The polarized light of the cosmic microwave background is sensitive to new physics that violates parity symmetry. For example, the interaction of photons with the fields of elusive dark matter and dark energy could cause a uniform rotation of the plane of linear polarization across the sky, an effect known as cosmic birefringence. We extract the cosmological rotation angle, $\beta$, using Bayesian analysis of parity-violating correlations, $EB$ and $TB$, of polarization data from the Atacama Cosmology Telescope (ACT) Data Release 6. We use prior probabilities for instrumental miscalibration angles derived from the optics model for the ACT telescope and instruments, and marginalize over a residual intensity-to-polarization leakage. We measure $\beta = 0.215^\circ\pm 0.074^\circ$ (68\% confidence level), which excludes $\beta=0$ with a statistical significance of $2.9\sigma$. Although there remain systematics in the ACT data that are not understood and do not allow us to draw strong cosmological conclusions, this result is consistent with previous independent results from the \wmap\ and \planck\ missions. It is suggestive that independent data sets and analyses using different methodologies have yielded the same sign and comparable magnitudes for $\beta$.

The hot and dense core formed in the collapse of a massive star is a powerful source of hypothetical feebly-interacting particles such as sterile neutrinos, dark photons, axion-like particles (ALPs), and others. Radiative decays such as $a\to2\gamma$ deposit this energy in the surrounding material if the mean free path is less than the radius of the progenitor star. For the first time, we use a supernova (SN) population with particularly low explosion energies as the most sensitive calorimeters to constrain this possibility. These SNe are observationally identified as low-luminosity events with low ejecta velocities and low masses of ejected $^{56}$Ni. Their low energies limit the energy deposition from particle decays to less than about 0.1 B, where $1~{\rm B~(bethe)}=10^{51}~{\rm erg}$. For 1-500 MeV-mass ALPs, this generic argument excludes ALP-photon couplings $G_{a\gamma\gamma}$ in the $10^{-10}$-$10^{-8}~{\rm GeV}^{-1}$ range.

We study magnetic conversion of ultra-relativistic axion-like particles (ALPs) into photons in compact-star environments, focusing on the hot, transient conditions of core-collapse supernova (SN) remnants and neutron-star mergers (NSMs). We address previously overlooked uncertainties, particularly the suppression caused by ejected matter near the stellar surface, a region crucial to the conversion process. We derive analytical expressions for the transition rate; they reveal the influence of key parameters and their uncertainties. We update constraints using historical gamma-ray data from SN~1987A and find g_{a\gamma}<5\times10^{-12}~{\rm GeV}^{-1} for $m_a\lesssim10^{-9}$ meV. We also forecast sensitivities for a future Galactic SN and for NSMs, assuming observations with Fermi-LAT or similar gamma-ray instruments. We distinguish ALPs -- defined as coupling only to photons and produced via Primakoff scattering -- from axions, which also couple to nucleons and emerge through nuclear bremsstrahlung. We omit pionic axion production due to its large uncertainties and inconsistencies, though it could contribute comparably to bremsstrahlung under optimistic assumptions. For the compact sources, we adopt time-averaged one-zone models, guided by numerical simulations, to enable clear and reproducible parametric studies.

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 Abell 399-Abell 401 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 $\lesssim$600 km s$^{-1}$, which are competitive with current state-of-the-art measurements. Additionally, we measure the optical depth of the filament component with a signal-to-noise of 8.5$\sigma$ and reveal hints of its morphology. This modular approach is well-suited for application to future instruments across a wide range of millimeter and sub-millimeter wavebands.

We present primordial non-Gaussianity predictions from a new high-precision code for simulating axion-U(1) inflation on a discrete lattice. We measure the primordial scalar curvature power spectrum and bispectrum from our simulations, determining their dependence on both scale and axion-gauge coupling strength. Both the gauge-sourced power spectrum and the bispectrum exhibit a strong blue tilt due to our choice of an $\alpha$-attractor inflaton potential. We provide fitting functions for the power spectrum and bispectrum that accurately reproduce these statistics across a wide range of scales and coupling strengths. While our fitting function for the bispectrum has a separable form, results from high-resolution simulations demonstrate that the full shape is not separable. Thus, our simulations generate realizations of primordial curvature perturbations with nontrivial correlators that cannot be generated using standard techniques for primordial non-Gaussianity. We derive bounds on the axion-gauge coupling strength based on the bispectrum constraints from the cosmic microwave background, demonstrating a new method for constraining inflationary primordial non-Gaussianity by simulating the nonlinear dynamics.

The core of a massive star (M > 8 Msun) eventually collapses. This implosion usually triggers a supernova (SN) explosion that ejects most of the stellar envelope and leaves behind a neutron star (NS) with a mass of up to about 2 Msun. Sometimes the explosion fails and a black hole forms instead. The NS radiates its immense binding energy (some 10% of its rest mass or $2-4\times10^{53}$ erg) almost entirely as neutrinos and antineutrinos of all flavors with typical energies of some 10 MeV. This makes core-collapse SNe the most powerful neutrino factories in the Universe. Such a signal was observed once - with limited statistics - from SN 1987A in the Large Magellanic Cloud. Today, however, many large neutrino detectors act as SN observatories and would register a high-statistics signal. A future Galactic SN, though rare (1-3 per century), would produce a wealth of astrophysical and particle-physics information, including possible signatures for new particles. Neutrinos are key to SN dynamics in the framework of the Bethe-Wilson delayed explosion paradigm. After collapse, they are trapped in the core for a few seconds, forming a dense neutrino plasma that can exhibit collective flavor evolution caused by the weak interaction, a subject of intense theoretical research.

The Population III.1 theory for supermassive black hole (SMBH) formation predicts a very early ($z\sim20-25$), transient phase, ``The Flash'', of cosmic reionization powered by supermassive stars that are SMBH progenitors. The universe then quickly recombined to become mostly neutral, with this state persisting until galaxies begin to reionize intergalactic gas again at $z\sim 10$. The overall Thomson scattering optical depth, $\tau$, from The Flash has been shown to be $\tau_{\rm PopIII.1}\sim0.03$, leading to a total $\tau\sim0.08-0.09$. Such a value, while significantly larger than that previously inferred from {\it Planck} observations of the low-$l$ $EE$ polarization power spectrum of the CMB, can help relieve several ``tensions'' faced by the standard $\Lambda$CDM cosmological model, especially the preference for negative neutrino masses and dynamic dark energy. Here we compute $EE$ power spectra of example models of The Flash. We find that, because of its very high redshift, the contribution to $l\lesssim8$ modes is dramatically reduced compared to usual low-$z$ reionization models for the same value of $\tau$, while the power at $l\gtrsim8$ is boosted. Thus the Pop III.1 reionization scenario provides a natural way to increase $\tau$, while remaining closer to the latest CMB low-$l$ polarization observations.

We present optical + near-infrared (NIR) + mid-infrared (MIR) observations of the normal Type Ia supernovae (SN Ia) 2022aaiq and 2024gy in the nebular phase, continuously spanning 0.35-28 microns. Medium-resolution JWST spectroscopy reveals novel narrow (v_{\mathrm{FWHM}}<1500 km s$^{-1}$) [Ni II] 1.94 and 6.64 micron cores in both events. The MIR [Ni II] 6.64 micron line exhibits a distinct narrow core atop a broader base, indicating a central enhancement of stable Ni. This structure points to high central densities consistent with a near-Chandrasekhar-mass ($M_{Ch}$) progenitor or a high-metallicity sub-$M_{Ch}$ progenitor. From detailed line-profile inversions of SN 2024gy, we derive emissivity profiles for stable iron-group elements (IGEs), radioactive material, and intermediate-mass elements (IMEs), revealing spatially distinct ejecta zones. The [Ni III] 7.35 micron line shows a shallow-to-steep slope transition -- a "broken-slope" morphology -- that matches predictions for delayed detonation explosions with separated deflagration and detonation ashes. We also reanalyze and compare to archival JWST spectra of SN 2021aefx and the subluminous SN 2022xkq. We estimate a stable $^{58}$Ni mass of $\sim0.1$ M$_\odot$ for SN 2024gy, consistent with delayed detonation models, and $\sim0.01$ M$_\odot$ for SN 2022xkq, favoring sub-$M_{Ch}$ scenarios. These results demonstrate that resolved line profiles, now accessible with JWST, provide powerful diagnostics of explosion geometry, central density, and progenitor mass in SN Ia.

Simulation-based inference (SBI) allows fast Bayesian inference for simulators encoding implicit likelihoods. However, some explicit likelihoods cannot be easily reformulated as simulators, hindering their integration into combined analyses within SBI frameworks. One key example in cosmology is given by the Planck CMB likelihoods. We present a simple method to construct an effective simulator for any explicit likelihood using samples from a previously converged Markov Chain Monte Carlo (MCMC) run. This effective simulator can subsequently be combined with any forward simulator. To illustrate this method, we combine the full Planck CMB likelihoods with a 3x2pt simulator (cosmic shear, galaxy clustering and their cross-correlation) for a Stage IV survey like Euclid, and test evolving dark energy parameterized by the $w_0w_a$ equation-of-state. Assuming the $w_0w_a$CDM cosmology hinted by DESI BAO DR2 + Planck 2018 + PantheonPlus SNIa datasets, we find that future 3x2pt data alone could detect evolving dark energy at $5\sigma$, while its combination with current CMB, BAO and SNIa datasets could raise the detection to almost $7\sigma$. Moreover, thanks to simulation reuse enabled by SBI, we show that our joint analysis is in excellent agreement with MCMC while requiring zero Boltzmann solver calls. This result opens up the possibility of performing massive global scans combining explicit and implicit likelihoods in a highly efficient way.

The developments summarized with the name ``weak gravitational lensing'' have led to exciting possibilities to study the (statistical properties of the) dark matter distribution in the Universe. Concentrating on those aspects which require deep wide-field imaging surveys, we describe the basic principles of weak lensing and discuss its future applications in view of NGST (a) to determine the statistical properties of the dark matter halos of individual galaxies, (b) to determine the mass and the mass profile of very low-mass clusters and groups at medium redshift and/or of more massive clusters at very high redshift, and (c) to measure the power spectrum of the matter distribution in the Universe in the non-linear regime, thereby also obtaining a mass-selected sample of halos and providing a means to investigate the scale- and redshift dependence of the bias `factor'.

We study magnetic conversion of ultra-relativistic axion-like particles (ALPs) into photons in compact-star environments, focusing on the hot, transient conditions of core-collapse supernova (SN) remnants and neutron-star mergers (NSMs). We address previously overlooked uncertainties, particularly the suppression caused by ejected matter near the stellar surface, a region crucial to the conversion process. We derive analytical expressions for the transition rate; they reveal the influence of key parameters and their uncertainties. We update constraints using historical gamma-ray data from SN~1987A and find g_{a\gamma}<5\times10^{-12}~{\rm GeV}^{-1} for $m_a\lesssim10^{-9}$ meV. We also forecast sensitivities for a future Galactic SN and for NSMs, assuming observations with Fermi-LAT or similar gamma-ray instruments. We distinguish ALPs -- defined as coupling only to photons and produced via Primakoff scattering -- from axions, which also couple to nucleons and emerge through nuclear bremsstrahlung. We omit pionic axion production due to its large uncertainties and inconsistencies, though it could contribute comparably to bremsstrahlung under optimistic assumptions. For the compact sources, we adopt time-averaged one-zone models, guided by numerical simulations, to enable clear and reproducible parametric studies.

Constraints on local primordial non-Gaussianity (LPnG) obtained from galaxy power spectra are limited by the perfect degeneracy between the LPnG parameter $f_{\rm NL}$ and the bias parameter $b_{\phi}$ which encodes the response of galaxy clustering to a change in the amplitude of primordial curvature fluctuations. For galaxies observed by galaxy surveys, the relation between $b_{\phi}$ and the galaxy bias $b_{g}$ is poorly understood and differs significantly from the universal mass function ansatz. In this paper, we investigate this non-universality in the context of dark-matter halos using the Separate Universe framework, focussing on dark-matter halos selected by mass and/or concentration. We show that the Separate Universe framework provides a natural explanation of the observed universality in the bias of dark-matter halos selected purely by their mass, independent of the spherical collapse picture of halo formation. We further propose an explanation for the observed non-universality in halos selected by concentration and corroborate it with $N$-body simulations in scale-free (EdS) and $\Lambda\text{CDM}$ cosmologies. In particular, we show that the relation between $b_{\phi}$ and halo bias $b_{h}$ for halos selected by concentration in matter-dominated cosmologies tends towards universality at the highest halo masses due to such halos gravitationally dominating their environment throughout their evolution. We also argue that concentration-selected halos of lower masses exhibit non-universality due to their mass accretion being significantly affected by the gravitational influence of neighbouring, more massive halos. Our results suggest that any non-universality in high redshift ($z\gtrsim 3$), high-bias objects observed by realistic galaxy surveys is entirely an artifact of the associated selection function.

Signal inference problems with non-Gaussian posteriors can be hard to tackle. Through using the concept of Gibbs free energy these posteriors are rephrased as Gaussian posteriors for the price of computing various expectation values with respect to a Gaussian distribution. We present a new way of translating these expectation values to a language of operators which is similar to that in quantum mechanics. This simplifies many calculations, for instance such involving log-normal priors. The operator calculus is illustrated by deriving a novel self-calibrating algorithm which is tested with mock data.

We present relations between X-ray luminosity and velocity dispersion (L-sigma), X-ray luminosity and gas mass (L-Mgas), and cluster radius and velocity dispersion (r500-sigma) for 62 galaxy clusters in the HIFLUGCS, an X-ray flux-limited sample minimizing bias toward any cluster morphology. Our analysis in total is based on ~1.3Ms of clean X-ray XMM-Newton data and 13439 cluster member galaxies with redshifts. Cool cores are among the major contributors to the scatter in the L-sigma relation. When the cool-core-corrected X-ray luminosity is used the intrinsic scatter decreases to 0.27 dex. Even after the X-ray luminosity is corrected for the cool core, the scatter caused by the presence of cool cores dominates for the low-mass systems. The scatter caused by the non-cool-core clusters does not strongly depend on the mass range, and becomes dominant in the high-mass regime. The observed L-sigma relation agrees with the self-similar prediction, matches that of a simulated sample with AGN feedback disregarding six clusters with <45 cluster members with spectroscopic redshifts, and shows a common trend of increasing scatter toward the low-mass end, i.e., systems with sigma<500km/s. A comparison of observations with simulations indicates an AGN-feedback-driven impact in the low-mass regime. The best fits to the $L-M_{\rm gas}$ relations for the disturbed clusters and undisturbed clusters in the observational sample closely match those of the simulated samples with and without AGN feedback, respectively. This suggests that one main cause of the scatter is AGN activity providing feedback in different phases, e.g., during a feedback cycle. The slope and scatter in the observed r500-sigma relation is similar to that of the simulated sample with AGN feedback except for a small offset but still within the scatter.

We propose a novel method for the description of spatial patterns formed by a coverage of point sets representing galaxy samples. This method is based on a complete family of morphological measures known as Minkowski functionals, which includes the topological Euler characteristic and geometric descriptors to specify the content, shape and connectivity of spatial sets. The method is numerically robust even for small samples, independent of statistical assumptions, and yields global as well as local morphological information. We illustrate the method by applying it to a Poisson process, a `double-Poisson' process, and to the Abell catalogue of galaxy clusters.

We introduce xCOLD GASS, a legacy survey providing a census of molecular gas in the local Universe. Building upon the original COLD GASS survey, we present here the full sample of 532 galaxies with CO(1-0) measurements from the IRAM-30m telescope. The sample is mass-selected in the redshift interval $0.0110^9$M$_{\odot}$. The CO(1-0) flux measurements are complemented by observations of the CO(2-1) line with both the IRAM-30m and APEX telescopes, HI observations from Arecibo, and photometry from SDSS, WISE and GALEX. Combining the IRAM and APEX data, we find that the CO(2-1) to CO(1-0) luminosity ratio for integrated measurements is $r_{21}=0.79\pm0.03$, with no systematic variations across the sample. The CO(1-0) luminosity function is constructed and best fit with a Schechter function with parameters {$L_{\mathrm{CO}}^* = (7.77\pm2.11) \times 10^9\,\mathrm{K\,km\,s^{-1}\, pc^{2}}$, $\phi^{*} = (9.84\pm5.41) \times 10^{-4} \, \mathrm{Mpc^{-3}}$ and $\alpha = -1.19\pm0.05$}. With the sample now complete down to stellar masses of $10^9$M$_{\odot}$, we are able to extend our study of gas scaling relations and confirm that both molecular gas fraction and depletion timescale vary with specific star formation rate (or offset from the star-formation main sequence) much more strongly than they depend on stellar mass. Comparing the xCOLD GASS results with outputs from hydrodynamic and semi-analytic models, we highlight the constraining power of cold gas scaling relations on models of galaxy formation.

The dispersion measures (DMs) of Fast Radio Bursts (FRBs) arise predominantly from free electrons in the large-scale structure of the Universe. The increasing number of FRB observations have started to empirically constrain the distribution of cosmic baryons, making it crucial to accurately forward model their propagation within cosmological simulations. In this work, we present a method for measuring FRB DMs in IllustrisTNG that continuously traces rays through the simulation while reconstructing all traversed line segments within the underlying Voronoi mesh. Leveraging this technique, we create over $20$ publicly available DM catalogs, including a full-sky DM map observed from a Milky Way-like environment. Our method addresses a problem in previous TNG-based studies, in which a sparse snapshot sampling in the path integral leads to a misestimation of the standard deviation and higher moments of the DM distribution $p(\rm{DM}|z)$ by over $50\%$. We show that our results are consistent with the most recent observational data from the DSA-110, ASKAP, and CHIME. We offer a functional form for $p(\rm{DM}| z)$ that provides very good fits across all redshifts from $0$ to $5.5$, showing that the previously proposed log-normal distribution is not well matched to the data. We find that using simulation box sizes smaller than $35\,h^{-1}\,$Mpc or resolutions with baryonic masses of less than $5\times10^{8}\,h^{-1}\,M_{\odot}$ can distort the derived DM signal by more than $8\%$. Our findings provide new insights into how the cosmic web shapes FRB signals, and highlight the importance of accurate methodology when comparing cosmological simulations against observations.

We introduce the Virgo Consortium's EAGLE project, a suite of hydrodynamical simulations that follow the formation of galaxies and black holes in representative volumes. We discuss the limitations of such simulations in light of their finite resolution and poorly constrained subgrid physics, and how these affect their predictive power. One major improvement is our treatment of feedback from massive stars and AGN in which thermal energy is injected into the gas without the need to turn off cooling or hydrodynamical forces, allowing winds to develop without predetermined speed or mass loading factors. Because the feedback efficiencies cannot be predicted from first principles, we calibrate them to the z~0 galaxy stellar mass function and the amplitude of the galaxy-central black hole mass relation, also taking galaxy sizes into account. The observed galaxy mass function is reproduced to $\lesssim 0.2$ dex over the full mass range, 10^8 &lt; M_*/M_\odot \lesssim 10^{11}, a level of agreement close to that attained by semi-analytic models, and unprecedented for hydrodynamical simulations. We compare our results to a representative set of low-redshift observables not considered in the calibration, and find good agreement with the observed galaxy specific star formation rates, passive fractions, Tully-Fisher relation, total stellar luminosities of galaxy clusters, and column density distributions of intergalactic CIV and OVI. While the mass-metallicity relations for gas and stars are consistent with observations for $M_* \gtrsim 10^9 M_\odot$, they are insufficiently steep at lower masses. The gas fractions and temperatures are too high for clusters of galaxies, but for groups these discrepancies can be resolved by adopting a higher heating temperature in the subgrid prescription for AGN feedback. EAGLE constitutes a valuable new resource for studies of galaxy formation.