Gravitational physics is the branch of physics which aims to unveil as much as possible about the fundamental laws of the gravitational force. It is always done in synergy with neighboring areas of physics and astronomy represented in ARCO. Einstein’s theory of General Relativity (GR) is a very successful huge leap in this direction. It was verified in numerous tests, experiments and observations from the solar system tests of the early days, until the recent compact merger events whose discovery became possible thanks to the new window of gravitational-wave astronomy. Gravitational waves themselves are also a seminal prediction of GR. In addition, the Lambda-CDM cosmological model is consistent with all present observations, thus providing a robust description and explanation to the evolution of the universe since the big bang. However, it is widely accepted that GR cannot be considered as the final fundamental formulation of gravity theory and there is still a long way to go. There are a few clouds which overshadow GR which will not be listed here in detail due to space considerations. But the conclusion is the need to extend and generalize GR. There are countless generalizations and extensions of GR, motivated from many various directions. As just a few examples, we mention scalar-tensor theories, f(R) theories, Horndeski vector-tensor theories, Einstein-Proca theories all adding to the tensorial gravitational Einsteinian degrees of freedom, scalar or vector degrees of freedom. Furthermore, there are supersymmetric extensions adding Fermion fields, like Supergravity and the radical Superstring theories. All these theories are studied in the framework of gravitational physics from all relevant possible aspects like black hole solutions and their properties as well as those of other compact objects, cosmological structure formation, cosmological models and their stability, propagation of gravitational waves, relation to the problems of Dark Energy and Dark Matter, Inflation in these theories, and a large number of other equivalently important issues. Comparison with the analogous GR systems is crucial in order to identify observational methods in order to separate GR from its extensions and the extensions of each other. Less direct comparisons is also of major importance since new concepts which emerge more naturally in one family of theories may also enhance the understanding of the others.

Bouncing Cosmological Isotropic Solutions in Scalar – Tensor Gravity

In collaboration with: A. A. Starobinsky (Landau Inst.) and D. Polarski (U. Montpellier) Bouncing non-singular isotropic cosmological solutions are investigated in a simple model of scalar-tensor gravity. New families of such solutions are found and their properties are presented and analyzed using an effective potential as the main tool. Bouncing solutions are shown to exist for a Higgs-like self-interaction potential which is bounded from below, in contrast to previous solutions that appeared in the literature based on potentials which were unbounded from below. In the simplest version of a scalar field with the quartic potential and conformal coupling to gravity, bouncing spatially flat solutions either have the Hubble function diverging in the past before the bounce, but with a well-behaved future, or are globally regular but unstable with respect to anisotropic or inhomogeneous perturbations at some finite values of the scalar field and curvature. Regular solutions can only exist in the part of the parameter space where the maximum of the effective potential is larger than the first zero of the potential, and gravity becomes repulsive at the bounce. Bouncing Cosmological Isotropic Solutions in Scalar - Tensor Gravity Several representative “time-profiles” (scalar field Φ(t) and Hubble function H(t)) of cosmological bounce solutions for the conformally coupled case (ξ=1/6). These bounce solutions violate the condition F(Φ)>0 at the bounce. Note that the symmetric Φ(t) solution has a maximum at the bounce. Note also a symmetric Φ(t) solution with a minimum at t=0. Bounce solutions are those that satisfy H(0)=0, H’(0)>0 and have no future singularities. Reference: Bouncing Cosmological Isotropic Solutions in Scalar-Tensor Gravity, D. Polarski, A.A. Starobinsky and Y. Verbin, JCAP 01 (2022) 052;  e-Print: 2111.07319 [gr-qc]  


Scalarized electric and magnetic black holes in the vector-tensor Horndeski gravity

In collaboration with Y. Brihaye (Mons U) Study of the Horndeski-Reissner-Nordstrom black hole (HRNBH) solutions. The electric and magnetic types of these BHs differ from each other, since there is no duality symmetry between electric and magnetic fields in the vector-tensor Horndeski gravity. The magnetic black hole solutions are especially simple and can be written explicitly in terms of hypergeometric functions. It is natural to expect that these black holes undergo spontaneous scalarization and it is indeed the case, if we add a massless real scalar field which is coupled to the vector-tensor Horndeski term. The single coupling constant of the Horndeski term is promoted now to a scalar field-dependent coupling function. This system contains (ordinary) Reissner-Nordstrom black holes with a vanishing scalar field, but on a certain “bifurcation surface” in parameter space these solutions become unstable and scalarized HRNBHs appear. The “double-faced” solutions of dyonic scalarized BHs exist in a certain domain of parameter space which is bounded by a critical surface on which the nature of the limiting solutions is determined by the proximity to the lines of vanishing electric or magnetic charges. Scalarized electric and magnetic black holes in the vector-tensor Horndeski gravity The dependence on the horizon value of the scalar field of the: mass (M), electric charge (Q), scalar charge (D) and Hawking temperature (T) of scalarized Horndeski Black holes with magnetic charges P=0, 0.5, 0.85. Left: mass and temperature, Right: electric and scalar charges. References: Y. Brihaye and Y. Verbin, Scalarized compact objects in the vector-tensor Horndeski gravity, Phys. Rev. D102 124021 (2020). Y. Verbin, Magnetic black holes in the vector-tensor Horndeski theory, arXiv: 2011.02515. Y. Brihaye and Y. Verbin, Scalarized dyonic black holes in the vector-tensor Horndeski gravity, Phys. Rev. D104, 024047 (2021).

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