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Register a new userNatural inflation with a nonminimal coupling to gravity
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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 inflations predictions to a good agreement with the Planck data. Depending on values of the coupling constant $xi$ and the symmetry breaking scale $f$, we find that with $|xi|sim 10^{-3}$ and $fgtrsim 2.0 M_{mathrm{pl}}$ predictions of the model stay inside $68%$ CL allowed region until $f$ increases up to $7.7 M_{mathrm{pl}}$, 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 $xi$. Successful reheating can also be realized in this model.
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We have found a successful model of chaotic inflation with an inflaton coupled nonminimally with gravity. The nonminimal coupling constant $xi$ runs with the evolution of the inflaton. The running nature of the coupling leads naturally to the situations where the coupling becomes small enough to have sufficient period of the inflation to resolve the cosmological puzzles.
We show that gravitational theories with a nonminimal coupling (NMC) to the matter fields lead to a violation of Etheringtons distance-duality relation, which relates the luminosity and angular diameter distances. We derive constraints on power-law and exponential NMC models using existing measurements of type Ia supernovae and baryon acoustic oscillations throughout the redshift range $0<z<1.5$. These complement previous constrains derived from cosmic-microwave background radiation and big-bang nucleosynthesis data.
We perform a phase space analysis of a generalized modified gravity theory with nonminimally coupling between geometry and matter. We apply the dynamical system approach to this generalized model and find that in the cosmological context, different choices of Lagrangian density will apparently result in different phases of the Universe. By carefully choosing the variables, we prove that there is an attractor solution to describe the late time accelerating universe when the modified gravity is chosen in a simple power-law form of the curvature scalar. We further examine the temperature evolution based on the thermodynamic understanding of the model. Confronting the model with supernova type Ia data sets, we find that the nonminimally coupled theory of gravity is a viable model to describe the late time Universe acceleration.
This is the second in a series of papers on preheating in inflationary models comprised of multiple scalar fields coupled nonminimally to gravity. In this paper, we work in the rigid-spacetime approximation and consider field trajectories within the single-field attractor, which is a generic feature of these models. We construct the Floquet charts to find regions of parameter space in which particle production is efficient for both the adiabatic and isocurvature modes, and analyze the resonance structure using analytic and semi-analytic techniques. Particle production in the adiabatic direction is characterized by the existence of an asymptotic scaling solution at large values of the nonminimal couplings, $xi_I gg 1$, in which the dominant instability band arises in the long-wavelength limit, for comoving wavenumbers $k rightarrow 0$. However, the large-$xi_I$ regime is not reached until $xi_I geq {cal O} (100)$. In the intermediate regime, with $xi_I sim {cal O}(1 - 10)$, the resonance structure depends strongly on wavenumber and couplings. The resonance structure for isocurvature perturbations is distinct and more complicated than its adiabatic counterpart. An intermediate regime, for $xi_I sim {cal O} (1 - 10)$, is again evident. For large values of $xi_I$, the Floquet chart consists of densely spaced, nearly parallel instability bands, suggesting a very efficient preheating behavior. The increased efficiency arises from features of the nontrivial field-space manifold in the Einstein frame, which itself arises from the fields nonminimal couplings in the Jordan frame, and has no analogue in models with minimal couplings. Quantitatively, the approach to the large-$xi_I$ asymptotic solution for isocurvature modes is slower than in the case of the adiabatic modes.
We consider a subclass of Horndeski theories for studying cosmic inflation. In particular, we investigate models of inflation in which the derivative self-interaction of the scalar field and the non-minimal derivative coupling to gravity are present in the action and equally important during inflation. In order to control contributions of each term as well as to approach the single-term limit, we introduce a special relation between the derivative interaction and the coupling to gravity. By calculating observable quantities including the power spectra and spectral tilts of scalar and tensor perturbation modes, and the tensor-to-scalar ratio, we found that the tensor-to-scalar ratio is suppressed by a factor of $(1+1/gamma)$, where $gamma$ reflects the strength of the derivative self-interaction of the inflaton field with respect to the derivative coupling gravity. We placed observational constraints on the chaotic and natural inflation models and showed that the models are consistent with the current observational data mainly due to the suppressed tensor-to-scalar ratio.
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