We propose a novel and natural mechanism for cosmic acceleration driven by primordial black holes (PBHs) exhibiting repulsive behavior. Using a new “Swiss Cheese” cosmological approach, we demonstrate that this cosmic acceleration mechanism is a general phenomenon by examining three regular black hole spacetimes - namely the Hayward, the Bardeen and the Dymnikova spacetimes - as well as the singular de Sitter-Schwarzschild spacetime. Interestingly, by matching these black hole spacetimes with an isotropic and homogeneous expanding Universe, we obtain a phase of cosmic acceleration that ends at an energy scale characteristic to the black hole parameters or due to black hole evaporation. This cosmic acceleration mechanism can be relevant either to an inflationary phase with a graceful exit and reheating or to an early dark energy type of contribution pertinent to the Hubble tension. Remarkably, we find that ultra-light PBHs with masses <math altimg='si11.svg'><mrow><mi>m</mi><mo linebreak='goodbreak'>&lt;</mo><mn>5</mn><mo linebreak='goodbreak'>&#215;</mo><msup><mn>10</mn><mn>8</mn></msup><mrow><mi mathvariant='normal'>g</mi></mrow></mrow></math> dominating the energy content of the Univese before Big Bang Nucleosynthesis, can drive a successful inflationary expansion era without the use of an inflaton field. Additionally, PBHs with masses <math altimg='si12.svg'><mrow><mi>m</mi><mo>&#8764;</mo><msup><mn>10</mn><mn>12</mn></msup><mrow><mi mathvariant='normal'>g</mi></mrow></mrow></math> and abundances <math altimg='si13.svg'><mrow><mn>0.107</mn><mo linebreak='goodbreak'>&lt;</mo><msubsup><mstyle mathvariant='normal'><mi>&#937;</mi></mstyle><mrow><mi mathvariant='normal'>PBH</mi></mrow><mrow><mi mathvariant='normal'>eq</mi></mrow></msubsup><mo linebreak='goodbreak'>&lt;</mo><mn>0.5</mn></mrow></math>, slightly before matter-radiation equality, can produce a substantial amount of early dark energy, helping to alleviate the <math altimg='si14.svg'><msub><mi>H</mi><mn>0</mn></msub></math> tension.

Romania

Konstantinos Dialektopoulos

Italy

Theodoros Papanikolaou

Greece

Vasilios Zarikas

Primordial black holes as cosmic expansion accelerators

In this work, we reconstruct the Hubble diagram using various data sets, including correlated ones, in artificial neural networks (ANN). Using ReFANN, that was built for data sets with independent uncertainties, we expand it to include non-Guassian data points, as well as data sets with covariance matrices among others. Furthermore, we compare our results with the existing ones derived from Gaussian processes and we also perform null tests in order to test the validity of the concordance model of cosmology.

India

Purba Mukherjee

Malta

Jackson Said

Jurgen Mifsud

Neural network reconstruction of cosmology using the Pantheon compilation

Testing gravity theories and their parameters using observations is an important issue in relativistic astrophysics. In this context, we investigate the motion of test particles and their harmonic oscillations in the spacetime of non-rotating hairy black holes (BHs) in Hordeski gravity, together with astrophysical applications of quasiperiodic oscillations (QPOs). We show possible values of upper and lower frequencies of twin-peak QPOs which may occur in the orbits from innermost stable circular orbits to infinity for various values of the Horndeski parameter q in relativistic precession, warped disk models, and three different sub-models of the epicyclic resonant model. We also study the behaviour of the QPO orbits and their position relative to innermost stable circular orbits (ISCOs) with respect to different values of the parameter q. It is obtained that at a critical value of the Horndeski parameter ISCO radius takes 6M which has been in the pure Schwarzschild case. Finally, we obtain mass constraints of the central BH of microquasars GRS 1915+105 and XTE 1550-564 at the GR limit and the possible value of the Horndeski parameter in the frame of the above-mentioned QPO models. The analysis of orbits of twin peak QPOs with the ratio of upper and lower frequencies 3:2, around the BHs in the frame of relativistic precession (RP) and epicyclic resonance (ER4) QPO models have shown that the orbits locate close to the ISCO. It is obtained that the distance between QPO orbits and ISCO is less than the error of the observations.

Uzbekistan

Javlon Rayimbaev

Kazakhstan

Furkat Sarikulov

Ahmadjon Abdujabbarov

Quasiperiodic oscillations around hairy black holes in Horndeski 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

Japan

Sebastian Bahamonde

Manuel Hohmann

Germany

Christian Pfeifer

China

Emmanuel Saridakis

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

Teleparallel Horndeski theory offers an avenue through which to circumvent the speed constraint of gravitational waves in an efficient manner. However, this provides an even larger plethora of models due to the increase in action terms. In this work we explore these models in the context of cosmological systems. Using Noether point symmetries, we classify the dynamical systems that emerge from teleparallel Horndeski cosmologies. This approach is very effective at selecting specific models in the general class of second-order teleparallel scalar–tensor theories, as well as for deriving exact solutions within a cosmological context. By iterating through the Lagrangians selected through the Noether symmetries, we solve for a number of cosmological systems which provides new cosmological systems to be studied.

Zinovia Oikonomopoulou

Classification of teleparallel Horndeski cosmology via Noether symmetries

We consider a general theory of all possible quadratic, first-order derivative terms of the non-metricity tensor in the framework of Symmetric Teleparallel Geometry. We apply the Noether Symmetry Approach to classify those models that are invariant under point transformations in a cosmological background and we use the symmetries of these models to reduce the dynamics of the system in order to find analytical solutions.

Sweden

Tomi Koivisto

Russia

Salvatore Capozziello

Noether symmetries in symmetric teleparallel cosmology

Adopting Noether point symmetries, we classify and integrate dynamical systems coming from Horndeski cosmologies. The method is particularly effective both to select the form of Horndeski models and to derive exact cosmological solutions. Starting from the Lagrangians selected by the Noether symmetries, it is possible to derive several modified theories of gravity like f(R) gravity, Brans–Dicke gravity, string inspired gravity and so on. In any case, exact solutions are found out.

Sergey Sushkov

Classification of the Horndeski cosmologies via Noether symmetries