Research

The reionization is an inhomogeneous process but its knowledge is very limited. CMB photons are scattered by the electrons produced by the reionization, and the statistics of the CMB anisotropies are altered. Namikawa (2017b) provides new constraints on the anisotropies of the reionization by measuring the trispectrum of the Planck temperature fluctuations. The constraint presented by this work implies that the B modes from the patchy reionization is at most several percent of the lensing B modes in power. Thus, the patchy reionization contamination is not significant if we wan to detect the primordial gravitational waves with r=0.01 which is the goal of the ongoing and near term CMB experiments.

Topological defects such as cosmic strings could be produced by the spontaneous symmetry breaking at the early universe. Constraints on cosmic strings give an informative picture of the early universe and have implications for particle physics. While cosmic strings have been constrained by many observations, Namikawa et al. (2012) proposes a new way to constrain cosmic strings using weak lensing measurements. Observables in weak lensing measurements (cosmic shear, deflection angle) are in general decomposed into two components by the parity symmetry (called E/B modes, or gradient/curl modes). The curl mode is not generated by the scalar perturbation at linear theory but could be produced by, e.g., the cosmic string. We first formulate the fullsky curl-mode estimator in the same way as in the gradient mode as a quadratic in CMB anisotropies. Then we discuss future prospects of constraining cosmic strings. Namikawa et al. (2013b) constrains for the first time the tension and reconnection probability P of the cosmic string using curl modes from 1) the Planck 2013 data release and 2) the ACT 2008 season data. At P<10^{-2}, our constraint becomes tighter than that from the Planck temperature power spectrum

The lensing-mass map reconstructed from CMB observation is a powerful probe in cosmology. However, not only ``precision'' but also ``accuracy'' should be seriously taken into account in ongoing and near future lensing measurements. Namikawa et al. (2013a) proposes new methods for the lensing reconstruction from CMB temperature fluctuations which are less sensitive than standard estimators to several potential experimental systematics. These estimators are more reliable than standard estimators when applied to realistic CMB observations. The estimator for the mean-field bias is useful for a cross-check on standard results. Namikawa & Takahashi (2014) generalized the estimators to the case with polarization data.

Inflationary scenario predicts the presence of the GWs originated from quantum fluctuations. Detection of the primordial GWs is known as a smoking gun of inflation. Measurements of the rotational pattern in the CMB polarization (B-mode) on angular scales larger than a few dozen arc-minutes have been considered as the best avenue to probe the primordial GWs. However, on large angular scales the GW B modes are expected to be dominated over 1) Galactic foreground, and 2) Gravitational lensing. These contaminants are needed to be removed in the future CMB analysis. I have studied to realize the removal of lensing (delensing). For example, LiteBIRD plans to observe the B mode in nearly fullsky, and delensing requires a fullsky lensing observation. To make a precise lensing template, the experiment should have high-angular resolution to resolve small-scale fluctuations perturbed by lensing, i.e., LiteBIRD should be a very large satellite. However, such large satellite is too expensive to realize, and a more economical way is needed to measure both the large-scale B-modes and lensing map. Namikawa & Nagata (2014) discuss possibility of constructing the lensing template from a patchwork of polarization maps obtained from ground-based experiments and find that the lensing template can be reconstructed in an unbiased manner. Namikawa & Nagata (2015) showed that the delensed B mode is still nearly a Gaussian field for Simons Array like experiment. On the other hand, the delensed B-modes obtained from higher-sensitivity experiments have still non-Gaussian behavior. Namikawa (2017b) presents a new way of delensing B modes of the CMB using a lensing potential reconstructed from the same realization of the CMB polarization (CMB internal delensing). Compared to the previous methods, we find that the RD estimator corrects the biases without degradation of the delensing efficiency.

In ongoing and future CMB experiments, alternative way of the delensing is to use lensing-mass tracers at high redshifts such as galaxy/haloes. Namikawa et al. (2016b) found that radio galaxies observed by the Square Kilometre Array Radio Continuum Survey is quite useful for LiteBIRD delensing. The lensing B modes are known to be a non-Gaussian field. If the delensed B modes also deviate from a Gaussian field, the statistical significance of detecting the GW B modes is degraded compared to the case if the delensed B mode is a Gaussian field. Since the studies of delensing have assumed a Gaussian field of lensing potential and/or mass tracers, Namikawa & Takahashi (2018) found that the impact of the nonlinear evolution of LSS on delensing is not significant.

The nonlinear growth of the LSS produces bispectrum and higher-order correlations of the lensing potential. Extracting this signals provides additional information on dark energy, dark matter and massive neutrinos. Namikawa et al. (2019) tests the fitting formula of the matter bispectrum by measuring the matter/lensing bispectra from the ray-tracying simulation. We found that the accuracy of the fitting formula depends on scale and configurations. For example, the fitting formula does not accurately predict the squeed bispectrum from the simulation.

The power spectra of the CMB and galaxy lensing are useful to constrain cosmology. Namikawa et al. (2010) shows a forecast study for the joint lensing analysis between Subaru Hyper Sprime-Cam (HSC) and ACTPol. We then point out that the constraint on massive neutrinos suffers from the degeneracy between the matter energy density and sum of neutrino masses. Namikawa et al. (2011) presents forecast studies for constraining the primordial non-Gaussianity from photometric surveys through a modification of the galaxy clustering amplitudes. I particularly focused on the magnification effect in estimating the non-Gaussian parameter, fNL. I found that the magnification effect in the galaxy clustering could mimic the modification by the primordial non-Gaussianity, and could cause a significant systematic bias in estimating fNL, especially for deep imaging surveys such as Subaru HSC and Large Synoptic Survey Telescope (LSST). However, if weak lensing measurements are further combined, the bias could be reduced well within the 1 sigma statistical error.

The nonlinear growth of the LSS produces bispectrum and higher-order correlations of the lensing potential. Extracting this signals provides additional information on dark energy, dark matter and massive neutrinos. Also, the impact of the nonlinear growth should be taken into account in the analysis of CMB lensing measurements. Namikawa (2016) shows that bispectrum of the CMB lensing potential is detectable from ongoing and near term CMB experiments. Namikawa et al. (2018) discusses constraints on gravity theories via the CMB lensing bi-spectrum.

The primordial GWs lead to the gravitational lensing effect on CMB / galaxies and produce a rotational pattern in the deflection vector / galaxy shear (curl mode). The curl mode therefore can be a probe of the primordial GWs. Namikawa et al. (2015) showed that, using the optimal (maximum likelihood) method, the GW-induced lensing is detectable in CMB experiments with noise levels necessary to confirm the consistency relation of the primordial GWs. Further, we point out that the curl mode is useful to constrain GWs generated after the recombination epoch. Namikawa et al. (2019) constrains the stochastic GW background at Mpc scales using the CMB temperature, B-mode and lensing curl mode data. This constraint is the most stringent to date at Mpc scale.

Compact binary stars at cosmological distances are promising sources for GWs, and these are thought to be powerful cosmological probes, referred to as the GW standard sirens. Since the redshift identification for each GW source at high redshift is challenging, Namikawa et al. (2016a) presented a new method for GW standard sirens without redshift measurements. The first detection of the GW event from the black-hole (BH) binary indicates a higher merger rate, and enlarges future prospects for GW cosmology using BH binaries. Namikawa et al. (2016c) shows that, even with the second-generation GW detector network, we can test whether BH binaries are a good tracer of the dark matter inhomogeneities. This test is important not only to understand the properties of their host galaxy, but also to show the possibility of applying GW sources to cosmology in the future.