Viable Palatini- f ( R ) cosmologies with generalized dark matter

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179

168

Abstract. We investigate cosmologies where the accelerated expansion of the Universe is driven by a field with an anisotropic equation of state. We model such scenarios within the Bianchi I framework, introducing two skewness parameters to quantify the deviation of pressure from isotropy. We study the dynamics of the background expansion in these models. A special case of anisotropic cosmological constant is analyzed in detail. The anisotropic expansion is then confronted with the redshift and angular distribution of the supernovae type Ia. In addition, we investigate the effects on the cosmic microwave background (CMB) anisotropies for which the main signature appears to be a quadrupole contribution. We find that the two skewness parameters can be very well constrained. Tightest bounds are imposed by the CMB quadrupole, but there are anisotropic models which avoid this bound completely. Within these bounds, the anisotropy can be beneficial as a potential explanation of various anomalous cosmological observations, especially in the CMB at the largest angles. We also consider the dynamics of linear perturbations in these models. The covariant approach is used to derive the general evolution equations for cosmological perturbations taking into account imperfect sources in an anisotropic background. The implications for the galaxy formation are then studied. These results might help to make contact between the observed anomalies in CMB and large scale structure and fundamental theories exhibiting Lorentz violation.

Section: Case I: Small Anisotropymentioning

confidence: 80%

Anisotropic dark energy: dynamics of the background and perturbations

Koivisto

Mota

2008

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179

168

“…In particular, the observational viability of some functional forms of f (R) gravity have been studied using mainly SNe Ia, CMB radiation and Large Scale Structure (LSS) data [41,54,42,43].…”

Section: Introductionmentioning

confidence: 99%

Cosmological constraints from the Hubble parameter onf(R) cosmologies

Carvalho

Santos

Alcaniz

et al. 2008

Abstract. Modified f (R) gravity in the Palatini approach has been presently applied to Cosmology as a realistic alternative to dark energy. In this concern, a number of authors have searched for observational constraints on several f (R) gravity functional forms using mainly data of type Ia supenovae (SNe Ia), Cosmic Microwave Background (CMB) radiation and Large Scale Structure (LSS). In this paper, by considering a homogeneous and isotropic flat universe, we use determinations of the Hubble function H(z), which are based on differential age method, to place bounds on the free parameters of the f (R) = R − β/R n functional form. We also combine the H(z) data with constraints from Baryon Acoustic Oscillations (BAO) and CMB measurements, obtaining ranges of values for n and β in agreement with other independent analyses. We find that, for some intervals of n and β, models based on f (R) = R − β/R n gravity in the Palatini approach, unlike the metric formalism, can produce the sequence of radiation-dominated, matter-dominated, and accelerating periods without need of dark energy.

“…Koivisto (2006) who used large scale structure cosmological data from Sloan Digital Sky Survey (SDSS). However, these tight constraints on f (R) gravity are loosened when dark matter with inherent stresses (generalized dark matter, GDM) is allowed (Koivisto 2007).…”

Section: Gravito-magnetic Field In Kerr-de Sitter Metricmentioning

confidence: 99%

Gravitomagnetic effects in Kerr-de Sitter space-time

Iorio

Ruggiero

2009

We explicitly worked out the orbital effects induced on the trajectory of a test particle by the the weak-field approximation of the Kerr-de Sitter metric.It results that the node, the pericentre and the mean anomaly undergo secular precessions proportional to k, which is a measure of the non linearity of the theory.We used such theoretical predictions and the latest observational determinations of the non-standard precessions of the perihelia of the inner planets of the Solar System to put a bound on k getting k ≤ 10 −29 m −2 . The node rate of the LAGEOS Earth's satellite yields k ≤ 10 −26 m −2 . The periastron precession of the double pulsar PSR J0737-3039A/B allows to obtain k ≤ 3 × 10Interpreting k as a cosmological constant Λ, it turns out that such constraints are weaker than those obtained from the Schwarzschild-de Sitter metric.