Graduated dark energy: Observational hints of a spontaneous sign switch in the cosmological constant

Abstract

We study the cosmological constant (Λ) in the standard ΛCDM model by introducing the graduated dark energy (gDE) characterised by a minimal dynamical deviation from the null inertial mass density of the Λ in the form ρinert ∝ ρ λ < 0 with λ < 1 being a ratio of two odd integers, for which its energy density ρ dynamically takes negative values in the finite past. For large negative values of λ, it creates a phenomenological model described by a smooth function that approximately describes the Λ spontaneously switching sign in the late universe to become positive today. We confront the model with the latest combined observational data sets of PLK+BAO+SN+H. It is striking that the data predict bimodal posterior probability distributions for the parameters of the model along with large negative λ values; the new maximum significantly excludes the Λ, and the old maximum contains the Λ. The improvement in the goodness of the fit for the Λ reaches highly significant levels, ∆χ 2 min = 6.4, for the new maxima, while it remains at insignificant levels, ∆χ 2 min 0.02, for the old maxima. We show that, in contrast to the old maxima, which do not distinguish from the Λ, the new maxima agree with the model-independent H0 measurements, high-precision Ly-α data, and model-independent Omh 2 diagnostic estimates. Our results provide strong hints of a spontaneous sign switch in the cosmological constant and lead us to conjecture that the universe has transitioned from AdS vacua to dS vacua, at a redshift z ≈ 2.32 and triggered the late-time acceleration, and suggests looking for such mechanisms in string theory constructions.

Rastall gravity extension of the standard $$\Lambda $$CDM model: theoretical features and observational constraints

“…On the other hand, this enhanced amount of the usual vacuum is completely screened by the effective source at z * = 2.4 [see (26) and Fig. 1], which is pretty close to the values suggested in [33,34,42] for relaxing a number of persistent low-redshift tensions, including the H 0 tension, that arise within the standard CDM model. It is worth noting that, as may be seen in the same figure, for z * > 2.4, the dust energy density assumes values larger than the total energy density of the Universe.…”

“…This situation achieved for > 0 is of particular interest and tempting for its further theoretical and observational investigation, as it has recently been reported in [31][32][33][34][35][36][37][38][39][40][41][42][43][44] that a number of persistent low-redshift tensions, including the H 0 tension, may be alleviated by a dynamical DE that assumes negative energy density values (or in cosmological models wherein the cosmological constant is dynamically screened) at finite redshift (see Sect. 1).…”

“…This leads to unrealistic values for the key parameters relevant to the early Universe, namely, z eq = 15303 and k eq = 0.04470 Mpc −1 , which are extremely different than the values z eq = 3391 and k eq = 0.01045 Mpc −1 obtained in the case of the standard CDM model ( = 0). This situation signals that the Rastall-CDM model with = 0.06, which is tempting as it leads to z * = 2.4 in line with [33,34,42], is not well behaved at large redshifts. Thus, it is conceivable that the high redshift cosmological data would not allow such large positive values of .…”

“…This tension becomes even more compelling as it worsens (relieves only partially) when the is replaced by the simplest minimally coupled single-field quintessence (phantom or quintom models), see [27][28][29][30] and [31] for further references. Surprisingly, it has been reported that the H 0 tension -as well as a number of other persistent low-redshift tensions -may be alleviated by a dynamical dark energy (DE) whose density can assume negative values or vanish at high redshifts [31][32][33][34][35][36][37][38][39][40][41][42][43][44]. The fact that the CMB Planck data favor positive spatial curvature -which imitates a negative energy density source -on top of the standard CDM model in contrast to the inflationary paradigm might also be signaling a need for such DE sources [5] (see also [45][46][47]).…”

“…[59][60][61][62] suggesting larger H 0 values in the light of observational analyses when the Brans-Dicke theory or its extensions are considered. There is a wide range of examples, related to these three possibilities, that exist (i) in theories in which relaxes from a large initial value via an adjustment mechanism [63][64][65], (ii) in the models in which itself spontaneously switches sign [42,66], (iii) in cosmological models based on Gauss-Bonnet gravity [67], (iv) in braneworld models [68,69], (v) in loop quantum cosmology [70,71], (vi) in higher-dimensional cosmologies that accommodate dynamical reduction of the internal space [72][73][74][75], (vii) in generalisations of the form of the matter Lagrangian in a non-linear way [76][77][78], (viii) in some constructions within the unimodular gravity violating the local energy-momentum conservation law [44].…”