Focus point on tensions in cosmology from early to late universe: the value of the Hubble constant and the question of dark energy

Abstract

Papers included in this Focus Point collection are devoted to one of the hot topics of modern cosmology: the Hubble constant tension. It emerged from observations revealing, with a high degree of confidence, a discrepancy between the value of the Hubble constant, as measured by galaxy distance surveys and standard candles, and the value derived from the Cosmic Microwave Background data. Tension emerges assuming the standard ΛCDM cosmological model [1][2][3]. This discrepancy is attracting numerous research groups, and it is resulting in hundreds of publications aimed to fix it. From a general point of view, this discrepancy and other ones seem to point out a tension between the current picture of late and early Universe.A broad range of topics can be considered in searching for a solution of this puzzle. A bunch of them has been considered in this Focus Point. They range from the detailed methods of observational data analysis, up to dark matter and dark energy models dealing with modifications of ΛCDM, general relativity, basic physical principles, or variation of physical constants, including of the gravitational constant (see [4] for a recent review).The puzzle is, essentially, twofold: from one side, the tension could be addressed by improving measurements and datasets like Cepheids, type Ia supernovae (SNeIa), galaxy sky surveys, gravitational lensing, baryon acoustic oscillations, Big Bang nucleosynthesis, etc. From the other side, the effort is to search for new physics, including possible modifications of general relativity.Let us illustrate the diversity of the approaches considered in this Focus Point.There are studies on the very nature of the main observational parameter, the redshift. In the framework of nonlinearelectromagnetism involving the Lorentz-Poincaré symmetry violation, the possibility of additional cosmological expansionindependent frequency shift has been discussed in [5,6]. The results are compared with observational Pantheon data of SNeIa. Such results allow to conclude that the frequency shift can support an alternative to accelerated expansion accommodating the SNeIa data. Importantly, laboratory tests to investigate additional shifts from extended Electromagnetism can be taken into account. In another study, systematic errors in the cosmological redshift for SNeIa measurements have been studied [7]. As shown, the reduction of the Hubble constant local value is possible up to 5%.The observational constraints for inflationary models derived from f (R) gravity, including the Starobinski and higher-order Lagrangians, have been derived using Planck satellite and Keck datasets [8]. Remarkably, it appears that although f (R) constant-roll models adopt observationally acceptable values of the parameter r, they do not predict favorable values of the spectral index. In another paper [9], f (R) gravity in Palatini approach is studied as an inverse problem: specifically, models are reconstructed starting from an effective equation of state to be constrained by data. In particular, the Starobinsk...