Correlation functions in stochastic inflation Vennin, Vincent; Starobinsky, Alexei A.
The European physical journal. C, Particles and fields,
09/2015, Letnik:
75, Številka:
9
Journal Article
Recenzirano
Odprti dostop
Combining the stochastic and
δ
N
formalisms, we derive non-perturbative analytical expressions for all correlation functions of scalar perturbations in single-field, slow-roll inflation. The ...standard, classical formulas are recovered as saddle-point limits of the full results. This yields a classicality criterion that shows that stochastic effects are small only if the potential is sub-Planckian and not too flat. The saddle-point approximation also provides an expansion scheme for calculating stochastic corrections to observable quantities perturbatively in this regime. In the opposite regime, we show that a strong suppression in the power spectrum is generically obtained, and we comment on the physical implications of this effect.
f(R) constant-roll inflation Motohashi, Hayato; Starobinsky, Alexei A.
The European physical journal. C, Particles and fields,
08/2017, Letnik:
77, Številka:
8
Journal Article
Recenzirano
Odprti dostop
The previously introduced class of two-parametric phenomenological inflationary models in general relativity in which the slow-roll assumption is replaced by the more general, constant-roll condition ...is generalized to the case of
f
(
R
) gravity. A simple constant-roll condition is defined in the original Jordan frame, and exact expressions for a scalaron potential in the Einstein frame, for a function
f
(
R
) (in the parametric form) and for inflationary dynamics are obtained. The region of the model parameters permitted by the latest observational constraints on the scalar spectral index and the tensor-to-scalar ratio of primordial metric perturbations generated during inflation is determined.
Baryon acoustic oscillations (BAOs) allow us to determine the expansion history of the universe, thereby shedding light on the nature of dark energy. Recent observations of BAOs in the Sloan Digital ...Sky Survey (SDSS) DR9 and DR11 have provided us with statistically independent measurements of H(z) at redshifts of 0.57 and 2.34, respectively. We show that these measurements can be used to test the cosmological constant hypothesis in a model-independent manner by means of an improved version of the Om diagnostic. Our results indicate that the SDSS DR11 measurement of H(z) = 222 + or - 7 km s super(-1) Mpc super(-1) at z = 2.34, when taken in tandem with measurements of H(z) at lower redshifts, imply considerable tension with the standard ACDM model. Our estimation of the new diagnostic Omh super(2) from SDSS DR9 and DR11 data, namely, Omh super(2) approximate0.122 + or - 0.01, which is equivalent to Omega sub(0)mh super(2) for the spatially flat ACDM model, is in tension with the value Omega sub(0)mh super(2) = 0.1426 + or - 0.0025 determined for LambdaCDM from Planck+WP. This tension is alleviated in models in which the cosmological constant was dynamically screened (compensated) in the past. Such evolving dark energy models display a pole in the effective equation of state of dark energy at high redshifts, which emerges as a smoking gun test for these theories.
We study the cosmological constraints on the variation of Newton's constant and on post-Newtonian parameters for simple models of the scalar-tensor theory of gravity beyond the extended ...Jordan-Brans-Dicke theory. We restrict ourselves to an effectively massless scalar field with a potential V∝F2, where F(σ)=Npl2+ξσ2 is the coupling to the Ricci scalar considered. We derive the theoretical predictions for cosmic microwave background anisotropies and matter power spectra by requiring that the effective gravitational strength at present is compatible with the one measured in a Cavendish-like experiment and by assuming an adiabatic initial condition for scalar fluctuations. When comparing these models with Planck 2015 and a compilation of baryonic acoustic oscillations data, all these models accommodate a marginalized value for H0 higher than in ΛCDM. We find no evidence for a statistically significant deviation from Einstein's general relativity. We find ξ<0.064 (|ξ|<0.011) at 95% CL for ξ>0 (for ξ<0, ξ≠−1/6). In terms of post-Newtonian parameters, we find 0.995<γPN<1 and 0.99987<βPN<1 (0.997<γPN<1 and 1<βPN<1.000011) for ξ>0 (for ξ<0). For the particular case of the conformal coupling, i.e., ξ=−1/6, we find constraints on the post-Newtonian parameters of similar precision to those within the Solar System.
We consider an inflationary model motivated by quantum effects of gravitational and matter fields near the Planck scale. Our Lagrangian is a resummed version of the effective Lagrangian recently ...obtained by Demmel, Saueressig, and Zanusso A proper fixed functional for four-dimensional quantum Einstein gravity, J. High Energy Phys. 08 (2015) 113. in the context of gravity as an asymptotically safe theory. It represents a refined Starobinsky model, Leff=MP2R/2+(a/2)R2/1+bln(R/μ2), where R is the Ricci scalar, a and b are constants, and μ is an energy scale. By implementing the COBE normalization and the Planck constraint on the scalar spectrum, we show that increasing b leads to an increased value of both the scalar spectral index ns and the tensor-to-scalar ratio r. Requiring ns to be consistent with the Planck Collaboration upper limit, we find that r can be as large as r≃0.01, the value possibly measurable by Stage IV CMB ground experiments and certainly from future dedicated space missions. The predicted running of the scalar spectral index α=dns/dln(k) is still of the order −5×10−4 (as in the Starobinsky model), about 1 order of magnitude smaller than the current observational bound.
PDF
