A Note on Agegraphic Dark Energy

TL;DR

This paper examines the agegraphic dark energy model based on quantum fluctuations, showing it can match current observations only with specific parameter choices and discussing potential modifications and scalar-field correspondences.

Contribution

It analyzes the viability of the agegraphic dark energy model, identifying parameter constraints and proposing modifications to address early universe bounds.

Findings

Model can fit current dark energy density with large n or nonzero coupling.

Constraints from nucleosynthesis require small n, challenging the model.

Proposes modifications to relax early universe bounds.

Abstract

Recently a new model of dynamical dark energy, or time-varying $\Lambda$, was proposed by Cai [arXiv:0707.4049] by relating the energy density of quantum fluctuations in a Minkowski space-time, namely $\rho_q \equiv 3 n^2 m_P^2/t^2$, where $n\sim {\cal O}(1)$ and t is the cosmic time, to the present day dark energy density. In this note, we show that the model can be adjusted to the present values of dark energy density parameter $\Omega_q$ ($\simeq 0.73$) and the equation of state ${\rm w}\Z{q}$ ($\simeq -1$) only if the numerical coefficient $n$ takes a reasonably large value ($n\gtrsim 3$) or the present value of the gravitational coupling of q-field to (dark) matter is also nonzero, namely, $\tilde{Q}\simeq \frac{2}{n}(\Omega\Z{q0})^{3/2}>0$ where $\Omega\Z{q0}$ is the present value of dark energy density fraction. We also discuss some of the difficulties of this proposal as a viable dark energy model with a constant $n$; especially, the bound imposed on the dark energy density parameter $\Omega_q <0.1$ during big bang nucleosynthesis (BBN) requires $n< 1/6$. To overcome this drawback, we outline a few modifications where such constraints can be weakened or relaxed. Finally, by establishing a correspondence between the agegraphic dark energy scenario and the standard scalar-field model, we also point out some interesting features of an agegraphic quintessence model.