### Conveners

#### Extended Theories of Gravity and Quantum Cosmology: Block 1

- Yi-Fu Cai (University of Science and Technology of China)

#### Extended Theories of Gravity and Quantum Cosmology: Block 2

- Yi-Fu Cai (University of Science and Technology of China)

#### Extended Theories of Gravity and Quantum Cosmology: Block 3

- Yi-Fu Cai (University of Science and Technology of China)

### Description

The current understanding of gravitation is based on Albert Einstein's classical theory of General Relativity. While the study of gravitational waves in recent years have brought tremendous success to the classical General Relativity, this description is incomplete when describing several phenomena such as the singularity and event horizon of a black hole, the origin of the universe, fundamental understanding of dark energy, etc. Facing those conceptual issues in our universe, there is an increasing demand for the study on extended theories of gravity and quantum cosmology. In this parallel session, we invite researchers to pay attention to the aforementioned issues and welcome oral talks to report their progress on the latest study.

On one hand, the formalism developed in thermodynamics of spacetime allows a derivation of Einstein equations from the proportionality of entropy to the area. On the other hand, low energy quantum gravity effects imply a modified entropy formula with an additional term logarithmic in the area. Combining both concepts, I will introduce the derivation of quantum modified gravitational dynamics...

We consider static and cylindrically symmetric interior string type solutions in the scalar-tensor representation of the hybrid metric-Palatini modified theory of gravity. As a first step in our study,we obtain the gravitational field equations and further simplify the analysis by imposing Lorentz invariance along the t and z axes, which reduces the number of unknown metric tensor components...

We study the effect of compact extra dimensions on the gravitational wave luminosity and waveform. We consider a toy model, with a compactified fifth dimension, and matter confined on a brane. We work in the context of five dimensional (5d) general relativity, though we do make connections with the corresponding Kaluza-Klein effective 4d theory. We show that the luminosity of gravitational...

Primordial black hole (PBH) is a kind of important Dark Matter candidate of cosmological origin. And it is also a potential seed of supermassive black holes. However, the formation and the astrophysical effects of PBH still remain unclear. From theoretical perspective, the speaker and his collaborators proposed sound speed resonance （SSR） mechanism as an efficient novel effect to produce PBH....

General Relativity is an extremely successful theory, at least for weak gravitational fields, however, it breaks down at very high energies, such as in correspondence of the initial singularity. Quantum Gravity is expected to provide more physical insights concerning this open question. Indeed, one alternative scenario to the Big Bang, that manages to completely avoid the singularity, is...

The Covariant Canonical Gauge theory of Gravity is generalized by including at the Lagrangian level all possible quadratic curvature invariants. In this approach, the covariant Hamiltonian principle and the canonical transformation framework are applied to derive a Palatini type gauge theory of gravity. The metric gµν, the

affine connection γλµν and their respective conjugate momenta, kµνσ...

From the theory of the multiverse cosmology，it is possible that our universe collides with other universes locally in its history，which may result in local changes of the curvature of the spacetime．In this paper，we propose a method to probe the multiverse using gravitational wave observations for the first time．Our method firstly makes triangles using two detected gravitational wave sources...

We consider inflationary scenarios of the supersymmetric quantum cosmology of FRLW models with a scalar field. We use the superfield formalism with a superpotential for the scalar superfield. From the probability amplitude solution of the supersymmetric Wheeler-DeWitt equation, we compute an effective probability density from which we get mean trajectories that are parametrized by the scalar....

We discuss the emergence of black hole shadow and photon-sphere inthe context of f(R) gravity. It is shown that the shadow is exponentially sensitive to linear instabilities of metric coming from some f(R) solutions. Thus, the instabilities of photon circular trajectories, delimiting the black hole photon-sphere, are double exponentialized. Specifically we individuate two Lyapunov exponents,...

During the early phase of in-spiral of a binary system, the tidal heating of a compact object due to its companion plays a significant role in the determination of the orbital evolution of the binary. The phenomenon depends crucially on the `hairs', as well as on the nature of the compact object. It turns out that the presence of extra dimension affects both these properties, by incorporating...

We wish to investigate if we can extend the insights provided by the publication "Can stellar mass black holes be quark stars?" by Z. Kovacs,et.al. in MNRAS, 2009 as to answering their question via examining what may happen in a quark star - black hole binary in its Gravitational wave generation. In doing so, we also examine how to Use this idea to explore the idea of a black hole as a...

Along with the accumulated cosmological observations, while the standard paradigm of modern cosmology has been verified with high precision, some new challenges such as the $H_0$ tension appear. To address these issues, we in this article reconstruct the free function of $f(T)$ gravity in a model-independent manner using the combined observational data. The obtained function is consistent with...

We reduplicate the Book “Dark Energy” by M. Li, X-D. Li, and Y. Wang, zero-point energy calculation with an unexpected “length’ added to the ‘width’ of a graviton wavefunction just prior to the entrance of ‘gravitons’ to a small region of space-time prior to a nonsingular start to the universe. We compare this to a solution worked out using Klauder Enhanced quantization, for the same given...

Gravitational-wave (GW) memory effects are lasting changes in the GW strain and its time integrals following bursts of GWs. They are closely related to the symmetries of asymptotically flat spacetimes and their corresponding conserved charges. There are three types of GW memory effects (displacement, spin, and center-of-mass) that are related to different conserved charges and have different...

We discuss the status of observables and operator ordering ambiguities in the quantum cosmology model with Brown Kuchař dust as the matter field. In order to study the dynamics of the FRW universe, Hubble parameter and Ricci scalar are expressed as a function of phase space variables. As these functions exhibit operator ordering ambiguity, several Hermitian extensions corresponding to these...

We generate two anisotropic solutions for a static sphere filled with quark matter in the framework of self-interacting Brans-Dicke theory. For this purpose, we add an anisotropic source in the seed distribution and decouple the field equations through deformation in the radial metric function. As a result of this transformation, the field equations are disintegrated into two systems which...

We review various theoretical models and scenarios based on teleparallel gravity and torsional modifications of gravity. Then we apply them in a cosmological framework, discussing the early and late time solutions as well as the observational constraints using various datasets. Finally we present the recent possibility of using multi-messenger astronomy, in order to investigate torsional...

A semiclassical state is studied as it approaches a cosmological Bianchi II singularity, by means of the evolution of its moments. Classically this system undergoes a transition between two Bianchi I models, with a specific and well-known transition law, which is derived based on the conservation of certain physical quantities. In the quantum theory, however, these constants of motion are...

A class of modified gravity theories with higher order derivative terms of a function of the matter Lagrangian $f(L_m)$ is considered. We will consider the Newtonian limit of the theory and show that the model predicts the standard Poisson equation for a massive test particle due to the higher order nature of the derivative matter coupling. Generally the energy momentum tensor is not...

We analyze the Bianchi I cosmology in the presence of a massless scalar

field and describe its dynamics via a semiclassical and quantum polymer

approach. We investigate the morphology of the emerging Big Bounce

by adopting three different sets of configurational variables:

the natural Ashtekar connections, the Universe volume plus two anisotropy

coordinates and a set of anisotropic volume-like...

In this work we study an "ungravity" origin to the cosmological constant. We derive the modified Friedmann equations from the first law of thermodynamics and the "ungravity" entropy-area relationship. From the modified Friedmann equations (in the late time regime) we find an effective cosmological constant.

In this talk I will discuss the possibility to extend the notion of the Nieh-Yan topological invariant to generic metric-affine geometries, where both torsion and nonmetricity are taken into account. Notably, the properties of projective invariance and topologicity can be independently accommodated by a suitable choice of the parameters featuring this new Nieh-Yan term. Considering a special...

In this talk we will argue that, in generic metric-affine theories of gravity, non-linear $R_{(\mu\nu)}$ terms in the action do not introduce new degrees of freedom, but rather couple the rest of the degrees of freedom of the theory among themselves. At a perturbative level, this leads to a set of effective interactions among the matter degrees of freedom suppressed by a heavy mass scale that...

We study the motion of a particle in the Bogoslovsky-Finsler space-time, where a Lorentz violation takes place due to a non-zero continuous parameter in the action. We demonstrate that the broken Lorentz symmetries are substituted by a different type of symmetry. The new symmetry vectors are generators of higher order (or hidden) symmetries that are related to integrals of motion which are...

We present the precanonical Hamiltonian analysis and quantization of the Palatini formulation of the teleparallel equivalent of General Relativity in vielbein variables. It leads to more sophisticated fundamental Dirac brackets than in the case of vielbein General Relativity and it naturally leads to a covariant precanonical Schroedinger equation of quantum graviity for a...