Hints of dynamical vacuum energy in the expanding Universe

TL;DR

The paper tests dynamical vacuum energy models in which $Λ$ depends on the Hubble rate and its derivative via $Λ(H)=C_0+C_H H^2+C_{\ abla\dot{H}} \dot{H}$ while allowing $G$ to vary slowly to preserve matter conservation. Background cosmology is solved for two realizations, G1 and G2, introducing parameters $ν$ and $α$ and showing that $G(a)\propto a^{4(1-ξ')}$ with $ξ,ξ'$ encoding the running, and that ΛCDM is recovered when $ν,α\to0$. A comprehensive fit is performed against expansion-history data (Omh^2, SNIa, BAO, CMB) plus growth data and BBN/CMB bounds on $ΔG/G$, using AIC for model comparison; the dynamical vacuum models yield smaller $\,χ^2$ and are favored over ΛCDM, especially G2, with ΔAIC values indicating significant evidence. Linear perturbation analysis shows modified growth via the slowly varying $G(a)$ leads to $f(z)$ and $fσ_8(z)$ histories compatible with observations, strengthening the case for dynamical vacuum energy as a viable alternative to a rigid cosmological constant.

Abstract

Recently there have been claims on model-independent evidence of dynamical dark energy. Herein we consider a fairly general class of cosmological models with a time-evolving cosmological term of the form $Λ(H)=C_0+C_H H^2+C_{\dot{H}} \dot{H}$, where $H$ is the Hubble rate. These models are well motivated from the theoretical point of view since they can be related to the general form of the effective action of quantum field theory in curved spacetime. Consistency with matter conservation can be achieved by letting the Newtonian coupling $G$ change very slowly with the expansion. We solve these dynamical vacuum models and fit them to the wealth of expansion history and linear structure formation data. The results of our analysis show a significantly better agreement as compared to the concordance $Λ$CDM model, thus supporting the possibility of a dynamical cosmic vacuum.

Figures (3)

• Figure 1: Left: Evolution of the effective EoS $\omega_D(z)$, Eq. (\ref{['eq:effEoS']}), for the models under consideration: Right: The corresponding evolution of the effective DE density $\Omega_{D}(z)$ normalized to the critical density (see text).
• Figure 2: Likelihood contours in the $(\Omega_m,\nu_{\rm eff})$ plane (for the values $-2\ln\mathcal{L}/\mathcal{L}_{max}=2.30$, $6.16, 11.81$, corresponding to 1$\sigma$, 2$\sigma$ and $3\sigma$ confidence levels for the G2 model using the full data analysis indicated in Table 2. The $\nu_{\rm eff}=0$ region ($\Lambda$CDM) is disfavored at $\sim 3\sigma$.
• Figure 3: Left: Comparison of the observed data ֠with error bars (in green) ֠and the theoretical evolution of the weighted growth rate of clustering $f(z)\sigma_8(z)$ for each dynamical vacuum model and the $\Lambda$CDM. Right: The corresponding evolution of the linear growth index $\gamma(z)$.