Jens Chluba

Dark photons are a gauge boson of a hypothetical dark sector, representing one of the most-studied minimal extensions of the Standard Model, with wide-ranging theoretical and observational implications. Here, we consider scenarios in which an initially unpopulated dark photon sector is populated via resonant photon to dark photon conversions. This process leads to observable spectral distortions in the cosmic microwave background (CMB), that can be used to constrain these models. We extend previous spectral distortion studies of the monopole spectrum to anisotropic spectral distortions, using the newly developed Frequency Hierarchy (FH) framework of CosmoTherm . We illustrate the physics by presenting detailed computations of the photon transfer functions and distortion cross power spectra throughout the dark photon parameter space. We find that the dark photon mass explicitly controls the shape (i.e., multipole-dependence) of the signal power spectra, while the overall amplitude of the signal is determined by the kinetic mixing parameter of the model. Using these results, we place complementary limits on the minimal dark photon model using data from Planck , finding that the constraints are only marginally weaker than those obtained with COBE/FIRAS data for the average (monopole) distortion. In addition, we compute the corrections to the standard temperature field, arguing that conversions at z ≳ 2 × 10 ⁶ may add iso-curvature type perturbations, which could lead to novel constraints in regimes where distortion anisotropies thermalize.

Spectral distortion anisotropies of the cosmic microwave background (CMB) provide a new probe of the early Universe that can be accessed using traditional CMB imaging techniques. It is possible to compute the creation and evolution of anisotropic signals for various scenarios using the frequency hierarchy method recently developed for CosmoTherm . However, the current treatment is not perfect and some approximations had to be made. Here, we carefully construct a modified form for the evolution equations that has the full equilibrium solutions built into the formulation. We improve the formalism to account for i) additional stimulated scattering effects, ii) kinematic corrections to the thermalization terms, iii) corrections to the standard perturbation variables and iv) direct photon sources. These effects could not be captured with the original formulation of the frequency hierarchy method, but are indeed important for cleanly separating real distortions from temperature signals. However, we show that previous results are not altered significantly when compared to the improved formulation presented here. As a new worked example, which could not be treated before, we also illustrate how possible changes in the temperature-redshift relation would create spectral distortion anisotropies in the pre-recombination era. The theoretical methods presented here are also an important step towards being able to consistently predict the CMB spectral distortion anisotropies in photon-dark photon and photon-axion conversion scenarios.

The repeated scattering of photons by thermal electrons at low temperatures is described by the Kompaneets equation and its generalized forms that include anisotropies and higher order temperature corrections. In this work, we use the boost operator approach to derive the related expressions in a transparent way that showcases the generality of the formalism and its application to radiative transfer problems. We consider the simplest form of the Kompaneets equation for the scattering in isotropic media at the leading order in the electron temperature and then include anisotropies in the photon field, reproducing previously obtained expressions for the evolution equations. For this we use expressions for the scattering operator in the electron rest frame up to first order in the electron recoil, 𝒪( hν / m _e c ² ), but then work at all orders in the electron momentum, p, as easily obtained with the boost operator approach. This shows how specific transformation rules can be formulated that allow simplification of the otherwise cumbersome and repetitive calculations. We also confirm the expressions for higher order temperature corrections in isotropic media, highlighting the validity of the approach presented here. As part of the derivation, we find expressions for the boost operator in general boost directions which we believe will also be useful in other applications of the formalism.

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.

Secondary anisotropies in the cosmic microwave background (CMB) contain information that can be used to test both cosmological models and models of galaxy formation. Starting from light-cone-based healpix maps and catalogues, we present a new set of mock CMB maps constructed in a self-consistent manner from the flamingo suite of cosmological hydrodynamical simulations, including CMB lensing, thermal and kinetic Sunyaev–Zel’dovich effects, cosmic infrared background, radio point source and anisotropic screening maps. We show that these simulations reproduce a wide range of observational constraints. We also compare our simulations with previous predictions based on dark matter-only simulations which generally model the secondary anisotropies independently from one another, concluding that our hydrodynamical simulation mocks perform at least as well as previous mocks in matching the observations whilst retaining self-consistency in the predictions of the different components. Using the model variations in flamingo, we further explore how the signals depend on cosmology and feedback modelling, and we predict cross-correlations between some of the signals that differ significantly from those in previous mocks. The mock CMB maps should provide a valuable resource for exploring correlations between different secondary anisotropies and other large-scale structure tracers, and can be applied to forecasts for upcoming surveys.

Recent observations of anomalous microwave emission (AME) reveal spectral features that are not readily reproduced by spinning dust models. We examine how dust grain distributions and environmental parameters determine the peak frequency and spectral width of AME spectral energy distribution (SED). Using Monte Carlo sampling and global sensitivity analysis, we find that AME features are dominantly controlled by three parameters: grain size, shape, and a phase-dependent environmental parameter. We also quantify the effects of SED broadening from ensembles of these dominant parameters, finding that the level of tension with observations is strongly phase dependent: molecular cloud (MC) is fully consistent, dark cloud (DC) shows minor deviations, and H ii regions exhibit significant offsets in peak frequency. The discrepancy in H ii echoes the observed depletion of small dust grains, particularly polycyclic aromatic hydrocarbons, in H ii regions. However, an observational H ii region may still have AME originating from nearby non-H ii clouds. In this case, model calculations for H ii regions would be inappropriate. Reproducing MC and DC AME features requires ensemble variations in both grain size and environmental parameters are required to reproduce the observed spread in peak frequency and spectral width. We further propose moment expansion and emulation-based inference methods for future AME spectral analysis.

We extend the recent boost operator formalism for relativistic Compton scattering calculations to also account for polarization. This allows us to provide general, exact expressions for the polarized Sunyaev–Zeldovich (SZ) effect sourced both kinematically and from intrinsic anisotropies of the Cosmic Microwave Background. The results are given in terms of rational operator functions that can be used to generate distortion spectra that describe the general SZ signal, reproducing the classical polarized SZ results in the appropriate limits. Our derivation allows for clear separation of physical effects in the generation of polarized SZ, and beyond the SZ application provides a general description of the Compton collision term in the Doppler-dominated regime. Through direct computation of important example cases we further illustrate the power of this new method.

The Sunyaev–Zeldovich (SZ) effect provides a powerful cosmological probe. We demonstrate that the corresponding relativistic SZ signal can be accurately calculated using the recently developed boost operator approach. We obtain formally exact expressions for the required differential operator that can be used to generate relativistic temperature and velocity correction functions to any order. Many of the otherwise cumbersome intermediate steps can be avoided and the required boost operator elements can be quickly generated using recurrence relations for the underlying aberration kernel. We confirm previous analytic expressions describing the relativistic SZ effect and give new expressions at third order in the cluster’s peculiar velocity, demonstrating the feasibility of the boost operator method. Our derivation also highlights general properties of the boost operator and showcases its application to radiative transfer problems of broader interest.