First evidence of running cosmic vacuum: challenging the concordance model

TL;DR

This study presents evidence that models with a dynamical vacuum energy density, which varies with the Hubble rate, fit cosmological data better than the standard cosmological constant model, challenging the traditional $bc$CDM paradigm.

Contribution

The paper introduces and tests running vacuum models with a Hubble-dependent vacuum energy, showing they outperform the standard $bc$CDM in fitting observational data.

Findings

Running vacuum models are favored over $bc$CDM at >4.2 sigma significance.

Dynamical vacuum models are strongly preferred by AIC and BIC criteria.

Results suggest the need to reconsider the cosmological constant as a fixed parameter.

Abstract

Despite the fact that a rigid $\Lambda$-term is a fundamental building block of the concordance $\Lambda$CDM model, we show that a large class of cosmological scenarios with dynamical vacuum energy density $\rho_{\Lambda}$ and/or gravitational coupling $G$, together with a possible non-conservation of matter, are capable of seriously challenging the traditional phenomenological success of the $\Lambda$CDM. In this paper, we discuss these "running vacuum models" (RVM's), in which $\rho_{\Lambda}=\rho_{\Lambda}(H)$ consists of a nonvanishing constant term and a series of powers of the Hubble rate. Such generic structure is potentially linked to the quantum field theoretical description of the expanding Universe. By performing an overall fit to the cosmological observables $SNIa+BAO+H(z)+LSS+BBN+CMB$ (in which the WMAP9, Planck 2013 and Planck 2015 data are taken into account), we find that the class of RVM's appears significantly more favored than the $\Lambda$CDM, namely at an unprecedented level of $\gtrsim4.2\sigma$. Furthermore, the Akaike and Bayesian information criteria confirm that the dynamical RVM's are strongly preferred as compared to the conventional rigid $\Lambda$-picture of the cosmic evolution.