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David Polarski

Publications and source records attributed to David Polarski.

At least 37 records · Page 2Linked to original sources
arXiv2008

The growth of matter perturbations in some scalar-tensor DE models

We consider asymptotically stable scalar-tensor dark energy (DE) models for which the equation of state parameter $w_{DE}$ tends to zero in the past. The viable models are of the phantom type today, however this phantomness is milder than in General Relativity if we take into account the varying gravitational constant when dealing with the SNIa data. We study further the growth of matter perturbations and we find a scaling behaviour on large redshifts which could provide an important constraint. In particular the growth of matter perturbations on large redshifts in our scalar-tensor models is close to the standard behaviour $δ_m \propto a$, while it is substantially different for the best-fit model in General Relativity for the same parametrization of the background expansion. As for the growth of matter perturbations on small redshifts, we show that in these models the parameter $γ'_0\equiv γ'(z=0)$ can take absolute values much larger than in models inside General Relativity. Assuming a constant $γ$ when $γ'_0$ is large would lead to a poor fit of the growth function $f$. This provides another characteristic discriminative signature for these models.

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arXiv2007

Power-laws f(R) theories are cosmologically unacceptable

In a recent paper [1] (PRL 98,131302,2007) we have shown that f(R)=R + mu R^{n} modified gravity dark energy models are not cosmologically viable because during the matter era that precedes the accelerated stage the cosmic expansion is given by a sim t^{1/2} rather than a sim t^{2/3}, where a is a scale factor and t is the cosmic time. A recent work [2] (PLB 639,135-143,2006) by Capozziello et al. criticised our results presenting some apparent counter-examples to our claim in f(R)= mu R^{n} models. We show here that those particular R^{n} models can produce an expansion as a sim t^{2/3} but this does not connect to a late-time acceleration. Hence, though acceptable f(R) dark energy models may exist, the R^{n} models presented in Capozziello et al. are not cosmologically viable, confirming our previous results in Ref. [1].

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arXiv2007

On the growth of linear perturbations

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 $Λ$CDM $γ'_0\equiv \frac{dγ}{dz}_0$ lies in a very narrow interval $-0.0195 \le γ'_0 \le -0.0157$ for $0.2 \le Ω_{m,0}\le 0.35$. 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 $γ'_0\approx -0.02$ 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.

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arXiv2007

Conditions for the cosmological viability of f(R) dark energy models

We derive the conditions under which dark energy models whose Lagrangian densities f are written in terms of the Ricci scalar R are cosmologically viable. We show that the cosmological behavior of f(R) models can be understood by a geometrical approach consisting in studying the m(r) curve on the (r, m) plane, where m=Rf_{,RR}/f_{,R} and r=-Rf_{,R}/f with f_{,R}=df/dR. This allows us to classify the f(R) models into four general classes, depending on the existence of a standard matter epoch and on the final accelerated stage. The existence of a viable matter dominated epoch prior to a late-time acceleration requires that the variable m satisfies the conditions m(r) approx+0 and dm/dr>-1 at r approx-1. For the existence of a viable late-time acceleration we require instead either (i) m=-r-1, (sqrt{3}-1)/2 0 and n<-1 and are thus cosmologically unacceptable. Similar conclusions can be reached for many other examples discussed in the text. In most cases the standard matter era is replaced by a cosmic expansion with scale factor a propto t^{1/2}. We also find that f(R) models can have a strongly phantom attractor but in this case there is no acceptable matter era.

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arXiv2007

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 that behave as a power of $R$ 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 behaviour is grossly inconsistent with cosmological observations (e.g. WMAP), thereby ruling out these models even if they pass the supernovae test and can escape the local gravity constraints.

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arXiv2006

Scalar-Tensor Models of Normal and Phantom Dark Energy

We consider the viability of dark energy (DE) models in the framework of the scalar-tensor theory of gravity, including the possibility to have a phantom DE at small redshifts $z$ as admitted by supernova luminosity-distance data. For small $z$, the generic solution for these models is constructed in the form of a power series in $z$ without any approximation. Necessary constraints for DE to be phantom today and to cross the phantom divide line $p=-ρ$ at small $z$ are presented. Considering the Solar System constraints, we find for the post-Newtonian parameters that $γ_{PN}<1$ and $γ_{PN,0}\approx 1$ for the model to be viable, and $β_{PN,0}>1$ (but very close to 1) if the model has a significantly phantom DE today. However, prospects to establish the phantom behaviour of DE are much better with cosmological data than with Solar System experiments. Earlier obtained results for a $Λ$-dominated universe with the vanishing scalar field potential are extended to a more general DE equation of state confirming that the cosmological evolution of these models rule them out. Models of currently fantom DE which are viable for small $z$ can be easily constructed with a constant potential; however, they generically become singular at some higher $z$. With a growing potential, viable models exist up to an arbitrary high redshift.

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arXiv2005

Can Lightcone Fluctuations be Probed with Cosmological Backgrounds?

Finding signatures of quantum gravity in cosmological observations is now actively pursued both from the theoretical and the experimental side. Recent work has concentrated on finding signatures of light-cone fluctuations in the CMB. Because in inflationary scenarios a Gravitational Wave Background (GWB) is always emitted much before the CMB, we can ask, in the hypothesis where this GWB could be observed, what is the imprint of light cone fluctuations on this GWB. We show that due to the flat nature of the GWB spectrum, the effect of lightcone fluctuations are negligible.

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arXiv2002

Accurate results for primordial black holes from spectra with a distinguished scale

We perform an accurate computation of the production rate for primordial black holes (PBHs). The reason is that the underlying mass variance had been overestimated systematically, as was shown recently. For scale-free powerlaw primordial spectra, and for a Universe with critical density, the mass variance is less than 34% of its value thought earlier for the spectral index in the range $1\leq n\leq 1.3$. We then extend our study to spectra with a characteristic scale and find the accurate shape of the corresponding mass variance. For a pure step in the primordial spectrum, the step in the variance is smoothed around the characteristic scale $k_s$. For a spectrum with large oscillations near $k_s$, we find a pronounced bump in the variance. This could yield a significant part of the cold dark matter in the form of PBHs with mass $M$ in the range $5\times 10^{15} {\rm g}\lesssim M \lesssim 10^{21} {\rm g}$.

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arXiv2002

Can Primordial Black Holes be a Significant Part of Dark Matter ?

The computation of PBH (primordial black hole) production from primordial perturbations has recently been improved by considering a more accurate relation between the primordial power spectrum and the PBH mass variance. We present here exact expressions which are valid for primordial spectra of arbitrary shape and which allow accurate numerical calculations. We then consider the possibility to have a significant part of dark matter in the form of PBHs produced by a primordial spectrum of inflationary origin possessing a characteristic scale. We show that in this model the relevant PBH mass is constrained to lie in the range $5\times 10^{15} {\rm g}\lesssim M \lesssim 10^{21} {\rm g}$. This is much less than the mass range coming from the QCD phase transition, allowing the two mechanisms to be easily distinguished.

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arXiv2002

Primordial Black Holes in an Accelerating Universe

General expressions are given for the generation of Primordial Black Holes (PBH) in a universe with a presently accelerated expansion due to a(n effective) cosmological constant. We give expressions both for a powerlaw scalefree primordial spectrum and for spectra which are not of that type. Specializing to the case of a pure cosmological constant $Λ$ and assuming flatness, we show that a comological constant with $Ω_{Λ,0}=0.7$ will decrease the mass variance at the PBH formation time by about 15% compared with a critical density universe.

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arXiv2001

Primordial Black Holes from inflationary models with and without Broken Scale Invariance

We review the formalism of primordial black holes (PBHs) production and show that the mass variance at horizon crossing has been systematically overestimated in previous studies. We derive the correct expression. The difference is maximal at the earliest formation times and still very significant for PBH masses $\sim 10^{15}$g, an accurate estimate requiring numerical calculations. In particular, this would lead to weaker constraints on the spectral index $n$. We then derive constraints on inflationary models from the fact that primordial black holes must not overclose the Universe. This is done both for the scale-free case of the power spectrum studied earlier and for the case where a step in the mass variance is superimposed. In the former case we find various constraints on $n$, depending on the parameters. In the latter case these limits can be much more strengthened, so that one could find from an observational limit on $n$ a constraint on the allowed height of the step.

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arXiv2001

Classicality of primordial fluctuations and Primordial Black Holes

The production of Primordial Black Holes (PBH) from inflationary perturbations provides a physical process where the effective classicality of the fluctuations does not hold for certain scales. For adiabatic perturbations produced during inflation, this range of scales corresponds to PBH with masses $M\ll 10^{15}$ g. For PBH with masses $M\sim M_H(t_e)$, the horizon mass at the end of inflation, the generation process during the preheating stage could be classical as well, in contrast to the formation of PBH on these scales by adiabatic inflationary perturbations. For the non evaporated PBH, the generation process is essentially classical.

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arXiv2001

Accelerated Expansion: Theory and Observations

The present paradigm in cosmology is the usual Big-Bang Cosmology in which two stages of accelerated expansion are incorporated: the inflationary phase in the very early universe which produces the classical inhomogeneities observed in the universe, and a second stage of acceleration at the present time as the latest Supernovae observations seem to imply. Both stages could be produced by a scalar field and observations will strongly constrain the microscopic lagrangian of any proposed model.

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arXiv1998

Primordial Fluctuations from Inflation: a Consistent Histories Approach

We show how the quantum-to-classical transition of the cosmological fluctuations produced during an inflationary stage can be described using the consistent histories approach. We identify the corresponding histories in the limit of infinite squeezing. To take the decaying mode into account, we propose an extension to coarse-grained histories.

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arXiv1998

On the Phase-Space Volume of Primordial Cosmological Perturbations

We show how to determine the typical phase space volume $Γ$ for primordial gravitational waves produced during an inflationary stage, which is invariant under squeezing. An expression for $Γ$ is found in the long wavelength regime. The quasi-classical entropy of a pure vacuum initial state defined as the logarithm of $Γ$ modulo a constant remains zero in spite of the generation of fluctuations (creation of real gravitons).

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arXiv1998

The Coherence of Primordial Fluctuations Produced During Inflation

The behaviour of quantum metric perturbations produced during inflation is considered at the stage after the second Hubble radius crossing. It is shown that the classical correlation between amplitude and momentum of a perturbation mode, previously shown to emerge in the course of an effective quantum-to-classical transition, is maintained for a sufficiently long time, and we present the explicit form in which it takes place using the Wigner function. We further show with a simple diffraction experiment that quantum interference, non-expressible in terms of a classical stochastic description of the perturbations, is essentially suppressed. Rescattering of the perturbations leads to a comparatively slow decay of this correlation and to a complete stochastization of the system.

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arXiv1997

CMB anisotropy predictions for a model of double inflation

We consider a double-inflationary model with two massive scalar fields interacting only gravitationally in the context of a flat cold dark matter (CDM) Universe. The cosmic microwave background (CMB) temperature anisotropies produced in this theory are investigated in great details for a window of parameters where the density fluctuation power spectrum P(k) is in good agreement with observations. The first Doppler (``acoustic'') peak is a crucial test for this model as well as for other models. For the ``standard'' values of the cosmological parameters of CDM, our model is excluded if the height of the Doppler peak is sensibly higher than about three times the Sachs-Wolfe plateau.

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