We show a gravitational spin-orbit interaction that can potentially modify the space-time geometry naturally emerges in the framework of Poincaré gauge theory. For this purpose, we derive the field equations of a particular model with cubic order invariants and demonstrate the existence of analytical solutions which display an interaction between the intrinsic and extrinsic angular momentum parameters in the gravitational action, in analogy to the spin-orbit interaction arising from atomic and nuclear systems. Due to the highly nonlinear character of the field equations under stationary and axisymmetric conditions, we focus on a degenerate case which simplifies their complexity, at the cost of constraining the geometry to the Kerr space-time. Thereby, our results indicate more general solutions with a spin-orbit interaction beyond the Kerr space-time are expected to arise in the nondegenerate models of Poincaré gauge theory.

Japan

South Korea

Sebastian Bahamonde

Jorge Gigante Valcarcel

A gravitational spin-orbit interaction in Poincaré gauge theory

We present a new exact black hole solution of a 5-dimensional Weyl-geometry Gauss-Bonnet theory of gravity. The Euclidean sector defines a fully regular metric coupled to the Weyl vector field. The Euclidean action and entropy are computed, with the latter following the simple <math altimg='si1.svg'><mrow><mi>A</mi><mo linebreak='goodbreak'>/</mo><mn>4</mn></mrow></math> form plus a term linear in the horizon radius, characteristic of Gauss-Bonnet couplings.

Chile

Máximo Bañados

An exact five dimensional Weyl-geometry Gauss-Bonnet black hole

Kyosuke Tomonari

Erratum to: Dirac–Bergmann analysis and degrees of freedom of coincident f(Q)-gravity

We investigate the propagating degrees of freedom of f(Q)-gravity in a 4-dimensional space-time under the imposition of the coincident gauge by performing the Dirac–Bergmann analysis. In this work, we start with a top-down reconstruction of the metric-affine gauge theory of gravity based only on the concept of a vector bundle. Then, the so-called geometrical trinity of gravity is introduced and the role of the coincident GR is clarified. After that, we reveal relationships between the boundary terms in the variational principle and the symplectic structure of the theory in order to confirm the validity of the analysis for our studied theories. Then, as examples, we revisit the analysis of GR and its  $$f({\mathop {R}\limits ^{\circ }})$$  <math> <mrow> <mi>f</mi> <mo>(</mo> <mover> <mi>R</mi> <mo>∘</mo> </mover> <mo>)</mo> </mrow> </math>  -extensions. Finally, after reviewing the Dirac–Bergmann analysis of the coincident GR and that of f(T)-gravity, we perform the analysis of coincident f(Q)-gravity. Under the imposition of appropriate spatial boundary conditions, we find that, as a generic case, the theory has five primary, three secondary, and two tertiary constraint densities and all these constraint densities are classified into second-class constraint density; the number six is the propagating degrees of freedom of the theory and there are no longer any remaining gauge degrees of freedom. We also discuss the condition of providing seven pDoF as a generic case. The violation of diffeomorphism invariance of coincident f(Q)-gravity make it possible to emerge such several sectors.

Dirac–Bergmann analysis and degrees of freedom of coincident f(Q)-gravity

The study of cosmological perturbation theory in f(T) gravity is a topic of great interest in teleparallel gravity since this is one of the simplest generalizations of the theory that modifies the teleparallel equivalent of general relativity. In this work, we explore the possibility of a non-flat FLRW background solution and perform perturbations for positively as well as negatively curved spatial geometries, together with a comparison to the flat case. We determine the generalized behaviour of the perturbative modes for this non-flat FLRW setting for arbitrary f(T) models, when the most general homogeneous and isotropic background tetrads are used. We also identify propagating modes in this setup, and relate this with the case of a flat cosmology.

Estonia

Greece

Kazakhstan

Konstantinos Dialektopoulos

Manuel Hohmann

Malta

Jackson Said

Germany

Christian Pfeifer

China

Emmanuel Saridakis

Perturbations in non-flat cosmology for f(T) gravity

We use observational data from the S2 star orbiting around the Galactic Center to constrain a black hole solution of extended teleparallel gravity models. Subsequently, we construct the shadow images of Sgr  $$\hbox {A}^{\star }$$  <math> <msup> <mtext>A</mtext> <mo>⋆</mo> </msup> </math>   black hole. In particular, we constrain the parameter  $$\alpha =1/\lambda $$  <math> <mrow> <mi>α</mi> <mo>=</mo> <mn>1</mn> <mo>/</mo> <mi>λ</mi> </mrow> </math>   which appears in the Born–Infeld f(T) model. In the strong gravity regime we find that the shadow radius increases with the increase of the parameter  $$\alpha $$  <math> <mi>α</mi> </math>  . Specifically, from the S2 star observations, we find within  $$1\sigma $$  <math> <mrow> <mn>1</mn> <mi>σ</mi> </mrow> </math>   that the parameter must lie between  $$0 \le \alpha /M^2 \le 6 \times 10^{-4}$$  <math> <mrow> <mn>0</mn> <mo>≤</mo> <mi>α</mi> <mo>/</mo> <msup> <mi>M</mi> <mn>2</mn> </msup> <mo>≤</mo> <mn>6</mn> <mo>×</mo> <msup> <mn>10</mn> <mrow> <mo>-</mo> <mn>4</mn> </mrow> </msup> </mrow> </math>  . Consequently, we used the best fit parameters to model the shadow images of Sgr  $$\hbox {A}^{\star }$$  <math> <msup> <mtext>A</mtext> <mo>⋆</mo> </msup> </math>   black hole and then using the Gauss-Bonnet theorem we analysed the deflection angle for leading order expansions of the parameter  $$\alpha $$  <math> <mi>α</mi> </math>  . It is found that within the parameter range, these observables are very close to the Schwarzschild case. Furthermore, using the best fit parameters for the Born–Infeld f(T) model we show that the angular diameter is consistent with recent observations for the Sgr  $$\hbox {A}^{\star }$$  <math> <msup> <mtext>A</mtext> <mo>⋆</mo> </msup> </math>   with angular diameter  $$(51.8 \pm 2.3) \mu $$  <math> <mrow> <mo>(</mo> <mn>51.8</mn> <mo>±</mo> <mn>2.3</mn> <mo>)</mo> <mi>μ</mi> </mrow> </math>  arcsec and difficult to be distinguished from the GR. For the deflection angle of light, in leading order terms, we find that the deflection angle expressed in the ADM mass coincides with the GR, but the ADM mass in the Born–Infeld f(T) gravity increases with the increase of  $$\alpha $$  <math> <mi>α</mi> </math>   and the overall deflection angle is expected to me greater in f(T) gravity. As a consequence of this fact, we have shown that the electromagnetic intensity observed in shadow images is smaller compared to GR.

North Macedonia

Kimet Jusufi

Italy

Salvatore Capozziello

Pakistan

Mubasher Jamil

Testing Born–Infeld f(T) teleparallel gravity through Sgr  $$\hbox {A}^\star $$  <math> <msup> <mtext>A</mtext> <mo>⋆</mo> </msup> </math>   observations

We derive the main classical gravitational tests for a recently found vacuum solution with spin and dilation charges in the framework of Metric-Affine gauge theory of gravity. Using the results of the perihelion precession of the star S2 by the GRAVITY collaboration and the gravitational redshift of Sirius B white dwarf we constrain the corrections provided by the torsion and nonmetricity fields for these effects.

Russia

Jorge Valcarcel

Observational constraints in metric-affine gravity

Teleparallel gravity offers a new avenue in which to construct gravitational models beyond general relativity. While teleparallel gravity can be framed in a way to be dynamically equivalent to general relativity, its modifications are mostly not equivalent to the traditional route to modified gravity. f(T, B) gravity is one such gravitational theory where the second and fourth order contributions to the field equations are decoupled. In this work, we explore the all important cosmological perturbations of this new framework of gravity. We derive the gravitational propagation equation, its vector perturbation stability conditions, and its scalar perturbations. Together with the matter perturbations, we derive the effective gravitational constant in this framework, and find an interesting branching behaviour that depends on the particular gravitational models being probed. We close with a discussion on the relation of these results with other gravitational theories.

Viktor Gakis

Stella Kiorpelidi

Tomi Koivisto

Jackson Levi Said

Cosmological perturbations in modified teleparallel gravity models: boundary term extension

In this paper, we analyze the inflationary cosmology using string field theory. This is done by using the zero level contribution from string field theory, which is a non-local tachyonic action. We will use the non-local Friedmann equations for this model based on string field theory, and calculate the slow-roll parameters for this model. We will then explicitly obtain the scalar and tensorial power spectrum, their related indices, and the tensor-to-scalar ratio for this model. Finally, we use cosmological data from Planck 2013 to 2018 to constrain the free parameters in this model, and find that string field theory is compatible with them.

Iran

South Africa

Haidar Sheikhahmadi

Canada

Mir Faizal

Ali Aghamohammadi

Saheb Soroushfar

Inflation in string field theory

The present paper represents an attempt for a very generic string inspired theory of gravitation, based on a stringy action in the teleparallel gravity which includes a specific functional which depends on the scalar field and its kinetic energy, as well as the torsion and boundary terms, embedding also possible effects from the teleparallel Gauss–Bonnet invariants. We focus our study in FLRW cosmology. After we deduce the cosmological equations for the associated generic theory of gravitation, we focus on string inspired couplings which are studied by considering different analytical techniques. The first analytical technique is based on the linear stability theory, by introducing proper dimensionless variables which enables us to study the structure of the phase space and the associated physical effects. In this case we have obtained different cosmological solutions which corresponds to matter and dark energy dominated solutions, achieving a possible transition between matter and dark energy dominated epochs. For each type of cosmological solutions we have discussed the corresponding physical features, attaining viable constraints for the coupling constants due to dynamical effects. The dynamical study of the physical features included also a numerical analysis by fine–tuning the initial conditions deep into the matter era, obtaining possible trajectories for the effective equation of state for specific coupling functions.