Teruaki Suyama

Non-Gaussianity in the modulated reheating scenario

We investigate the non-Gaussianity of primordial curvature perturbations in the modulated reheating scenario where the primordial perturbation is generated due to the spatial fluctuation of the rate of the inflaton decay to radiation. We use the {delta}N formalism to evaluate the trispectrum of the curvature perturbation as well as its bispectrum. We give expressions for three nonlinear parameters f{sub NL}, {tau}{sub NL}, and g{sub NL} in the modulated reheating scenario. If both the intrinsic non-Gaussianity of scalar field fluctuations and third derivative of the decay rate with respect to the scalar fields are negligibly small, g{sub NL} has at least the same order of magnitude as f{sub NL}. We also give a general inequality between f{sub NL} and {tau}{sub NL}, which is true for other inflationary scenarios as long as the primordial non-Gaussianity comes from superhorizon evolution.

Primordial Black Hole Scenario for the Gravitational-Wave Event GW150914

We point out that the gravitational-wave event GW150914 observed by the LIGO detectors can be explained by the coalescence of primordial black holes (PBHs). It is found that the expected PBH merger rate would exceed the rate estimated by the LIGO Scientific Collaboration and the Virgo Collaboration if PBHs were the dominant component of dark matter, while it can be made compatible if PBHs constitute a fraction of dark matter. Intriguingly, the abundance of PBHs required to explain the suggested lower bound on the event rate, >2  events  Gpc^{-3} yr^{-1}, roughly coincides with the existing upper limit set by the nondetection of the cosmic microwave background spectral distortion. This implies that the proposed PBH scenario may be tested in the not-too-distant future.

Non-Gaussianity, Spectral Index and Tensor Modes in Mixed Inflaton and Curvaton Models

We study non-Gaussianity, the spectral index of primordial scalar fluctuations, and tensor modes in models where fluctuations from the inflaton and the curvaton can both contribute to the present cosmic density fluctuations. Even though simple single-field inflation models generate only tiny non-Gaussianity, if we consider such a mixed scenario, large non-Gaussianity can be produced. Furthermore, we study the inflationary parameters such as the spectral index and the tensor-to-scalar ratio in this kind of models and discuss in what cases models predict the spectral index and tensor modes allowed by the current data while generating large non-Gaussianity, which may have many implications for model-buildings of the inflationary universe.

Primordial non-Gaussianity in multi-scalar slow-roll inflation

We analyse the non-Gaussianity for primordial curvature perturbations generated in a multi-scalar slow-roll inflation model including the model with non-separable potential by making use of δN formalism. Many authors have investigated the possibility of large non-Gaussianity for the models with separable potential, and they have found that the non-linear parameter, fNL, is suppressed by the slow-roll parameters. We show that for the non-separable models fNL is given by the product of a factor which is suppressed by the slow-roll parameters and a possible enhancement factor which is given by exponentials of quantities of O(1).

Ultra Slow-Roll Inflation and the non-Gaussianity Consistency Relation

Ultra slow-roll inflation has recently been used to challenge the non-Gaussianity consistency relation. We show that this inflationary scenario belongs to a one parameter class of models and we study its properties and observational predictions. We demonstrate that the power spectrum remains scale-invariant and that the bi-spectrum is of the local type with fnl=5(3-ns)/4 which, indeed, represents a modification of the consistency relation. However, we also show that the system is unstable and suffers from many physical problems among which is the difficulty to correctly WMAP normalize the model. We conclude that ultra slow-roll inflation remains a very peculiar case, the physical relevance of which is probably not sufficient to call into question the validity of the consistency relation.

A General Analysis of Non-Gaussianity from Isocurvature Perturbations

Light scalars may be ubiquitous in nature, and their quantum fluctuations can produce large non-Gaussianity in the cosmic microwave background temperature anisotropy. The non-Gaussianity may be accompanied with a small admixture of isocurvature perturbations, which often have correlations with the curvature perturbations. We present a general method to calculate the non-Gaussianity in the adiabatic and isocurvature perturbations with and without correlations, and see how it works in several explicit examples. We also show that they leave distinct signatures on the bispectrum of the cosmic microwave background temperature fluctuations.

CMB temperature bispectrum induced by cosmic strings

The Cosmic Microwave Background (CMB) bispectrum of the temperature anisotropies induced by a network of cosmic strings is derived for small angular scales, under the assumption that the principal cause of temperature fluctuations is the Gott-Kaiser-Stebbins (GKS) effect. We provide analytical expressions for all isosceles triangle configurations in Fourier space. Their overall amplitude is amplified as the inverse cube of the angle and diverges for flat triangles. The isosceles configurations generically lead to a negative bispectrum with a power law decay l^(-6) for large multipole l. However, collapsed triangles are found to be associated with a positive bispectrum whereas the squeezed triangles still exhibit negative values. We then compare our analytical estimates to a direct computation of the bispectrum from a set of 300 statistically independent temperature maps obtained from Nambu-Goto cosmic string simulations in a Friedmann-Lemaitre-Robertson-Walker (FLRW) universe. We find good agreement for the overall amplitude, the power law behaviour and angle dependency of the various triangle configurations. At l~500 the cosmic string GKS effect contributes approximately the same equilateral CMB bispectrum amplitude as an inflationary model with |fNL|~10^3, if the strings contribute about 10% of the temperature power spectrum at l=10. Current bounds on fNL are not derived using cosmic string bispectrum templates, and so our fNL estimate cannot be used to derive bounds on strings. However it does suggest that string bispectrum templates should be included in the search of CMB non-Gaussianities. Comment: 15 pages, 12 figures, uses RevTex. References and physical discussion added. Matches published version

Healthy degenerate theories with higher derivatives

In the context of classical mechanics, we study the conditions under which higher-order derivative theories can evade the so-called Ostrogradsky instability. More precisely, we consider general Lagrangians with second order time derivatives, of the form

Vacuum structure for scalar cosmological perturbations in modified gravity models

L(\ddot\phi^a,\dot\phi^a,\phi^a;\dot q^i,q^i)

Black hole perturbation in the most general scalar-tensor theory with second-order field equations. II. The even-parity sector

with