Brans–Dicke cosmology with a Λ-term: a possible solution to ΛCDM tensions*

Abstract

We present a full-fledged analysis of Brans–Dicke cosmology with a cosmological constant and cold dark matter (BD-ΛCDM for short). We extend the scenarios where the current cosmological value of the BD-field is restricted by the local astrophysical domain to scenarios where that value is fixed only by the cosmological observations, which should be more natural in view of the possible existence of local screening mechanism. Our analysis includes both the background and perturbations equations in different gauges. We find that the BD-ΛCDM is favored by the overall cosmological data as compared to the concordance GR-ΛCDM model, namely data on distant supernovae, cosmic chronometers, local measurements of the Hubble parameter, baryonic acoustic oscillations, large-scale structure formation and the cosmic microwave background under full Planck 2018 CMB likelihood. We also test the impact of strong and weak-lensing data on our results, which can be significant. We find that the BD-ΛCDM can mimic effective quintessence with a significance of about 3.0–3.5σ c.l. (depending on the lensing datasets). The fact that the BD-ΛCDM behaves effectively as a running vacuum model (RVM) when viewed from the GR perspective helps to alleviate some of the existing tensions with the data, such as the σ 8 excess predicted by GR-ΛCDM. On the other hand, the BD-ΛCDM model has a crucial bearing on the acute H 0-tension with the local measurements, which is rendered virtually harmless owing to the small increase of the effective value of the gravitational constant with the expansion. The simultaneous alleviation of the two tensions is a most remarkable feature of BD-gravity with a cosmological constant in the light of the current observations, and hence goes in support of BD-ΛCDM against GR-ΛCDM.

Friedmann cosmology with decaying vacuum density in Brans–Dicke theory

“…The aim of the work is to find a cosmological scenario of the model in BD theory with varying cosmological term which would be capable to link the dynamics of the early Universe with that of our late Universe. This work extends the successful approach recently presented on BD cosmology with a rigid cosmological term [61,62], and reinforces the idea that dynamical models of the vacuum energy can be very helpful to improve the fit to cosmological and cosmographical observations [63][64][65][66][67]. In particular, they help alleviating the so-called H 0 and σ 8 tensions [68][69][70][71], see e.g.…”

Stringy-running-vacuum-model inflation: from primordial gravitational waves and stiff axion matter to dynamical dark energy

“…cosmological observations (and in alleviating the tensions afflicting the latter) provided the effective value of ν eff is positive and of order ν eff = O(10 −3 ) > 0 [22][23][24][25][26][27][28][29]31,32]. Amazingly, this value is also of the order of the predicted one in (92).…”

“…Let us, however, note that one may also obtain an effective RVM evolution in a Brans-Dicke (BD) context with a cosmological constant, hence within a gravity paradigm different from GR. It turns out that in this latter form (in which there is an evolution of the effective gravitational coupling as well) the RVM is particularly efficient in solving the main tensions, above all the one associated with the local value of the Hubble parameter, H 0 [31,32]. In either framework (GR or BD), the dynamical vacuum energy density associated with the RVM is based on the following general renormalisation-group (RG)-like form of the vacuum energy density in terms of powers of the Hubble parameter H(t) =ȧ/a, which is a function of the cosmic time t, and its cosmic-time derivativeḢ [12][13][14][15]: (1) where the coefficients A i , B i are dimensionless, whereas the C i ones and higher are dimensionful.…”

Early modified gravity in light of the H0 tension and LSS data

“…The former ones, however, seem to be preferred over since reduce the value of the comoving size of the sound horizon at baryon drag r s , without spoiling the fit to CMB and BAO data [30][31][32]. Two well studied frameworks to modify the early time dynamics of the Universe and inject the required energy into the cosmic fluid to lower r s with respect to the ΛCDM one are modified gravity (MG) [33][34][35][36][37][38][39][40][41][42][43][44][45] and early dark energy (EDE) models [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63]. 1 In this paper, we extend the model with a scalar field on-minimally coupled to the Ricci scalar of the form FðσÞ ¼ M 2 pl þ ξσ 2 in presence of a cosmological constant Λ [41,42], by providing it with a small effective mass.…”