arXiv subjects

Mehdi Shokri

Publications and source records attributed to Mehdi Shokri.

20 records · Page 2Linked to original sources

arXiv2020

What is the amplitude of the Gravitational Waves background expected in the Starobinsky model ?

The inflationary model proposed by Starobinski in 1979 predicts an amplitude of the spectrum of primordial gravitational waves, parametrized by the tensor to scalar ratio, of $r=0.0037$ in case of a scalar spectral index of $n_S=0.965$. This amplitude is currently used as a target value in the design of future CMB experiments with the ultimate goal of measuring it at more than five standard deviations. Here we evaluate how stable are the predictions of the Starobinski model on $r$ considering the experimental uncertainties on $n_S$ and the assumption of $Λ$CDM. We also consider inflationary models where the $R^2$ term in Starobinsky action is generalized to a $R^{2p}$ term with index $p$ close to unity. We found that current data place a lower limit of $r>0.0013$ at $95 \%$ C.L. for the classic Starobinski model, and predict also a running of the scalar index different from zero at more than three standard deviation in the range $dn/dlnk=-0.0006_{-0.0001}^{+0.0002}$. A level of gravitational waves of $r\sim0.001$ is therefore possible in the Starobinski scenario and it will not be clearly detectable by future CMB missions as LiteBIRD and CMB-S4. When assuming a more general $R^{2p}$ inflation we found no expected lower limit on $r$, and a running consistent with zero. We found that current data are able to place a tight constraints on the index of $R^{2p}$ models at $95\%$ C.L. i.e. $p= 0.99^{+0.02}_{-0.03}$.

astro-ph.CO↗

arXiv2019

Cosmic Microwave Background constraints on non-minimal couplings in inflationary models with power law potentials

Inflationary models with power-law potentials are starting to be severely constrained by the recent measurements of Cosmic Microwave Background anisotropies provided by the Planck Satellite and by the BICEP2 telescope. In particular, models with power-law potentials $V(φ)\propto φ^n$ with $n \ge 2$ are strongly disfavored by present data since they predict a sizable contribution of gravitational waves with a tensor/scalar ratio of $r\sim0.15$ that is at odds with current limits. A non-minimal coupling to gravity has been proposed as a physical mechanism to lower the predictions for $r$. In this paper we further investigate the issue, presenting constraints on non-minimal couplings from current CMB data under the assumption of power-law potentials. We found that models with $n>2$ show a statistically significant indication (above $95 \%$ C.L.) for a non minimal coupling. Non minimal coupling is also preferred by models with $n<2$ albeit just at about $68 \%$ C.L.. Interestingly, all the models considered show a non-zero running of the spectral index, $ n_{\rm run}$, consistent with the 2018 Planck release value of $-0.007 \pm 0.0068$. We point out how future accurate measurement of $ n_{\rm run}$ would be necessary to significantly constraint these models and eventually rule out some or all of them. The combination of Planck data with the Bicep/Keck dataset strengthen these considerations.

astro-ph.CO↗