851 citations · 5.2k across the 133 of their papers we have counts for
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Limits on Isocurvature Perturbations from Non-Gaussianity in WMAP Temperature Anisotropies
Chiaki Hikage, Kazuya Koyama, Takahiko Matsubara +2
We study the effect of primordial isocurvature perturbations on non-Gaussian properties of CMB temperature anisotropies. We consider generic forms of the non-linearity of isocurvat…
Constraints on Neutrino Masses from Weak Lensing
Kiyotomo Ichiki, Masahiro Takada, Tomo Takahashi
The weak lensing (WL) distortions of distant galaxy images are sensitive to neutrino masses by probing the suppression effect on clustering strengths of total matter in large-scale…
Adiabatic fluctuations from cosmic strings in a contracting universe
Robert H. Brandenberger, Tomo Takahashi, Masahide Yamaguchi
We show that adiabatic, super-Hubble, and almost scale invariant density fluctuations are produced by cosmic strings in a contracting universe. An essential point is that isocurvat…
Primordial Curvature Fluctuation and Its Non-Gaussianity in Models with Modulated Reheating
Kazuhide Ichikawa, Teruaki Suyama, Tomo Takahashi +1
We investigate non-Gaussianity in the modulated reheating scenario where fluctuations of the decay rate of the inflaton generate adiabatic perturbations, paying particular attentio…
Signatures of Non-Gaussianity in the Curvaton Model
Kari Enqvist, Tomo Takahashi
We discuss the signatures of non-Gaussianity in the curvaton model where the potential includes also a non-quadratic term. In such a case the non-linearity parameter f_NL can becom…
Probing the Effective Number of Neutrino Species with Cosmic Microwave Background
Kazuhide Ichikawa, Toyokazu Sekiguchi, Tomo Takahashi
We discuss how much we can probe the effective number of neutrino species N_nu with cosmic microwave background alone. Using the data of WMAP, ACBAR, CBI and BOOMERANG experiments,…
Non-Gaussianity, Spectral Index and Tensor Modes in Mixed Inflaton and Curvaton Models
Kazuhide Ichikawa, Teruaki Suyama, Tomo Takahashi +1
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…
Primordial Helium Abundance from CMB: a constraint from recent observations and a forecast
Kazuhide Ichikawa, Toyokazu Sekiguchi, Tomo Takahashi
We studied a constraint on the primordial helium abundance Y_p from current and future observations of CMB. Using the currently available data from WMAP, ACBAR, CBI and BOOMERANG,…
The Hubble constant and dark energy from cosmological distance measures
Kazuhide Ichikawa, Tomo Takahashi
We study how the determination of the Hubble constant from cosmological distance measures is affected by models of dark energy and vice versa. For this purpose, constraints on the…
Constraints on Generalized Dark Energy from Recent Observations
Kiyotomo Ichiki, Tomo Takahashi
Effects of a generalized dark energy fluid is investigated on cosmic density fluctuations such as cosmic microwave background. As a general dark energy fluid, we take into consider…
Dark Energy Parametrizations and the Curvature of the Universe
Kazuhide Ichikawa, Tomo Takahashi
We investigate observational constraints on the curvature of the universe not restricting ourselves to a cosmological constant as dark energy, in particular allowing a dark energy…
Implication of Dark Energy Parametrizations on the Determination of the Curvature of the Universe
Kazuhide Ichikawa, Masahiro Kawasaki, Toyokazu Sekiguchi +1
We investigate how the nature of dark energy affects the determination of the curvature of the universe from recent observations. For this purpose, we consider the constraints on t…
Revisiting the Constraint on the Helium Abundance from CMB
Kazuhide Ichikawa, Tomo Takahashi
We revisit the constraint on the primordial helium mass fraction Yp from observations of cosmic microwave background (CMB) alone. By minimizing chi square of recent CMB experiments…
Dark Energy Evolution and the Curvature of the Universe from Recent Observations
Kazuhide Ichikawa, Tomo Takahashi
We discuss the constraints on the time-varying equation of state for dark energy and the curvature of the universe using observations of type Ia supernovae from Riess et al. and th…
On the Determination of Neutrino Masses and Dark Energy Evolution from the Cross Correlation of CMB and LSS
Kazuhide Ichikawa, Tomo Takahashi
We discuss the possibilities of the simultaneous determination of the neutrino masses and the evolution of dark energy from future cosmological observations such as cosmic microwav…
Implications of the Curvaton on Inflationary Cosmology
Takeo Moroi, Tomo Takahashi
We study implications of the curvaton, a late-decaying light scalar field, on inflationary cosmology, paying particular attentions to modifications of observable quantities such as…
Making waves on CMB power spectrum and inflaton dynamics
Masahiro Kawasaki, Fuminobu Takahashi, Tomo Takahashi
We discuss cosmic microwave background anisotropies in models with an unconventional primordial power spectrum. In particular, we consider an initial power spectrum with some ``spi…
Bigger Rip with No Dark Energy
Paul H. Frampton, Tomo Takahashi
By studying a modified Friedmann equation which arises in an extension of general relativity which accommodates a time-dependent fundamental length , we consider cosmological…
Correlated Isocurvature Fluctuation in Quintessence and Suppressed CMB Anisotropies at Low Multipoles
Takeo Moroi, Tomo Takahashi
We consider cosmic microwave background (CMB) anisotropy in models with quintessence taking into account of isocurvature fluctuation in the quintessence. It is shown that, if the p…
Cosmic Degeneracy with Dark Energy Equation of State
James L. Crooks, James O. Dunn, Paul H. Frampton +2
We discuss a degeneracy between the geometry of the universe and the dark energy equation of state which exists in the power spectrum of the cosmic microwave background…
The Fate of Dark Energy
Paul H. Frampton, Tomo Takahashi
By studying the present cosmological data, particularly on CMB, SNeIA and LSS, we find that the future fate of the universe, for simple linear models of the dark energy equation-of…
Cosmic Microwave Background Anisotropy in models with Quintessence
Tomo Takahashi
We study the Cosmic Microwave Background (CMB) anisotropies produced by cosine-type quintessence models. In our analysis, effects of the adiabatic and isocurvature fluctuations are…
Isocurvature Fluctuations in Tracker Quintessence Models
Masahiro Kawasaki, Takeo Moroi, Tomo Takahashi
We consider effects of the isocurvature perturbation in the framework of the tracker-type quintessence models. During the inflation, fluctuations in the amplitude of the quintessen…
Cosmic Microwave Background Anisotropy with Cosine-Type Quintessence
Masahiro Kawasaki, Takeo Moroi, Tomo Takahashi
We study the Cosmic Microwave Background (CMB) anisotropies produced by cosine-type quintessence models. In our analysis, effects of the adiabatic and isocurvature fluctuations are…