Density perturbations for running vacuum: a successful approach to structure formation and to the σ8-tension

Abstract

Recent studies suggest that dynamical dark energy (DDE) provides a better fit to the rising affluence of modern cosmological observations than the concordance model (ΛCDM) with a rigid cosmological constant, Λ. Such is the case with the running vacuum models (RVMs) and to some extent also with a simple XCDM parametrization. Apart from the cosmic microwave background (CMB) anisotropies, the most crucial datasets potentially carrying the DDE signature are: i) baryonic acoustic oscillations (BAO), and ii) direct large scale structure (LSS) formation data (i.e. the observations on f (z)σ 8 (z) at different redshifts). As it turns out, analyses mainly focusing on CMB and with insufficient BAO+LSS input, or those just making use of gravitational weaklensing data for the description of structure formation, generally fail to capture the DDE signature, whereas the few existing studies using a rich set of CMB+BAO+LSS data (see in particular Solà, Gómez-Valent & de Cruz Pérez 2015and Zhao et al. 2017) do converge to the remarkable conclusion that DDE might well be encoded in the current cosmological observations. Being the issue so pressing, here we explain both analytically and numerically the origin of the possible hints of DDE in the context of RVMs, which arise at a significance level of 3 − 4σ. By performing a detailed study on the matter and vacuum perturbations within the RVMs, and comparing with the XCDM, we show why the running vacuum fully relaxes the existing σ 8 -tension and accounts for the LSS formation data much better than the concordance model.

Gravitational and chiral anomalies in the running vacuum universe and matter-antimatter asymmetry

“…As a matter of fact, this is also what makes possible for the DE in our epoch to inherit a "relic" dynamical H 2 -component as part of the observed DE contribution to the current energy budget of the Cosmos. Therefore, in the context of the scenario described in the present article, we naturally predict dynamical DE, which, as argued above, seems to be favoured by current observations [14][15][16][17]45].…”

“…For simplicity, let us hereafter stick to the simplest association µ = H. Taking into account thatḢ = −(q + 1)H 2 , where q is the deceleration parameter, which assumes the values q = 1, 1/2, −1, as we move from the radiation-into the matter-and DE-dominated epochs, respectively, we can see that the modification introduced byḢ is not very important and we can pick up the main effect already with the canonical association µ = H -this is indeed substantiated in the practical analyses, see e.g. [14][15][16][17]. In this situation, we have a 1 = a 3 = a 5 = 0 in (4).…”

“…In fact, the RVM agrees excellently with the current cosmological data at large scales [1], but also makes important predictions [14][15][16][17] that could alleviate current tensions in the data, concerning, for instance, the so-called σ 8 tension and an associated improvement in describing large-scale structure formation, compared to the ΛCDM paradigm. The model also provides better insight into the discrepancy with the (local) value of H 0 between measurements by the Hubble Space Telescope, based on Cepheid observations [30], and those by the Planck Collaboration, based on Cosmic Microwave Background (CMB) studies [1].…”

“…Numerous studies of that model on its cosmological evolution from the early universe to the present day can be found in [8][9][10][11][12][13]. Furthermore, detailed confrontations with the recent cosmological data has been presented in [14][15][16], which extend the analyses of [17][18][19] and of older works [20].…”

“…In [14][15][16] it is argued that the presence of the index ν in the RVM evolution of the Hubble parameter (11), which affects the scaling of the vacuum energy density (5) and, thus, differentiates it from the standard ΛCDM case, leads to combined fits to SnIA+BAO+H(z)+CMB data that favour a lower value of σ 8 .…”