Curvaton Scenario Research Articles

Chain inflation reconsidered

We investigate density perturbations in the chain inflation scenario, where the inflaton undergoes successive tunneling transitions along one field direction. First we show that when the bubble walls associated with such a phase transition meet, they induce the next phase transition and continue to propagate as phase interfaces. Then we present an analytical calculation of the density fluctuations and an estimate of non-Gaussianities such as fNL, which we find to be small (of order 1). To get the right amplitude for the power spectrum, there have to be 108 phase transitions per Hubble time, a significant model building challenge. We find that working models of chain inflation must be rather strongly coupled, and thus have a very limited range of validity as effective field theories. We discuss generalizations to the multiple field case, the curvaton scenario, and noncanonical kinetic terms.

Supercurvaton

We discuss observational consequences of the curvaton scenario, which naturally appears in the context of the simplest model of chaotic inflation in supergravity. The non-gaussianity parameter fNL in this scenario is very sensitive to the choice of the model parameters; it can be either very small or very large. Under certain conditions, this parameter can take values in the observationally interesting range from O(10) to O(100). We point out that local fNL can take different values in different parts of the universe. The regions where fNL is particularly large form a curvaton web resembling a net of thick domain walls, strings, or global monopoles.

Non-Gaussianity and gravitational waves from a quadratic and self-interacting curvaton

In this paper we consider how non-Gaussianity of the primordial density perturbation and the amplitude of gravitational waves from inflation can be used to determine parameters of the curvaton scenario for the origin of structure. We show that in the simplest quadratic model, where the curvaton evolves as a free scalar field, measurement of the bispectrum relative to the power spectrum, ${f}_{\mathrm{NL}}$, and the tensor-to-scalar ratio can determine both the expectation value of the curvaton field during inflation and its dimensionless decay rate relative to the curvaton mass. We show how these predictions are altered by the introduction of self-interactions, in models where higher-order corrections are determined by a characteristic mass scale and discuss how additional information about primordial non-Gaussianity and scale dependence may constrain curvaton interactions.

Optimizing future experimental probes of inflation

The discovery of many novel realizations of the inflationary universe paradigm has led to a degeneracy problem: many different inflationary Lagrangians generate the same perturbation spectra. Resolving this problem requires the future discovery of additional observables beyond the scalar adiabatic and tensor two-point functions on CMB scales. One important source of degeneracy arises in models where the density perturbation is generated by a noninflationary degree of freedom, for example, through curvatons or modulated reheating. We consider the curvaton scenario as representative of this class, and analyze the degeneracy with single field, canonical inflation that results if the curvaton goes undetected by future observations. We perform Monte Carlo potential reconstructions in the absence of distinguishing observables, such as non-Gaussiantities or isocurvature modes. The resulting degeneracy is considerable and the improved measurements of spectral parameters from future probes like CMBPol offer little to better the situation. Given a degeneracy-breaking observation, the observables must still be inverted to obtain the inflationary potential, with different observations resulting in reconstructions of varying quality. We find that a future detection of isocurvature modes or a precision measurement of the tensor spectral index will enable the most successful reconstructions in the presence of curvatons.

The curvaton scenario in brane cosmology: model parameters and their constraints

We have studied the curvaton scenario in brane world cosmology in an intermediate inflationary scenario. This approach has allowed us to find some constraints on different parameters that appear in the model.

Large nonlocal non-Gaussianity from a curvaton brane

We use a generalized delta N formalism to study the generation of the primordial curvature perturbation in the curvaton brane scenario inspired by stringy compactifications. We note that the non-Gaussian features, especially the trispectra, crucially depend on the decay mechanism in a general curvaton scenario. Specifically, we study the bispectra and trispectra of the curvaton brane model in detail to illustrate the importance of curvaton decay in generating nonlinear fluctuations. When the curvaton brane moves nonrelativistically during inflation, the shape of non-Gaussianity is local, but the corresponding size is different from that in the standard curvaton scenario. When the curvaton brane moves relativistically in inflationary stage, the shape of non-Gaussianity is of equilateral type.

How the curvaton scenario, modulated reheating and an inhomogeneous end of inflation are related

In this paper we analyse three models of the early universe, for whichthe respective mechanisms for generating the curvature perturbation are considereddisparate. We find that in fact the mechanisms are very similar, and henceexplain why they give rise to a large non-gaussianity.We show that the mechanism for generating the primordialcurvature perturbation, and hence the observable non-gaussianity, issimilar in both the Curvaton and Modulated Reheating models. In bothcases the model can be written in terms of an energy transfer betweenthe constituting fluids. We then show that this is also true for themechanism of generating the curvature perturbation by symmetrybreaking the end of inflation. We then relate this to the non-gaussiancontribution to the curvature perturbation and find that it isinversely proportional to the efficiency with which the curvatureperturbation is transferred between the fluids. For the first time, we generalise modelsof modulated reheating to allow for a non-linear energy transfer rate.

Non-gaussianity with Lagrange multiplier field in the curvaton scenario

In this paper, we will use δ\U0001d4a9-formalism to calculate the primordial curvature perturbation for the curvaton model with a Lagrange multiplier field. We calculate the non-linearity parameters fNL and gNL in the sudden-decay approximation in this kind of model, and we find that one could get a large non-Gaussinity even if the curvaton dominates the total energy density before it decays, and this property will make the curvaton model much richer. We also calculate the probability density function of the primordial curvature perturbation in the sudden-decay approximation, as well as some moments of it.

RIGHT-HANDED SNEUTRINO CURVATON AND NON-GAUSSIANITY

In this paper, we explore the parameter space for a Right-Handed (RH) sneutrino curvaton that can generate large non-Gaussianity without assuming any particular inflation sector. The mass of the RH sneutrino is suggested from a discussion on the initial condition of the curvaton field. It is shown that a small Yukawa coupling is generally required for a successful RH sneutrino curvaton. However, the Yukawa coupling can be larger if we consider the braneworld scenario. Some general discussion about the spectral index in curvaton scenario is also provided.

Non-standard primordial fluctuations and nongaussianity in string inflation

Inflationary scenarios in string theory often involve a large number of light scalar fields, whose presence can enrich the post-inflationary evolution of primordial fluctuations generated during the inflationary epoch. We provide a simple example of such post-inflationary processing within an explicit string-inflationary construction, using a Kahler modulus as the inflaton within the framework of LARGE Volume Type-IIB string flux compactifications. We argue that inflationary models within this broad category often have a selection of scalars that are light enough to be cosmologically relevant, whose contributions to the primordial fluctuation spectrum can compete with those generated in the standard way by the inflaton. These models consequently often predict nongaussianity at a level, f_NL ~ O(10), potentially observable by the Planck satellite, with a bi-spectrum maximized by triangles with squeezed shape in a string realisation of the curvaton scenario. We argue that the observation of such a signal would robustly prefer string cosmologies such as these that predict a multi-field dynamics during the very early universe.

Quadra-spectrum and quint-spectrum from inflation and curvaton models

We calculate the quadra-spectrum and quint-spectrum, corresponding to five and six point correlation functions of the curvature perturbation. For single field inflation with standard kinetic term, the quadra-spectrum and quint-spectrum are small, which are suppressed by slow roll parameters. The calculation can be generalized to multiple fields. When there is no entropy perturbation, the quadra-spectrum and quint-spectrum are suppressed as well. With the presence of entropy perturbation, the quadra-spectrum and quint-spectrum can get boosted. We illustrate this boost in the multi-brid inflation model. For the curvaton scenario, the quadra-spectrum and quint-spectrum are also large in the small r limit. We also calculate representative terms of quadra-spectrum and quint-spectrum for inflation with generalized kinetic terms, and estimate their order of magnitude for quasi-single field inflation.

Inflaton versus Curvaton in Higher-Dimensional Gauge Theories

We construct a model of cosmological inflation and perturbation based on the higher-dimensional gauge theory. The inflaton and curvaton are the scalar fields arising from the extra space components of the gauge field living in more than four dimensions. We take the six-dimensional (6D) Yang-Mills theory compactified on $T^2$ as a toy model, and apply the one-loop effective potential of the inflaton and the curvaton to the curvaton scenario. We have found that the curvaton is subdominant for the linear curvature perturbation, but that a significant non-Gaussianity and a sizable tensor to scalar ratio are generated.

Curvaton decay into relativistic matter

We consider an inflationary curvaton scenario, where the curvaton decays into two non-interacting relativistic fluids and later during the cosmological evolution one of them becomes non-relativistic, forming dark matter component of the universe. We study the thermic properties and the generation of non-gaussianity in this three fluid curvaton model. By solving the evolution of the system and using several cosmological conditions we find that the allowed parameter space is strongly constrained. The naturalness of this curvaton scenario is also discussed.

Non-Gaussianity from false vacuum inflation: old curvaton scenario

We calculate the three-point correlation function of the comoving curvature perturbationgenerated during an inflationary epoch driven by false vacuum energy. We get a novelfalse vacuum shape bispectrum, which peaks in the equilateral limit. Using this result,we propose a scenario which we call ``old curvaton''. The shape of the resultingbispectrum lies between the local and the false vacuum shapes. In addition we have alarge running of the spectral index.

Non-Gaussianity from resonant curvaton decay

We calculate curvature perturbations in the scenario in which thecurvaton field decays into another scalar field viaparametric resonance. As a result of a nonlinear stage at the end of the resonance, standardperturbative calculation techniques fail in this case.Instead, we use lattice field theory simulations and the separate universeapproximation to calculate the curvature perturbation as a nonlinear function ofthe curvaton field.For the parameters tested, the generated perturbations are highly non-Gaussianandnot well approximated by the usual fNL parameterisation.Resonant decay plays an important role in the curvaton scenario and can have a substantial effect on the resulting perturbations.

Review of Local Non‐Gaussianity from Multifield Inflation

We review models which generate a large non‐Gaussianity of the local form. We first briefly consider three models which generate the non‐Gaussianity either at or after the end of inflation; the curvaton scenario, modulated (p)reheating, and an inhomogeneous end of inflation. We then focus on ways of generating the non‐Gaussianity during inflation. We derive general conditions which a product or sum separable potential must satisfy in order to generate a large local bispectrum during slow‐roll inflation. As an application, we consider two‐field hybrid inflation. We then derive a formalism not based on slow roll which can be applied to models in which the slow‐roll parameters become large before inflation ends. An exactly soluble two‐field model is given in which this happens. Finally, we also consider further non‐Gaussian observables, a scale dependence of fNL and the trispectrum.

Gravitational wave background and non-Gaussianity as a probe of the curvaton scenario

We study observational implications of thestochastic gravitational wave backgroundand a non-Gaussian feature of scalar perturbations on the curvatonmechanism of the generation of density/curvature fluctuations,and show that they can determine the properties of the curvatonin a complementary manner to each other. Therefore even if Planckcould not detect any non-Gaussianity, future space-based laserinterferometers such as DECIGO or BBO could practically exhaustits parameter space.

A scale-dependent power asymmetry from isocurvature perturbations

If the hemispherical power asymmetry observed in the cosmic microwave background (CMB) on large angular scales is attributable to a superhorizon curvaton fluctuation, then the simplest model predicts that the primordial density fluctuations should be similarly asymmetric on all smaller scales. The distribution of high-redshift quasars was recently used to constrain the power asymmetry on scales $k\ensuremath{\simeq}1.5h\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$, and the upper bound on the amplitude of the asymmetry was found to be a factor of 6 smaller than the amplitude of the asymmetry in the CMB. We show that it is not possible to generate an asymmetry with this scale dependence by changing the relative contributions of the inflaton and curvaton to the adiabatic power spectrum. Instead, we consider curvaton scenarios in which the curvaton decays after dark matter freezes out, thus generating isocurvature perturbations. If there is a superhorizon fluctuation in the curvaton field, then the rms amplitude of these perturbations will be asymmetric, and the asymmetry will be most apparent on large angular scales in the CMB. We find that it is only possible to generate the observed asymmetry in the CMB while satisfying the quasar constraint if the curvaton's contribution to the total dark matter density is small, but nonzero. The model also requires that the majority of the primordial power comes from fluctuations in the inflaton field. Future observations and analyses of the CMB will test this model because the power asymmetry generated by this model has a specific spectrum, and the model requires that the current upper bounds on isocurvature power are nearly saturated.

Primordial non-Gaussianity in models with dark matter isocurvature fluctuations

We investigate primordial non-Gaussianity and dark matter isocurvature fluctuations in the modulated reheating and the curvaton scenarios. In these scenarios, a large non-Gaussianity can be generated; on the other hand, depending on how dark matter is produced, too large isocurvature fluctuations can also arise, which is inconsistent with current observations. In this paper, we study this issue in a mixed scenario where the curvature fluctuations can also be produced from the inflaton fluctuations as well as those from a light scalar field such as the modulus and the curvaton. We show that primordial fluctuations can be highly non-Gaussian without conflicting with the current constraint on isocurvature fluctuations for such mixed scenarios. However, if the constraint on isocurvature fluctuations becomes severer as expected by the Planck satellite, ${f}_{\mathrm{NL}}$, a nonlinearity parameter for adiabatic fluctuations, should be very small as ${f}_{\mathrm{NL}}\ensuremath{\lesssim}3$, which would give interesting implications for the generation mechanism of dark matter. Non-Gaussianity from isocurvature fluctuations is also discussed in these scenarios.

Constraints on cosmic hemispherical power anomalies from quasars

Recent analyses of the cosmic microwave background (CMB) maps from the WMAP satellite have uncovered evidence for a hemispherical power anomaly, i.e. a dipole modulation of the CMB power spectrum at large angular scales with an amplitude of ±14 percent. Erickcek et al have put forward an inflationary model to explain this anomaly. Their scenario is a variation on the curvaton scenario in which the curvaton possesses a large-scale spatial gradient that modulates the amplitude of CMB fluctuations. We show that this scenario would also lead to a spatial gradient in the amplitude of perturbations σ8, and hence to a dipole asymmetry in any highly biased tracer of the underlying density field. Using the high-redshift quasars from the Sloan Digital Sky Survey, we find an upper limit on such a gradient of |∇σ8|/σ8 < 0.027rlss−1 (99% posterior probability), where rlss is the comoving distance to the last-scattering surface. This rules out the simplest version of the curvaton spatial gradient scenario.