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Register a new userCurvaton reheating in logamediate inflationary model
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In a logamediate inflationary universe model we introduce the curvaton field in order to bring this inflationary model to an end. In this approach we determine the reheating temperature. We also outline some interesting constraints on the parameters that describe our models. Thus, we give the parameter space in this scenario.
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We discuss the possibility of explaining the recent NANOGrav results by inflationary gravitational waves (IGWs) with a blue-tilted primordial spectrum. Although such IGWs can account for the NANOGrav signal without contradicting the upper bound on the tensor-to-scalar ratio at the cosmic microwave background scale, the predicted spectrum is in strong tension with the upper bound on the amplitude of the stochastic gravitational wave background by big-bang nucleosynthesis (BBN) and the second LIGO-Virgo observation run. However, the thermal history of the Universe, such as reheating and late-time entropy production, affects the spectral shape of IGWs at high frequencies and permits evading the upper bounds. We show that, for the standard reheating scenario, when the reheating temperature is relatively low, a blue tensor spectrum can explain the recent NANOGrav signal without contradicting the BBN and the LIGO-Virgo constraints. We further find that, when one considers a late-time entropy production, the NANOGrav signal can be explained even for an instant reheating scenario.
We investigate two-field inflationary models in which scalar cosmological pertubations are generated via a spectator field nonminimally coupled to gravity, with the particular emphasis on curvaton scenarios. The principal advantage of these models is in the possibility to tune the spectator spectral index via the nonminimal coupling. Our models naturally yield red spectrum of the adiabatic perturbation demanded by observations. We study how the nonminimal coupling affects the spectrum of the curvature perturbation generated in the curvaton scenarios. In particular we find that for small, negative nonminimal couplings the spectral index gets a contribution that is negative and linear in the nonminimal coupling. Since in this way the curvature spectrum becomes redder, some of curvaton scenarios can be saved, which would otherwise be ruled out. In the power law inflation we find that a large nonminimal coupling is excluded since it gives the principal slow roll parameter that is of the order of unity. Finally, we point out that nonminimal coupling can affect the postinflationary growth of the spectator perturbation, and in this way the effectiveness of the curvaton mechanism.
We derive a simple model-independent upper bound on the strength of magnetic fields obtained in inflationary and post-inflationary magnetogenesis taking into account the constraints imposed by the condition of weak coupling, back-reaction and Schwinger effect. This bound turns out to be rather low for cosmologically interesting spatial scales. Somewhat higher upper bound is obtained if one assumes that some unknown mechanism suppresses the Schwinger effect in the early universe. Incidentally, we correct our previous estimates for this case.
Inspired by cite{Jiang:2018uce}, we propose a similar curvaton mechanism whose realization occurs in preheating process, in which the effective mass is running (its potential consists of coupling part and exponential part whose contribution is subdominant comparing to the coupling part). The production of curvaton contains the cases of narrow resonance and broad resonances whose criteria comes via the spectral index of curvaton. Since the inflationary potential is chaotic inflation (quadratic potential), it could smoothly transit into the preheating process. Once the entropy perturbation transferred into curvature perturbation, we will use $delta N$ formalism to investigate its validity. By neglecting the contribution of exponential potential of curvaton, we calculate power spectrum $P_zeta$ and non linear Non-Gaussian parameter $f_{NL}$. Our calculation analytically shows that these two observables are independent of potential of inflaton. Finally, as the curvaton almost decay (inflaton field vanishes), the exponential potential will be approaching a constant of order of cosmological constant, which may play a role of dark energy.
We revise the Non-Gaussianity of canonical curvaton scenario with a generalized $delta N$ formalism, in which it could handle the generic potentials. In various curvaton models, the energy density is dominant in different period including the secondary inflation of curvaton, matter domination and radiation domination. Our method could unify to deal with these periods since the non-linearity parameter $f_{rm NL}$ associated with Non-Gaussianity is a function of equation of state $w$. We firstly investigate the most simple curvaton scenario, namely the chaotic curvaton with quadratic potential. Our study shows that most parameter space satisfies with observational constraints. And our formula will nicely recover the well-known value of $f_{rm NL}$ in the absence of non-linear evolution. From the micro origin of curvaton, we also investigate the Pseudo-Nambu-Goldstone curvaton. Our result clearly indicates that the second short inflationary process for Pseudo-Nambu-Goldstone curvaton is ruled out in light of observations. Finally, our method sheds a new way for investigating the Non-Gaussianity of curvaton mechanism, espeically for exploring the Non-Gaussianity in MSSM curvaton model.
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