arXiv subjects

W. L. Kimmy Wu

Publications and source records attributed to W. L. Kimmy Wu.

17 recordsLinked to original sources

arXiv2026

Disentangling geometry & growth in cosmic neutrino mass constraints: a more physical split and diagnostic

Recent cosmological data hint at a neutrino mass sum below the minimum value allowed by the neutrino mass oscillation experiments, with a preference for the opposite of massive neutrinos ("negative" neutrino masses). Since massive neutrinos impact the expansion history of the Universe, which in turn modifies the growth of structure, we ask whether such a preference originates from the observed geometry or structure growth, and whether the two agree. In this paper, we introduce a more physical split of $\sum m_ν$ into parameters $\sum m_ν^{geo}$ and $\sum m_ν^{growth}$ based on their physical effects, with extension to negative values. We split the expansion rate at the Boltzmann-code level: $\sum m_ν^{geo}$ sets $H(z)$ for distance computation, and $\sum m_ν^{growth}$ sets $H(z)$ for perturbation evolution. Combining Planck primary CMB, Planck+ACT+SPT CMB lensing and DESI BAO data, we find similar sensitivity and "negative" preference from geometry and growth with $\sum m_ν^{geo} = -0.07\pm0.10$ eV (68% C.L.) and $\sum m_ν^{growth} = -0.13^{+0.10}_{-0.12}$ eV (68% C.L.). The difference of the two is a null test of the $Λ$CDM model, passed by current data at $0.4σ$ level. Forecasts on Advanced SO CMB and DESI DR3 BAO show that $0.058$ eV would be rejected at $3.0σ$ from the geometry channel alone, if the negative mean of $\sum m_ν^{geo} = -0.07$ eV persists. The two channels can also provide a powerful new diagnostic between possible origins of the neutrino mass tension. With future CMB and BAO data, our Fisher forecasts show that an incorrect dark energy model displaces $\sum m_ν^{geo}$ from $0.058$ eV by $3.1$-$4.2σ$ and $\sum m_ν^{growth}$ by $1.1$-$1.3σ$, while an underestimated reionization optical depth reverses the pattern, $2.7$-$3.6σ$ in $\sum m_ν^{growth}$ against at most $0.23σ$ in $\sum m_ν^{geo}$. (Abridged)

astro-ph.CO↗

arXiv2026

Measuring Cosmic Neutrino Masses Independently of Dark Energy

Neutrino oscillations establish that neutrinos are massive, providing the only laboratory detection of physics beyond the Standard Model. Direct kinematic experiments bound the electron-neutrino mass to $m_{ν_e} < 0.45$ eV (KATRIN, 90% CL), implying $\sum m_ν\lesssim 1.3$ eV. Conversely, cosmology within $Λ$CDM is highly constraining: Planck CMB, CMB lensing, and DESI DR2 BAO yield $\sum m_ν< 0.056$ eV (95% CL), in 2-3$σ$ tension with the inverted-ordering floor (0.10 eV). However, this bound relies on $Λ$CDM, while data hint at an evolving dark energy. To determine the model dependence of cosmic neutrino mass bounds, we deconstruct each probe's sensitivity to late-time physics and pursue two robust routes to a $\sum m_ν$ bound: (i) The existing dark-energy-marginalized route, retaining all data and marginalizing over $(w_0, w_a)$, is shown to also be immune to flexible binned and cubic $w(a)$ histories, yielding $\sum m_ν< 0.152$ eV, sharpening to $σ(\sum m_ν) \approx 0.043$ eV with Simons Observatory lensing and Spec-S5 BAO. (ii) A new late-Universe-free route combines primary CMB, marginalizing over acoustic-peak smoothing via $A_{\rm lens}$, with the reconstructed lensing spectrum $C_L^{κκ}$, removing late-time expansion dependence by construction. This yields $\sum m_ν< 0.41$ eV today, tightening to 0.31 eV (Simons Observatory) and 0.28 eV (cosmic-variance limit) across all tested dark-energy models. These relaxed bounds trade statistical power for model independence. Interestingly, they land in the sensitivity range targeted by next-generation laboratory experiments like Project 8 ($m_{ν_e} \sim 0.1$ eV), motivating vital synergies between future cosmological and terrestrial neutrino measurements.

astro-ph.CO↗

arXiv2026

Backlighting the Cosmic Web with Fast Radio Bursts: An Anthology of Dispersion Measure Cross-Correlations with Large-Scale Structure and Baryon Tracers

Fast Radio Bursts (FRBs) probe baryons permeating the cosmic web through their dispersion measures (DMs), which encode the integrated electron density along cosmological sightlines. Using 3,455 unique FRB sources from CHIME/FRB with $\sim 15$ arcmin localizations, we present an anthology of DM correlations with tracers of large-scale structure and baryonic matter at redshifts $z \lesssim 1.5$. We measure statistically significant correlations at $2.6-5σ$ with ten probes, including galaxies ($2.8σ$), weak gravitational lensing ($2.6σ$), cosmic infrared background ($4.0σ$), cosmic microwave background (CMB) lensing ($3.3σ$), thermal Sunyaev Zel'dovich (tSZ) effect ($3.8σ$), X-ray emission tracing galaxy clusters ($5.0σ$) and superclusters ($3.3σ$), soft X-ray background (SXRB, $4.1σ$), and radio continuum emission ($3.2σ$). These measurements reveal a consistent picture in which FRB sightlines intersecting overdense environments carry systematically larger DMs. Correlations with hot-gas tracers provide additional leverage on the strength of feedback, as they are strongly weighted towards the dense, bound gas. The measured amplitude of tSZ$\times$DM and SXRB$\times$DM correlations are consistent with theoretical predictions of baryon distribution from a DM-$z$ relation-inferred model with moderate feedback at $\sim 0.5σ$ level. Weaker feedback scenario is ruled out at $\sim 3.5σ$ by the SXRB$\times$DM correlation. Taken together, these measurements constitute a quantitative multi-tracer foundation for a new era in which FRBs from next generation facilities, such as BURSTT, CHORD, DSA, and SKA, in harmony with other probes, will map the baryon content of the full extent of the cosmic web.

astro-ph.CO↗

arXiv2026

Foreground Mitigation for CMB Lensing with the Global Minimum Variance Quadratic Estimator

Weak gravitational lensing of the cosmic microwave background (CMB) is a powerful probe of cosmology, providing insight into structure formation and the evolution of the universe. Current and upcoming CMB experiments such as SPT-3G and the Simons Observatory (SO) provide high-resolution, low-noise temperature and polarization maps that are ideal for lensing reconstruction. The global minimum variance (GMV) quadratic estimator for CMB lensing reduces reconstruction noise over the standard quadratic estimator (SQE). In this work, we extend the GMV framework to incorporate the tSZ-deproj and cross-ILC foreground mitigation techniques, which enhance robustness against contamination from astrophysical sources. For a simulation study using SPT-3G Ext-10k and SO Extended configurations at $\ell_{\mathrm{max}}^T = 3500$, the lensing bias at $L < 1000$ is reduced from $\sim4\%$ with standard GMV and SQE to $2\%$ with tSZ-deproj, and to $< 1\%$ with cross-ILC. These methods enable the construction of foreground-cleaned lensing maps suitable for cross-correlation analyses, with direct relevance for current and future surveys.

astro-ph.CO↗

arXiv2026

Unified and consistent structure growth measurements from joint ACT, SPT and \textit{Planck} CMB lensing

We present the tightest cosmic microwave background (CMB) lensing constraints to date on the growth of structure by combining CMB lensing measurements from the Atacama Cosmology Telescope (ACT), the South Pole Telescope (SPT) and \textit{Planck}. Each of these surveys individually provides lensing measurements with similarly high statistical power, achieving signal-to-noise ratios of approximately 40. The combined lensing bandpowers represent the most precise CMB lensing power spectrum measurement to date with a signal-to-noise ratio of 61 and an amplitude of $A_\mathrm{lens}^\mathrm{recon} = 1.025 \pm 0.017$ with respect to the theory prediction from the best-fit CMB \textit{Planck}-ACT cosmology. The bandpowers from all three lensing datasets, analyzed jointly, yield a $1.6\%$ measurement of the parameter combination $S_8^\mathrm{CMBL} \equiv σ_8\,(Ω_m/0.3)^{0.25} = 0.825^{+0.015}_{-0.013}$. Including Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillation (BAO) data improves the constraint on the amplitude of matter fluctuations to $σ_8 = 0.829 \pm 0.009$ (a $1.1\%$ determination). When combining with uncalibrated supernovae from \texttt{Pantheon+}, we present a $4\%$ sound-horizon-independent estimate of $H_0=66.4\pm2.5\,\mathrm{km\,s^{-1}\,Mpc^{-1}} $. The joint lensing constraints on structure growth and present-day Hubble rate are fully consistent with a $Λ$CDM model fit to the primary CMB data from \textit{Planck} and ACT. While the precise upper limit is sensitive to the choice of data and underlying model assumptions, when varying the neutrino mass sum within the $Λ\mathrm{CDM}$ cosmological model, the combination of primary CMB, BAO and CMB lensing drives the probable upper limit for the mass sum towards lower values, comparable to the minimum mass prior required by neutrino oscillation experiments.

astro-ph.CO↗

arXiv2025

CMB-S4: Foreground-Cleaning Pipeline Comparison for Measuring Primordial Gravitational Waves

We compare multiple foreground-cleaning pipelines for estimating the tensor-to-scalar ratio, $r$, using simulated maps of the planned CMB-S4 experiment within the context of the South Pole Deep Patch. To evaluate robustness, we analyze bias and uncertainty on $r$ across various foreground suites using map-based simulations. The foreground-cleaning methods include: a parametric maximum likelihood approach applied to auto- and cross-power spectra between frequency maps; a map-based parametric maximum-likelihood method; and a harmonic-space internal linear combination using frequency maps. We summarize the conceptual basis of each method to highlight their similarities and differences. To better probe the impact of foreground residuals, we implement an iterative internal delensing step, leveraging a map-based pipeline to generate a lensing $B$-mode template from the Large Aperture Telescope frequency maps. Our results show that the performance of the three approaches is comparable for simple and intermediate-complexity foregrounds, with $σ(r)$ ranging from 3 to 5 $\times 10^{-4}$. However, biases at the $1-2σ$ level appear when analyzing more complex forms of foreground emission. By extending the baseline pipelines to marginalize over foreground residuals, we demonstrate that contamination can be reduced to within statistical uncertainties, albeit with a pipeline-dependent impact on $σ(r)$, which translates to a detection significance between 2 and 4$σ$ for an input value of $r = 0.003$. These findings suggest varying levels of maturity among the tested pipelines, with the auto- and cross-spectra-based approach demonstrating the best stability and overall performance. Moreover, given the extremely low noise levels, mutual validation of independent foreground-cleaning pipelines is essential to ensure the robustness of any potential detection.

astro-ph.CO↗

arXiv2022

Inflation: Theory and Observations

Cosmic inflation provides a window to the highest energy densities accessible in nature, far beyond those achievable in any realistic terrestrial experiment. Theoretical insights into the inflationary era and its observational probes may therefore shed unique light on the physical laws underlying our universe. This white paper describes our current theoretical understanding of the inflationary era, with a focus on the statistical properties of primordial fluctuations. In particular, we survey observational targets for three important signatures of inflation: primordial gravitational waves, primordial non-Gaussianity and primordial features. With the requisite advancements in analysis techniques, the tremendous increase in the raw sensitivities of upcoming and planned surveys will translate to leaps in our understanding of the inflationary paradigm and could open new frontiers for cosmology and particle physics. The combination of future theoretical and observational developments therefore offer the potential for a dramatic discovery about the nature of cosmic acceleration in the very early universe and physics on the smallest scales.

astro-ph.CO↗

arXiv2022

Snowmass Theory Frontier: Astrophysics and Cosmology

We summarize progress made in theoretical astrophysics and cosmology over the past decade and areas of interest for the coming decade. This Report is prepared as the TF09 "Astrophysics and Cosmology" topical group summary for the Theory Frontier as part of the Snowmass 2021 process.

hep-ph↗

arXiv2022

Bias on Tensor-to-Scalar Ratio Inference With Estimated Covariance Matrices

We investigate simulation-based bandpower covariance matrices commonly used in cosmological parameter inferences such as the estimation of the tensor-to-scalar ratio $r$. We find that upper limits on $r$ can be biased low by tens of percent. The underestimation of the upper limit is most severe when the number of simulation realizations is similar to the number of observables. Convergence of the covariance-matrix estimation can require a number of simulations an order of magnitude larger than the number of observables, which could mean $\mathcal{O}(10\ 000)$ simulations. This is found to be caused by an additional scatter in the posterior probability of $r$ due to Monte Carlo noise in the estimated bandpower covariance matrix, in particular, by spurious non-zero off-diagonal elements. We show that matrix conditioning can be a viable mitigation strategy in the case that legitimate covariance assumptions can be made.

astro-ph.CO↗

arXiv2022

Forecasting ground-based sensitivity to the Rayleigh scattering of the CMB in the presence of astrophysical foregrounds

The Rayleigh scattering of cosmic microwave background (CMB) photons off the neutral hydrogen produced during recombination effectively creates an additional scattering surface after recombination that encodes new cosmological information, including the expansion and ionization history of the universe. A first detection of Rayleigh scattering is a tantalizing target for next-generation CMB experiments. We have developed a Rayleigh scattering forecasting pipeline that includes instrumental effects, atmospheric noise, and astrophysical foregrounds (e.g., Galactic dust, cosmic infrared background, or CIB, and the thermal Sunyaev-Zel'dovich effect). We forecast the Rayleigh scattering detection significance for several upcoming ground-based experiments, including SPT-3G+, Simons Observatory, CCAT-prime, and CMB-S4, and examine the limitations from atmospheric and astrophysical foregrounds as well as potential mitigation strategies. When combined with Planck data, we estimate that the ground-based experiments will detect Rayleigh scattering with a significance between 1.6 and 3.7, primarily limited by atmospheric noise and the CIB.

astro-ph.CO↗

arXiv2021

DeepSZ: Identification of Sunyaev-Zel'dovich Galaxy Clusters using Deep Learning

Galaxy clusters identified from the Sunyaev Zel'dovich (SZ) effect are a key ingredient in multi-wavelength cluster-based cosmology. We present a comparison between two methods of cluster identification: the standard Matched Filter (MF) method in SZ cluster finding and a method using Convolutional Neural Networks (CNN). We further implement and show results for a `combined' identifier. We apply the methods to simulated millimeter maps for several observing frequencies for an SPT-3G-like survey. There are some key differences between the methods. The MF method requires image pre-processing to remove point sources and a model for the noise, while the CNN method requires very little pre-processing of images. Additionally, the CNN requires tuning of hyperparameters in the model and takes as input, cutout images of the sky. Specifically, we use the CNN to classify whether or not an 8 arcmin $\times$ 8 arcmin cutout of the sky contains a cluster. We compare differences in purity and completeness. The MF signal-to-noise ratio depends on both mass and redshift. Our CNN, trained for a given mass threshold, captures a different set of clusters than the MF, some of which have SNR below the MF detection threshold. However, the CNN tends to mis-classify cutouts whose clusters are located near the edge of the cutout, which can be mitigated with staggered cutouts. We leverage the complementarity of the two methods, combining the scores from each method for identification. The purity and completeness of the MF alone are both 0.61, assuming a standard detection threshold. The purity and completeness of the CNN alone are 0.59 and 0.61. The combined classification method yields 0.60 and 0.77, a significant increase for completeness with a modest decrease in purity. We advocate for combined methods that increase the confidence of many lower signal-to-noise clusters.

astro-ph.CO↗

arXiv2021

Breaking the degeneracy between polarization efficiency and cosmological parameters in CMB experiments

Accurate cosmological parameter estimates using polarization data of the cosmic microwave background (CMB) put stringent requirements on map calibration, as highlighted in the recent results from the Planck satellite. In this paper, we point out that a model-dependent determination of polarization calibration can be achieved by the joint fit of the TE and EE CMB power spectra. This provides a valuable cross-check to band-averaged polarization efficiency measurements determined using other approaches. We demonstrate that, in $Λ$CDM, the combination of the TE and EE constrain polarization calibration with sub-percent uncertainty with Planck data and 2% uncertainty with SPTpol data. We arrive at similar conclusions when extending $Λ$CDM to include the amplitude of lensing $A_{\rm L}$, the number of relativistic species $N_{\rm eff}$, or the sum of the neutrino masses $\sum m_ν$. The uncertainties on cosmological parameters are minimally impacted when marginalizing over polarization calibration, except, as can be expected, for the uncertainty on the amplitude of the primordial scalar power spectrum $\ln(10^{10} A_{\rm s})$, which increases by $20-50$%. However, this information can be fully recovered by adding TT data. For current and future ground-based experiments, SPT-3G and CMB-S4, we forecast the cosmological parameter uncertainties to be minimally degraded when marginalizing over polarization calibration parameters. In addition, CMB-S4 could constrain its polarization calibration at the level of $\sim$0.2% by combining TE and EE, and reach $\sim$0.06% by also including TT. We therefore conclude that relying on calibrating against Planck polarization maps, whose statistical uncertainty is limited to $\sim$0.5%, would be insufficient for upcoming experiments.

astro-ph.CO↗

arXiv2021

Hubble constant difference between CMB lensing and BAO measurements

We apply a tension metric $Q_\textrm{UDM}$, the update difference in mean parameters, to understand the source of the difference in the measured Hubble constant $H_0$ inferred with cosmic microwave background lensing measurements from the Planck satellite ($H_0=67.9^{+1.1}_{-1.3}\, \mathrm{km/s/Mpc}$) and from the South Pole Telescope ($H_0=72.0^{+2.1}_{-2.5}\, \mathrm{km/s/Mpc}$) when both are combined with baryon acoustic oscillation (BAO) measurements with priors on the baryon density (BBN). $Q_\textrm{UDM}$ isolates the relevant parameter directions for tension or concordance where the two data sets are both informative, and aids in the identification of subsets of data that source the observed tension. With $Q_\textrm{UDM}$, we uncover that the difference in $H_0$ is driven by the tension between Planck lensing and BAO+BBN, at probability-to-exceed of 6.6%. Most of this mild tension comes from the galaxy BAO measurements parallel to the line of sight. The redshift dependence of the parallel BAOs pulls both the matter density $Ω_m$ and $H_0$ high in $Λ$CDM, but these parameter anomalies are usually hidden when the BAO measurements are combined with other cosmological data sets with much stronger $Ω_m$ constraints.

astro-ph.CO↗

arXiv2020

DeepCMB: Lensing Reconstruction of the Cosmic Microwave Background with Deep Neural Networks

Next-generation cosmic microwave background (CMB) experiments will have lower noise and therefore increased sensitivity, enabling improved constraints on fundamental physics parameters such as the sum of neutrino masses and the tensor-to-scalar ratio r. Achieving competitive constraints on these parameters requires high signal-to-noise extraction of the projected gravitational potential from the CMB maps. Standard methods for reconstructing the lensing potential employ the quadratic estimator (QE). However, the QE performs suboptimally at the low noise levels expected in upcoming experiments. Other methods, like maximum likelihood estimators (MLE), are under active development. In this work, we demonstrate reconstruction of the CMB lensing potential with deep convolutional neural networks (CNN) - ie, a ResUNet. The network is trained and tested on simulated data, and otherwise has no physical parametrization related to the physical processes of the CMB and gravitational lensing. We show that, over a wide range of angular scales, ResUNets recover the input gravitational potential with a higher signal-to-noise ratio than the QE method, reaching levels comparable to analytic approximations of MLE methods. We demonstrate that the network outputs quantifiably different lensing maps when given input CMB maps generated with different cosmologies. We also show we can use the reconstructed lensing map for cosmological parameter estimation. This application of CNN provides a few innovations at the intersection of cosmology and machine learning. First, while training and regressing on images, we predict a continuous-variable field rather than discrete classes. Second, we are able to establish uncertainty measures for the network output that are analogous to standard methods. We expect this approach to excel in capturing hard-to-model non-Gaussian astrophysical foreground and noise contributions.

astro-ph.CO↗

arXiv2019

Sounds Discordant: Classical Distance Ladder & $Λ$CDM -based Determinations of the Cosmological Sound Horizon

Type Ia Supernovae, calibrated by classical distance ladder methods, can be used, in conjunction with galaxy survey two-point correlation functions, to empirically determine the size of the sound horizon $r_{\rm s}$. Assumption of the $Λ$CDM model, together with data to constrain its parameters, can also be used to determine the size of the sound horizon. Using a variety of cosmic microwave background (CMB) datasets to constrain $Λ$CDM parameters, we find the model-based sound horizon to be larger than the empirically-determined one with a statistical significance of between 2 and 3$σ$, depending on the dataset. If reconciliation requires a change to the cosmological model, we argue that change is likely to be important in the two decades of scale factor evolution prior to recombination. Future CMB observations will therefore likely be able to test any such adjustments; e.g., a third generation CMB survey like SPT-3G can achieve a three-fold improvement in the constraints on $r_{\rm s}$ in the $Λ$CDM model extended to allow additional light degrees of freedom.

astro-ph.CO↗

arXiv2016

CMB-S4 Science Book, First Edition

This book lays out the scientific goals to be addressed by the next-generation ground-based cosmic microwave background experiment, CMB-S4, envisioned to consist of dedicated telescopes at the South Pole, the high Chilean Atacama plateau and possibly a northern hemisphere site, all equipped with new superconducting cameras. CMB-S4 will dramatically advance cosmological studies by crossing critical thresholds in the search for the B-mode polarization signature of primordial gravitational waves, in the determination of the number and masses of the neutrinos, in the search for evidence of new light relics, in constraining the nature of dark energy, and in testing general relativity on large scales.

astro-ph.CO↗

arXiv2013

Freezing Out Early Dark Energy

A phenomenological model of dark energy that tracks the baryonic and cold dark matter at early times but resembles a cosmological constant at late times is explored. In the transition between these two regimes, the dark energy density drops rapidly as if it were a relic species that freezes out, during which time the equation of state peaks at +1. Such an adjustment in the dark energy density, as it shifts from scaling to potential-domination, could be the signature of a trigger mechanism that helps explain the late-time cosmic acceleration. We show that the non-negligible dark energy density at early times, and the subsequent peak in the equation of state at the transition, leave an imprint on the cosmic microwave background anisotropy pattern and the rate of growth of large scale structure. The model introduces two new parameters, consisting of the present-day equation of state and the redshift of the freeze-out transition. A Monte Carlo Markov Chain analysis of a ten-dimensional parameter space is performed to compare the model with pre-Planck cosmic microwave background, large scale structure and supernova data and measurements of the Hubble constant. We find that the transition described by this model could have taken place as late as a redshift z~400. We explore the capability of future cosmic microwave background and weak lensing experiments to put tighter constraints on this model. The viability of this model may suggest new directions in dark-energy model building that address the coincidence problem.

astro-ph.CO↗