arXiv subjects

Toshiya Namikawa

Publications and source records attributed to Toshiya Namikawa.

86 records · Page 5Linked to original sources

arXiv2016

CMB Lensing Bispectrum from Nonlinear Growth of the Large Scale Structure

We discuss detectability of the nonlinear growth of the large-scale structure in the cosmic microwave background (CMB) lensing. Lensing signals involved in CMB anisotropies have been measured from multiple CMB experiments, such as Atacama Cosmology Telescope (ACT), Planck, POLARBEAR, and South Pole Telescope (SPT). Reconstructed lensing signals are useful to constrain cosmology via their angular power spectrum, while detectability and cosmological application of their bispectrum induced by the nonlinear evolution are not well studied. Extending the analytic estimate of the galaxy lensing bispectrum presented by Takada and Jain (2004) to the CMB case, we show that even near term CMB experiments such as Advanced ACT, Simons Array and SPT3G could detect the CMB lensing bispectrum induced by the nonlinear growth of the large-scale structure. In the case of the CMB Stage-IV, we find that the lensing bispectrum is detectable at $\gtrsim 50σ$ statistical significance. This precisely measured lensing bispectrum has rich cosmological information, and could be used to constrain cosmology, e.g., the sum of the neutrino masses and the dark-energy properties.

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arXiv2016

Constraining higher-order parameters for primordial non-Gaussianities from power spectra and bispectra of imaging survey

We investigate the statistical power of higher-order statistics and cross-correlation statistics to constrain the primordial non-Gaussianity from the imaging surveys. In particular, we consider the local-type primordial non- Gaussianity and discuss how well one can tightly constrain the higher-order non-Gaussian parameters ($g_{\rm NL}$ and $τ_{\rm NL}$) as well as the leading order parameter $f_{\rm NL}$ from the halo/galaxy clustering and weak gravitational lensing measurements. Making use of a strong scale-dependent behavior in the galaxy/halo clustering, Fisher matrix analysis reveals that the bispectra can break the degeneracy between non-Gaussian parameters ($f_{\rm NL}$, $g_{\rm NL}$ and $τ_{\rm NL}$) and this will give simultaneous constraints on those three parameters. The combination of cross-correlation statistics further improves the constraints by factor of 2. As a result, upcoming imaging surveys like the Large Synoptic Survey Telescope have the potential to improve the constraints on the primordial non-Gaussianity much tighter than those obtained from the CMB measurement by Planck, giving us an opportunity to test the single-sourced consistency relation, $τ_{\rm NL} \ge (36/25) f_{\rm NL}^2$.

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arXiv2016

Anisotropies of gravitational-wave standard sirens as a new cosmological probe without redshift information

Gravitational waves (GWs) from compact binary stars at cosmological distances are promising and powerful cosmological probes, referred to as the GW standard sirens. With future GW detectors, we will be able to precisely measure source luminosity distances out to a redshift $z\sim5$. To extract cosmological information, previously proposed cosmological studies using the GW standard sirens rely on source redshift information obtained through an extensive electromagnetic follow-up campaign. However, the redshift identification is typically time consuming and rather challenging. Here, we propose a novel method for cosmology with the GW standard sirens free from the redshift measurements. Utilizing the anisotropies of the number density and luminosity distances of compact binaries originated from the large-scale structure, we show that, once GW observations will be well established in the future, (i) these anisotropies can be measured even at very high redshifts ($z\geq 2$), where the identification of the electromagnetic counterpart is difficult, (ii) the expected constraints on the primordial non-Gaussianity with the Einstein Telescope would be comparable to or even better than the other large-scale structure probes at the same epoch, and (iii) the cross-correlation with other cosmological observations is found to have high-statistical significance, providing additional cosmological information at very high redshifts.

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arXiv2016

Delensing Cosmic Microwave Background B-modes with the Square Kilometre Array Radio Continuum Survey

We explore the potential use of the Radio Continuum (RC) survey conducted by the Square Kilometre Array (SKA) to remove (delens) the lensing-induced B-mode polarization and thus enhance future cosmic microwave background (CMB) searches for inflationary gravitational waves. Measurements of large-scale B-modes of the CMB are considered to be the best method for probing gravitational waves from the cosmic inflation. Future CMB experiments will, however, suffer from contamination by non-primordial B-modes, one source of which is the lensing B-modes. Delensing, therefore, will be required for further improvement of the detection sensitivity for gravitational waves. Analyzing the use of the two-dimensional map of galaxy distribution provided by the SKA RC survey as a lensing mass tracer, we find that joint delensing using near future CMB experiments and the SKA phase 1 will improve the constraints on the tensor-to-scalar ratio by more than a factor of $\sim 2$ compared to those without the delensing analysis. Compared to the use of CMB data alone, the inclusion of the SKA phase 1 data will increase the significance of the constraints on the tensor-to-scalar ratio by a factor $1.2$-$1.6$. For LiteBIRD combined with a ground-based experiment such as Simons Array and Advanced ACT, the constraint on the tensor-to-scalar ratio when adding SKA phase 2 data is improved by a factor of $2.3$-$2.7$, whereas delensing with CMB data alone improves the constraints by only a factor $1.3$-$1.7$. We conclude that the use of SKA data is a promising method for delensing upcoming CMB experiments such as LiteBIRD.

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arXiv2015

Non-Gaussian Structure of B-mode Polarization after Delensing

The B-mode polarization of the cosmic microwave background on large scales has been considered as a probe of gravitational waves from the cosmic inflation. Ongoing and future experiments will, however, suffer from contamination due to the B-modes of non-primordial origins, one of which is the lensing induced B-mode polarization. Subtraction of the lensing B-modes, usually referred to as delensing, will be required for further improvement of detection sensitivity of the gravitational waves. In such experiments, knowledge of statistical properties of the B-modes after delensing is indispensable to likelihood analysis particularly because the lensing B-modes are known to be non-Gaussian. In this paper, we study non-Gaussian structure of the delensed B-modes on large scales, comparing them with those of the lensing B-modes. In particular, we investigate the power spectrum correlation matrix and the probability distribution function (PDF) of the power spectrum amplitude. Assuming an experiment in which the quadratic delensing is an almost optimal method, we find that delensing reduces correlations of the lensing B-mode power spectra between different multipoles, and that the PDF of the power spectrum amplitude is well described as a normal distribution function with a variance larger than that in the case of a Gaussian field. These features are well captured by an analytic model based on the 4th order Edgeworth expansion. As a consequence of the non-Gaussianity, the constraint on the tensor-to-scalar ratio after delensing is degraded within approximately a few percent, which depends on the multipole range included in the analysis.

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arXiv2015

Full-sky lensing reconstruction of gradient and curl modes from CMB maps

We present a method of lensing reconstruction on the full sky, by extending the optimal quadratic estimator proposed by Okamoto & Hu (2003) to the case including the curl mode of deflection angle. The curl mode is induced by the vector and tensor metric perturbations, and the reconstruction of the curl mode would be a powerful tool to not only check systematics in the estimated gradient mode but also probe any vector and tensor sources. We find that the gradient and curl modes can be reconstructed separately, thanks to the distinctive feature in the parity symmetry between the gradient and curl modes. We compare our estimator with the flat-sky estimator proposed by Cooray et al (2005). Based on the new formalism, the expected signal-to-noise ratio of the curl mode produced by the primordial gravitational-waves and a specific model of cosmic strings are estimated, and prospects for future observations are discussed.

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arXiv2015

Future detectability of gravitational-wave induced lensing from high-sensitivity CMB experiments

We discuss the future detectability of gravitational-wave induced lensing from high-sensitivity cosmic microwave background (CMB) experiments. Gravitational waves can induce a rotational component of the weak-lensing deflection angle, usually referred to as the curl mode, which would be imprinted on the CMB maps. Using the technique of reconstructing lensing signals involved in CMB maps, this curl mode can be measured in an unbiased manner, offering an independent confirmation of the gravitational waves complementary to the B-mode polarization experiments. Based on the Fisher matrix analysis, we first show that with the noise levels necessary to confirm the consistency relation for the primordial gravitational waves, the future CMB experiments will be able to detect the gravitational-wave induced lensing signals. For a tensor-to-scalar ratio of $r < 0.1$, even if the consistency relation is difficult to confirm with a high significance, the gravitational-wave induced lensing would be detected at more than $3\,σ$ significance level. Further, we point out that high-sensitivity experiments will be also powerful to constrain the gravitational waves generated after the recombination epoch. Compared to the B-mode polarization, the curl mode is particularly sensitive to gravitational waves generated at low redshifts ($z < 10$) with a low frequency ($k < 10^{-3}$ Mpc$^{-1}$), and it could give a much tighter constraint on their energy density $Ω_{\rm GW}$ by more than three orders of magnitude.

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arXiv2014

Lensing reconstruction from a patchwork of polarization maps

The lensing signals involved in CMB polarization maps have already been measured with ground-based experiments such as SPTpol and POLARBEAR, and would become important as a probe of cosmological and astrophysical issues in the near future. Sizes of polarization maps from ground-based experiments are, however, limited by contamination of long wavelength modes of observational noise. To further extract the lensing signals, we explore feasibility of measuring lensing signals from a collection of small sky maps each of which is observed separately by a ground-based large telescope, i.e., lensing reconstruction from a patchwork map of large sky coverage organized from small sky patches. We show that, although the B-mode power spectrum obtained from the patchwork map is biased due to baseline uncertainty, bias on the lensing potential would be negligible if the B-mode on scales larger than the blowup scale of $1/f$ noise is removed in the lensing reconstruction. As examples of cosmological applications, we also show 1) the cross-correlations between the reconstructed lensing potential and full-sky temperature/polarization maps from satellite missions such as PLANCK and LiteBIRD, and 2) the use of the reconstructed potential for delensing B-mode polarization of LiteBIRD observation.

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arXiv2014

Cosmology from weak lensing of CMB

The weak lensing effect on the cosmic microwave background (CMB) induces distortions in spatial pattern of CMB anisotropies, and statistical properties of CMB anisotropies become a weakly non-Gaussian field. We first summarize the weak lensing effect on the CMB (CMB lensing) in the presence of scalar, vector and tensor perturbations. Then we focus on the lensing effect on CMB statistics and methods to estimate deflection angles and their power spectrum. We end by summarizing recent observational progress and future prospect.

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arXiv2013

Constraining cosmic string parameters with curl mode of CMB lensing

We present constraints on a cosmic string network with a measurement of weak gravitational lensing from CMB temperature map. The cosmic string network between observer and last scattering surface of CMB photons generates vector and/or tensor metric perturbations, and the deflection of CMB photons by these gravitational fields has curl mode which is not produced by the scalar metric perturbations. In this paper, we use the power spectrum of curl mode obtained from Planck to constrain the string tension, Gμ, and the reconnection probability, P. In demonstrating the parameter constraints with Planck curl mode, we also measure the lensing power spectrum from the Atacama Cosmology Telescope (ACT) 2008 season data, which have better angular resolution with lower instrumental noise on a much smaller chunk of the sky. Assuming P=1, the upper bound on tension is Gμ= 6.6\times 10^{-5} with 2σ(95% C.L.), using curl mode from Planck, which is weaker than that from the small-scale temperature power spectrum. For small values of P, however, the constraint from curl mode becomes tighter compared to that from temperature power spectrum. For P\lsim 10^{-2}, we obtain the constraint on the combination of the string parameters as GμP^{-1}\leq 3.4\times 10^{-5} at more than 2σ(95% C.L.).

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arXiv2013

Full-sky formulae for weak lensing power spectra from total angular momentum method

We systematically derive full-sky formulae for the weak lensing power spectra generated by scalar, vector and tensor perturbations from the total angular momentum (TAM) method. Based on both the geodesic and geodesic deviation equations, we first give the gauge-invariant expressions for the deflection angle and Jacobi map as observables of the CMB lensing and cosmic shear experiments. We then apply the TAM method, originally developed in the theoretical studies of CMB, to a systematic derivation of the angular power spectra. The TAM representation, which characterizes the total angular dependence of the spatial modes projected along a line-of-sight, can carry all the information of the lensing modes generated by scalar, vector, and tensor metric perturbations. This greatly simplifies the calculation, and we present a complete set of the full-sky formulae for angular power spectra in both the E-/B-mode cosmic shear and gradient-/curl-mode lensing potential of deflection angle. Based on the formulae, we give illustrative examples of non-vanishing B-mode cosmic shear and curl-mode of deflection angle in the presence of the vector and tensor perturbations, and explicitly compute the power spectra.

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arXiv2013

Bias-Hardened CMB Lensing

We present new methods for lensing reconstruction from CMB temperature fluctuations which have smaller mean-field and reconstruction noise bias corrections than current lensing estimators, with minimal loss of signal-to-noise. These biases are usually corrected using Monte Carlo simulations, and to the extent that these simulations do not perfectly mimic the underlying sky there are uncertainties in the bias corrections. The bias-hardened estimators which we present can have reduced sensitivity to such uncertainties, and provide a desirable cross-check on standard results. To test our approach, we also show the results of lensing reconstruction from simulated temperature maps given on 100 deg^2, and confirm that our approach works well to reduce biases for a typical masked map in which 70 square masks each having 10 arcminute on a side exist, covering 2% of the simulated map, which is similar to the masks used in the current SPT lensing analysis.

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arXiv2012

Weak lensing generated by vector perturbations and detectability of cosmic strings

We study the observational signature of vector metric perturbations through the effect of weak gravitational lensing. In the presence of vector perturbations, the non-vanishing signals for B-mode cosmic shear and curl-mode deflection angle, which have never appeared in the case of scalar metric perturbations, naturally arise. Solving the geodesic and geodesic deviation equations, we drive the full-sky formulas for angular power spectra of weak lensing signals, and give the explicit expressions for E-/B-mode cosmic shear and gradient-/curl-mode deflection angle. As a possible source for seeding vector perturbations, we then consider a cosmic string network, and discuss its detectability from upcoming weak lensing and CMB measurements. Based on the formulas and a simple model for cosmic string network, we calculate the angular power spectra and expected signal-to-noise ratios for the B-mode cosmic shear and curl-mode deflection angle. We find that the weak lensing signals are enhanced for a smaller intercommuting probability of the string network, $P$, and they are potentially detectable from the upcoming cosmic shear and CMB lensing observations. For $P\sim 10^{-1}$, the minimum detectable tension of the cosmic string will be down to $Gμ\sim 5\times 10^{-8}$. With a theoretically inferred smallest value $P\sim 10^{-3}$, we could even detect the string with $Gμ\sim 5\times 10^{-10}$.

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arXiv2011

Magnification effect on the detection of primordial non-Gaussianity from photometric surveys

We present forecast results for constraining the primordial non-Gaussianity from photometric surveys through a large-scale enhancement of the galaxy clustering amplitude. In photometric surveys, the distribution of observed galaxies at high redshifts suffers from the gravitational-lensing magnification, which systematically alters the number density for magnitude-limited galaxy samples. We estimate size of the systematic bias in the best-fit cosmological parameters caused by the magnification effect, particularly focusing on the primordial non-Gaussianity. For upcoming deep and/or wide photometric surveys like HSC, DES and LSST, the best-fit value of the non-Gaussian parameter, fNL, obtained from the galaxy count data is highly biased, and the true values of fNL would typically go outside the 3-sigma error of the biased confidence region, if we ignore the magnification effect in the theoretical template of angular power spectrum. The additional information from cosmic shear data helps not only to improve the constraint, but also to reduce the systematic bias. As a result, the size of systematic bias on fNL would become small enough compared to the expected 1-sigma error for HSC and DES, but it would be still serious for deep surveys with z_m > 1.5, like LSST. Tomographic technique improves the constraint on fNL by a factor of 2-3 compared to the one without tomography, but the systematic bias would increase.

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