Ajay Gill

We present the weak gravitational lensing dataset from the Super-pressure Balloon-Borne Imaging Telescope (SuperBIT), which imaged 30 galaxy clusters during its 45 night flight in 2023 April–May. SuperBIT is a first-of-its-kind balloon-borne imaging telescope that achieved near-diffraction-limited observations in near-space conditions above 98% of the Earth’s atmosphere. We use the metacalibration algorithm to obtain calibrated galaxy shapes for our target clusters and several calibration fields, enabling unbiased reconstruction of the weak-lensing signal. We employ several diagnostics throughout the pipeline, including assessments of point-spread function (PSF) modeling residuals and their impact on weak-lensing measurements, as well as tests for correlations between galaxy shapes and measured galaxy and PSF properties. To assess the multiplicative shear bias of the pipeline, we analyze a parallel set of simulated images that incorporate the real observing conditions from the flight, including measured SuperBIT PSFs, observed sky backgrounds, and detector noise, yielding a bias of (1.54% ± 6.00%).

We present a multifrequency and multi-instrument methodology to study the physical properties of galaxy clusters and cosmic filaments using cosmic microwave background observations. Our approach enables simultaneous measurement of both the thermal (tSZ) and kinematic Sunyaev–Zeldovich (kSZ) effects, incorporates relativistic corrections, and models astrophysical foregrounds such as thermal dust emission. We do this by jointly fitting a single physical model across multiple maps from multiple instruments at different frequencies, rather than fitting a model to a single Compton- y map. We demonstrate the success of this method by fitting the A399–A401 galaxy cluster pair and filament system using archival data from the Planck satellite and new, targeted deep data from the Atacama Cosmology Telescope, covering 11 different frequencies over 14 maps from 30 GHz to 545 GHz. Our tSZ results are consistent with previous work using Compton- y maps. We measure the line-of-sight peculiar velocities of the cluster–filament system using the kSZ effect and find statistical uncertainties on individual cluster peculiar velocities of ≲600 km s ⁻¹ , which are competitive with current state-of-the-art measurements. Additionally, we measure the optical depth of the filament component with a signal-to-noise of 8.5 σ and reveal hints of its morphology. This modular approach is well-suited for application to future instruments across a wide range of millimeter and submillimeter wavebands.

We present updated forecasts for the scientific performance of the degree-scale (0.5 deg FWHM at 93 GHz), deep-field survey to be conducted by the Simons Observatory (SO). By 2027, the SO Small Aperture Telescope (SAT) complement will be doubled from three to six telescopes, including a doubling of the detector count in the 93 GHz and 145 GHz channels to 48,160 detectors. Combined with a planned extension of the survey duration to 2035, this expansion will significantly enhance SO's search for a B -mode signal in the polarisation of the cosmic microwave background, a potential signature of gravitational waves produced in the very early Universe. Assuming a 1/ f noise model with knee multipole ℓ _knee = 50 and a moderately complex model for Galactic foregrounds, we forecast a 1 σ (or 68% confidence level) constraint on the tensor-to-scalar ratio r of σ _r = 1.2 × 10 ^-3 , assuming no primordial B-modes are present. This forecast assumes that 70% of the B -mode lensing signal can ultimately be removed using high resolution observations from the SO Large Aperture Telescope (LAT) and overlapping large-scale structure surveys. For more optimistic assumptions regarding foregrounds and noise, and assuming the same level of delensing, this forecast constraint improves to σ _r = 7 × 10 ^-4 . These forecasts represent a major improvement in SO's constraining power, being a factor of around 2.5 times better than what could be achieved with the originally planned campaign, which assumed the existing three SATs would conduct a five-year survey.

We present power spectra of the cosmic microwave background (CMB) anisotropy in temperature and polarization, measured from the Data Release 6 maps made from Atacama Cosmology Telescope (ACT) data. These cover 19,000 deg ² of sky in bands centered at 98, 150 and 220 GHz, with white noise levels three times lower than Planck in polarization. We find that the ACT angular power spectra estimated over 10,000 deg ² , and measured to arcminute scales in TT, TE and EE, are well fit by the sum of CMB and foregrounds, where the CMB spectra are described by the ΛCDM model. Combining ACT with larger-scale Planck data, the joint P-ACT dataset provides tight limits on the ingredients, expansion rate, and initial conditions of the universe. We find similar constraining power, and consistent results, from either the Planck power spectra or from ACT combined with WMAP data, as well as from either temperature or polarization in the joint P-ACT dataset. When combined with CMB lensing from ACT and Planck , and baryon acoustic oscillation data from the Dark Energy Spectroscopic Instrument (DESI DR1), we measure a baryon density of Ω _b h ² = 0.0226 ± 0.0001, a cold dark matter density of Ω _c h ² = 0.118 ± 0.001, a Hubble constant of H ₀ = 68.22 ± 0.36 km/s/Mpc, a spectral index of n _s = 0.974 ± 0.003, and an amplitude of density fluctuations of σ ₈ = 0.813 ± 0.005. Including the DESI DR2 data tightens the Hubble constant to H ₀ = 68.43 ± 0.27 km/s/Mpc; ΛCDM parameters agree between the P-ACT and DESI DR2 data at the 1.6 σ level. We find no evidence for excess lensing in the power spectrum, and no departure from spatial flatness. The contribution from Sunyaev-Zel'dovich (SZ) anisotropy is detected at high significance; we find evidence for a tilt with suppressed small-scale power compared to our baseline SZ template spectrum, consistent with hydrodynamical simulations with feedback.

We present Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) maps of the Cosmic Microwave Background temperature and polarization anisotropy at arcminute resolution over three frequency bands centered on 98, 150 and 220 GHz. The maps are based on data collected with the AdvancedACT camera over the period 2017–2022 and cover 19,000 square degrees with a median combined depth of 10 μK arcmin. We describe the instrument, mapmaking and map properties and illustrate them with a number of figures and tables. The ACT DR6 maps and derived products are available on LAMBDA at https://lambda.gsfc.nasa.gov/product/act/actadv_prod_table.html . We also provide an interactive web atlas at https://phy-act1.princeton.edu/public/snaess/actpol/dr6/atlas and HiPS data sets in Aladin (e.g. https://alasky.cds.unistra.fr/ACT/DR4DR6/color_CMB ).

We use new cosmic microwave background (CMB) primary temperature and polarization anisotropy measurements from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) to test foundational assumptions of the standard cosmological model, ΛCDM, and set constraints on extensions to it. We derive constraints from the ACT DR6 power spectra alone, as well as in combination with legacy data from the Planck mission. To break geometric degeneracies, we include ACT and Planck CMB lensing data and baryon acoustic oscillation data from DESI Year-1. To test the dependence of our results on non-ACT data, we also explore combinations replacing Planck with WMAP and DESI with BOSS, and further add supernovae measurements from Pantheon+ for models that affect the late-time expansion history. We verify the near-scale-invariance (running of the spectral index dn _s / d ln k = 0.0062 ± 0.0052) and adiabaticity of the primordial perturbations. Neutrino properties are consistent with Standard Model predictions: we find no evidence for new light, relativistic species that are free-streaming ( N _eff = 2.86 ± 0.13, which combined with astrophysical measurements of primordial helium and deuterium abundances becomes N _eff = 2.89 ± 0.11), for non-zero neutrino masses (∑ m _ν < 0.089 eV at 95% CL), or for neutrino self-interactions. We also find no evidence for self-interacting dark radiation ( N _idr < 0.134), or for early-universe variation of fundamental constants, including the fine-structure constant ( α _EM / α _EM,0 = 1.0043 ± 0.0017) and the electron mass ( m _e / m _e,0 = 1.0063 ± 0.0056). Our data are consistent with standard big bang nucleosynthesis (we find Y _p = 0.2312 ± 0.0092), the COBE/FIRAS -inferred CMB temperature (we find T _CMB = 2.698 ± 0.016 K), a dark matter component that is collisionless and with only a small fraction allowed as axion-like particles, a cosmological constant ( w = -0.986 ± 0.025), and the late-time growth rate predicted by general relativity ( γ = 0.663 ± 0.052). We find no statistically significant preference for a departure from the baseline ΛCDM model. In fits to models invoking early dark energy, primordial magnetic fields, or an arbitrary modified recombination history, we find H ₀ = 69.9 ^+0.8 _-1.5 , 69.1 ± 0.5, or 69.6 ± 1.0 km/s/Mpc, respectively; using BOSS instead of DESI BAO data reduces the central values of these constraints by 1–1.5 km/s/Mpc while only slightly increasing the error bars. In general, models introduced to increase the Hubble constant or to decrease the amplitude of density fluctuations inferred from the primary CMB are not favored over ΛCDM by our data.

Optical communication can revolutionize satellite networks by enabling high-data-rate, power-efficient links. The CubeSat Laser Infrared CrosslinK (CLICK) mission demonstrates optical crosslinks between small spacecraft using commercially available components in a low size, weight, and power (SWaP) 3U CubeSat platform. The mission consists of CLICK-A, which demonstrated an optical downlink from low-Earth orbit in 2022, and CLICKB/C, which will establish full-duplex optical crosslink communication at 1550 nm, and data rates of up to 20 Mbps over 25–450 km. CLICK-B/C’s fine-pointing control combines spacecraft body pointing with a fine steering mirror using MEMS technology. This work focuses on the alignment of the CLICK-B/C full-duplex crosslink optical terminals. The payloads use two beams, a beacon beam at 980 nm and the data transmit beam at 1537 and 1563 nm, each transmit wavelength corresponding to a specific payload. In order to meet the mission data rate and range requirements, the alignment objective for the optical terminal is to achieve a 0.121 mrad transmit beam divergence and 39 µrad pointing accuracy. Optical link budgets for CLICK are used to assess the impact of optical alignment on mission performance, modeling as-built conditions including optical power, beam divergence, pointing accuracy, and receiver sensitivity. Methods for alignment and divergence testing have been developed, with measured flight telescope divergences of 0.125 ± 0.007 mrad. Optical alignment of the full terminals and their integration with the spacecraft are also underway. In this work, we discuss the alignment and evaluation of the as-built CLICK optical terminal, and use a Python optical link budget model with updated parameters such as divergence, alignment, pointing losses, and inter-satellite distance and assess effects on the mission signal-to-noise ratio (SNR) and bit error rate (BER). Initial investigation into the link budget shows that, with the achieved divergence the operational link range exceeds 350 km and has potential to close beyond 450 km. These results will inform final payload integration and advance high-data-rate optical communication for future small satellite constellations.

According to cosmic microwave background (CMB) measurements, baryonic matter constitutes about 5% of the mass-energy density of the universe. A significant population of these baryons, for a long time referred to as “missing”, resides in a low density, warm-hot intergalactic medium (WHIM) outside galaxy clusters, tracing the “cosmic web”, a network of large scale dark matter filaments. Various studies have detected this inter-cluster gas, both by stacking and by observing individual filaments in compact, massive systems. In this paper, we study short filaments (< 10 Mpc) connecting massive clusters (〈M ₅₀₀〉 ≈ 3 × 10¹⁴ M_⊙) detected by the Atacama Cosmology Telescope (ACT) using the scattering of CMB light off the ionised gas, a phenomenon known as the thermal Sunyaev-Zeldovich (tSZ) effect. The first part of this work is a search for suitable candidates for high resolution follow-up tSZ observations. We identify four cluster pairs with an intercluster signal above the noise floor (S/N > 2), including two with a tentative > 2σ statistical significance for an intercluster bridge from the ACT data alone. In the second part of this work, starting from the same cluster sample, we directly stack on ∼100 cluster pairs and observe an excess SZ signal between the stacked clusters of y = (7.2^+2.3 _-2.5) × 10^-7 with a significance of 3.3σ. It is the first tSZ measurement of hot gas between clusters in this range of masses at moderate redshift (〈z〉 ≈ 0.5). We compare this to the signal from simulated cluster pairs with similar redshifts and separations in the THE300 and MAGNETICUM Pathfinder cosmological simulations and find broad consistency. Additionally, we show that our measurement is consistent with scaling relations between filament parameters and mass of the embedded halos identified in simulations.

Directly imaging and detecting exoplanets with a high-contrast imaging instrument must overcome residual starlight and quasi-static wavefront errors which pose significant challenges for real-time detection. In this work, we combine an Extended Kalman Filter (EKF) and a Convolutional Neural Network (CNN) to enhance exoplanet detection in high-contrast images. The EKF estimates the real and imaginary components of the open-loop electric field as well as the incoherent intensity for each pixel. The EKF is assembled using blocks or cells of pixels in the dark zone where each cell is the size of a potential planet point spread function (PSF). This aids the estimator as the properties of the planet PSF pixels should be related. The cell mapping for the EKF is initialized randomly assuming no prior knowledge of the potential planet location. The CNN is trained and tested on images from the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed at the Space Telescope Science Institute. HiCAT does not have an off-axis light source to emulate a planet on the testbed, so planets are injected via software into the raw testbed images. The CNN is trained to distinguish planetary signals from noise, such as starlight leakage, and to identify potential planets within specific grid cells in an image. Once the CNN identifies a possible planetary signal, the cell mapping and all appropriate EKF components are adjusted to center the detected planet within a single cell. This enables the EKF to refine the estimation of coherent and incoherent light at the updated grid coordinates, allowing for a more dynamic adaptation to changing wavefront errors and helping to ensure optimal suppression of residual starlight. This CNN-EKF approach provides a pathway for integrating machine learning and statistical modeling for real-time, autonomous exoplanet detection. It highlights the potential for advancing the performance of future space-based high-contrast imaging missions. The scope of this paper demonstrated that all major components of the autonomous detection pipeline are functional without causing delays or contrast degradation.