A model of accelerating dark energy in decelerating gravity

TL;DR

This paper explores a cosmological model combining Chaplygin gas dark energy with DGP braneworld gravity, showing it fits supernova data well without fine-tuning and may suggest a five-dimensional universe.

Contribution

It introduces a novel model integrating Chaplygin gas with DGP gravity that fits supernova data without fine-tuning and addresses the coincidence problem.

Findings

Self-decelerating branch fits supernova data as well as ΛCDM.

Model requires only one additional parameter.

Potential indication of a five-dimensional universe.

Abstract

The expansion of the Universe is accelerated as testified by observations of SNeIa at varying redshifts. Explanations of this acceleration are of two kinds: modifications of Einstein gravity or new forms of energy. An example of modified gravity is the braneworld Dvali-Gabadadze-Porrati (DGP) model, an example of dark energy is Chaplygin gas. Both are characterized by a cross-over length scale $r_c$ which marks the transition between physics occurring on our four-dimensional brane, and in a five-dimensional bulk space. Assuming that the scales $r_c$ in the two models are the same, we study Chaplygin gas dark energy in flat DGP geometries. The self-accelerating branch does not give a viable model, it causes too much acceleration. We derive the Hubble function and the luminosity distance for the self-decelerating branch, and then fit a compilation of 192 SNeIa magnitudes and redshifts in the space of the three parameters of the model. Our model with the self-decelerating branch fits the supernova data as successfully as does the $\Lambda CDM$ model, and with only one additional parameter. In contrast to the $\Lambda CDM$ model, this model needs no fine-tuning, and it can explain the coincidence problem. It is unique in the sense that it cannot be reduced to a cosmological constant model in any other limit of the parameter space than in the distant future. If later tests with other cosmological data are successful, we have here a first indication that we live in a five-dimensional braneworld.