Curvaton Scenario Research Articles

Primordial black holes from a curvaton scenario with strongly non-Gaussian perturbations

We investigate the production of primordial black holes (PBHs) in a mixed inflaton-curvaton scenario with a quadratic curvaton potential, assuming the curvaton is in de Sitter equilibrium during inflation with 〈χ〉 = 0. In this setup, the curvature perturbation sourced by the curvaton is strongly non-Gaussian, containing no leading Gaussian term. We show that for m 2/H 2 ≳ 0.3, the curvaton contribution to the spectrum of primordial perturbations on CMB scales can be kept negligible but on small scales the curvaton can source PBHs. In particular, PBHs in the asteroid mass range 10-16 M ⊙ ≲ M ≲ 10-10 M ⊙ with an abundance reaching F PBH = 1 can be produced when the inflationary Hubble scale H ≳ 1012 GeV and the curvaton decay occurs in the window from slightly before the electroweak transition to around the QCD transition.

Growth of curvature perturbations for PBH formation & detectable GWs in non-minimal curvaton scenario revisited

We revisit the growth of curvature perturbations in non-minimal curvaton scenario with a non-trivial field metric λ(ϕ) where ϕ is an inflaton field, and incorporate the effect from the non-uniform onset of curvaton's oscillation in terms of an axion-like potential. The field metric λ(ϕ) plays a central role in the enhancement of curvaton field perturbation δχ, serving as an effective friction term which can be either positive or negative, depending on the first derivative λ ,ϕ . Our analysis reveals that δχ undergoes the superhorizon growth when the condition η eff ≡ -2 √2ϵ M Pl λ ,ϕ /λ < -3 is satisfied. This is analogous to the mechanism responsible for the amplification of curvature perturbations in the context of ultra-slow-roll inflation, namely the growing modes dominate curvature perturbations. As a case study, we examine the impact of a Gaussian dip in λ(ϕ) and conduct a thorough investigation of both the analytical and numerical aspects of the inflationary dynamics. Our findings indicate that the enhancement of curvaton perturbations during inflation is not solely determined by the depth of the dip in λ(ϕ). Rather, the first derivative λ ,ϕ also plays a significant role, a feature that has not been previously highlighted in the literature. Utilizing the δ𝒩 formalism, we derive analytical expressions for both the final curvature power spectrum and the non-linear parameter f NL in terms of an axion-like curvaton's potential leading to the non-uniform curvaton's oscillation. Additionally, the resulting primordial black hole abundance and scalar-induced gravitational waves are calculated, which provide observational windows for PBHs.

New universal property of cosmological gravitational wave anisotropies

The anisotropies of the stochastic gravitational wave background, as produced in the early phases of cosmological evolution, can act as a key probe of the primordial universe particle content. We point out a new universal property of gravitational wave anisotropies of cosmological origin: for adiabatic initial conditions, their angular power spectrum is insensitive to the equation of state of the cosmic fluid driving the expansion before big-bang nucleosynthesis. Any deviation from this universal behaviour points to the presence of non-adiabatic sources of primordial fluctuations. Such scenarios can be tested by gravitational wave detectors operating at a frequency range which is fully complementary to CMB experiments. In this work we prove this general result, and we illustrate its consequences for a representative realisation of initial conditions based on the curvaton scenario. In the case of the simplest curvaton setup, we also find a significant cross-correlation between gravitational wave anisotropies and the CMB temperature fluctuations. There is a fourfold enhancement vis-\`{a}-vis the purely adiabatic scenario. We discuss the implications of our findings for identifying the origin of the (cosmological) gravitational wave background when, as is often the case, this cannot be determined solely on the basis of its spectral shape.

Curvaton and quantum gravity effect on the tensor-to-scalar ratio of the chaotic inflation

The expected tensor-to-scalar ratio estimate of the upcoming CMB mission probe measurements may establish a lower value of the ratio than the currently obtained value. It can be described in terms of a single field chaotic inflation model along with the curvaton or quantum gravity or their combined effect. Consequently, the role of quantum gravity or curvaton in the dynamics of the early universe may not be ruled out. The curvaton scenario and quantum gravity effect can be tested experimentally. The upcoming CMB missions can validate the curvaton scenario and quantum gravity experimentally.

Non-minimally coupled curvaton

We investigate two-field inflationary models in which scalar cosmological pertubations are generated viaa spectator field nonminimallycoupled to gravity, with the particular emphasis oncurvaton scenarios.The principal advantage of these models is in the possibility to tunethe 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 perturbationgenerated in the curvaton scenarios. In particular we find that for small, negative nonminimal couplingsthe 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 canbe saved, which would otherwise be ruled out.In the power law inflation we find that a large nonminimal coupling is excluded sinceit gives the principal slow-roll parameter that is of the order of unity.Finally, we point out that nonminimal coupling can affectthe postinflationary growth of the spectator perturbation,and in this way the effectiveness of the curvaton mechanism.

Revised fNL parameter in a curvaton scenario

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.

Curvaton: Perturbations and reheating

We study chaotic and runaway potential inflationary models in the curvaton scenario. In particular, we address the issue of large tensor-to-scalar ratio and red-tilted spectrum in chaotic models and reheating in runaway model in the light of latest Planck results. We show that curvaton can easily circumvent these problems and is well applicable to both type of models. For chaotic models, the observable non-Gaussianity put strong constraints on the decay epoch of curvaton as well as on its field value around the horizon exit. Besides, it can also explain the observed red-tilt in the spectrum as a consequence of its negative mass-squared value. As for the runaway inflationary models, curvaton by sudden decay into the background radiation provides an efficient reheating mechanism, whereas the inflaton rolls down from its potential and enters into the kinetic regime. To this effect, we consider the generalized exponential potential and obtain the allowed parametric space for model parameters. From the estimates on inflaton parameters, we constrained the curvaton mass and then the reheating temperature. We constrained the latter for both dominating and sub-dominating case and show that it agrees with the nucleosynthesis constraint.

Cosmological collider physics and the curvaton

Primordial non-Gaussianity signatures of extremely heavy particles are re- examined within a simple alternative to the standard inflationary paradigm, in which the primordial fluctuations and the inflationary spacetime expansion are sourced by two different fields. The curvaton scenario provides an example of this in which the distinct roles are played by the curvaton and the inflaton fields, respectively. We study couplings of the curvaton to heavy particles with masses of order the inflationary Hubble scale, and show that they can lead to non-Gaussian signals orders of magnitude larger than those in standard inflation, consistent with explicit effective field theory control of inflationary dynamics. This brings various motivated particle physics signatures, such as loops of heavy gauge-charged scalars and fermions, within future observational reach.

Preheating in the nonminimal derivative coupling curvaton model with nonstandard kinetic matter term

The curvaton scenario received much attention recently. It provides an alternative way to generate scale-invariant perturbations for the observed universe, besides the standard inflation paradigm. This paper studies the preheating in the nonminimal derivative coupling curvaton model with a nonstandard kinetic matter term, as a consequence of the large scale power spectrums of curvature perturbation. It is found that the nonstandard kinetic matter term is able to modify the behavior of the preheating, compared to the nonminimal derivative coupling curvaton model with standard kinetic matter term. Our result may provide an insight to the preheating process for bouncing cosmology or brane-inspired cosmology.

Refined study of isocurvature fluctuations in the curvaton scenario

We revisit the generation of dark matter isocurvature perturbations in the curvaton model in greater detail, both analytically and numerically. As concrete examples, we investigate the cases of thermally decoupled dark matter and axionic dark matter. We show that the radiation produced by the decay of the curvaton, which has not been taken into account in previous analytical studies, can significantly affect the amplitude of isocurvature perturbations. In particular, we find that they are drastically suppressed even when the dark matter freeze-out (or the onset of the axion oscillations for axionic dark matter) occurs before the curvaton decays, provided the freeze-out takes place deep in the curvaton-dominated Universe. As a consequence, we show that the current observational isocurvature constraints on the curvaton parameters are not as severe as usually thought.

Where does curvaton reside? Differences between bulk and brane frames

Some classes of inflationary models naturally introduce two distinct metrics/frames, and their equivalence in terms of observables has often been put in question. D-brane inflation proposes candidates for an inflaton embedded in the string theory and possesses descriptions on the brane and bulk metrics/frames, which are connected by a conformal/disformal transformation that depends on the inflaton and its derivatives. It has been shown that curvature perturbations generated by the inflaton are identical in both frames, meaning that observables such as the spectrum of cosmic microwave background (CMB) anisotropies are independent of whether matter fields---including those in the standard model of particle physics---minimally couple to the brane or the bulk metric/frame. This is true despite the fact that the observables are eventually measured by the matter fields and that the total action including the matter fields is different in the two cases. In contrast, in curvaton scenarios, the observables depend on the frame to which the curvaton minimally couples. Among all inflationary scenarios, we focus on two models motivated by the KKLMMT fine-tuning problem: a slow-roll inflation with an inflection-point potential and a model of a rapidly rolling inflaton that conformally couples to gravity. In the first model, the difference between the frames in which the curvaton resides is encoded in the spectral index of the curvature perturbations, depicting the nature of the frame transformation. In the second model, the curvaton on the brane induces a spectral index significantly different from that in the bulk and is even falsified by the observations. This work thus demonstrates that two frames connected by a conformal/disformal transformation lead to different physical observables such as CMB anisotropies in curvaton models.

Curvaton as dark matter with secondary inflation

We consider a novel cosmological scenario in which a curvaton is long-lived and plays the role of cold dark matter (CDM) in the presence of a short, secondary inflation. Non-trivial evolution of the large scale cosmological perturbation in the curvaton scenario can affect the duration of the short term inflation, resulting in the inhomogeneous end of inflation. Non-linear parameters of the curvature perturbation are predicted to be fNL ≈ 5/4 and gNL ≈ 0. The curvaton abundance can be well diluted by the short-term inflation and accordingly, it does not have to decay into the Standard Model particles. Then the curvaton can account for the present CDM with the isocurvature perturbation being sufficiently suppressed because both the adiabatic and CDM isocurvature perturbations have the same origin. As an explicit example, we consider the thermal inflation scenario and a string axion as a candidate for this curvaton-dark matter. We further discuss possibilities to identify the curvaton-dark matter with the QCD axion.

Hubble induced mass after inflation in spectator field models

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 the curvaton, such as the reheating temperature and the inflation scale. We comment on the implication of our discussion for baryogenesis in the curvaton scenario. In the modulated reheating, the predictions of models e.g. the non-gaussianity can be considerably altered. Furthermore, we propose a new model of the modulated reheating utilizing the Hubble induced mass which realizes a wide range of the local non-gaussianity parameter.

Probing a panoply of curvaton-decay scenarios using CMB data

In the curvaton scenario, primordial curvature perturbations are produced by a second field that is sub-dominant during inflation. Depending on how the curvaton decays [possibly producing baryon number, lepton number, or cold dark matter (CDM)], mixtures of correlated isocurvature perturbations are produced, allowing the curvaton scenario to be tested using cosmic microwave background (CMB) data. Here, a full range of 27 curvaton-decay scenarios is compared with CMB data, placing limits on the curvaton fraction at decay, $r_D$, and the lepton asymmetry, $\xi_{\rm lep}$. If baryon number is generated by curvaton decay and CDM before (or vice-versa), these limits imply specific predictions for non-Gaussian signatures testable by future CMB experiments and upcoming large-scale-structure surveys.

Curvaton scenarios with inflaton decays into curvatons

We consider the possible decay of the inflaton into curvaton particles during reheating and analyse its effect on curvaton scenarios. Typical decay curvatons are initially relativistic then become non-relativistic, changing the background history of the Universe. We show that this change to the background is the only way in which observational predictions of the scenario are modified. Moreover, once the required amplitude of perturbations is fixed by observation there are no signatures of such decays in other cosmological observables. The decay curvatons can prevent the Universe from becoming dominated by the curvaton condensate, making it impossible to match observations in parts of parameter space. This constrains the branching ratio of the inflaton to curvaton to be less than of order $0.1$ typically. If the branching ratio is below about $10^{-4}$ it has negligible impact on the model parameter space and can be ignored.

Lensing bias to CMB measurements of compensated isocurvature perturbations

Compensated isocurvature perturbations (CIPs) are modes in which the baryon and dark matter density fluctuations cancel. They arise in the curvaton scenario as well as some models of baryogenesis. While they leave no observable effects on the cosmic microwave background (CMB) at linear order, they do spatially modulate two-point CMB statistics and can be reconstructed in a manner similar to gravitational lensing. Due to the similarity between the effects of CMB lensing and CIPs, lensing contributes nearly Gaussian random noise to the CIP estimator that approximately doubles the reconstruction noise power. Additionally, the cross correlation between lensing and the integrated Sachs-Wolfe (ISW) effect generates a correlation between the CIP estimator and the temperature field even in the absence of a correlated CIP signal. For cosmic-variance limited temperature measurements out to multipoles $l \leq 2500$, subtracting a fixed lensing bias degrades the detection threshold for CIPs by a factor of $1.3$, whether or not they are correlated with the adiabatic mode.

Compensated isocurvature perturbations in the curvaton model

Primordial fluctuations in the relative number densities of particles, or isocurvature perturbations, are generally well constrained by cosmic microwave background (CMB) data. A less probed mode is the compensated isocurvature perturbation (CIP), a fluctuation in the relative number densities of cold dark matter and baryons. In the curvaton model, a subdominant field during inflation later sets the primordial curvature fluctuation $\zeta$. In some curvaton-decay scenarios, the baryon and cold dark matter isocurvature fluctuations nearly cancel, leaving a large CIP correlated with $\zeta$. This correlation can be used to probe these CIPs more sensitively than the uncorrelated CIPs considered in past work, essentially by measuring the squeezed bispectrum of the CMB for triangles whose shortest side is limited by the sound horizon. Here, the sensitivity of existing and future CMB experiments to correlated CIPs is assessed, with an eye towards testing specific curvaton-decay scenarios. The planned CMB Stage 4 experiment could detect the largest CIPs attainable in curvaton scenarios with more than 3$\sigma$ significance. The significance could improve if small-scale CMB polarization foregrounds can be effectively subtracted. As a result, future CMB observations could discriminate between some curvaton-decay scenarios in which baryon number and dark matter are produced during different epochs relative to curvaton decay. Independent of the specific motivation for the origin of a correlated CIP perturbation, cross-correlation of CIP reconstructions with the primary CMB can improve the signal-to-noise ratio of a CIP detection. For fully correlated CIPs the improvement is a factor of $\sim$2$-$3.

Bayesian evidence of the post-Planck curvaton

We perform a Bayesian model comparison for scenarios within the quadratic curvaton model, determining the degree to which both are disfavoured with respect to the ΛCDM concordance model and single-field quadratic inflation, using the recent Planck data release. Despite having three additional model parameters, the simplest curvaton scenario is not disfavoured relative to single-field quadratic inflation, and it becomes favoured against this single-field model when we include the joint BICEP/Keck/Planck analysis. In all cases we assume an instantaneous inflaton decay and no surviving isocurvature perturbations. Despite the success of Planck reaching its forecast measurement accuracy, we show that the current constraints on local non-Gaussianity are insufficiently precise to have any significant impact on the evidence ratios so far. We also determine the precision σ(fNL) required by future measurements assuming a fiducial value of fNL=−5/4 or 10.8 to no longer disfavour the curvaton against the ΛCDM parametrisation, and we discuss the effect that the predicted increase in precision from future measurements on fNL may have. We show that our results are not very sensitive to our choice of priors.

Scale-dependent non-Gaussianity and the CMB power asymmetry

We introduce an alternative parametrisation for the scale dependence of the non-linearity parameter fNL in quasi-local models of non-Gaussianity. Our parametrisation remains valid when fNL changes sign, unlike the commonly adopted power law ansatz fNL(k)∝ knfNL. We motivate our alternative parametrisation by appealing to the self-interacting curvaton scenario, and as an application, we apply it to the CMB power asymmetry. Explaining the power asymmetry requires a strongly scale dependent non-Gaussianity. We show that regimes of model parameter space where fNL is strongly scale dependent are typically associated with a large gNL and quadrupolar power asymmetry, which can be ruled out by existing observational constraints.

Planck constraints on neutrino isocurvature density perturbations

The recent Cosmic Microwave Background data from the Planck satellite experiment, when combined with HST determinations of the Hubble constant, are compatible with a larger, non-standard, number of relativistic degrees of freedom at recombination, parametrized by the neutrino effective number $N_{eff}$. In the curvaton scenario, a larger value for $N_{eff}$ could arise from a non-zero neutrino chemical potential connected to residual neutrino isocurvature density (NID) perturbations after the decay of the curvaton field, parametrized by the amplitude $\alpha^{NID}$. Here we present new constraints on $N_{eff}$ and $\alpha^{NID}$ from an analysis of recent cosmological data. We found that the Planck+WP dataset does not show any indication for a neutrino isocurvature component, severly constraining its amplitude, and that current indications for a non-standard $N_{eff}$ are further relaxed.