Reheating in tachyonic inflationary models: Effects on the large scale curvature perturbations

We investigate the problem of perturbative reheating and its effects on the evolution of the curvature perturbations in tachyonic inflationary models. We derive the equations governing the evolution of the scalar perturbations for a system consisting of a tachyon and a perfect fluid. Assuming the perfect fluid to be radiation, we solve the coupled equations for the system numerically and study the evolution of the perturbations from the sub-Hubble to the super-Hubble scales. In particular, we analyze the effects of the transition from tachyon driven inflation to the radiation dominated epoch on the evolution of the large scale curvature and non-adiabatic pressure perturbations. We consider two different potentials to describe the tachyon and study the effects of two possible types of decay of the tachyon into radiation. We plot the spectrum of curvature perturbations at the end of inflation as well as at the early stages of the radiation dominated epoch. We find that reheating does not affect the amplitude of the curvature perturbations in any of these cases. These results corroborate similar conclusions that have been arrived at earlier based on the study of the evolution of the perturbations in the super-Hubble limit. We illustrate that, before the transition to the radiation dominated epoch, the relative non-adiabatic pressure perturbation between the tachyon and radiation decays in a fashion very similar to that of the intrinsic entropy perturbation associated with the tachyon. Moreover, we show that, after the transition, the relative non-adiabatic pressure perturbation dies down extremely rapidly during the early stages of the radiation dominated epoch. It is these behavior which ensure that the amplitude of the curvature perturbations remain unaffected during reheating. We also discuss the corresponding results for the popular chaotic inflation model in the case of the canonical scalar field.     

Inflation and reheating in the Starobinsky model with conformal HiggsField

This is a talk presented by A.A. Tokareva at Baikal summer school on physics of elementary particles and astrophysics 2012. We studied the reheating after the Starobinsky inflation and have found that the main process is the inflaton decay to SM gauge fields due to the conformal anomaly. The reheating temperature is low leading to the possibility to detect the gravity wave signal from inflation and evaporation of structures formed after inflation in DECIGO and BBO experiments. Also we give predictions for the parameters of scalar perturbation spectrum at the next-to-leading order of slow roll and obtain a bound on the Higgs mass.     

Baryon number generation in a flipped SU(5) × U(1) model

We consider the possibilities for generating a baryon asymmetry in the early universe in a flipped SU(5) × U(1) model inspired by the superstring. Depending on the temperature of the radiation background after inflation we can distinguish between two scenarios for baryogenesis: (1) after reheating the original SU(5) × U(1) symmetry is restored, or there was no inflation at all; (2) reheating after inflation is rather weak and SU(5) × U(1) is broken. In either case the asymmetry is generated by the out-of-equilibrium decays of a massive SU(3) × SU(2) × U(1) singlet field φ_m. In the flipped SU(5) × U(1) model, gauge symmetry breaking is triggered by strong coupling phenomena, and is in general accompanied by the production of entropy. We examine constraints on the reheating temperature and the strong coupling scale in each of the scenarios.     

Primordial curvature perturbation during and at the end of multi-field inflation

We study the generation of the primordial curvature perturbation in multi-field inflation. Considering both the evolution of the perturbation during inflation and the effects generated at the end of inflation, we present a general formula for the curvature perturbation. We provide the analytic expressions of the power spectrum, spectral tilt and non-Gaussianity for the separable potentials of two inflaton scalars, and apply them to some specific models.     

Reheating and entropy perturbations in fibre inflation

We study reheating in some one and two field realizations of fibre inflation.We find that reheating begins with a phase of preheating in which long wavelength fluctuation modes are excited.In two field models there is a danger that the parametric amplification of infrared fluctuations in the second scalar field-associated with an entropy mode-might induce an instability of the curvature fluctuations.We show that,at least in the models we consider,the entropy mode has a sufficiently large mass to prevent this instability.Hence,from the point of view of reheating the models we consider are well-behaved.     

Cosmic inflation and model comparison

We analyze the implications for inflation of the recently released Planck Cosmic Microwave Background data and explain why the single-field slow-roll scenarios with minimal kinetic terms are favored. Within this class of models, we show how Bayesian model comparison can be used to further exclude about one third of the inflationary scenarios. We also study the end of inflation and show that Planck can already constrain the reheating phase. Finally, we conclude by discussing how future missions will be able to improve our knowledge of the inflationary mechanism.     

Generating the curvature perturbation at the end of inflation in string theory

In brane inflationary scenarios, the cosmological perturbations are supposed to originate from the vacuum fluctuations of the inflaton field corresponding to the position of the brane. We show that a significant, and possibly dominant, contribution to the curvature perturbation is generated at the end of inflation through the vacuum fluctuations of fields, other than the inflaton, which are light during the inflationary trajectory and become heavy at the brane-antibrane annihilation. These fields appear generically in string compactifications where the background geometry has exact or approximate isometries and parametrize the internal angular directions of the brane.     

Reheating a multi-throat universe by brane motion

We propose a mechanism of reheating after inflation in multi-throat scenarios of warped extra dimensions. Validity of an effective field theory on the standard model (SM) brane requires that the position of the SM brane during inflation be different from the position after inflation. The latter is supposed to be near the tip of the SM throat but the former is not. After inflation, when the Hubble expansion rate becomes sufficiently low, the SM brane starts moving towards the tip and eventually oscillates. The SM fields are excited by the brane motion and the universe is reheated. Since interaction between the brane position modulus and the SM fields is suppressed only by the local string scale, the modulus effectively decays into the SM fields.     

Modulated inflation

We have studied modulated inflation that generates curvature perturbation from light-field fluctuation. As discussed in previous works, even if the fluctuation of the inflaton itself does not generate the curvature perturbation, fluctuation of a light field may induce fluctuation for the end-line of inflation and this may lead to generation of cosmological perturbation “at the end of the inflation”. Our scenario is different from this kind of modulated scenario, as clearly explained in this Letter by using δN formalism. We also explain the crucial difference from the standard multi-field inflation model. We show concrete examples of the modulated inflation scenario in which large non-gaussianity can be generated. We also discuss the running of the non-gaussianity parameter.     

G-Curvaton

In this Letter, we study a curvaton model where the curvaton is acted by Galileon field. We calculate the power spectrum of fluctuation of G-Curvaton during inflation and discuss how it converts to the curvature perturbation after the end of inflation. We estimate the bispectrum of curvature perturbation induced, and show the dependence of non-Gaussianity on the parameters of model. It is found that our model can have sizable local and equilateral non-Gaussianities to up to O(¹⁰²), which is illustrated by an explicit example.     

Curvature perturbation from supersymmetric flat directions

We show that a contribution to the total curvature perturbation may be due to the presence of flat directions in supersymmetric models. It is generated at the first oscillation of the flat direction condensate when the latter relaxes to the minimum of its potential after the end of inflation. We also point out that, if the contribution to the total curvature perturbation from supersymmetric flat direction is the dominant one, then a significant level of non-Gaussianity in the cosmological perturbation is also naturally expected.     

More flipped SU(5)×U(1) baryosynthesis

We supplement a previous discussion of baryosynthesis in flipped SU(5)×U(1) GUTs by including (1) the large incoherent field energy density which is likely SU(5) is broken, and (2) the possibility of additional Higgs triplet fields suggested by four dimensional string model-building. We consider strong (weak) reheating scenarios in which the Universe is (is not) SU(5) symmetric after inflation. We find an adequate baryon asymmetry subsequent to strong reheating, whatever the number of Higgs triplets (although beware of possible difficulties with quasi-stable relic particles), whereas weak reheating requires at least two Higgs triplets.     

Spectra in Coherent States with Excited-de Sitter Mode during Inflation

We use an excited-de Sitter mode as the fundamental mode function for the far past time limit during inflation, to study the corrections of spectra of curvature perturbation. Excited-de Sitter mode is actually the approximate solution of the inflaton field equation that asymptotically approaches to the de Sitter mode function in the first approximation. We build coherent state over excited-de Sitter mode. Then, we compute spectrum of the curvature perturbation with this coherent state as the initial state. We show that in this case, the spectrum of curvature perturbation is scale dependent. As a important result of using this coherent state, we find a non-zero non-Gaussian one-point function as a possible tiny source for generation of anisotropy in CMB from the initial mode in the string or Planck scale.     

The kinematics of cosmic reheating

We calculate the relaxation rate of a scalar field in a plasma of other scalars and fermions with gauge interactions using thermal quantum field theory. It yields the rate of cosmic reheating and thereby determines the temperature of the “hot big bang” in inflationary cosmology. The total rate originates from various processes, including decays and inverse decays as well as Landau damping by scatterings. It involves quantum statistical effects and off-shell transport. Its temperature dependence can be highly non-trivial, making it impossible to express the reheating temperature in terms of the model parameters in a simple way. We pay special attention to the temperature dependence of the phase space due to the modified dispersion relations in the plasma. We find that it can have a drastic effect on the efficiency of perturbative reheating, which depends on the way particles in the primordial plasma interact. For some interactions thermal masses can effectively close the phase space for the dominant dissipative processes and thereby impose an upper bound on the reheating temperature. In other cases they open up new channels of dissipation, hence increase the reheating temperature. At high temperatures we find that the universe can even be heated through couplings to fermions, which are often assumed to be negligible due to Pauli-blocking. These effects may also be relevant for baryogenesis, dark matter production, the fate of moduli and in scenarios of warm inflation.     

Hybrid quintessential inflation

A model is presented in which a single scalar field is responsible for both primordial inflation at early times and then dark energy at late times. This field is coupled to a second scalar field which becomes unstable and starts to oscillate after primordial inflation, thus driving a reheating phase that can create a high post-inflation temperature. This model easily avoids overproduction of gravity waves, which is a problem in the original quintessential inflation model in which reheating occurs via gravitational particle production.     

Non-thermal leptogenesis and baryon asymmetry in different neutrino mass models

In the present work we study non-thermal leptogenesis and baryon asymmetry in the universe in different neutrino mass models discussed recently. For each model we obtain a formula relating the reheating temperature after inflation to the inflaton mass. It is shown that all but four cases are excluded and that in the cases which survive the inflaton mass and the reheating temperature after inflation are bounded from below and from above.     

On the Locality of the No Hair Conjecture and the Measure of the Universe

We reanalize the recently proposed proof by Jensen and Stein-Schabes [1] of the No Hair Theorem for inhomogeneous spacetimes, putting a special emphasis on the asymptotic behaviour of the shear and curvature. We conclude that the theorem only holds locally and estimate the minimum size a region should be in order for it to inflate. We discuss in some detail the assumptions used in the theorem. In the last section we speculate about the possible measure of the set of spacetimes that would undergo inflation.     

Primordial perturbations and non-gaussianities in Ho?ava-Lifshitz gravity

We investigate primordial perturbations and non-gaussianities in the Ho?ava-Lifshitz theory of gravitation. In the UV limit, the scalar perturbation in the Ho?ava theory is naturally scale-invariant, ignoring the details of the expansion of the Universe. One may thus relax the exponential inflation and the slow-roll conditions for the inflaton field. As a result, it is possible that the primordial non-gaussianities, which are " slow-roll suppressed” in the standard scenarios, become large. We calculate the non-gaussianities from the bispectrum of the perturbation and find that the equilateral-type non-gaussianity is of the order of unity, while the local-type non-gaussianity remains small, as in the usual single-field slow-roll inflation model in general relativity. Our result is a new constraint on Ho?ava-Lifshitz gravity.     

Gauge fields and inflation

The isotropy and homogeneity of the cosmic microwave background (CMB) favors “scalar driven” early Universe inflationary models. However, gauge fields and other non-scalar fields are far more common at all energy scales, in particular at high energies seemingly relevant to inflation models. Hence, in this review we consider the role and consequences, theoretical and observational, that gauge fields can have during the inflationary era. Gauge fields may be turned on in the background during inflation, or may become relevant at the level of cosmic perturbations. There have been two main classes of models with gauge fields in the background, models which show violation of the cosmic no-hair theorem and those which lead to isotropic FLRW cosmology, respecting the cosmic no-hair theorem. Models in which gauge fields are only turned on at the cosmic perturbation level, may source primordial magnetic fields. We also review specific observational features of these models on the CMB and/or the primordial cosmic magnetic fields. Our discussions will be mainly focused on the inflation period, with only a brief discussion on the post inflationary (p)reheating era.