Natural inflation with a nonminimal coupling to gravity

Abstract

Although natural inflation is a theoretically well-motivated model for cosmic inflation, it is in tension with recent Planck cosmic microwave background anisotropy measurements. We present a way to alleviate this tension by considering a very weak nonminimal coupling of the inflaton field to gravity in both contexts of metric and Palatini formulations of general relativity. We start our discussions with a generic form of the inflaton coupling to the Ricci scalar, then focus on a simple form to do phenomenological study. Our results show that such an extension can bring natural inflation's predictions to a good agreement with the Planck data. Depending on values of the coupling constant ξ and the symmetry breaking scale f, we find that with |ξ|∼ 10-3 and f≳ 2.0 Mpl predictions of the model stay inside 68% CL allowed region until f increases up to 7.7 Mpl, then only inside 95% CL region after f exceeds the latter value. The predictions from the metric and the Palatini theories are very similar due to the simple form of the coupling function we use and the small magnitude of the coupling ξ. Successful reheating can also be realized in this model.

Natural Inflation with non minimal coupling to gravity in $R^2$ gravity under the Palatini formalism

“…for comparative purposes we we reproduce the results of ref [10] The effect of adding the term αR 2 can be realized in figure (4b). The same domain for the parameters f and ξ are scanned but now for non-zero α = 0.1.…”

“…On the other hand, current observational constraints on non-minimal coupling are pretty loose, for example|ξ| ≥ 10 15 from the LHC's result [9]. NI with NMC to gravity has been studied [10], The study shows that this extension can bring natural inflation's predictions into good agreement with Planck data, Depending on values of the coupling constant ξ and the symmetry breaking scale f .…”

Slow-roll inflation in $f(R,T)$ gravity with a $RT$ mixing term

“…There are several other attempts to bring natural inflation into agreement with Planck results, such as with a warm dissipative effect [53], or other forms of modified gravity, e.g., in a UV complete quadratic gravity [54], a non-minimal coupling to Ricci scalar [29], f (R, T ) gravity [33]. It is worth noticing that the simple form of f (R, T ) considered in Ref.…”

“…A natural question then arises: Is it possible to reconsider those inflationary models that have been ruled out by observations after assuming that the inflaton fields are non-minimally coupled to gravity? Some indications toward a positive answer have been put forward in the literature [24,25,26,27,28,29,30].…”

Constraining non-minimally coupled $β$-exponential inflation with CMB data

“…In the observational perspective, it has also been shown that in some cases the introduction of a non-minimal coupling may improve the description of the data (see e.g. [24][25][26][27][28][29][30][31][32] and references therein).…”