The 10th Anniversary of Universe: Standard Cosmological Models, and Modified Gravity and Cosmological Tensions

Dear Colleagues,

The Standard Cosmological Model is considered to be a description of our understanding of the Universe and its contents. The ΛCDM model, as the current standard cosmological model, is consistent with most astronomical observations at present, but it is not perfect; there are still many theoretical difficulties and tensions. The significant discrepancy between the H₀ values measured from a local distance and from the cosmic microwave background, i.e., the Hubble tension, is the most crucial subject of the standard ΛCDM model. The observations of galaxies at epochs of 200 millions years after the big bang is another problem for the model. In addition, taking into account the abundance and nature of dark energy and dark matter, the nature of inflation, cosmological tensions such as the H₀ and S₈, the possible values of local anisotropy in the evolution of the universe, and the theoretical problems of the cosmological constant, the validity range of the ΛCDM model might be restricted. The absent observations of dark matter has brought several researchers to think that dark matter could not exist and that the indirect observation of its existence through several effects may be due to the fact that gravity is different from eneral Relativity.

Furthermore, if we consider that gravity geometrically accommodates in a metrical spacetime of accelerations, its description can be developed in the framework of a modified gravitation theory. In this case, anisotropic cosmological models have been developed when the underlining geometry of spacetime has a more generalized metric structure than the Riemannian one. Also, anisotropic models have been proposed to solve the Hubble tension.

Modified gravity does not exclude standard general relativity, but it has its boundaries. Modified theories of gravity extend the form of general relativity through various methods, leading to different field equations and thus to different cosmological implications. They play an essential role and contribute to modern cosmology, providing a foundation for the current understanding of physical phenomena of the Universe. Experiments with gravitational wave detectors, LIGO, VIRGO, and KAGRA amplify the hopes of modified gravity researchers that g-wave perturbations will eventually emerge, extending the models beyond the general relativity. Observations of EUCLID could also help to understand the nature of dark energy and gravity. Arrangements and solutions of H₀ tension, based on the latest research, might imply new physics beyond the ΛCDM model.  

Prof. Dr. Panayiotis Stavrinos
Prof. Dr. Antonino Del Popolo
Prof. Dr. Hermano Velten
Prof. Dr. Tina Kahniashvili
Prof. Dr. Jean-Michel Alimi
Prof. Dr. Yi-Fu Cai
Guest Editors

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