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Register a new userHemispherical Variance Anomaly and Reionization Optical Depth
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CMB full-sky temperature data show a hemispherical asymmetry in power nearly aligned with the Ecliptic. In real space, this anomaly can be quantified by the temperature variance in the northern and southern Ecliptic hemispheres, with the north displaying an anomalously low variance while the south appears consistent with expectations from the best-fitting theory, LCDM. While this is a well-established result in temperature, the low signal-to-noise ratio in current polarization data prevents a similar comparison. Even though temperature and polarization are correlated, polarization realizations constrained by temperature data show that the lack of variance is not expected to be present in polarization data. Therefore, a natural way of testing whether the temperature result is a fluke is to measure the variance of CMB polarization components. In anticipation of future CMB experiments that will allow for high-precision large-scale polarization measurements, we study how variance of polarization depends on LCDM parameters uncertainties by forecasting polarization maps with Plancks MCMC chains. We find that, unlike temperature variance, polarization variance is noticeably sensitive to present uncertainties in cosmological parameters. This comes mainly from the current poor constraints on the reionization optical depth, tau, and the fact that tau drives variance at low multipoles. In this work we show how the variance of polarization maps generically depends on the cosmological parameters. We demonstrate how the improvement in the tau measurement seen between Plancks two latest data releases results in a tighter constraint on polarization variance expectations. Finally, we consider even smaller uncertainties on tau and how more precise measurements of tau can drive the expectation for polarization variance in a hemisphere close to that of the cosmic-variance-limited distribution.
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We investigate if the hemispherical asymmetry in the CMB is produced from asymmetric excited initial condition. We show that in the limit where the deviations from the Bunch-Davies vacuum is large and the scale of new physics is maximally separated from the inflationary Hubble parameter, the primordial power spectrum is modulated only by position dependent dipole and quadrupole terms. Requiring the dipole contribution in the power spectrum to account for the observed power asymmetry, $A=0.07pm0.022$, we show that the amount of quadrupole terms is roughly equal to $A^2$. The {it mean} local bispectrum, which gets enhanced for the excited initial state, is within the $1sigma$ bound of Planck 2015 results for a large field model, $f_{rm NL}simeq 4.17$, but is reachable by future CMB experiments. The amplitude of the local non-gaussianity modulates around this mean value, depending on the angle that the correlated patches on the 2d CMB surface make with the preferred direction. The amount of variation minimizes for the configuration in which the short and long wavelengths modes are around the preferred pole and $|vec k_3|approx |vec k_{lapprox10}|ll |vec k_1|approx |vec k_2|approx |vec k_{lapprox2500}|$ with $f_{rm NL}^{rm min}approx 3.64 $. The maximum occurs when these modes are at the antipode of the preferred pole, $f_{rm NL}^{rm max}approx 4.81$ . The difference of non-gaussianity between these two configurations is as large as $simeq 1.17$ which can be used to distinguish this scenario from other scenarios that try to explain the observed hemispherical asymmetry.
We analyze simulated maps of the Cosmology Large Angular Scale Surveyor (CLASS) experiment and recover a nearly cosmic-variance limited estimate of the reionization optical depth $tau$. We use a power spectrum-based likelihood to simultaneously clean foregrounds and estimate cosmological parameters in multipole space. Using software specifically designed to constrain $tau$, the amplitude of scalar fluctuations $A_s$, and the tensor-to-scalar ratio $r$, we demonstrate that the CLASS experiment will be able to estimate $tau$ within a factor of two of the full-sky cosmic variance limit allowed by cosmic microwave background polarization measurements. Additionally, we discuss the role of CLASSs $tau$ constraint in conjunction with gravitational lensing of the CMB on obtaining a $gtrsim4sigma$ measurement of the sum of the neutrino masses.
The Epoch of Reionization (EoR) depends on the complex astrophysics governing the birth and evolution of the first galaxies and structures in the intergalactic medium. EoR models rely on cosmic microwave background (CMB) observations, and in particular the large-scale E-mode polarization power spectra (EE PS), to help constrain their highly uncertain parameters. However, rather than directly forward-modelling the EE PS, most EoR models are constrained using a summary statistic -- the Thompson scattering optical depth, $tau_e$. Compressing CMB observations to $tau_e$ requires adopting a basis set for the EoR history. The common choice is the unphysical, redshift-symmetric hyperbolic tangent (Tanh) function, which differs in shape from physical EoR models based on hierarchical structure formation. Combining public EoR and CMB codes, 21cmFAST and CLASS, here we quantify how inference using the $tau_e$ summary statistic impacts the resulting constraints on galaxy properties and EoR histories. Using the last Planck 2018 data release, we show that the marginalized constraints on the EoR history are more sensitive to the choice of the basis set (Tanh vs physical model) than to the CMB likelihood statistic ($tau_e$ vs PS). For example, EoR histories implied by the growth of structure show a small tail of partial reionization extending to higher redshifts. However, biases in inference using $tau_e$ are negligible for the Planck 2018 data. Using EoR constraints from high-redshift observations including the quasar dark fraction, galaxy UV luminosity functions and CMB EE PS, our physical model recovers $tau_e=0.0569^{+0.0081}_{-0.0066}$.
We present an estimation of the reionization optical depth $tau$ from an improved analysis of the High Frequency Instrument (HFI) data of Planck satellite. By using an improved version of the HFI map-making code, we greatly reduce the residual large scale contamination affecting the data, characterized, but not fully removed, in the Planck 2018 legacy release. This brings the dipole distortion systematic effect, contaminating the very low multipoles, below the noise level. On large scale polarization only data, we measure $tau=0.0566_{-0.0062}^{+0.0053}$ at $68%$ C.L., reducing the Planck 2018 legacy release uncertainty by $sim40%$. Within the $Lambda$CDM model, in combination with the Planck large scale temperature likelihood, and the high-$ell$ temperature and polarization likelihood, we measure $tau=0.059pm0.006$ at $68%$ C.L. which corresponds to a mid-point reionization redshift of $z_{rm re}=8.14pm0.61$ at $68%$ C.L.. This estimation of the reionization optical depth with $10%$ accuracy is the strongest constraint to date.
This paper explores methods for constructing low multipole temperature and polarisation likelihoods from maps of the cosmic microwave background anisotropies that have complex noise properties and partial sky coverage. We use Planck 2018 High Frequency Instrument (HFI) and updated SRoll2 temperature and polarisation maps to test our methods. We present three likelihood approximations based on quadratic cross spectrum estimators: (i) a variant of the simulation-based likelihood (SimBaL) techniques used in the Planck legacy papers to produce a low multipole EE likelihood; (ii) a semi-analytical likelihood approximation (momento) based on the principle of maximum entropy; (iii) a density-estimation `likelihood-free scheme (DELFI). Approaches (ii) and (iii) can be generalised to produce low multipole joint temperature-polarisation (TTTEEE) likelihoods. We present extensive tests of these methods on simulations with realistic correlated noise. We then analyse the Planck data and confirm the robustness of our method and likelihoods on multiple inter- and intra-frequency detector set combinations of SRoll2 maps. The three likelihood techniques give consistent results and support a low value of the optical depth to reoinization, tau, from the HFI. Our best estimate of tau comes from combining the low multipole SRoll2 momento (TTTEEE) likelihood with the CamSpec high multipole likelihood and is tau = 0.0627+0.0050-0.0058. This is consistent with the SRoll2 teams determination of tau, though slightly higher by 0.5 sigma, mainly because of our joint treatment of temperature and polarisation.
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