arXiv subjects

Siavash Yasini

Publications and source records attributed to Siavash Yasini.

6 recordsLinked to original sources

arXiv2021

Effects of boosting on extragalactic components: methods and statistical studies

In this work we examine the impact of our motion with respect to the CMB rest frame on statistics of CMB maps by examining the one-, two-, three- and four- point statistics of simulated maps of the CMB and Sunyaev-Zeldovich (SZ) effects. We validate boosting codes by comparing their outcomes for temperature and polarization power spectra up to $\ell \simeq 6000$. We derive and validate a new analytical formula for the computation of the boosted power spectrum of a signal with a generic frequency dependence. As an example we show how this increases the boosting correction to the power spectrum of CMB intensity measurements by $\sim 30\%$ at 150 GHz. We examine the effect of boosting on thermal and kinetic SZ power spectra from semianalytical and hydrodynamical simulations; the boosting correction is generally small for both simulations, except when considering frequencies near the tSZ null. For the non-Gaussian statistics, in general we find that boosting has no impact with two exceptions. We find that, whilst the statistics of the CMB convergence field are unaffected, quadratic estimators that are used to measure this field can become biased at the $O(1)\%$ level by boosting effects. We present a simple modification to the standard estimators that removes this bias. Second, bispectrum estimators can receive a systematic bias from the Doppler induced quadrupole when there is anisotropy in the sky -- in practice this anisotropy comes from masking and inhomogenous noise. This effect is unobservable and already removed by existing analysis methods.

astro-ph.CO↗

arXiv2020

Footprints of Doppler and Aberration Effects in CMB Experiments: Statistical and Cosmological Implications

In the frame of the Solar System, the Doppler and aberration effects cause distortions in the form of mode couplings in the cosmic microwave background (CMB) temperature and polarization power spectra and hence impose biases on the statistics derived by the moving observer. We explore several aspects of such biases and pay close attention to their effects on CMB polarization which previously have not been examined in detail. A potentially important bias that we introduce here is $\textit{boost variance}$---an additional term in cosmic variance, induced by the observer's motion. Although this additional term is negligible for whole-sky experiments, in partial-sky experiments it can reach $10\%$ (temperature) to $20\%$ (polarization) of the standard cosmic variance ($σ$). Furthermore, we investigate the significance of motion-induced $\textit{power}$ and $\textit{parity}$ asymmetries in TT, EE, and TE as well as potential biases induced in cosmological parameter estimation performed with whole-sky TTTEEE. Using Planck-like simulations, we find that our local motion induces $\sim1-2 \%$ hemispherical asymmetry in a wide range of angular scales in the CMB temperature and polarization power spectra; however, it does not imply any significant amount of parity asymmetry or shift in cosmological parameters. Finally, we examine the prospects of measuring the velocity of the Solar System w.r.t. the CMB with future experiments via the mode coupling induced by the Doppler and aberration effects. Using the CMB TT, EE, and TE power spectra up to $\ell=4000$, SO and CMB-S4 can make a dipole-independent measurement of our local velocity respectively at $8.5σ$ and $20σ$.

astro-ph.CO↗

arXiv2019

Pairwise Transverse Velocity Measurement with the Rees-Sciama Effect

We introduce a new estimator for the mean pairwise velocities of galaxy clusters, which is based on the measurement of the clusters' $\textit{transverse}$ velocity components. The Rees-Sciama (RS) effect offers an opportunity to measure transverse peculiar velocities through its distinct dipolar signature around the halo centers in the Cosmic Microwave Background (CMB) temperature map. We exploit this dipolar structure to extract the magnitude and direction of the transverse velocity vectors from CMB maps simulated with the expected characteristics of future surveys like CMB-S4. Although in the presence of lensed CMB and instrumental noise individual velocities are not reliably reconstructed, we demonstrate that the mean pairwise velocity measurement obtained using the estimator yields a signal-to-noise ratio of $5.2$ for $\sim21,000$ halos with $M > 7\times10^{13}\rm M_\odot$ in a $40\times40$ [deg$^2$] patch at $z=0.5$. While the proposed estimator carries promising prospects for measuring pairwise velocities through the RS effect in CMB stage IV experiments, its applications extend to any other potential probe of transverse velocities.

astro-ph.CO↗

arXiv2017

Beyond the Boost: Measuring the intrinsic dipole of the CMB using the spectral distortions of the monopole and quadrupole

We present a general framework for accurate spectral modeling of the low multipoles of the cosmic microwave background (CMB) as observed in a boosted frame. In particular, we demonstrate how spectral measurements of the low multipoles can be used to separate the motion-induced dipole of the CMB from a possible intrinsic dipole component. In a moving frame, the leakage of an intrinsic dipole moment into the CMB monopole and quadrupole induces spectral distortions with distinct frequency functions that respectively peak at 337 GHz and 276 GHz. The leakage into the quadrupole moment also induces a geometrical distortion to the spatial morphology of this mode. The combination of these effects can be used to lift the degeneracy between the motion-induced dipole and any intrinsic dipole that the CMB might possess. Assuming the current peculiar velocity measurements, the leakage of an intrinsic dipole with an amplitude of $ΔT = 30μ$K into the monopole and quadrupole moments will be detectable by a PIXIE--like experiment at $\sim 40~$nK ($2.5σ$) and $\sim 130~$nK ($11σ$) level at their respective peak frequencies.

astro-ph.CO↗

arXiv2017

A Generalized Doppler and Aberration Kernel for Frequency-Dependent Cosmological Observables

We introduce a $\textit{frequency-dependent}$ Doppler and aberration transformation kernel for the harmonic multipoles of a general cosmological observable with spin weight $s$, Doppler weight $d$ and arbitrary frequency spectrum. In the context of Cosmic Microwave Background (CMB) studies, the frequency-dependent formalism allows to correct for the motion-induced aberration and Doppler effects on individual frequency maps with different masks. It also permits to deboost background radiations with non-blackbody frequency spectra, like extragalactic foregrounds and CMB spectra with primordial spectral distortions. The formalism can also be used to correct individual E and B polarization modes and account for motion-induced E/B mixing of polarized observables with $d\neq1$ at different frequencies. We apply the generalized aberration kernel on polarized and unpolarized CMB specific intensity at 100 and 217 GHz and show that the motion-induced effects typically increase with the frequency of observation. In all-sky CMB experiments, the frequency-dependence of the motion-induced effects for a blackbody spectrum are overall negligible. However in a cut-sky analysis, ignoring the frequency dependence can lead to percent level error in the polarized and unpolarized power spectra over all angular scales. In the specific cut-sky used in our analysis ($b > 45^\circ, f_\text{sky}\simeq14\%$), and for the dipole-inferred velocity $β=0.00123$ typically attributed to our peculiar motion, the Doppler and aberration effects can change polarized and unpolarized power spectra of specific intensity in the CMB rest frame by $1-2\%$, but we find the polarization cross-leakage between E and B modes to be negligible.

astro-ph.CO↗

arXiv2016

Kinetic Sunyaev Zeldovich effect in an anisotropic CMB model: measuring low multipoles of the CMB at higher redshifts using intensity and polarization spectral distortions

We present a novel mathematical formalism that allows to easily compute the expected kinetic Sunyaev Zeldovich (kSZ) signal in intensity and polarization due to an anisotropic primordial Cosmic Microwave Background (CMB). We derive the expected intensity and polarization distortions in the direction of non-moving galaxy clusters and then we generalize our calculations for non-zero peculiar velocity. We show that, in the direction of moving clusters, low CMB multipoles impose intensity and polarization spectral distortions with different frequency dependences. The polarization signal primarily probes the quadrupole moment of the CMB, with a significant contribution from the primordial dipole and octupole moments. For a typical cluster velocity of 1000 km/s, corrections to the quadrupole-induced polarization of a non-moving cluster are of the order of 2-10% between 200-600 GHz, and depend on cluster's position on the sky, velocity magnitude and direction of motion. We also find that the angular dependence of the signal varies with frequency of observation. The distinct frequency and angular dependences of the polarization induced by the primordial dipole and octupole can be exploited to measure them despite other physical effects and foregrounds. Contrary to polarization, intensity distortions are affected by all the CMB multipoles, so they cannot be readily used to probe the low multipoles at higher redshifts. However, correlations between intensity and polarization signals, can be used to enhance the signal to noise ratio for the measurements of the primordial dipole, quadrupole and octupole. The more general calculation of the aberration kernel presented in this work has applications reaching beyond the SZ cluster science addressed here. For example, it can be exploited to the deboost/deaberrate CMB multipoles as observed in our local frame.

astro-ph.CO↗