arXiv subjects

Hiroyuki Tashiro

Publications and source records attributed to Hiroyuki Tashiro.

At least 73 records · Page 4Linked to original sources

arXiv2014

CMB $μ$ distortion from primordial gravitational waves

We propose a new mechanism of generating the $μ$ distortion in cosmic microwave background (CMB) originated from primordial gravitational waves. Such $μ$ distortion is generated by the damping of the temperature anisotropies through the Thomson scattering, even on scales larger than that of Silk damping. This mechanism is in sharp contrast with that from the primordial curvature (scalar) perturbations, in which the temperature anisotropies mainly decay by Silk damping effects. We estimate the size of the $μ$ distortion from the new mechanism, which can be used to constrain the amplitude of primordial gravitational waves on smaller scales independently from the CMB anisotropies, giving more wide-range constraint on their spectral index by combining the amplitude from the CMB anisotropies.

astro-ph.CO↗

arXiv2014

21cm fluctuations from primordial magnetic fields

Recent observations of magnetic fields in intergalactic void regions and in high redshift galaxies may indicate that large scale magnetic fields have a primordial origin. If primordial magnetic fields were present soon after the recombination epoch, they would have induced density fluctuations on the one hand and dissipated their energy into the primordial gas on the other, and thereby significantly alter the thermal history of the Universe. Here we consider both the effects and calculate the brightness temperature fluctuations of the 21cm line using simple Monte Carlo simulations. We find that the fluctuations of the 21cm line from the energy dissipation appear only on very small scales and those from the density fluctuations always dominate on observationally relevant angular scales.

astro-ph.CO↗

arXiv2013

Cosmological signatures of tilted isocurvature perturbations: reionization and 21cm fluctuations

We investigate cosmological signatures of uncorrelated isocurvature perturbations whose power spectrum is blue-tilted with spectral index 2<p<4. Such an isocurvature power spectrum can promote early formation of small-scale structure, notably dark matter halos and galaxies, and may thereby resolve the shortage of ionizing photons suggested by observations of galaxies at high redshifts (z~7-8) but that are required to reionize the universe at z~10. We mainly focus on how the formation of dark matter halos can be modified, and explore the connection between the spectral shape of CMB anisotropies and the reionization optical depth as a powerful probe of a highly blue-tilted isocurvature primordial power spectrum. We also study the consequences for 21cm line fluctuations due to neutral hydrogens in minihalos. Combination of measurements of the reionization optical depth and 21cm line fluctuations will provide complementary probes of a highly blue-tilted isocurvature power spectrum.

astro-ph.CO↗

arXiv2013

The effect of primordial black holes on 21 cm fluctuations

The 21 cm signal produced by non-evaporating primordial black holes (PBHs) is investigated. X-ray photons emitted by accretion of matter onto a PBH ionize and heat the intergalactic medium (IGM) gas near the PBH. Using a simple analytic model, we show that this X-ray heating can produce an observable differential 21 cm brightness temperature. The region of the observable 21 cm brightness temperature can extend to 1-10 Mpc comoving distance from a PBH depending on the PBH mass. The angular power spectrum of 21 cm fluctuations due to PBHs is also calculated. The peak position of the angular spectrum depends on PBH mass, while the amplitude is independent of PBH mass. Comparing this angular power spectrum with the angular power spectrum caused by primordial density fluctuations, it is found that both of them become comparable if $Ω_{\rm PBH} = 10^{-11} (M/10^{3} M_\odot)^{-0.2}$ at $z=30$ and $10^{-12} (M/10^{3} M_\odot)^{-0.2}$ at $z=20$ for the PBH mass from $10 M_\odot $ to $10^8 M_\odot $. Finally we find that the Square Kilometer Array can detect the signal due to PBHs up to $Ω_{\rm PBH}=10^{-5} (M/10^{3} M_\odot)^{-0.2}$ at $z=30$ and $10^{-7} (M/10^{3} M_\odot)^{-0.2}$ at $z=20$ for PBHs with mass from $10^2 M_\odot $ to $10^8 M_\odot $.

astro-ph.CO↗

arXiv2013

High redshift signatures in the 21 cm forest due to cosmic string wakes

Cosmic strings induce minihalo formation in the early universe. The resultant minihalos cluster in string wakes and create a "21 cm forest" against the cosmic microwave background (CMB) spectrum. Such a 21 cm forest can contribute to angular fluctuations of redshifted 21 cm signals integrated along the line of sight. We calculate the root-mean-square amplitude of the 21 cm fluctuations due to strings and show that these fluctuations can dominate signals from minihalos due to primordial density fluctuations at high redshift ($z \gtrsim 10$), even if the string tension is below the current upper bound, $G μ< 1.5 \times 10^{-7}$. Our results also predict that the Square Kilometre Array (SKA) can potentially detect the 21 cm fluctuations due to strings with $G μ\approx 7.5 \times 10^{-8}$ for the single frequency band case and $4.0 \times 10^{-8}$ for the multi-frequency band case.

astro-ph.CO↗

arXiv2013

CMB distortion anisotropies due to the decay of primordial magnetic fields

We investigate the power spectrum of the distortion of Cosmic Microwave Background (CMB) due to the decay of the primordial magnetic fields. It is known that there are two-types of the CMB distortions, so-called μ- and y-types and we find that the signal of the y-type distortion becomes larger than that of the μ-type one. We also discuss cross power spectra between the CMB distortions and the CMB temperature anisotropy, which are naturally generated due to the existence of the primordial magnetic fields. We find that such cross power spectra have small amplitudes compared with the auto-power spectra of the CMB distortions because of the Silk damping effect of the temperature anisotropy. We also investigate the possibility of detecting such signal in the future CMB experiments, including not only absolutely calibrated experiments such as PIXIE but also relatively calibrated experiments such as LiteBIRD and CMBpol.

astro-ph.CO↗

arXiv2013

Constraints on primordial magnetic fields from CMB distortions in the axiverse

Measuring spectral distortions of the cosmic microwave background (CMB) is attracting considerable attention as a probe of high energy particle physics in the cosmological context, since PIXIE and PRISM have recently been proposed. In this paper, CMB distortions due to resonant conversions between CMB photons and light axion like particles (ALPs) are investigated, motivated by the string axiverse scenario which suggests the presence of a plenitude of light axion particles. Since these resonant conversions depend on the strength of primordial magnetic fields, constraints on CMB distortions can provide an upper limit on the product of the photon-ALP coupling constant g and the comoving strength of primordial magnetic fields B. Potentially feasible constraints from PIXIE/PRISM can set a limit g B < 10^{-16} GeV^{-1} nG for ALP mass, m_ϕ< 10^{-14} eV. Although this result is not a direct constraint on g and B, it is significantly tighter than the product of the current upper limits on g and B.

astro-ph.CO↗

arXiv2013

CMB Distortions from Damping of Acoustic Waves Produced by Cosmic Strings

We study diffusion damping of acoustic waves in the photon-baryon fluid due to cosmic strings, and calculate the induced $μ$- and $y$-type spectral distortions of the cosmic microwave background. For cosmic strings with tension within current bounds, their contribution to the spectral distortions is subdominant compared to the distortions from primordial density perturbations.

astro-ph.CO↗

arXiv2013

Galactic Faraday rotation effect on polarization of 21 cm lines from the epoch of reionization

Redshifted 21 cm signal from neutral hydrogen is one of the most competitive probes of the epoch of reionization (EoR). Unpolarized 21 cm radiation acquires a certain level of linear polarization during the EoR due to Thompson scattering. This linear polarization, if measured, could probe important information about the EoR. We study the effect of Galactic Faraday rotation (FR) in the detection of polarization of 21 cm radiation. Since the effect of FR is strong due to the large wavelength nature of redshifted 21 cm signal, the initial E-mode signal carried by 21 cm photons is modified in its entirety. We show that a 99 % accuracy on rotation measure (RM) data is necessary to recover the initial E-mode signal. We conclude that, given the current status of RM observations, it is not possible to retrieve the initial polarization signal of 21 cm from the EoR. However, we are optimistic that it may be possible from the next generation radio surveys.

astro-ph.CO↗

arXiv2013

Cosmological magnetic field correlators from blazar induced cascade

TeV blazars offer an exciting prospect for discovering cosmological magnetic fields and for probing high energy processes, including CP violation, in the early universe. We propose a method for reconstructing both the non-helical and the helical magnetic field correlators using observations of cascade photons from TeV blazars.

astro-ph.CO↗

arXiv2013

Constraining primordial non-Gaussianity with CMB-21cm cross-correlations?

We investigate the effect of primordial non-Gaussianity on the cross-correlation between the CMB anisotropies and the 21 cm fluctuations from the epoch of reionization. We assume an analytic reionization model and an ionization fraction with $f_{\rm NL}$ induced scale dependent bias. We estimate the angular power spectrum of the cross-correlation of the CMB and 21 cm. In order to evaluate the detectability, the signal-to-noise (S/N) ratio for only a single redshift slice is also calculated for current and future observations, such as CMB observations by Planck satellite and 21 cm observations by Omniscope. The existence of the $f_{\rm NL}$ increases the signal of the cross-correlation at large scales and the amplification does not depend on the reionization parameters in our reionization model. However the cosmic variance is significant on such scales and the S/N ratio is suppressed. The obtained S/N ratio is 2.8 (2.4) for $f_{\rm NL}=10$ (100) in our fiducial reionization model. Our work suggests in the absence of significant foregrounds and systematics, the auto-correlations of 21 cm is a better probe of $f_{\rm NL}$ than the cross-correlations (as expected since it depends on $b^2$), while the cross-correlations contain only one factor of $b$. Nevertheless, it is interesting to examine the cross-correlations between 21 cm and CMB, as the signal-to-noise ratio is not negligible and it is more likely we can rid ourselves of systematics and foregrounds that are common to both CMB and 21 cm experiments than completely clean 21 cm of all of the possible foregrounds and systematics in large scales.

astro-ph.CO↗

arXiv2013

21 cm Angular Spectrum of Cosmic String Loops

The 21 cm signatures induced by moving cosmic string loops are investigated. Moving cosmic string loops seed filamentary nonlinear objects. We analytically evaluate the differential 21 cm brightness temperature from these objects. We show that the brightness temperature reaches 200 mK for a loop whose tension is about the current upper limit, $Gμ\sim10^{-7}$. We also calculate the angular power spectrum, assuming scaling in loop distribution. We find that the angular power spectrum for $Gμ>10^{-8}$ at $z=30$ or $Gμ>10^{-10}$ at $z=20$ can dominate the spectrum of the primordial density fluctuations. Finally we show that a future SKA-like observation has the potential to detect the power spectrum due to loops with $Gμ=10^{-8}$ at $z=20$.

astro-ph.CO↗

arXiv2013

Background reionization history from omniscopes

The measurements of the 21-cm brightness temperature fluctuations from the neutral hydrogen at the Epoch of Reionization (EoR) should inaugurate the next generation of cosmological observables. In this respect, many works have concentrated on the disambiguation of the cosmological signals from the dominant reionization foregrounds. However, even after perfect foregrounds removal, our ignorance on the background reionization history can significantly affect the cosmological parameter estimation. In particular, the interdependence between the hydrogen ionized fraction, the baryon density and the optical depth to the redshift of observation induce nontrivial degeneracies between the cosmological parameters that have not been considered so far. Using a simple, but consistent reionization model, we revisit their expected constraints for a futuristic giant 21-cm omniscope by using for the first time Markov Chain Monte Carlo (MCMC) methods on multiredshift full sky simulated data. Our results agree well with the usual Fisher matrix analysis on the three-dimensional flat sky power spectrum but only when the above-mentioned degeneracies are kept under control. In the opposite situation, Fisher results can be inaccurate. We show that these conditions can be fulfilled by combining cosmic microwave background measurements with multiple observation redshifts probing the beginning of EoR. This allows a precise reconstruction of the total optical depth, reionization duration and maximal spin temperature. Finally, we discuss the robustness of these results in presence of unresolved ionizing sources. Although most of the standard cosmological parameters remain weakly affected, we find a significant degradation of the background reionization parameter estimation in presence of nuisance ionizing sources.

astro-ph.CO↗

arXiv2012

CMB Distortions from Superconducting Cosmic Strings

We reconsider the effect of electromagnetic radiation from superconducting strings on cosmic microwave background (CMB) mu- and y-distortions and derive present (COBE-FIRAS) and future (PIXIE) constraints on the string tension, mu_s, and electric current, I. We show that absence of distortions of the CMB in PIXIE will impose strong constraints on mu_s and I, leaving the possibility of light strings (G mu_s < 10^{-18}) or relatively weak currents (I < 10 TeV).

astro-ph.CO↗

arXiv2012

Cosmological constraints from CMB distortion

We examine bounds on adiabatic and isocurvature density fluctuations from $μ$-type spectral distortions of the cosmic microwave background (CMB). Studies of such distortion are complementary to CMB measurements of the spectral index and its running, and will help to constrain these parameters on significantly smaller scales. We show that a detection on the order of $μ\sim 10^{-7}$ would strongly be at odds with the standard cosmological model of a nearly scale-invariant spectrum of adiabatic perturbations. Further, we find that given the current CMB constraints on the isocurvature mode amplitude, a nearly scale-invariant isocurvature mode (common in many curvaton models) cannot produce significant $μ$-distortion. Finally, we show that future experiments will strongly constrain the amplitude of the isocurvature modes with a highly blue spectrum as predicted by certain axion models.

astro-ph.CO↗

arXiv2012

Chiral Effects and Cosmic Magnetic Fields

In the presence of cosmic chiral asymmetry, chiral-vorticity and chiral-magnetic effects can play an important role in the generation and evolution of magnetic fields in the early universe. We include these chiral effects in the magnetic field equations and find solutions under simplifying assumptions. Our numerical and analytical results show the presence of an attractor solution in which chiral effects produce a strong, narrow, Gaussian peak in the magnetic spectrum and the magnetic field becomes maximally helical. The peak in the spectrum shifts to longer length scales and becomes sharper with evolution. We also find that the dynamics may become non-linear for certain parameters, pointing to the necessity of a more complete analysis.

astro-ph.CO↗

arXiv2012

Constraints on Superconducting Cosmic Strings from Early Reionization

Electromagnetic radiation from superconducting cosmic string loops can reionize neutral hydrogen in the universe at very early epochs, and affect the cosmic microwave background (CMB) temperature and polarization correlation functions at large angular scales. We constrain the string tension and current using WMAP7 data, and compare with earlier constraints that employed CMB spectral distortions. Over a wide range of string tensions, the current on the string has to be less than 10^7 GeV.

astro-ph.CO↗

arXiv2012

Primordial magnetic fields with X-ray and S-Z cluster survey

The effect of primordial magnetic fields on X-ray or S-Z galaxy cluster survey is investigated. After recombination, the primordial magnetic fields generate additional density fluctuations. Such density fluctuations enhance the number of galaxy clusters. Taking into account the density fluctuations generated by primordial magnetic fields, we calculate the number of galaxy clusters based on the Press-Schechter formalism. Comparing with the results of Chandra X-ray galaxy cluster survey, we found that the existence of primordial magnetic fields with amplitude larger than 10 nGuass would be inconsistent. Moreover, we show that S-Z cluster surveys also have a sensitivity to constrain primordial magnetic fields. Especially SPT S-Z cluster survey has a potential to constrain the primordial magnetic fields with several nano Gauss.

astro-ph.CO↗