David Polarski

Are f ( R ) Dark Energy Models Cosmologically Viable

All f(R) modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a strong coupling. This coupling induces a cosmological evolution radically different from standard cosmology. We find that, in all f(R) theories where a power of R is dominant at large or small R (which include most of those proposed so far in the literature), the scale factor during the matter phase grows as t(1/2) instead of the standard law t(2/3). This behavior is grossly inconsistent with cosmological observations (e.g., Wilkinson Microwave Anisotropy Probe), thereby ruling out these models even if they pass the supernovae test and can escape the local gravity constraints.

Scalar-tensor gravity in an accelerating universe

We consider scalar-tensor theories of gravity in an accelerating universe. The equations for the background evolution and the perturbations are given in full generality for any parametrization of the Lagrangian, and we stress that apparent singularities are sometimes artifacts of a pathological choice of variables. Adopting a phenomenological viewpoint, i.e., from the observations back to the theory, we show that the knowledge of the luminosity distance as a function of redshift up to z ~ (1-2), which is expected in the near future, severely constrains the viable subclasses of scalar-tensor theories. This is due to the requirement of positive energy for both the graviton and the scalar partner. Assuming a particular form for the Hubble diagram, consistent with present experimental data, we reconstruct the microscopic Lagrangian for various scalar-tensor models, and find that the most natural ones are obtained if the universe is (marginally) closed.

On the growth of linear perturbations

Abstract We consider the linear growth of matter perturbations in various dark energy (DE) models. We show the existence of a constraint valid at z = 0 between the background and dark energy parameters and the matter perturbations growth parameters. For ΛCDM γ 0 ′ ≡ d γ d z | 0 lies in a very narrow interval − 0.0195 ⩽ γ 0 ′ ⩽ − 0.0157 for 0.2 ⩽ Ω m , 0 ⩽ 0.35 . Models with a constant equation of state inside General Relativity (GR) are characterized by a quasi-constant γ 0 ′ , for Ω m , 0 = 0.3 for example we have γ 0 ′ ≈ − 0.02 while γ 0 can have a nonnegligible variation. A smoothly varying equation of state inside GR does not produce either | γ 0 ′ | > 0.02 . A measurement of γ ( z ) on small redshifts could help discriminate between various DE models even if their γ 0 is close, a possibility interesting for DE models outside GR for which a significant γ 0 ′ can be obtained.

Quantum to classical transition for fluctuations in the early universe

According to the inflationary scenario for the very early Universe, all inhomogeneities in the Universe are of genuine quantum origin. On the other hand, looking at these inhomogeneities and measuring them, clearly no specific quantum mechanical properties are observed. We show how the transition from their inherent quantum gravitational nature to classical behavior comes about — a transition whereby none of the successful quantitative predictions of the inflationary scenario for the present-day universe is changed. This is made possible by two properties. First, the quantum state for the spacetime metric perturbations produced by quantum gravitational effects in the early Universe becomes very special (highly squeezed) as a result of the expansion of the Universe (as long as the wavelength of the perturbations exceeds the Hubble radius). Second, decoherence through the environment distinguishes the field amplitude basis as being the pointer basis. This renders the perturbations presently indistinguishable from stochastic classical inhomogeneities.

Pointer states for primordial fluctuations in inflationary cosmology

Primordial fluctuations in inflationary cosmology acquire classical properties through decoherence when their wavelengths become larger than the Hubble scale. Although decoherence is effective, it is not complete, so a significant part of primordial correlations remains up to the present moment. We address the issue of the pointer states which provide a classical basis for the fluctuations with respect to the influence by an environment (other fields). Applying methods from the quantum theory of open systems (the Lindblad equation), we show that this basis is given by narrow Gaussians that approximate eigenstates of field amplitudes. We calculate both the von Neumann and linear entropy of the fluctuations. Their ratio to the maximal entropy per field mode defines a degree of partial decoherence in the entropy sense. We also determine the time of partial decoherence making the Wigner function positive everywhere which, for super-Hubble modes during inflation, is virtually independent of coupling to the environment and is only slightly larger than the Hubble time. On the other hand, assuming a representative environment (a photon bath), the decoherence time for sub-Hubble modes is finite only if some real dissipation exists.

Dispersion of growth of matter perturbations in f ( R ) gravity

We study the growth of matter density perturbations

POWER-LAWS f(R) THEORIES ARE COSMOLOGICALLY UNACCEPTABLE

{\ensuremath{\delta}}_{m}

Generalizing the running vacuum energy model and comparing with the entropic-force models

for a number of viable

On the equation of state of dark energy

f(R)

Chameleon dark energy models with characteristic signatures

gravity models that satisfy both cosmological and local gravity constraints, where the Lagrangian density