Nonstatic Reissner-Nordstr\"om metric in the perturbative $f(R)$ theory: Embedding in the background of the FLRW cosmology, uniqueness of solutions, the TOV equation

TL;DR

This paper develops a nonstatic Reissner-Nordström metric within perturbative $f(R)$ gravity, revealing new effects like gravitational wave emission from spherically symmetric sources and embedding these solutions in FLRW cosmology.

Contribution

It introduces a nonstatic charged black hole solution in $f(R)$ gravity, demonstrating time-dependent metrics and gravitational wave emission, and presents a new embedding method in cosmological backgrounds.

Findings

Time-dependent metrics outside gravitational sources

Potential gravitational wave emission from spherically symmetric sources

A perturbative TOV equation in $f(R)$ gravity

Abstract

This article introduces a nonstatic Reissner-Nordstr\"om metric, a metric that does not emit electromagnetic waves but can emit gravitational waves. We first use the GR theory to study a charged spherically symmetric gravitational source (CSSGS), the obtained results are further improved in comparison with the previous studies. In particular, this article considers that the field is not necessarily static. The metric tensors $ g_{\mu\nu} $ are considered both outside and inside the gravitational source (the results show that in the first case $ g_{\mu\nu} $ are time independent, in the latter case they are time dependent). The gravitational acceleration and the event horizon of a charged black hole are investigated. The results prove that the gravitational field is always attractive. We then use the perturbative $ f(R) $ theory to consider CSSGS. The obtained results not only correct the solution of Einstein's equation in magnitude (this will describe astronomical and cosmological quantities more accurately than Einstein's equation), but also reveal new effects. Outside the gravitational source, the metric tensors can depend on time, this makes it possible for a spherically symmetric gravitational source to emit gravitational waves (Einstein's equation cannot give this effect). However, a spherically symmetric field still does not emit electromagnetic waves. Next we present a new method for embedding the spherically symmetric metrics of a star (or a black hole) in the background of the FLRW cosmological. Finally, we discuss the uniqueness of the solutions of the f(R) theory. The perturbative TOV equation is also found.