Toyokazu Sekiguchi

Axion dark matter from topological defects

The cosmological scenario where the Peccei-Quinn symmetry is broken after inflation is investigated. In this scenario, topological defects such as strings and domain walls produce a large number of axions, which contribute to the cold dark matter of the universe. The previous estimations of the cold dark matter abundance are updated and refined based on the field-theoretic simulations with improved grid sizes. The possible uncertainties originated in the numerical calculations are also discussed. It is found that axions can be responsible for the cold dark matter in the mass range

A General Analysis of Non-Gaussianity from Isocurvature Perturbations

m_a=(0.8-1.3)\times 10^{-4}\mathrm{eV}

Axion cosmology with long-lived domain walls

for the models with the domain wall number

Improved estimation of radiated axions from cosmological axionic strings

N_{\rm DW}=1

Cosmological constraints on dark matter models with velocity-dependent annihilation cross section

, and

Probing the effective number of neutrino species with the cosmic microwave background

m_a\approx\mathcal{O}(10^{-4}-10^{-2})\mathrm{eV}

Cosmological Constraints on Isocurvature and Tensor Perturbations

with a mild tuning of parameters for the models with

Primordial Helium Abundance from CMB: a constraint from recent observations and a forecast

N_{\rm DW}>1

Implications of dark energy parametrizations for the determination of the curvature of the universe

. Such higher mass ranges can be probed in future experimental studies.

Precise measurements of primordial power spectrum with 21 cm fluctuations

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.