What is the amplitude of the gravitational waves background expected in the Starobinski model?

Publication date: Available online 24 December 2019Source: Physics of the Dark UniverseAuthor(s): Fabrizio Renzi, Mehdi Shokri, Alessandro MelchiorriAbstractThe 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 nS=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 nSand the assumption of ΛCDM. We also consider inflationary models where the R2 term in Starobinsky action is generalized to a R2p 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∼0.001 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 R2p 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 R2p models at...

Source: Physics of the Dark Universe - December 25, 2019 Category: Physics Source Type: research