We investigate scalar-induced stochastic gravitational waves from adiabatic curvature perturbations sourced by a spectator field via the modulated reheating mechanism. We consider a spectator scalar with Higgs-like couplings and inflaton decay via shift symmetric dimension-five operators. The spectator is assumed to be in the Sitter vacuum and it sources blue-tilted, strongly non-Gaussian curvature perturbations which can dominate the spectrum on small scales $k \gg \rm{Mpc}^{-1}$. We find that the setup could generate a gravitational wave signal testable by surveys like BBO and DECIGO but only for large coupling values not expected in low-energy particle physics setups that can be perturbatively extrapolated up to the inflationary scale.
Modulated reheating scenario is one of the most attractive models that predict possible detections of not only the primordial non-Gaussianity but also the tensor fluctuation through future CMB observations such as the Planck satellite, the PolarBeaR and the LiteBIRD satellite experiments. We study the baryonic-isocurvature fluctuations in the Affleck-Dine baryogenesis with the modulated reheating scenario. We show that the Affleck-Dine baryogenesis can be consistent with the modulated reheating scenario with respect to the current observational constraint on the baryonic-isocurvature fluctuati…
Reheating after R^2 inflation proceeds through gravitational particle production of conformally noninvariant fields. We argue that the nonvanishing expectation value of flat directions generic in supersymmetric theories break conformal invariance of the fields coupled to them in a position-dependent manner due to quantum fluctuations. As a result modulated reheating can occur after the supergravity R^2-inflation. The resultant curvature fluctuation is a mixture of the one produced during inflation and that produced by modulated reheating. The spectral index takes a value between n_s=0.960 and …
We propose a new model for the hemispherical power asymmetry of the CMB based on modulated reheating. Non-Gaussianity from modulated reheating can be small enough to satisfy the bound from Planck if the dominant modulation of the inflaton decay rate is linear in the modulating field $σ$. $σ$ must then acquire a spatially-modulated power spectrum with a red scale-dependence. This can be achieved if the primordial perturbation of $σ$ is generated via tachyonic growth of a complex scalar field. Modulated reheating due to $σ$ then produces a spatially modulated and scale-dependent sub-dominant con…
Within the framework of RG improved inflationary cosmology motivated by asymptotically safe gravity, we study the dynamics of a scalar field which can be interpreted as the Higgs field. The background trajectories of this model can provide sufficient inflationary e-folds and a graceful exit to a radiation dominated phase. We study the possibility of generating primordial curvature perturbations through the Standard Model Higgs boson. This can be achieved under finely tuned parameter choices by making use of the modulated reheating mechanism. The primordial non-gaussianity is expected to be siz…
There might be a light scalar field during inflation which is not responsible for the accelerating inflationary expansion. Then, its quantum fluctuation is stretched during inflation. This scalar field could be a curvaton, if it decays at a late time. In addition, if the inflaton decay rate depends on the light scalar field expectation value by interactions between them, density perturbations could be generated by the quantum fluctuation of the light field when the inflaton decays. This is modulated reheating mechanism. We study curvature perturbation in models where a light scalar field does …
In the modulated reheating scenario, the field that drives inflation has a spatially varying decay rate, and the resulting inhomogeneous reheating process generates adiabatic perturbations. We examine the statistical properties of the density perturbations generated in this scenario. Unlike earlier analyses, we include the dynamics of the field that determines the inflaton decay rate. We show that the dynamics of this modulus field can significantly alter the amplitude of the power spectrum and the bispectrum, even if the modulus field has a simple potential and its effective mass is smaller t…
We show that the modulated reheating mechanism can naturally account for dark radiation, whose existence is hinted by recent observations of the cosmic microwave background radiation and the primordial Helium abundance. In this mechanism, the inflaton decay rate depends on a light modulus which acquires almost scale-invariant quantum fluctuations during inflation. We find that the light modulus is generically produced by the inflaton decay and therefore a prime candidate for the dark radiation. Interestingly, an almost scale-invariant power spectrum predicted in the modulated reheating mechani…
In this paper we analyse three models of the early universe, for which the respective mechanisms for generating the curvature perturbation are considered disparate. We find that in fact the mechanisms are very similar, and hence explain why they give rise to a large non-gaussianity. We show that the mechanism for generating the primordial curvature perturbation, and hence the observable non-gaussianity, is similar in both the Curvaton and Modulated Reheating models. In both cases the model can be written in terms of an energy transfer between the constituting fluids. We then show that this is …
The purely gravitational dark matter (PGDM) which interacts with the standard model particles only by gravitational interaction has recently been discussed. Due to its feeble interaction, PGDM may be produced mainly by the gravitational particle creation, which plays an important role in the reheating after kinetically driven inflation and some potential-driven inflation without subsequent inflaton oscillating phase. Therefore, we consider the possibility of the gravitational reheating model which can also explain the present PGDM density at the same time. We consider a model where two massive…
It is shown that a positive non-minimal coupling of the Higgs field to gravity can solve the two problems in inflation models in which postinflationary universe is dominated by an energy with stiff equation of state such as a kination, namely, overproduction of gravitons in gravitational reheating scenario, and overproduction of curvature perturbation from Higgs condensation. Furthermore, we argue that the non-minimal coupling parameter can be constrained more stringently with the progress in observations of large-scale structure and cosmic microwave background.
Spectator field models such as the curvaton scenario and the modulated reheating are attractive scenarios for the generation of the cosmic curvature perturbation, as the constraints on inflation models are relaxed. In this paper, we discuss the effect of Hubble induced masses on the dynamics of spectator fields after inflation. We pay particular attention to the Hubble induced mass by the kinetic energy of an oscillating inflaton, which is generically unsuppressed but often overlooked. In the curvaton scenario, the Hubble induced mass relaxes the constraint on the property of the inflaton and …
We study density perturbations, including their non-Gaussianity, in models in which the decay rate of the curvaton depends on another light scalar field, denoted the modulaton. Although this model shares some similarities with the standard curvaton and modulated reheating scenarios, it exhibits interesting predictions for f_NL and g_NL that are specific to this model. We also discuss the possibility that both modulaton and curvaton fluctuations contribute to the final curvature perturbation. Our results naturally include the standard curvaton and modulated reheating scenarios as specific limit…
We investigate the imprint of reheating on the gravitational wave spectrum produced by self-ordering of multi-component scalar fields after a global phase transition. The equation of state of the Universe during reheating, which usually has different behaviour from that of a radiation-dominated Universe, affects the evolution of gravitational waves through the Hubble expansion term in the equations of motion. This gives rise to a different power-law behavior of frequency in the gravitational wave spectrum. The reheating history is therefore imprinted in the shape of the spectrum. We perform $5…
We revisit the gravitational leptogenesis scenario in which the inflaton is coupled to gravity by the Chern-Simons term and the lepton asymmetry is generated through the gravitational anomaly in the lepton number current during inflation. We constrain the possible model parameter space by requiring the absence of ghost modes below the Planck scale, which would suggest the breakdown of the effective theory, and evaluate the net baryon asymmetry for various reheating processes. We find that the mechanism with these requirements is insufficient to explain the observed baryon asymmetry of the Univ…
A new mechanism is presented which can reheat the Universe in non-oscillatory models of inflation, where the inflation period is followed by a period dominated by the kinetic density for the inflaton field (kination). The mechanism considers an auxiliary field non-minimally coupled to gravity. The auxiliary field is a spectator during inflation, rendered heavy by the non-minimal coupling to gravity. During kination however, the non-minimal coupling generates a tachyonic mass, which displaces the field, until its bare mass becomes important, leading to coherent oscillations. Then, the field dec…
Gravitational scalar production with a generic reheating scenario
School of Theoretical Physics, Dublin Institute for Advanced Studies（都柏林高等研究院理论物理学院）；School of Physics, Faculty of Basic Sciences, University of Shanghai for Science and Technology（上海理工大学理学院物理系）
We provide a detailed study of reheating in the kination regime in a scenario where the particle content is produced by gravitational production of massive scalars decaying into massless scalars and fermions which eventually reheat the Universe. A detailed calculation is given by using Boltzmann equations and decay rates obtained using formalism of quantum field theory in curved spacetime. By numerical calculations the reheating temperature is found to be in the $10^{9}$-$10^{13}$ GeV regime. Moreover, the fermionic channel of decay is found to be the dominant channel of decay when the mass $m…
A generic feature of the known string inflationary models is that the same physics that makes the inflaton lighter than the Hubble scale during inflation often also makes other scalars this light. These scalars can acquire isocurvature fluctuations during inflation, and given that their VEVs determine the mass spectrum and the coupling constants of the effective low-energy field theory, these fluctuations give rise to couplings and masses that are modulated from one Hubble patch to another. These seem just what is required to obtain primordial adiabatic fluctuations through conversion into den…
We study post-inflationary evolution in $α$-attractor T-models of inflation. We consider the dynamics of both scalar fields present in these models: the inflaton and the spectator, as a negative field-space curvature may lead to geometrical destabilization of the spectator. We perform state-of-the-art lattice simulations with a dedicated numerical code optimized for those models. We corroborate earlier findings that the perturbations of the spectator field are much more unstable than the perturbations of the inflaton field, so the dynamics of early stages of preheating is dominated by the evol…
We study primordial density perturbations generated by the late decay of a curvaton field whose decay rate may be modulated by the local value of another isocurvature field, analogous to models of modulated reheating at the end of inflation. We calculate the primordial density perturbation and its local-type non-Gaussianity using the sudden-decay approximation for the curvaton field, recovering standard curvaton and modulated reheating results as limiting cases. We verify the Suyama-Yamaguchi inequality between bispectrum and trispectrum parameters for the primordial density field generated by…
We present a model for viable gravitational reheating involving a scalar field directly coupled to the Ricci curvature scalar. Crucial to the model is a period of kination after inflation, which causes the Ricci scalar to change sign thus inducing a tachyonic effective mass $m^{2} \propto -H^2$ for the scalar field. The resulting tachyonic growth of the scalar field provides the energy for reheating, allowing for temperatures high enough for thermal leptogenesis. Additionally, the required period of kination necessarily leads to a blue-tilted primordial gravitational wave spectrum with the pot…
Reheating with Axion-SU(2) and Gravitational Chern-Simons Couplings
Trottier Space Institute, Department of Physics, McGill University（麦吉尔大学物理系特罗蒂尔空间研究所）；Department of Physics, Bu-Ali Sina (Avicenna) University（布阿里·西那（阿维森纳）大学物理系）；School of Continuing Studies, McGill University（麦吉尔大学继续教育学院）
Bridging inflation and reheating: chiral gravitational waves from aHz to GHz
Anhui Normal University（安徽师范大学）；Jiangsu University of Science and Technology（江苏科技大学）；The Hong Kong University of Science and Technology（香港科技大学）；Jockey Club Institute for Advanced Study, The Hong Kong University of Science and Technology（香港科技大学赛马会先进研究学院）
We show that curvature induced particle production at reheating generates adiabatic dark matter if there are non-minimally coupled spectator scalars weakly coupled to visible matter. The observed dark matter abundance implies an upper bound on spectator masses $m$ and non-minimal coupling values $ξ$. For example, assuming quadratic inflation, instant reheating and a single spectator scalar with only gravitational couplings, the observed dark matter abundance is obtained for $m\sim 0.1$ GeV and $ξ\sim 1$. Larger mass and coupling values of the spectator are excluded as they would lead to overpr…
We consider a model of preheating where the coupling of the inflaton to the preheat field is modulated by an additional scalar field which is light during inflation. We establish that such a model produces the observed curvature perturbation analogously to the modulated reheating scenario. The contribution of modulated preheating to the power spectrum and to non-Gaussianity can however be significantly larger compared to modulated perturbative reheating. We also consider the implications of the current constraints on isocurvature perturbations in case where the modulating field is responsible …
Imprints of Reheating Dynamics on Gravitational Waves from Phase Transitions
SRM University-AP（SRM大学-安得拉邦分校）；Uppsala University（乌普萨拉大学）；University of Warsaw（华沙大学）；Nicolaus Copernicus Astronomical Center Polish Academy of Sciences（波兰科学院尼古拉·哥白尼天文学中心）；Nagoya University（名古屋大学）
We provide a detailed study of reheating in the kination regime where particle content is created by gravitational production of massive scalars mutually interacting with a massless scalar field. The produced particles subsequently decay into massless particles eventually reheating the Universe. We aim for a more precise picture using Boltzmann equations and decay rates obtained by methods of quantum field theory in curved spacetime. By numerical calculations it is found that after inflation the Universe ends up being dominated by ordinary matter for a while before the radiation dominated era.…
We study the potential impact of detecting the inflationary gravitational wave background by the future space-based gravitational wave detectors, such as DECIGO and BBO. The signal-to-noise ratio of each experiment is calculated for chaotic/natural/hybrid inflation models by using the precise predictions of the gravitational wave spectrum based on numerical calculations. We investigate the dependence of each inflation model on the reheating temperature which influences the amplitude and shape of the spectrum, and find that the gravitational waves could be detected for chaotic/natural inflation…
Recently, a combined model of the primordial inflation and the present cosmic acceleration has been proposed in the context of f(R) gravity. This model is composed of the late-time acceleration term and an R2 term, which enables the model to avoid high curvature singularity and describe a quasi-de Sitter inflationary phase in the early Universe. An interesting feature of this model is that the reheating dynamics after the inflation is significantly modified, in contrast to the original R2 model, and affects the shape of a gravitational wave background (GWB) spectrum. Here we investigate the pr…
We consider the direct $s$-channel gravitational production of dark matter during the reheating process. Independent of the identity of the dark matter candidate or its non-gravitational interactions, the gravitational process is always present and provides a minimal production mechanism. During reheating, a thermal bath is quickly generated with a maximum temperature $T_{\rm max}$, and the temperature decreases as the inflaton continues to decay until the energy densities of radiation and inflaton oscillations are equal, at $T_{\rm RH}$. During these oscillations, $s$-channel gravitational pr…
We investigate the co-evolution of dark matter (DM) density perturbation and metric perturbation in the WIMP paradigm. Instead of adopting the conventional assumption that DM starts out in thermal equilibrium, we propose a simple phase of DM production for the WIMP paradigm and extend our analysis to this phase. Being free from the envelop of thermal equilibrium, an amplified perturbative resonance between DM density perturbation and scalar modes of metric perturbation takes place during the DM production phase, and consequently results in a suppression of the tensor-to-scalar ratio of metric …
A pseudoscalar inflaton $φ$, coupled to the topological charge density $F\tilde{F}$ of a non-Abelian sector, can decay to gauge bosons ($φ\to g g $), which may thermalize rapidly. The friction felt by $φ$ is then increased by non-Abelian "strong sphalerons", leading to a self-amplifying process that can efficiently heat up the medium. We determine a lower bound for the gravitational wave production rate from such a process, originating via hydrodynamic fluctuations and particle collisions, in terms of a minimal number of parameters. Only a moderate fraction of energy density is converted to gr…
We consider the generation of the baryon asymmetry in models with right-handed neutrinos produced through gravitational scattering of the inflaton during reheating. The right-handed neutrinos later decay and generate a lepton asymmetry, which is partially converted to a baryon asymmetry by Standard Model sphaleron processes. We find that a sufficient asymmetry can be generated for a wide range of right-handed neutrino masses and reheating temperatures. We also show that the same type of gravitational scattering produces Standard Model Higgs bosons, which can achieve inflationary reheating cons…
Primordial Gravitational Waves from Phase Transitions during Reheating
Chungbuk National University（忠北国立大学）；Research Institute for Nanoscale Science and Technology, Chungbuk National University（忠北国立大学纳米科学技术研究所）；New York University Abu Dhabi（纽约大学阿布扎比分校）；University of Oklahoma（俄克拉荷马大学）
We study the effects of the Gauss-Bonnet term on the energy spectrum of inflationary gravitational waves. The models of inflation are classified into two types based on their predictions for the tensor power spectrum: red-tilted ($n_T<0$) and blue-tilted spectra ($n_T>0$), respectively, and then the energy spectra of the gravitational waves are calculated for each type of model. We find that the gravitational wave spectra are enhanced depending on the model parameter if the predicted inflationary tensor spectra have a blue tilt, whereas they are suppressed for the spectra that have a red tilt.…
It is well known that the inflationary scenario often displays different sets of degeneracies in its predictions for CMB observables. These degeneracies usually arise either because multiple inflationary models predict similar values for the scalar spectral index $n_{_S}$ and the tensor-to-scalar ratio $r$, or because within the same model, the values of $\lbrace n_{_S}, r \rbrace$ are insensitive to some of the model parameters, making it difficult for CMB observations alone to constitute a unique probe of inflationary cosmology. We demonstrate that by taking into account constraints on the p…
