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1. Machine Learning-Driven Analysis of kSZ Maps to Predict CMB Optical Depth τ

   https://doi.org/10.1088/1538-3873/ae4069  

   Farhadi_Khouzani, Farshid; Shaw, Abinash Kumar; La_Plante, Paul; Shareef, Bryar Mustafa; Gewali, Laxmi (February 2026, Publications of the Astronomical Society of the Pacific)

   Abstract Upcoming measurements of the kinetic Sunyaev–Zel’dovich (kSZ) effect, which results from Cosmic Microwave Background (CMB) photons scattering off moving electrons, offer a powerful probe of the Epoch of Reionization (EoR). The kSZ signal contains key information about the timing, duration, and spatial structure of the EoR. A precise measurement of the CMB optical depthτ, a key parameter that characterizes the universe’s integrated electron density, would significantly constrain models of early structure formation. However, the weak kSZ signal is difficult to extract from CMB observations due to significant contamination from astrophysical foregrounds. We present a machine learning approach to extractτfrom simulated kSZ maps. We train advanced machine learning models, including swin transformers, on high-resolution seminumeric simulations of the kSZ signal. To robustly quantify prediction uncertainties ofτ, we employ the Laplace Approximation (LA). This approach provides an efficient and principled Gaussian approximation to the posterior distribution over the model’s weights, allowing for reliable error estimation. We investigate and compare two distinct application modes: a post-hoc LA applied to a pre-trained model, and an online LA where model weights and hyperparameters are optimized jointly by maximizing the marginal likelihood. This proof-of-concept approach provides a framework for constrainingτand its associated uncertainty, which could be adapted for analysis of upcoming CMB surveys like the Simons Observatory and CMB-S4. 

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2. Precision cosmology with primordial GW backgrounds in presence of astrophysical foregrounds

   https://doi.org/10.1088/1475-7516/2023/04/054  

   Racco, D.; Poletti, D. (April 2023, Journal of Cosmology and Astroparticle Physics)

   Abstract The era of Gravitational-Wave (GW) astronomy will grant the detection of the astrophysical GW background from unresolved mergers of binary black holes, and the prospect of probing the presence of primordial GW backgrounds. In particular, the low-frequency tail of the GW spectrum for causally-generated primordial signals (like a phase transition) offers an excellent opportunity to measure unambiguously cosmological parameters as the equation of state of the universe, or free-streaming particles at epochs well before recombination. We discuss whether this programme is jeopardised by the uncertainties on the astrophysical GW foregrounds that coexist with a primordial background. We detail the motivated assumptions under which the astrophysical foregrounds can be assumed to be known in shape, and only uncertain in their normalisation. In this case, the sensitivity to a primordial signal can be computed by a simple and numerically agile procedure, where the optimal filter function subtracts the components of the astrophysical foreground that are close in spectral shape to the signal. We show that the degradation of the sensitivity to the signal in presence of astrophysical foregrounds is limited to a factor of a few, and only around the frequencies where the signal is closer to the foregrounds. Our results highlight the importance of modelling the contributions of eccentric or intermediate-mass black hole binaries to the GW background, to consolidate the prospects to perform precision cosmology with primordial GW backgrounds. 

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3. The Cosmic Microwave Background Optical Depth Constrains the Duration of Reionization

   https://doi.org/10.3847/2041-8213/ade141  

   Cain, Christopher; Van_Engelen, Alexander; Croker, Kevin S; Kramer, Darby; D’Aloisio, Anson; Lopez, Garett (July 2025, The Astrophysical Journal Letters)

   Abstract Recently, it was pointed out that invoking a large value of the cosmic microwave background (CMB) optical depth,τ_CMB = 0.09, could help resolve tensions between Dark Energy Survey Instrument DR2 baryon acoustic oscillation data and the CMB. This is larger than the value ofτ_CMB = 0.058 measured from the Planck low-ℓpolarization data. Traditionally,τ_CMBis thought of as a constraint on reionization’s midpoint. However, recent observations and modeling of the Lyαforest of high-zquasars at 5 < z < 6 have tightly constrained the timing of the last 10%–20% of reionization, adding nuance to this interpretation. Here, we point out that fixing reionization’s endpoint, in accordance with the latest Lyαforest constraints, rendersτ_CMBa sensitive probe of the duration of reionization, as well as its midpoint. We compare low and high values ofτ_CMBto upper limits on the patchy kinematic Sunyaev–Zel'dovich (pkSZ) effect, another CMB observable that constrains reionization’s duration, and find that a value ofτ_CMB = 0.09 is in ≈2σtension with existing limits on the pkSZ from the South Pole Telescope (SPT). The strength of this tension is sensitive to the choices involved in modeling the other CMB foregrounds in the SPT measurement, and in the modeling of the pkSZ signal itself. 

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4. The Squeezed Bispectrum from CHIME H I Emission and Planck Cosmic Microwave Background Lensing: Current Sensitivity and Forecasts

   https://doi.org/10.3847/1538-4357/ae77e6  

   Chakraborty, Arnab; Dobbs, Matt; Foreman, Simon; Gray, Liam; Halpern, Mark; Hinshaw, Gary; Joseph, Albin; MacEachern, Joshua; Masui, Kiyoshi W; Mena-Parra, Juan; et al (June 2026, The Astrophysical Journal)

   Abstract Line intensity mapping using atomic hydrogen (HI) has the potential to efficiently map large volumes of the Universe if the signal can be successfully separated from overwhelmingly bright radio foreground emission. This motivates cross correlations, to ascertain the cosmological nature of measured HIfluctuations, and to study their connections with galaxies and the underlying matter density field. However, these same foregrounds render the cross correlation with projected fields such as the lensing of the cosmic microwave background (CMB) difficult. Indeed, the correlated Fourier modes vary slowly along the line of sight and are thus most contaminated by the smooth-spectrum radio continuum foregrounds. In this paper, we implement a method that avoids this issue by attempting to measure the nonlinear gravitational coupling of the small-scale 21 cm power from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with large-scale Planck CMB lensing. This measurement is a position-dependent power spectrum, i.e., a squeezed integrated bispectrum. Using 94 nights of CHIME data between 1.0 < z < 1.3 and aggressive foreground filtering, we find that the expected signal is 5 times smaller than the current noise. We forecast that incorporating the additional nights of CHIME data already collected would enable a signal-to-noise ratio of 3, without any further improvements in filtering for foreground cleaning. 

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5. Origin of the ring ellipticity in the black hole images of M87 ^*

   https://doi.org/10.1051/0004-6361/202555235  

   Dahale, Rohan; Cho, Ilje; Moriyama, Kotaro; Wiik, Kaj; Tiede, Paul; Gómez, José L; Chan, Chi-kwan; Gold, Roman; Bernshteyn, Vadim Y; Foschi, Marianna; et al (July 2025, Astronomy & Astrophysics)

   We investigate the origin of the elliptical ring structure observed in the images of the supermassive black hole M87^*, aiming to disentangle contributions from gravitational, astrophysical, and imaging effects. Leveraging the enhanced capabilities of the Event Horizon Telescope (EHT)'s 2018 array, including improved (u,v)-coverage from the Greenland Telescope, we measured the ring's ellipticity using five independent imaging methods, obtaining a consistent average value ofτ = 0.08_−0.02^+0.03with a position angle ofξ = 50.1_−7.6^+6.2 degrees. To interpret this measurement, we compared it to general relativistic magnetohydrodynamic (GRMHD) simulations spanning a wide range of physical parameters including the thermal or nonthermal electron distribution function, spins, and ion-to-electron temperature ratios in both low- and high-density regions. We find no statistically significant correlation between spin and ellipticity in GRMHD images. Instead, we identify a correlation between ellipticity and the fraction of non-ring emission, particularly in nonthermal models and models with higher jet emission. These results indicate that the ellipticity measured from the M87^*emission structure is consistent with that expected from simulations of turbulent accretion flows around black holes, where it is dominated by astrophysical effects rather than gravitational ones. Future high-resolution imaging, including space very long baseline interferometry and long-term monitoring, will be essential to isolate gravitational signatures from astrophysical effects. 

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