# 1st Pacific Summer School

# Theory and Observations in

Einstein’s Theory of Gravity and its Modifications

**January 17-23, 2024**

Universidad Central de Chile, Santiago

Universidad Central de Chile, Santiago

## SUMMER SCHOOL

The **1 ^{st} The 1st Pacific Summer School: Theory and Observations in Einstein’s Theory of Gravity and its Modifications**, will take place from January 17th to January 23rd in Santiago, Chile.

This school will be held at Vicente Kovacevic II building, Facultad de Ingeniería y Arquitectura, Universidad Central de Chile.

This inaugural edition marks the beginning of a series of schools to be held in the South Pacific region, which aims to convene local and foreign M.Sc. and Ph.D. students, postdoc researchers, and leading experts in gravitation. The primary focus is to discuss the remarkable recent developments in both theory and observations in cosmology, gravitational waves, holography, black holes, and other compact objects within the context of general relativity and modified theories of gravity. Participants, especially students, will have the opportunity to learn advanced solution-generating techniques for general relativity, as well as an introduction to scalar-tensor theories and their applications. This event will be a suitable opportunity to assess new areas of contact and facilitate ongoing collaborations while promoting new research partnerships.

**Organizers:** Cristián Erices, Eleftherios Papantonopoulos, Alberto Faraggi

## PLENARY INVITED SPEAKERS

TO BE ANNOUNCED SOON

## LECTURERS

TO BE ANNOUNCED SOON

## PARTICIPANTS LIST

Last Name | Affiliation | Country of affiliation |
---|---|---|

Alemán López Guillermo Enrique | Universidad Federeal Juiz de Fora | Brazil |

Anastasiou Georgios | Universidad Arturo Prat | Chile |

Astudillo Bastián | Universidad Andrés Bello | Chile |

Bravo Moises | Universidad católica del Maule | Chile |

Bahamondes Sebastian | Pontificia Universidad Católica de Chile | Chile |

Busnego Mairym | UNAB | Chile |

Cáceres Nicolás | Pontificia Universidad Católica de Chile | Chile |

Calero Nicolás | Escuela Politécnica Nacional | Ecuador |

Corral Cristóbal | UNAP | Chile |

Caro Perez Fabian | Universidad de Antofagasta | Chile |

Castillo-Felisola Oscar | Universidad Tecnica Federico Santa Maria | Chile |

Cruz Miguel | Universidad Veracruzana | Brazil |

Cruz Palma Ignacio | Pontificia Universidad Católica de Chile | Chile |

Cuadros-Melgar Bertha | University of Sao Paulo | Brazil |

Dhankar Praveen Kumar | G H Raisoni College of Engineering, Nagpur | India |

Fabian Caro Perez | Universidad de Antofagasta | Chile |

Fernández Jose | None | |

Flores Alfonso Daniel | Instituto de Ciencias Exactas y Naturales | Chile |

Garcia de Andrade Luiz Carlos | Dept de física teórica UERJ and institute for cosmology Croatia | Brazil |

Galvez Rafael | Universidad Andres Bello | Chile |

Guzmán Alejandro | Pontificia Universidad Católica de Valparaíso | Chile |

Gallardo Sebastian Luciano | Instituto Balseito | Argentina |

Hassan Puttasiddappa | Prajwal Universidade Federal do Espírito Santo | Brazil |

Hernández Benjamin | Universidad de Concepción | Chile |

Hosseini Hamideh | Diego Portales Unversity | Chile |

Kakkat Vishnu | University of South Africa | South Africa |

Kalita Dibash | Gauhati University | India |

Karna Charuchandra | Central University of Haryana | India |

Labrin Oriana | Pontificia Universidad Católica de Valparaíso | Chile |

Luis Guajardo | Universidad de Talca | Chile |

Mishra Sai Swagat | BITS PILANI, Hyderabad Campus | India |

Mora Alarcón Nicolás | Universidad de Concepción | Chile |

Moreno Javier Haifa | U-Technion | Israel |

Molina Martin | Pontificia Universidad Catolica de Valparaíso | Chile |

Núñez-Barra Camilo | Pontificia Universidad Católica de Chile | Chile |

Petridis Savvas | National technical university of Athens | Greece |

Pérez Claudio | Universidad Andrés Bello | Chile |

Peña-Donaire | Valentín Pontificia Universidad Católica de Chile | Chile |

Pundeer Naeem Ahmad | Jadavpur University, Kolkata | India |

Pinochet Nicolás | Pontificia Universidad Católica de Valparaíso | Chile |

Riquelme Simón | Universidad San Sebastián/CECs | |

Rivera Marco | UDEC- MIRO | Chile |

Rozas Marcelo | Pontificia universidad católica de Valparaíso | Chile |

Ramírez de Arellano Torres Cielo Estela | Universidad de Concepción | Chile |

Rubio Carlos | Universidad Adolfo Ibañez | Chile |

Sempe Matías Nicolás | Universidad Nacional de La Plata- CONICET | Argentina |

Sahoo Pradyumn | BITS-Pilani, Hyderabad Campus | India |

Santos Fabiano F. | Universidade Federal do Maranhão | Brazil |

Sanhueza Leonardo | Universidad de Concepción | Chile |

Saridakis Emmanuel | National Observatory of Athens | Greece |

Tlatelpa Mascote Brenda | Universidad Autónoma Metropolitana Unidad Iztapalapa | México |

Trigo Fabian | Universidad de Valparaiso | Chile |

Varas Alejandro | Pontificia Universidad Católica de Valparaíso | Chile |

Videla Nelson | Pontificia Universidad Católica de Valparaíso | Chile |

Villanueva J.R. | Universidad de Valparaíso | Chile |

Zamani Saboura sadat | University of Szczecin | Poland |

## SCHEDULE

TO BE ANNOUNCED SOON

## CLOUD OF ABSTRACTS

## Georgios Anastasiou

## TBA

TBA

## Cristóbal Corral

## Axial anomaly in nonlinear conformal electrodynamics

We study the axial anomaly of Dirac spinors on gravitational instanton backgrounds in the context of nonlinear electrodynamics. In order to do so, we consider Einstein gravity minimally coupled to a recently proposed conformal electrodynamics that enjoys duality transformation invariance. These symmetries allow us to generalize the Eguchi-Hanson configuration while preserving its geometry. We then compute the Dirac index of the nonlinearly charged Eguchi-Hanson and Taub-NUT configurations. We find that there is an excess of positive chiral Dirac fermions over the negative ones which triggers the anomaly.

## Oscar Castillo-Felisola

## Cosmological perturbations in affine gravaties

A basic premise in cosmology is that the Universe is isotropic and homogeneous. However, the most interesting observations might be explained only after considering perturbations departing from the initial conception. We propose a method that extend the well-known methods of cosmological perturbations in metric models of gravity to affine models of gravity.

## Nicolás Cáceres

## Holographic renormalization of Horndeski gravity

We study the renormalization of a particular sector of Horndeski theory. We focus on the nonminimal coupling of a scalar field to the Gauss-Bonnet term through an arbitrary function of the former plus a kinetic coupling to the Einstein tensor. In the asymptotically AdS sector of the theory, we perform a near-boundary expansion of the fields and we work out the asymptotic form of the action and its variation. By assuming a power expansion of the scalar coupling function and the Gauss-Bonnet term, we find specific conditions on their coefficients such that the action and charges are finite. To accomplish the latter, a finite set of intrinsic boundary terms has to be added. If the nonminimal kinetic coupling is absent, the trace of the holographic stress-energy tensor cannot be zero while the dual CFT remains unitary as the scalar mass lies outside the Breitenlohner-Freedman bound. However, if one considers the kinetic coupling to the Einstein tensor, we find that its contribution allows one to recover the unitarity of the dual CFT, motivating the introduction of that term from a holographic viewpoint

## Bertha Cuadros-Melgar

## Black Holes with Yukawa Potential: Stability and Shadows

We consider a Yukawa-modified potential in order to obtain a spherically symmetric black hole solution. The main modification that the potential introduces is the rescaling of the mass of the black hole mimicking the dark matter effects at large distances from the black hole. We test the dynamical stability of this solution with scalar, electromagnetic, and gravitational perturbations and study the correspondence of the eikonal limit of the quasinormal frequencies with the shadow radius of the black hole. In addition, we introduce astronomical parameters to determine the deviation of these quantities from a Schwarzschild solution. We also consider the thermodynamic stability and found the possibility of first order phase transitions.

## Alfonso Daniel Flores

## Geometric Topology and the Gravitational Cheshire Effect

Recently, it has been shown that the fundamental geometrical structures of three-dimensional manifolds are in fact solutions of massive gravity. In this talk, we discuss the intrinsically Lorentzian analogues of these structures framed within general relativity. Physically, the systems we discuss include gravitational waves, self-gravitating electromagnetic radiation and gravitational stealth fields.

## Luiz Carlos García de Andrade

## Inflationary hypermagnetogenesis in Einstein Cartan portal

Following the non minimal coupling with torsion trace and scalar fields we investigate gauge bosons in Einstein Cartan portal. It is show that the hypermagnetic field is generalised tô include an exponential factor of torsion trace potential which depending upon it’s chirality van give rise tô dynamo action or decay of hypermagnetic fields. Constraints in torsion trace are given from LHC data.

## Prajwal Hassan Puttasiddappa

## Probing Modified Gravity at the Galactic Center: Insights from Stellar Orbits

**Tentative Abstract**: I would like to outline our research results pertaining to constraints on scalar tensor theories and f(R,T) and f(R, L) theories. Utilizing the orbital dynamics of stars around the galactic center, we constrain the parameters induced by a fifth force in these theories. This force originates from either a scalar field or novel couplings between spacetime curvature and matter.

## Praveen Kumar Dhankar

## Holographic dark energy with scalar field in higher dimensional space-time

In this paper, we have generalized the work of Sheykhi (Phys. Rev. D, 84: 107302, 2011) by choosing Hubble radius as system’s IR cut-off and construct the analytical form of the potentials as a function of scalar field, namely V = V (\phi) as well as this dynamics of the scalar fields as a function of time, namely \phi = \phi(t) then we have implemented the connection between HDE and scalar elds model including quintessence, tachyon, K-essence and dilaton energy density in higher dimensional spacetime FRW universe. We analyzed the physical and geometrical interpretations of the cosmological model with reference to the higher dimensional spacetime.

## Vishnu Kakkat

## Application of harmonic maps to higher-dimensional black holes

Extending recent work in 5 dimensions, we prove the existence and uniqueness of solutions to the reduced Einstein equations for vacuum black holes in $(n+3)$-dimensional spacetimes admitting the isometry group $\mathbb{R}\times U(1)^{n}$, with Kaluza-Klein asymptotics for $n\geq3$. This is equivalent to establishing existence and uniqueness for singular harmonic maps $\varphi\colon \R^3\setminus\Gamma\to SL(n+1,\R)/SO(n+1)$ with prescribed blow-up along $\Gamma$, a subset of the $z$-axis in $\R^3$. We also analyze the topology of the domain of outer communication for these spacetimes, by developing an appropriate generalization of the plumbing construction used in the lower dimensional case.

## Javier Moreno

## Classifying generalized quasitopological gravities

Generalized quasitopological gravities (GQTGs) are higher-order extensions of Einstein gravity in D dimensions satisfying a number of interesting properties, such as possessing second-order linearized equations of motion on top of maximally symmetric backgrounds, admitting nonhairy generalizations of the Schwarzschild-Tangherlini black hole which are characterized by a single metric function or forming a perturbative spanning set of the space of effective theories of gravity. In this work, we classify all inequivalent GQTGs at all curvature orders n and spacetime dimension D≥4. This is achieved after the explicit construction of a dictionary that allows the uplift of expressions evaluated on a single-function static and spherically symmetric ansatz into fully covariant ones. On the one hand, applying such prescription for D≥5, we find the explicit covariant form of the unique inequivalent quasitopological gravity that exists at each n and, for the first time, the covariant expressions of the n-2 inequivalent proper GQTGs existing at every curvature order n. On the other hand, for D=4, we are able to provide the first rigorous proof of the fact that there is one, and only one, (proper) inequivalent GQTG at each curvature order n, deriving along the way a simple expression for such four-dimensional representative at every order n.

## Simón Riquelme

## TBA

TBA

## Marco Rivera

## Estimation of the Post-Newtonian parameters of Earth's gravitational field using a Hong-Ou-Mandel interferometer

The Hong-Ou-Mandel (HOM) effect is analyzed for photons in a modified Mach-Zehnder setup with two particles experiencing different gravitational potentials, which are later recombined using a beam-splitter. We find that the HOM effect depends directly on the relativistic time dilation between the arms of the setup. This temporal dilation can be used to estimate the $\gamma$ and $\beta$ parameters of the parameterized post-Newtonian formalism, by doing this we find relative uncertainties for the estimation of both parameters as measured by an observer at the endpoint of the array. Afterwards, we switch our strategy and consider estimating the parameters through the probability of the observable of detection in coincidence, which depends on the quantum states entering the array. Considering separable states and two-mode squeezing vacuum states yields significant improvements in the estimation of the $\gamma$ parameter, while the estimation of $\beta$ does not improve significantly. Finally, we implement Bayesian estimation for both approaches and find that the estimation of $\beta$ through measurement of time delays is not improved, while the estimation of $\gamma$ is dramatically enhanced. Also, a similar analysis shows that using coincidence detection probability outperforms all other methods and allows to make precise estimations for both parameters.

## Carlos Rubio

## Hubble tension and matter inhomogeneities: a theoretical perspective.

We have studied how local density perturbations could reconcile the Hubble tension. We reproduced a local void through a perturbed FLRW metric with a potential Phi which depends on both time and space. This method allowed us to obtain a perturbed luminosity distance, which is compared with both local and cosmological data. However, when constraining local cosmological parameters with previous results, we found that neither LambdaCDM nor omega(Lambda)CDM cannot solve the Hubble tension.

## Sai Swagat Mishra

## H0 tension in torsion-based modified gravity

The rising concern in the Hubble constant tension of the cosmological models motivates the scientific community to search for alternative cosmological scenarios that could resolve the tension. In this regard, we aim to work on a torsion-based modified theory of gravity which is an alternative description to the coherence model. We solve numerically for the Hubble parameter using two exponential Lagrangian functions of torsion T and a trace of energy-momentum tensor for the dust case. Further, we constrain the cosmological and model parameters; to do that, we use Hubble, SNe Ia, Baryon Acoustic Oscillations, Cosmic Microwave Background samples, and Markov Chain Monte Carlo (MCMC) simulation through Bayesian statistics. We obtain the values of Hubble constant for our model, and the outputs align with the recent observational measurements of H0. In addition, we check the deviation of our results from model-independent measurements of H0 from Planck2018, SH0ES, and LiCOW experiments. In contrast, our finding partially solved the tension but gave a new possible direction to alleviate the tension.

## Fabiano F. Santos

## AdS/BCFT holographic transport in Horndeski

We present an investigation of the thermodynamics and hydrodynamics of a five-dimensional black hole in the presence of an external magnetic field. The solution is the gravity dual to an Anti-de-Sitter/Boundary Conformal Field Theory (AdS/BCFT) correspondence. For this, we will establish the correspondence AdS$_{5}$/BCFT$_{4}$, and with it, we will study the properties of an anisotropic fluid with an external magnetic field. Using holographic renormalization we compute the free energy and holographic stress tensor residing on boundary Q. From the point of view of the fluid/gravity correspondence, we have a class of boundary extensions existing in boundary Q, for which the stress-energy tensor describes a conformal fluid.

## Pradyumn Sahoo

## H_0 H0 tension in torsion-based gravity

The rising concern in the Hubble constant tension ($H_0$ tension) of the cosmological models motivates the scientific community to search for alternative cosmological scenarios that could resolve the $H_0$ tension. In this regard, we aim to work on a torsion-based modified theory of gravity which is an alternative description to the coherence model. We solve numerically for the Hubble parameter using two exponential Lagrangian functions of torsion $T$ and a trace of energy-momentum tensor $\mathcal{T}$ for the dust case. Further, we constrain the cosmological and model parameters; to do that, we use Hubble, SNe Ia, Baryon Acoustic Oscillations, Cosmic Microwave Background samples, and Markov Chain Monte Carlo (MCMC) simulation through Bayesian statistics. We obtain the values of Hubble constant $H_0$ for our model, and the outputs align with the recent observational measurements of $H_0$. In addition, we check the deviation of our results from model-independent measurements of $H_0$ from Planck2018, S$H_0$ES, and $H_0$LiCOW experiments. In contrast, our finding partially solved the $H_0$ tension but gave a new possible direction to alleviate the $H_0$ tension.

## Brenda Tlatelpa Mascote

## La doble copia clásica

Una de las extensiones no lineales de las ecuaciones de Maxwell más reciente es la electrodinámica de ModMax, capaz de preservar simetría U (1) y ser invariante bajo transformaciones duales y conformes. Estas características hacen que sea un modelo interesante de estudio por sus aplicaciones en la mecánica cuántica, relatividad general, cosmología y materia condensada. En este trabajo estamos interesados en su aplicación en la Relatividad General, el principal interés es analizar la métrica de Einstein- ModMax. Posteriormente se plantea obtener una prescripción de esta solución en el formalismo de la doble copia clásica. El formalismo de la doble copia clásica es una rama de la física muy reciente, cuya idea principal es mapear soluciones de las ecuaciones de Einstein a soluciones de la electrodinámica de Maxwell. Planeamos aplicar este formalismo para obtener la doble copia de la electrodinámica no lineal ModMax.

## J.R. Viilanueva

## Adiabatic analysis of black holes

## Nelson Videla

## Reconstructing k-inflation from the scalar spectral index and reheating constraints

Inspired by the reconstruction scheme of the inflaton field potential $V(\phi)$ from the attractors $n_s(N)$, we investigate the viability of reconstructing the inflationary potential within the framework of k-inflation for a non-linear kinetic term $K(X)=k_{n+1}X^n$ through three expressions for the scalar spectral index $n_s(N)$, namely: (i) $n_s-1=-\frac{2}{N}$, (ii) $n_s-1=-\frac{p}{N}$, and (iii) $n_s-1=-\frac{\beta}{N^q}$. For each reconstructed potential, we determine the values of the parameter space which characterizes it by requiring that it must reproduce the observable parameters from PLANCK 2018 and BICEP/Keck results. Furthermore, we analyze the reheating era by assuming a constant equation of state, in which we derive the relations between the reheating duration, the temperature at the end of reheating together with the reheating epoch, and the number of $e$-folds during inflation. In this sense, we unify the inflationary observables in order to narrow the parameter space of each model within the framework of the reconstruction in k-inflation.

## VENUE AND CONTACT

**• Building**:

Vicente Kovacevic II

**• Address**:

Av. Santa Isabel 1278, Santiago

**• Contact**: