carries two magnetic charges that are completely fixed in terms of the parameters that appear in the Killing vector used for the gauging. In the second part of the paper, we extend the work of Bellucci et al. on black hole attractors in gauged supergravity to the case where also hypermultiplets are present. The attractors are shown to be governed by an effective potential $V_{\text{eff}}$, which is extremized on the horizon by all the scalar fields of the theory. Moreover, the entropy is given by the critical value of $V_{\text{eff}}$. In the limit of vanishing scalar potential, $V_{\text{eff}}$ reduces (up to a prefactor) to the usual black hole potential.

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arXiv2014

Rotating black holes in an expanding universe from fake supergravity

Using the recipe of arXiv:0902.4814, where all fake supersymmetric backgrounds of matter-coupled fake N=2, d=4 gauged supergravity were classified, we construct dynamical rotating black holes in an expanding FLRW universe. This is done for two different prepotentials that are both truncations of the stu model and correspond to just one vector multiplet. In this scenario, the cosmic expansion is driven by two U(1) gauge fields and by a complex scalar that rolls down its potential. Generically, the solutions of arXiv:0902.4814 are fibrations over a Gauduchon-Tod base space, and we make three different choices for this base, namely flat space, the three-sphere and the Berger sphere. In the first two cases, the black holes are determined by harmonic functions on the base, while in the last case they obey a deformed Laplace equation that contains the squashing parameter of the Berger sphere. This is the generalization to a cosmological context of the usual recipe in ungauged supergravity, where black holes are given in terms of harmonic functions on three-dimensional Euclidean space. The constructed solutions may be instrumental in addressing analytically questions like black hole collisions and violation of cosmic censorship.

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arXiv2013

Multi-centered black holes with a negative cosmological constant

We present a recipe that allows to construct multi-centered black holes embedded in an arbitrary FLRW universe. These solutions are completely determined by a function satisfying the conformal Laplace equation on the spatial slices $\text{E}^3$, $\text{S}^3$ or $\text{H}^3$. Since anti-de Sitter space can be written in FLRW coordinates, this includes as a special case multi-centered black holes in AdS, in the sense that, far away from the black holes, the energy density and the pressure approach the values given by a negative cosmological constant. We study in some detail the physical properties of the single-centered asymptotically AdS case, which does not coincide with the usual Reissner-Nordström-AdS black hole, but is highly dynamical. In particular, we determine the curvature singularities and trapping horizons of this solution, compute the surface gravity of the trapping horizons, and show that the generalized first law of black hole dynamics proposed by Hayward holds in this case. It turns out that the spurious big bang/big crunch singularities that appear when one writes AdS in FLRW form, become real in presence of these dynamical black holes. This implies that actually only one point of the usual conformal boundary of AdS survives in the solutions that we construct. Finally, a generalization to arbitary dimension is also presented.

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arXiv2012

Black holes in an expanding universe from fake supergravity

In arXiv:0902.4814, a general recipe to construct fake supersymmetric solutions to fake N=2, d=4 gauged supergravity coupled to abelian vector multiplets was presented. We use these results to find new multi-centered black hole solutions in an asymptotically FLRW universe. These satisfy the weak energy condition and are maximally charged under two U(1) gauge fields coupled to a scalar, which drives the cosmic expansion while rolling down its potential. As a special subcase, our black holes include the ones constructed previously by Gibbons and Maeda in arXiv:0912.2809. The latter contain two non-negative real numbers n_S, n_T obeying the constraint n_S+n_T=4, with the cases n_T=4 and n_T=1 corresponding to the Kastor-Traschen and the Maeda-Ohta-Uzawa solution respectively. We show that n_S, n_T arise directly as exponents in the prepotential of the fake supergravity theory, and that the above constraint stems from the fact that the prepotential must be a homogeneous function of degree two. Finally, some physical properties of the black holes, like asymptotic behaviour, curvature singularities and trapping horizons, are also discussed. Similar to other solutions that appeared previously in the literature, there is a symmetry enhancement near the event horizon, which becomes therefore a Killing horizon, in spite of the highly dynamical nature of the original spacetime. The temperature associated to this Killing horizon turns out to be nonvanishing.

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