Dark energy in conformal Killing gravity

TL;DR

This paper shows that a modified gravity theory called conformal Killing gravity can produce effects similar to dark energy without explicitly including it, potentially explaining cosmic acceleration and addressing the Hubble tension.

Contribution

It demonstrates that conformal Killing gravity naturally yields an effective dark energy component with a specific equation of state, offering a novel explanation for cosmic acceleration.

Findings

Effective dark energy with ω = -5/3 arises naturally in the theory.

The model can explain the accelerated expansion at low redshifts.

Potential to resolve the Hubble tension and coincidence problem.

Abstract

The Friedmann equation, augmented with an additional term that effectively takes on the role of dark energy, is demonstrated to be an exact solution to the recently proposed gravitational theory named "conformal Killing gravity." This theory does not explicitly incorporate dark energy. This finding suggests that there is no necessity to postulate the existence of dark energy as an independent physical entity. The dark energy derived from this theory is characterized by a specific equation of state parameter, denoted as $\omega$, which is uniquely determined to be $-5/3$. If this effective dark energy is present, typically around 5% of the total energy density at the present time, and under the assumption of density parameters for matter and the cosmological constant, $\Omega_{\rm m}\sim 0.25$ and $\Omega_\Lambda \sim 0.7$, respectively, the expansion of the universe at low redshifts ($z < 1.5$) can exceed expectations, while the expansion at $z > 1.5$ remains unchanged. This offers a potential solution to the Hubble tension problem. Alternatively, effective dark energy could be a dominant component in the present-day universe. In this scenario, there is also the potential to address the Hubble tension, and furthermore, it resolves the coincidence problem associated with the cosmological constant.