# 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 the «Salón Rojo» Auditorium (Red Saloon) of Gonzalo Hernández Uribe building, Faculty of Law and Humanities, 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

**Thomas Sotiriou**

*(Nottingham, United Kingdom)*

**Marco Astorino**

*(Milano, Italy)*

**Elcio Abdalla**

*(Sao Paulo, Brazil)*

**Amilcar Queiroz**

*(Campina Grande, Brazil)*

**Eloy Ayón-Beato**

*(Ciudad de México, México)*

**Sebastián Céspedes**

**(London, England)****Domenico Sapone**

**(Santiago, Chile)****Jorge Noreña**

*(Valparaíso, Chile)*

**Dumitru Astefanesei**

*(Valparaíso, Chile)*

**Juan Magaña**

*(La Serena, Chile)*

**Julio Oliva**

*(Concepción, Chile)*

**Fernando Izaurieta**

*(Concepción, Chile)*

**Giorgio Anastasiou**

*(Iquique, Chile)*

## LECTURERS

## Thomas Sotiriou

**Black holes beyond GR:**I will give an introduction to the structure and dynamics of black holes beyond General Relativity and the Standard Model. Using scalar fields as a case study, I will discuss no-hair theorems and their exceptions, under which conditions black holes can carry a scalar charge, how does this charge depend on the mass and/or spin of the black hole, and how can observation be used to detect it. If time permits I will also discuss the notion and properties of black holes in theories that exhibit violations of Lorentz symmetry and arbitrarily fast propagation. The lectures will be pedagogical but include recent research results and summarise the key open questions in this area as well.

## Marco Astorino

**(Black Holes generation from) Integrability of Einstein Equations:**The solution-generating technique for axisymmetric and stationary spacetimes in general relativity is presented. The Einstein-Maxwell field equations are shown to be equivalent to the Ernst equations. Infinitesimal and finite symmetries of the Ernst complex equations are found and analyzed. These symmetries make the system integrable, so all the solutions for the theory can be built from group transformations. The main symmetry transformations are studied and they are applied to build new solutions from old ones, particular emphasis is given to black hole spacetimes.

## PARTICIPANTS LIST

Last Name | First Name | Affiliation | Country |
---|---|---|---|

Abdalla | Elcio | Universidade de Sao Paulo | Brazil |

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

Anastasiou | Georgios | Universidad Arturo Prat | Chile |

Aragón | Almendra | Universidad Diego Portales | Chile |

Arratia | Esteban | Universidad de Concepción | Chile |

Araya | Ignacio | Universidad Arturo Prat | Chile |

Astefanesei | Dumitru | Pontificia Universidad Católica de Valparaíso | Chile |

Astorino | Marco | Laboratorio Italiano di Fisica Teorica | Italia |

Astudillo | Bastián | Universidad Andrés Bello | Chile |

Ayón-Beato | Eloy | CINVESTAV | Mexico |

Baake | Olaf | Universidad de Talca | Chile |

Bahamondes | Sebastian | Pontificia Universidad Católica de Chile | Chile |

Barrera-Hinojosa | Cristian | Universidad de Valparaíso | Chile |

Barriga Delgadillo | Francisco | Universidad de Concepción | Chile |

Becar | Ramon | Universidad Católica de Temuco | Chile |

Bravo | Martín | Universidad Arturo Prat | Chile |

Bravo | Moises | Universidad católica del Maule | Chile |

Britting | Svenja | Stockholm University | Sweden |

Busnego | Mairym | Universidad Andrés Bello | Chile |

Cáceres | Nicolás | Pontificia Universidad Católica de Chile | Chile |

Calero | Nicolás | Escuela Politécnica Nacional | Ecuador |

Caro | Fabián | Universidad de Antofagasta | Chile |

Castillo-Felisola | Oscar | Universidad Técnica Federico Santa María | Chile |

Cerda | Javiera | Universidad Andrés Bello | Chile |

Céspedes | Sebastián | Imperial College | England |

Colipí-Marchant | Francisco | Universidad de Chile | Chile |

Cruz | Miguel | Universidad Veracruzana | Mexico |

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

Cuevas-Tapia | Benjamín | Pontificia Universidad Católica de Chile | Chile |

Delgado | Evangelo | Universidad de Concepción | Chile |

Díaz | Jean | Universidad de La Serena | Chile |

Díaz | Mauricio | Universidad del Bío Bío | Chile |

Diez | Borja | Universidad Arturo Prat | Chile |

Dubey | Nitesh Kumar | Indian Institute of Astrophysics | India |

Faraggi | Alberto | Universidad Andrés Bello | Chile |

Fathi | Mohsen | Universidad de Santiago | Chile |

Fernández | Jose | None | None |

Flores | Daniel | Universidad Arturo Prat | Chile |

Gajardo Ruz | Leonardo | Universidad de Concepción | Chile |

Gallardo | Sebastián | Instituto Balseiro | Argentina |

Galvez | Rafael | Universidad Andrés Bello | Chile |

Gálvez | Sebastián | Pontificia Universidad Católica de Chile | Chile |

Garcia de Andrade | Luiz Carlos | UERJ and Institute for Cosmology Croatia | Brazil |

Gonzalez | Pablo | Universidad Diego Portales | Chile |

Guzmán | Alejandro | Pontificia Universidad Católica de Valparaíso | Chile |

Guzmán | Saúl | Instituto Politécnico Nacional | Mexico |

Hernández | Benjamin | Universidad de Concepción | Chile |

Hosseini | Hamideh | Universidad Diego Portales | Chile |

Huenupi | Javier | Universidad de Chile | Chile |

Hughes | Ellie | Universidad de Chile | Chile |

Ipinza | Mauricio | Universidad San Sebastián | Chile |

Izaurieta | Fernando | Universidad San Sebastián | Chile |

Jara Lobo | Francisco Andrés | Universidad de Concepción | Chile |

Jiménez Parra | Daniel | Universidad de Concepción | Chile |

Kalita | Dibash | Gauhati University | India |

Kariminezhaddahka | Mona | Universidad de Valparaíso | Chile |

Karna | Charuchandra | Central University of Haryana | India |

Kashi | Bhuvaneshwari | International Centre for Theoritical Sciences | India |

Kiorpelidi | Stella | National Technical University of Athens | Greece |

Kolhatkar | Ameya | BITS Pilani | India |

Labrin | Oriana | Pontificia Universidad Católica de Valparaíso | Chile |

Lepe | Samuel | Pontificia Universidad Católica de Valparaíso | Chile |

Luis | Guajardo | Universidad de Talca | Chile |

Maedler | Thomas | Universidad Diego Portales | Chile |

Magaña | Juan | Universidad Central | Chile |

Mantel | Chester | Harvard | United States |

Medina | Daniel | Universidad de Guanajuato | Mexico |

Miskovic | Olivera | Pontificia Universidad Católica de Valparaiso | Chile |

Molina | Martin | Pontificia Universidad Católica de Valparaíso | Chile |

Moncada | Felipe | Universidad Católica de Temuco | Chile |

Mora | Nicolás | Universidad de Concepción | Chile |

Moreno | Cristóbal | Pontificia Universidad Católica de Chile | Chile |

Moreno | Javier | Haifa U-Technion | Israel |

Neogi | Turmoli | Centro de Estudios Científicos | Chile |

Noreña | Jorge | Pontificia Universidad Católica de Valparaíso | Chile |

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

Olea | Rodrigo | Universidad Andrés Bello | Chile |

Oliva | Julio | Universidad de Concepción | Chile |

Olivares | Marco | Universidad Diego Portales | Chile |

Osorio Navarro | Loreto Soledad | Pontificia Universidad Católica de Chile | Chile |

Palma | Gonzalo | Universidad de Chile | Chile |

Paul | Aritrya | IIT(ISM)Dhanbad | India |

Pedreros | Vicente | Universidad de Chile | Chile |

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

Pérez | Claudio | Universidad Andrés Bello | Chile |

Petridis | Savvas | National Technical University of Athens | Greece |

Pino | Daniel | Universidad de Concepción | Chile |

Pino | Thaiz | Universidad de Concepción | Chile |

Pinochet | Nicolás | Pontificia Universidad Católica de Valparaíso | Chile |

Pundeer | Naeem Ahmad | Jadavpur University | India |

Queiroz | Amilcar | Universidade Federal de Campina Grande | Brazil |

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

Ramorino | Cristopher | Universidad de Santiago de Chile | Chile |

Reyes | Francisco | Universidad de Santiago de Chile | Chile |

Riquelme | Simón | Universidad San Sebastián/CECs | Chile |

Rivera | Marco | Universidad de Concepción | Chile |

Robledo | Joaquín | Universidad de La Serena | Chile |

Rojas Mejías | Raúl | Universidad de Concepción | Chile |

Rozas | Marcelo | Pontificia Universidad Católica de Valparaíso | Chile |

Rubio | Carlos | Universidad Adolfo Ibañez | Chile |

Sanhueza | Leonardo | Universidad de Concepción | Chile |

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

Sapone | Domenico | Universidad de Chile | Chile |

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

Sotiriou | Thomas | Nottingham University | England |

Tapia | Alonso | Pontificia Universidad Católica de Chile | Chile |

Tapia | Lilianne | Universidad de Concepción | Chile |

Tlatelpa Mascote | Brenda | Universidad Autónoma Metropolitana | Mexico |

Trigo | Fabian | Universidad de Valparaíso | Chile |

Tristán Álvarez | Diego Francisco | Universidad Autónoma de San Luis Potosí | Mexico |

Urrutia | Javier | Pontificia Universidad Católica de Valparaíso | Chile |

Varas | Alejandro | Pontificia Universidad Católica de Valparaíso | Chile |

Vasquez | Yerko | Universidad de La Serena | Chile |

Vidal | Jessica | Pontificia Universidad Católica de Chile | Chile |

Videla | Nelson | Pontificia Universidad Católica de Valparaíso | Chile |

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

Villalobos | Daniel | Universidad de La Serena | Chile |

Zamani | Saboura sadat | University of Szczecin | Poland |

Zúñiga | Benjamín | Universidad Andrés Bello | Chile |

## PROGRAM

## 09:00-09:30 Opening

Hour | 17-Jan | 18-Jan | 19-Jan | 22-Jan | 23-Jan |
---|---|---|---|---|---|

09:30-10:30 | M.Astorino 1 | M.Astorino 2 | M.Astorino 3 | M.Astorino | J.Oliva |

10:30-11:00 | Coffee break | Coffee break | Coffee break | Coffee break | Coffee break |

11:00-12:00 | E.Ayón-Beato | D.Astefanesei | T.Sotiriou 3 | T.Sotiriou 4 | S.Céspedes |

12:00-12:20 | T.Sotiriou 1 | G.Anastasiou | B.Zúñiga | E.Abdalla | J.Noreña |

12:20-12:40 | T.Sotiriou 1 | G.Anastasiou | B.Tlatelpa | E.Abdalla | J.Noreña |

12:40-13:00 | T.Sotiriou 1 | G.Anastasiou | F.Santos | E.Abdalla | J.Noreña |

13:00-13:10 | Lunch break | Lunch break | F.Santos | Lunch break | Lunch break |

13:10-15:00 | Lunch break | Lunch break | Free afternoon | Lunch break | Lunch break |

15:00-15:30 | D.Flores | T.Sotiriou 2 | Free afternoon | A.Queiroz | D.Sapone |

15:30-16:00 | R.Rojas | T.Sotiriou 2 | Free afternoon | A.Queiroz | D.Sapone |

16:00-16:30 | C.Rubio | J.Moreno | Free afternoon | O.Castillo-F. | S. Riquelme |

16:30-17:00 | Coffee break | Coffee break | Free afternoon | Coffee break | Coffee break |

17:00-17:30 | M.Fathi | M.Cruz | Free afternoon | F.Izaurieta | J.Magaña |

17:30-18:00 | N.Cáceres | N.Videla | Free afternoon | F.Izaurieta | J.Magaña |

18:00-18:20 | E.Arratia | Free afternoon | L.García | S.Kiorpelidi |

## CLOUD OF ABSTRACTS

## Elcio Abdalla

## Whispers of the Universe: theoretical and observational developments in the BINGO/ABDUS project

I present the theoretical and technical developments expected in the construction of the BINGO (BAO from Integrated Gas Observations) observatory, which is being supported by Brazil and China, including members from UK, South Africa, France, USA and Italy.

## Georgios Anastasiou

## Conformal Renormalization in Einstein-AdS gravity and scalar-tensor theories

In this talk, I will present the main aspects of the conformal renormalization prescription for metric fields and its recent extension to scalar-tensor theories. In particular, it will be shown how bulk conformal invariance dictates the counterterms of Einstein-AdS gravity in 4D and 6D. There will be provided concrete examples of topological black holes in 6D. It will be also discussed the generalization of this prescription in the presence of scalar fields, in a certain class of scalar-tensor theories.

## Esteban Arratia

## Degenerated Neutron Stars in D=4 Einsteinian Cubic Gravity

In this work we have built a perturbative model of stellar equilibrium for Einsteinian Cubic Gravity (ECG) in $D=4$ at first order in the cubic term coupling parameter $\lambda$. Here we consider that neutron stars are described by the Equation of State (EoS) of a fully degenerated Fermionic gas (0K), from which we obtain a family of solutions for different values of $\lambda$ and where each solution has a characteristic total mass-radius values as well as pressure and density profiles for the interior of the star. We compared our results with known solutions of General Relativity considering the same EoS and initial conditions. Interestingly, for low values of the central density, ECG solutions tend to those of GR, however, our results show that, in general, ECG admits denser stars.

## Dumitru Astefanesei

## Hairy Black Hole Holography

## Marco Astorino

## Accelerating type I black holes

Thanks to the Ehlers transformation of the Ernst equations, the gravitomagnetic mass can be added to the whole Plebanski-Demianski family of solutions. Thus we can efficiently generate a large class of black holes in general relativity, such as the accelerating Reissner-Nordstrom or the accelerating Kerr-Newman, endowed with the NUT parameter. The full rotating version carries a couple of independent NUT charges, one associated to the black hole and the other to the accelerating background. The two NUT parameters can be coupled to remove the axial irregularity which causes the Misner string. All the metrics we build are not of D-type according to the Petrov classification, but type-I. Similarly another Lie-point symmetry of the Ernst equations, the Harrison transformation, allows us to build a novel type I accelerating black holes, in the Einstein-Maxwell theory, such as the Schwarzschild or Reissner-Nordstrom, which differs and generalise the standard C-metrics. These exotic accelerating black holes can be interpreted as a limit of binary systems.

## Eloy Ayón-Beato

## New perspectives on conformal scalar fields

Recently, an extension of the standard four-dimensional scalar conformal action, yielding a second-order field equation that remains conformally invariant, was proposed. In spite of this, the corresponding action is not invariant under conformal transformations and this motivates us to define the notion of non-Noetherian conformal scalar field. In this work, we go further by determining the most general action in four dimensions that gives rise to a non-Noetherian conformal scalar field satisfying a second-order equation. This task is achieved by using the solution to the inverse problem of the calculus of variations. Surprisingly enough, the standard equation is shown to be extended by a non-Noetherian conformal piece involving a nonminimal coupling with a very particular combination of squared curvature terms, which is none other than the one defining the so-called Critical Gravity. We also prove that the most general second-order Euler-Lagrange equation for a conformal scalar field involves additional Noetherian conformal nonminimal couplings defined by an arbitrary function of the Weyl tensor. The recently proposed non-Noetherian conformal extension is recovered as a particular example of this function.

## 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.

## Oscar Castillo-Felisola

## Cosmological perturbations in affine gravities

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.

## Miguel Cruz

## P-V phase transitions in Einstein gravity

In the framework of Einstein gravity, we study the thermodynamic equation state, P=P(V,T), associated with a flat Friedmann-Lemaitre-Robertson-Walker universe with two dominant non-interacting fluids at late times related by the cosmological coincidence parameter. We find that under these circumstances, first-order P-V phase transitions are admitted if the dark energy density and the cosmological coincidence parameter can be related to the geometry of the apparent horizon.

## Mohsen Fathi

## Photon rings around black holes illuminated by thin accretion disks

In this presentation, I delve into significant concepts surrounding the formation of light rings around static black holes that draw their illumination from an accretion disk. I discuss how these rings are identified, considering the sequence of photon orbits around the black holes. Furthermore, I explore the impact of an additional gravitational potential in the spacetime metric and provide several illustrative examples to demonstrate these effects.

## 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 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.

## Fernando Izaurieta

## Hubble Tension, Torsion and Gravitational Faraday Effect

Allowing for a nonvanishing spin tensor and torsion for cold dark matter (ω=0) has the consequence of giving rise to an effective FLRW dynamics with a small negative barotropic constant for an effective dark matter density (−1/3<ωeff≤0). It alleviates the Hubble parameter tension, and it also gives rise to the gravitational analogue of the Faraday effect.

## Stella Kiorpelidi

## Scalarization of the Reissner-Nordsröm black hole with higher gauge field corrections

We discuss spontaneous scalarization of the Reissner-Nordström black hole in the presence of higher gauge field corrections that arise in the context of string, as well as higher-dimensional more fundamental gravity theories. Our theory admits the Reissner-Nordström solution at the scalar vacuum of the theory (φ=0) and we find that the higher order gauge field correction term always result in the tachyonic instability of our system once the coupling function satisfies the condition that its second derivative is positive at the scalar vacuum. We find that the branches do not end with an extremal black hole, rather with a singularity as indicated by the divergence of the Kretschmann scalar. The black holes can be overcharged in the sense that they may carry larger electric charge in comparison to their mass. Finally, these solutions possess larger entropy at the event horizon radius when compared to the Reissner-Nordström black hole, as well as to scalarized black holes without the higher order gauge field terms, indicating in this way the thermodynamic stability and prefer-ability of our system, when compared to existing literature, while they respect the energy conditions.

## Juan Magaña

## Taxonomy of cosmological models and their observational constraints

The current cosmological model postulates the existence of two exotic components, dark matter and dark energy under the general relativity framework. Although this so-called Lambda cold dark matter can explain several cosmological observations, it faces several problems related to the nature of such dark components. This has led to a growing exploration of alternative models attempting to solve those drawbacks. In this talk, I will briefly review the characteristics of a (non-exhaustive) list of cosmological (modified gravity) models as well as some of the most used cosmological samples such as: type Ia supernovae, observational Hubble data, cosmic microwave background, baryon acoustic oscillations among other. Finally, I summarize the status of dark energy and dark matter problems.

## 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.

## Julio Oliva

## Semiclassical effects in massive gravity in 2+1

In the semiclassical description of General Relativity, matter fields are quantized on a fixed classical geometry. This approach led, for example to the seminal discovery of black hole radiation by S. Hawking. Once the matter fields are quantized, a renormalized stress energy tensor can be computed and the backreaction on the geometry can be obtained. In this talk, I will consider how the presence of higher curvature terms may modify this simple picture. I will focus on the higher curvature terms that lead to New Massive Gravity, which is a healthy 2+1-dimensional theory, with propagating degrees of freedom, and I will comment as well on the extensions to theories containing powers of the Riemann up to 5, which are compatible with a holographic c-theorem an possess a conformal invariance at the linearized level.

## Sebastián Céspedes

## On the IR divergences in de Sitter: loops, resummation and the semi-classical wavefunction

Detecting local Non-Gaussianity provides valuable insights into the early universe’s particle composition. Interactions between the inflaton and light particles yield distinctive signatures, potentially observable in upcoming surveys. However, addressing IR divergences in light fields on de Sitter spacetimes requires careful treatment. Stochastic inflation offers a solution, but its relationship with perturbative computations remains unclear.

In this presentation, we establish a precise connection between perturbation theory and stochastic formalism using the wavefunction formalism. We extend this analysis to multifield inflation models and clarify recent non-perturbative findings from stochastic inflation through their compatibility with perturbation theory calculations.

## Amilcar Queiroz

## BINGO Radio Telescope: Cosmology and FRB

The BINGO (Baryon Acoustic Oscillations (BAO) from Integrated Neutral Gas Observations) radio telescope will map the Universe using neutral Hydrogen emission line in redshift range 0.13 ≤ z ≤ 0.45. The main expected result is to probe BAO. In addition, the instrument optical design and hardware configuration support the search for Fast Radio Bursts (FRBs).

Fast Radio Bursts (FRBs) are transient radio pulses discovered in 2007 using data from the Parkes telescope. They are very bright, with sub-second duration and their arrival time delay is characterized by a frequency dependency known as Dispersion Measure (DM). There are little less than a thousand FRBs measured, a few hundreds of which are repeaters (which can be periodic or not) and very few have their host galaxies localized (Petroff et al. 2022), with no apparent privileged sky distribution. The FRBs’ origin is still an open problem.

In this talk, we discuss FRBs, how they can be used to constrain Cosmological Parameters and the efficiency of the BINGO radio telescope to measure these transients.

## Simón Riquelme

## Ultralight Spin 2 Fields During Inflation

The inflaton background generates a preferred foliation of the FRWL spacetime endowing it with a “condensed matter” structure, where quasiparticles may emerge, leaving non-trivial signatures on the statistics of both quantum scalar curvature perturbations and graviton perturbations. We quickly review the systematics of the construction of effective field theories for the early universe, and report some work in progress as we consider the highly relevant case of a spin 2 field as matter content.

## 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.

## Raúl Rojas

## Thermodynamically stable asymptotically flat black holes in Einstein-Maxwell-scalar theory

We analize the thermodynamic stability of four-dimensional asymptotically flat hairy black holes by considering a general class of exact solutions in Einstein-Maxwell-scalar theory with a non-trivial scalar field potential. We find that, regardless of the values of the parameters of the theory, there always exists a sub-class of hairy black holes that are thermodynamically stable and have the extremal limit well defined. This generic feature that makes the equilibrium configurations locally stable should be related to the properties of the dilaton potential that is decaying towards the spatial infinity, but behaves as a box close to the horizon. We prove that these thermodynamically stable solutions are also dynamically stable under spherically symmetric perturbations.

## 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.

## Fabiano 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.

## Domenico Sapone

## Cosmological constraints from LSS surveys

Understanding the expansion of the universe is the top priority of all space agencies. In this talk, we will discuss the primary cosmological observables capable of shedding light on these mysterious phenomena. In particular, we will focus on the forthcoming large scale structures surveys and their ability to fundamental constrain cosmological parameters.

## 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.

## Benjamín Zuñiga

## Equivalencia entre Anomalía Multiplicativa y Energía de Casimir

Se deriva una fórmula explícita para calcular la anomalía multiplicativa o defecto entre productos zeta-regularizados de factores lineales, usando una parametrización de Feynman, que generaliza las fórmulas de Shintani y Mizuno. Como primera aplicación, se considera el caso de esferas en n = 2, 3, 4, 5 y 6 dimensiones, corroborando resultados conocidos en la literatura. Luego se trabaja en un universo cerrado de Einstein a temperatura finita, esto es, S1_β xS^n−1. En este contexto se muestra que la energía usual de Casimir para un operador GJMS es equivalente a la suma de la anomalía multiplicativa de cada Laplaciano que lo forma. Finalmente se presenta la noción de una energía de Casimir mejorada, que incluye la anomalía multiplicativa entre los Laplacianos.

## VENUE AND CONTACT

**• **Venue:

Gonzalo Hernández Uribe building, «Salón Rojo» Auditorium (Red Saloon)

**WIFI NETWORK:** Wifi_Ucentral**
PASSWORD:** UCEN2009

**• Address**:

Lord Cochrane 417, Santiago

**• Contact**: