Imperfect fluid description of modified gravities

Faraoni, Valerio; Côté, Jeremy

doi:10.1103/physrevd.98.084019

Cited by 60 publications

(78 citation statements)

References 132 publications

(120 reference statements)

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“…Depending on the characteristics of the detected expansion anisotropy, it could be illuminating to the nature of inflaton, dark energy (DE) and even gravitation. For instance, altering the stiff-fluid-like behavior of the expansion anisotropy could be possible mainly by either replacing minimally coupled scalar field models of inflaton or DE by a source yielding anisotropic pressure (e.g., vector fields; see [12] for a list of anisotropic stresses and their effects on the expansion anisotropy) or replacing GR by a modified gravity that can give rise to effective source yielding anisotropic pressure (e.g., Brans-Dicke theory [13][14][15]); see, for examples, [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33].…”

Section: Introductionmentioning

confidence: 99%

Constraints on a Bianchi type I spacetime extension of the standard ΛCDM model

et al. 2019

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We consider the simplest anisotropic generalization, as a correction, to the standard ΛCDM model, by replacing the spatially flat Robertson-Walker metric by the Bianchi type-I metric, which brings in a new term Ωσ0a −6 (mimicking the stiff fluid) in the average expansion rate H(a) of the Universe. From Hubble and Pantheon data, relevant to the late Universe (z 2.4), we obtain the constraint Ωσ0 10 −3 , in line with the model-independent constraints. When the baryonic acoustic oscillations and cosmic microwave background (CMB) data are included, the constraint improves by 12 orders of magnitude, i.e., Ωσ0 10 −15 . We find that this constraint could alter neither the matter-radiation equality redshift nor the peak of the matter perturbations. Demanding that the expansion anisotropy has no significant effect on the standard big bang nucleosynthesis (BBN), we find the constraint Ωσ0 10 −23 . We show explicitly that the constraint from BBN renders the expansion anisotropy irrelevant to make a significant change in the CMB quadrupole temperature, whereas the constraint from the cosmological data in our model provides the temperature change up to ∼ 11 mK, though it is much beyond the CMB quadrupole temperature.

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Section: Introductionmentioning

confidence: 99%

Constraints on a Bianchi type I spacetime extension of the standard ΛCDM model

et al. 2019

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“…Within this approach, the correspondence between general scalar-tensor theories and this effective fluid has been worked out explicitly first in [111] (see also [112] for the preceding work), and has shown that, in general, the corresponding effective fluid is imperfect. A recent work [113] extended and completed the correspondence showing how a symmetry of BD theory translates into a symmetry of this fluid such that this correspondence is valid for any spacetime geometries. Brans and Dicke's 1961 theory [106,110], motivated first by the implementation of Mach's principle in gravity, today provides a prototype of scalar-tensor theories [107][108][109].…”

Section: Introductionmentioning

confidence: 90%

Anisotropic massive Brans–Dicke gravity extension of the standard $$\Lambda $$CDM model

et al. 2020

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We present an explicit detailed theoretical and observational investigation of an anisotropic massive Brans-Dicke (BD) gravity extension of the standard ΛCDM model, wherein the extension is characterized by two additional degrees of freedom; the BD parameter, ω, and the present day density parameter corresponding to the shear scalar, Ω σ 2 ,0 . The BD parameter, determining the deviation from general relativity (GR), by alone characterizes both the dynamics of the effective dark energy (DE) and the redshift dependence of the shear scalar. These two affect each other depending on ω, namely, the shear scalar contributes to the dynamics of the effective DE, and its anisotropic stress -which does not exist in scalar field models of DE within GR-controls the dynamics of the shear scalar deviating from the usual ∝ (1 + z) 6 form in GR. We mainly confine the current work to non-negative ω values as it is the right sign -theoretically and observationally-for investigating the model as a correction to the ΛCDM. By considering the current cosmological observations, we find that ω 250, Ω σ 2 ,0 10 −23 and the contribution of the anisotropy of the effective DE to this value is insignificant. We conclude that the simplest anisotropic massive BD gravity extension of the standard ΛCDM model exhibits no significant deviations from it all the way to the Big Bang Nucleosynthesis. We also point out the interesting features of the model in the case of negative ω values; for instance, the constraints on Ω σ 2 ,0 could be relaxed considerably, the values of ω ∼ −1 (relevant to string theories) predict dramatically different dynamics for the expansion anisotropy.

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“…The entropy perturbation, e.g., is one of the two basic dynamical quantities in our context and neither velocity components nor metric functions do appear explicitly in the system of equations. An imperfect fluid description of scalar-tensor theories has been recently performed also in [21].…”

Section: E Perturbations Of (An-)isotropic Pressures and Energy Fluxmentioning

confidence: 99%

“…In the presence of an adequate timelike vector field, this energymomentum tensor is then characterized by an energy density, a scalar isotropic pressure, an anisotropic pressure and an energy flux. It has the structure of the energy-momentum tensor of an imperfect fluid [18][19][20][21]. Imperfect-fluid dynamics can provide an effective description for cosmological models beyond the standard model.…”

Section: Introductionmentioning

confidence: 99%

Matter Growth in Imperfect Fluid Cosmology

2019

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Extensions of Einstein's General Relativity (GR) can formally be given a GR structure in which additional geometric degrees of freedom are mapped on an effective energy-momentum tensor. The corresponding effective cosmic medium can then be modeled as an imperfect fluid within GR. The imperfect fluid structure allows us to include, on a phenomenological basis, anisotropic stresses and energy fluxes which are considered as potential signatures for deviations from the cosmological standard Λ-cold-dark-matter (ΛCDM) model. As an example, we consider the dynamics of a scalartensor extension of the standard model, the eΦΛCDM model. We constrain the magnitudes of anisotropic pressure and energy flux with the help of redshift-space distortion (RSD) data for the matter growth function f σ8.

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Imperfect fluid description of modified gravities

Cited by 60 publications

References 132 publications

Constraints on a Bianchi type I spacetime extension of the standard ΛCDM model

Constraints on a Bianchi type I spacetime extension of the standard ΛCDM model

Anisotropic massive Brans–Dicke gravity extension of the standard $$\Lambda $$CDM model

Matter Growth in Imperfect Fluid Cosmology

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