Gravitino dark matter from inflaton decay

We discuss a scenario that gravitinos produced non-thermally by an inflaton decay constitute dark matter in the present universe. We find that this scenario is realized for wide ranges of the inflaton mass and the vacuum expectation value. What is intriguing about this scenario is that the gravitino dark matter can have a relatively large free streaming length at matter-radiation equality, which can be probed by future observation on QSO-galaxy strong lens system.     

Radiative corrections set up inflation: dynamical models

We establish the possibility that the initial configuration of fields for an inflationary period in the early universe can be set up by radiative corrections to the tree-level potential. A maximum of the radiatively-corrected potential can occur at an energy scale just above the Planck scale; we consider that the scalar (inflaton}) field rolls down from this maximum. In a model, we find that the radiative corrections also induce a minimum in the potential at a non–zero value of the energy scale somewhat below the Planck scale.     

Reheating,thermalization and non-thermal gravitino production in MSSM inflation

In the framework of MSSM inflation, matter and gravitino production are here investigated through the decay of the fields which are coupled to the udd inflaton, a gauge-invariant combination of squarks. After the end of inflation, the flat direction oscillates about the minimum of its potential, losing at each oscillation about 56% of its energy into bursts of gauge/gaugino and scalar quanta when crossing the origin. These particles then acquire a large inflaton VEV-induced mass and decay perturbatively into the MSSM quanta and gravitinos, transferring the inflaton energy very efficiently via instant preheating. Regarding thermalization, we show that the MSSM degrees of freedom thermalize very quickly, yet not immediately by virtue of the large vacuum expectation value of the inflaton, which breaks the \(SU(3)_C\times U(1)_Y\) symmetry into a residual U(1). The energy transfer to the MSSM quanta is very efficient, since full thermalization is achieved after only \(\mathcal {O}(40)\) complete oscillations. The udd inflaton thus provides an extremely efficient reheating of the Universe, with a temperature \(T_{\text {reh}}=\mathcal {O}(10^8\,{\text {GeV}})\), which allows for instance several mechanisms of baryogenesis. We also compute the gravitino number density from the perturbative decay of the flat direction and of the SUSY multiplet. We find that the gravitinos are produced in negligible amount and satisfy cosmological bounds such as the Big Bang nucleosynthesis (BBN) and dark matter (DM) constraints.     

Adiabatic density perturbations and matter generation from the minimal supersymmetric standard model

We propose that the inflaton is coupled to ordinary matter only gravitationally and that it decays into a completely hidden sector. In this scenario both baryonic and dark matter originate from the decay of a flat direction of the minimal supersymmetric standard model, which is shown to generate the desired adiabatic perturbation spectrum via the curvaton mechanism. The requirement that the energy density along the flat direction dominates over the inflaton decay products fixes the flat direction almost uniquely. The present residual energy density in the hidden sector is typically shown to be small.     

Studies on modified power law inflation

In this study, we evaluate power law inflation (PLI) with a monomial potential and obtain a novel exact solution. It is well known that the conventional PLI with an exponential potential is inconsistent with the Planck data. Unlike the standard PLI, the present model does not encounter the graceful exit problem, and the results agree fairly well with recent observations. In our analysis, we calculate the spectral index and the tensor-to-scalar ratio, both of which agree very well with recent observational data and are comparable with those of other modified inflationary models. The employed technique reveals that the large cosmological constant decreases with the expansion of the universe in the case of the PLI. The coupling of the inflaton with gravitation is the primary factor in this technique. The basic assumption here is that the two metric tensors in the gravitational and inflaton parts correspond to different conformal frames, which contradicts with the conventional PLI, where the inflaton is directly coupled with the background metric tensor. This fact has direct applications to different dark energy models and the assisted quintessence theory.     

Dark energy,curvature, and cosmic coincidence

The fact that the energy densities of dark energy and matter are similar currently, known as the coincidence problem, is one of the main unsolved problems of cosmology. We present here a model in which a spatial curvature of the universe can lead to a transition in the present epoch from a matter dominated universe to a scaling dark energy dominance in a very natural way. In particular, we show that if the exponential potential of the dark energy field depends linearly on the spatial curvature density of a closed universe, the observed values of some cosmological parameters can be obtained assuming acceptable values for the present spatial curvature of the universe, and without fine tuning in the only parameter of the model. We also comment on possible variations of this model, and realistic scenarios in which it could arise.     

Late reheating, hadronic jets, and baryogenesis

If inflaton couples very weakly to ordinary matter, the reheating temperature of the Universe can be lower than the electroweak scale. In this Letter we show that the late reheating occurs in a highly nonuniform way, within narrow areas along the jets produced by ordinary particles originated from inflaton decays. Depending on inflaton mass and decay constant, the initial temperature inside the lumps of the overheated plasma may be large enough to trigger the unsuppressed sphaleron processes with baryon number nonconservation. This allows for efficient local electroweak baryogenesis at reheating temperatures TR approximately O(10) GeV.     

Warm tachyonic inflation in strong dissipative regime: A study with exp[-T2] potential

Warm inflation is an interesting possibility of describing the early universe. The basic idea of warm inflation is that a scalar field called inflaton is coupled to several other fields in the inflationary era. In this paper, we study the warm tachyonic inflation with exp[-T²] potential. In the present model the dissipation coefficient has been considered as constant. We find that the model can successfully predict the cosmological observables within the experimental bounds.     

Inflation with hyperbolic potential in the braneworld model

In this paper we study inflationary dynamics with a scalar field in an inverse coshyperbolic potential in the braneworld model. We note that a sufficient inflation may be obtained with the potential considering slow-roll approximation in the high energy limit. We determine the minimum values of the initial inflaton field required to obtain sufficient inflation and also determine the relevant inflationary parameters. The numerical values of spectral index of the scalar perturbation spectrum are determined by varying the number of e-foldings for different initial values of the inflaton field. The result obtained here is in good agreement with the current observational limits.      

Successful supersymmetric inflation

We reconsider the problems of cosmological inflation in effective supergravity theories. A singlet field in a hidden sector is demonstrated to yield an acceptable inflationary potential, without fine tuning. In the simplest such model, the requirement of generating the microwave background anisotropy measured by COBE fixes the inflationary scale to be about 10¹⁴ GeV, implying a reheat temperature of order 10⁵ GeV. This is low enough to solve gravitino problem but high enough to allow baryogenesis after inflation. Such consistency requires that the generation of gravitational waves be negligible and that the spectrum of scalar density perturbations depart significantly from scale invariance, thus improving the fit to large-scale structure in an universe dominated by cold dark matter. We also consider the problems associated with gravitino production through inflaton decay and with other weakly coupled fields such as the moduli encountered in (compactified) string theories.     

Baryon asymmetry, inflation and squeezed states

We use the general formalism of squeezed rotated states to calculate baryon asymmetry in the wake of inflation through parametric amplification. We base our analysis on a B and CP violating Lagrangian in an isotropically expanding universe. The B and CP violating terms originate from the coupling of complex fields with non-zero baryon number to a complex background inflaton field. We show that a differential amplification of particle and antiparticle modes gives rise to baryon asymmetry.     

Inflaton field governed universe from NKK theory of gravity: stochastic approach

We study a non-perturbative single field (inflaton) governed cosmological model from a 5D non-compact Kaluza-Klein (NKK) theory of gravity. The inflaton field fluctuations are estimated for different epochs of the evolution of the universe. We conclude that the inflaton field has been sliding down its (quadratic) potential hill along the whole evolution of the universe and a mass is involved of the order of the Hubble parameter. In the model here developed the only free parameter is the Hubble parameter, which could be reconstructed in the future from Super Nova Acceleration Probe (SNAP) data. Received: 17 August 2005, Revised: 12 September 2005, Published online: 14 October 2005 PACS: 04.20.Jb, 11.10.kk, 98.80.Cq     

Decaying cosmological parameter in the early universe from NKK theory of gravity

Using a formalism recently introduced we study the decaying of the cosmological parameter during the early evolution of an universe, whose evolution is governed by a vacuum equation of state. We use a stochastic approach in a nonperturbative treatment of the inflaton field from a Noncompact Kaluza–Klein (NKK) theory, to study the evolution of energy density fluctuations in the early universe.     

Reheating as a surface effect

We describe a new mechanism for reheating the Universe through evaporation of a surface charge of a fragmented inflaton condensate. We show that for a range of Yukawa coupling of the inflaton to the matter sector evaporation gives rise to a much smaller reheat temperature compared to the standard perturbative decay. As a consequence, reheating through a surface effect could solve the gravitino and moduli overproduction problem in inflationary models without fine tuning the Yukawa sector.     

Essay: Preheating and Turbulence: Echoes of a Not So Quiet Universe

We study the nonlinear decay of the inflaton which causes the reheating of the Universe in the transition from the inflationary phase to the radiation dominated phase, resulting in the creation of almost all matter constituting the present Universe. Our treatment allows us to follow the full dynamics of the system in a long time regime, and to describe not only the parametric resonance processes with nonlinear restructuring but also to characterize a final turbulent state in the dynamics by which the energy is nonlinearly transferred to all scales of the system with a consequent thermalization of the created matter.     

Some studies on dark energy related problems

In this work we perform some studies related to dark energy. Firstly, we propose a dynamical approach to explain the dark energy contents of the universe. We assume that a massless scalar field couples to the Hubble parameter with some Planck-mass suppressed interactions. This scalar field develops a Hubble parameter-dependent (thus time-dependent) vacuum expectation value, which renders a time-independent relative density for the dark energy and thus can explain the coincidence of the dark energy density of the universe. Furthermore, we assume that the dark matter particle is metastable and decays very late into the dark energy scalar field. Such a conversion of matter to dark energy can give an explanation for the starting time of the accelerating expansion of the universe. Secondly, we introduce multiple Affleck-Dine fields to the landscape scenario of dark energy in order to have the required baryon-asymmetrical universe. PACS: 95.36. + x, 95.35. + d     

Entropies of the various components of the universe

In this study, we investigate the entropies of photons, ideal gas-like dust (baryonic matter), and a special kind of dark energy in the context of cosmology. When these components expand freely with the universe, we calculate the entropy and specific entropy of each component from the perspective of statistics. Under specific assumptions and conditions, the entropies of these components can satisfy the second law of thermodynamics independently. Our calculations show that the specific entropy of matter cannot be a constant during the expansion of the universe, except for photons. When these components interact with the space-time background, particle production (annihilation) can occur. We study the influence of the interaction on the entropies of these components and obtain the conditions guaranteeing that the entropy of each component satisfies the second law of thermodynamics.     

Non-minimal gravitational coupling of phantom and big rip singularity

We consider a non-minimal coupling of a perfect fluid matter system with geometry, which the coupling function is taken to be an arbitrary function of the Ricci scalar. Due to such a coupling, the matter stress tensor is no longer conserved and there is an energy transfer between the two components. By solving the conservation equation and applying the second law of thermodynamics, we show that direction of the energy transfer depends on the equation of state of the matter fluid. In particular, a phantom fluid should loose energy with expansion of the universe. This energy reduction can avoid the universe to end with a cosmic doomsday.