Kiyotomo Ichiki

Cosmological Magnetic Field: A Fossil of Density Perturbations in the Early Universe

The origin of the substantial magnetic fields that are found in galaxies and on even larger scales, such as in clusters of galaxies, is yet unclear. If the second-order couplings between photons and electrons are considered, then cosmological density fluctuations, which explain the large-scale structure of the universe, can also produce magnetic fields on cosmological scales before the epoch of recombination. By evaluating the power spectrum of these cosmological magnetic fields on a range of scales, we show here that magnetic fields of 10–18.1 gauss are generated at a 1-megaparsec scale and can be even stronger at smaller scales (10–14.1 gauss at 10 kiloparsecs). These fields are large enough to seed magnetic fields in galaxies and may therefore have affected primordial star formation in the early universe.

Gravitational wave spectrum induced by primordial scalar perturbations

We derive the complete spectrum of gravitational waves induced by primordial scalar perturbations ranging over all observable wavelengths. This scalar-induced contribution can be computed directly from the observed scalar perturbations and general relativity and is, in this sense, independent of the cosmological model for generating the perturbations. The spectrum is scale invariant on small scales, but has an interesting scale dependence on large and intermediate scales, where scalar-induced gravitational waves do not redshift and are hence enhanced relative to the background density of the Universe. This contribution to the tensor spectrum is significantly different in form from the direct model-dependent primordial tensor spectrum and, although small in magnitude, it dominates the primordial signal for some cosmological models. We confirm our analytical results by direct numerical integration of the equations of motion.

Observational Constraints on Dark Radiation in Brane Cosmology

Center for Astrophysics, Department of Physics,University of Notre Dame, Notre Dame, IN 46556(Dated: February 5, 2008)We analyze the observational constraints on brane-world cosmology whereby the universe is de-scribed as a three-brane embedded in a five-dimensional anti-de Sitter space. In this brane-universecosmology, the Friedmann equation is modified by the appearance of extra terms which derive fromexistence of the extra dimensions. In the present work we concentrate on the “dark radiation”term which diminishes with cosmic scale factor as a

Magnetic field generation from cosmological perturbations.

In this Letter, we discuss the generation of magnetic field from cosmological perturbations. We consider the evolution of three component plasma (electron, proton, and photon) evaluating the collision term between electrons and photons up to the second order. The collision term is shown to induce electric current, which then generates magnetic field. There are three contributions, two of which can be evaluated from the first-order quantities, while the other one is fluid vorticity, which is purely second order. We estimate the magnitudes of the former contributions and show that the amplitude of the produced magnetic field is about approximately 10(-19) G at 10 Mpc comoving scale at the decoupling. Compared to astrophysical and inflationary mechanisms for seed-field generation, our study suffers from much less ambiguities concerning unknown physics and/or processes.

Probing polarization states of primordial gravitational waves with cosmic microwave background anisotropies

We discuss the polarization signature of primordial gravitational waves imprinted in cosmic microwave background (CMB) anisotropies. The high-energy physics motivated by superstring theory or M-theory generically yields parity violating terms, which may produce a circularly polarized gravitational wave background (GWB) during inflation. In contrast to the standard prediction of inflation with unpolarized GWB, circularly polarized GWB generates non-vanishing TB- and EB-mode power spectra of CMB anisotropies. We evaluate the TB- and EB-mode power spectra taking into account the secondary anisotropies induced by the reionization and investigate the dependence of cosmological parameters. We then discuss current constraints on the circularly polarized GWB from large angular scales () of the three year WMAP data. Prospects for future CMB experiments are also investigated based on a Monte Carlo analysis of parameter estimation, showing that the circular polarization degree, e, which is the asymmetry of the tensor power spectra between right- and left-handed modes normalized by the total amplitude, can be measured down to .

LOWER BOUNDS ON INTERGALACTIC MAGNETIC FIELDS FROM SIMULTANEOUSLY OBSERVED GeV-TeV LIGHT CURVES OF THE BLAZAR Mrk 501

We derive lower bounds on intergalactic magnetic fields (IGMFs) from upper limits on the pair echo emission from the blazar Mrk 501, that is, delayed GeV emission from secondary e{sup -}e{sup +} pairs produced via interactions of primary TeV gamma rays with the cosmic infrared background. Utilizing only simultaneous GeV-TeV light curves observed by VERITAS, MAGIC, and the Fermi Large Area Telescope during a multiwavelength campaign in 2009 that included a TeV flare, bounds are deduced on the IGMF strength of B {approx}> 10{sup -20} G at the 90% confidence level for a field coherence length of 1 kpc. Since our analysis is based firmly on the observational data alone and is nearly free of assumptions concerning the primary TeV flux in unobserved periods or spectral bands, our evaluation of the pair echo flux is conservative and the evidence for a non-zero IGMF is more robust compared to previous studies.

Constraints on neutrino masses from weak lensing

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 structure. We use the latest measurements of WL correlations, the Canada-France-Hawaii Telescope Legacy Survey data, to explore constraints on neutrino masses. We find that, while the WL data alone cannot place a stringent limit on neutrino masses due to parameter degeneracies, the constraint can be significantly improved when combined with other cosmological probes, such as the WMAP 5-year data (WMAP5) and the distance measurements of type-Ia supernovae (SNe) and baryon acoustic oscillations (BAO). The upper bounds on the sum of neutrino masses are , 0.76, and 0.54 eV (95% CL) for WL+WMAP5, WMAP5+SNe+BAO, and WL+WMAP5+SNe+BAO, respectively, assuming a flat {lambda}CDM model with finite-mass neutrinos. In deriving these constraints, our analysis includes the non-Gaussian covariances of the WL correlation functions to properly take into account significant correlations between different angles.

New constraints on the primordial magnetic field

We present the newest statistical and numerical analysis of the matter and cosmic microwave background power spectrum with effects of the primordial magnetic field (PMF) included. New limits to the PMF strength and power spectral index are obtained based upon the accumulated data for both the matter and CMB power spectra on small angular scales. We find that a maximum develops in the probability distribution for a magnitude of the PMF of

Cosmological constraints on Newton's constant

|{B}_{\ensuremath{\lambda}}|=0.85\ifmmode\pm\else\textpm\fi{}1.25(\ifmmode\pm\else\textpm\fi{}1\ensuremath{\sigma})\text{ }\text{ }\mathrm{nG}

The impact of massive neutrinos on the abundance of massive clusters

on a comoving scale of at 1 Mpc, corresponding to upper limits of