Can Dark Energy Emerge from a Varying G and Spacetime Geometry?

Ekim Taylan Hanımeli,Isaac Tutusaus,Alain Blanchard,Brahim Lamine

doi:10.3390/universe8030148

Abstract

The accelerated expansion of the universe implies the existence of an energy contribution known as dark energy. Associated with the cosmological constant in the standard model of cosmology, the nature of this dark energy is still unknown. We will discuss an alternative gravity model in which this dark energy contribution emerges naturally, as a result of allowing for a time-dependence on the gravitational constant, G, in Einstein’s field equations. With this modification, Bianchi’s identities require an additional tensor field to be introduced so that the usual conservation equation for matter and radiation is satisfied. The equation of state of this tensor field is obtained using additional constraints, coming from the assumption that this tensor field represents the space-time response to the variation of G. We will also present the predictions of this model for the late-universe data, and show that the energy contribution of this new tensor is able to explain the accelerated expansion of the universe without the addition of a cosmological constant. Unlike many other alternative gravities with varying gravitational strength, the predicted G evolution is also consistent with local observations and therefore this model does not require screening. We will finish by discussing possible other implications this approach might have for cosmology and some future prospects.

Highlights

Modern cosmology has increasingly become a precision science during the last 30 years, allowing the determination of many properties of the universe to a great degree of accuracy [1,2]
Dark energy is represented by a cosmological constant term, Λ, within Einstein’s equations
As our model is concerned with dark energy, we use late-universe probes, namely type-Ia supernovae (SNIa) and baryon acoustic oscillations (BAO)

Summary

Introduction

Modern cosmology has increasingly become a precision science during the last 30 years, allowing the determination of many properties of the universe to a great degree of accuracy [1,2]. It is possible to imagine that a theory of quantum gravity will have some sort of a mechanism allowing vacuum energy density to be set to a value that matches the cosmological observations.. Gravity does not work exactly as general relativity envisions, and there should be a modification in such a way to explain away the accelerated expansion In this view, dark energy is the artefact of a gravitational mechanism. As there is no fundamental reason that G should be a constant, this model can be said to lift that assumption This way, instead of offering a new theory of gravity, we study a phenomenological extension to general relativity. The discussion and results presented in the following are based on Ref. [12] by the present authors, and we refer the reader to this reference for further details

Varying G in Einstein’s Equations

Analysis and Results

Conclusions and Outlook