Cosmic Microwave Background Spectral Distortions Research Articles

New mass window for primordial black holes as dark matter from the memory burden effect

The mass ranges allowed for primordial black holes (PBHs) to constitute all of dark matter (DM) are broadly constrained. However, these constraints rely on the standard semiclassical approximation which assumes that the evaporation process is self-similar. Quantum effects such as memory burden take the evaporation process out of the semiclassical regime latest by the time the black hole loses half of its mass. What happens beyond this time is currently not known. However, theoretical evidence based on prototype models indicates that the evaporation slows down, thereby extending the lifetime of a black hole. This modifies the mass ranges constrained, in particular, by big bang nucleosynthesis (BBN) and cosmic microwave background spectral distortions. We show that previous constraints are largely relaxed when the PBH lifetime is extended, making it possible for PBHs to constitute all of DM in previously excluded mass ranges. In particular, this is the case for PBHs lighter than 109 g that enter the memory burden stage before BBN and are still present today as DM. Published by the American Physical Society 2024

CMB spectral distortions from enhanced primordial perturbations: the role of spectator axions

Primordial scalar and tensor modes can induce Cosmic Microwave Background spectral distortions during horizon re-entry. We investigate a specific mechanism proposed for this purpose, characterized by the coupling of an SU(2) gauge field to an axion undergoing a momentary stage of rapid evolution during inflation. Examining in details the perturbations produced by this model, we find that spectral distortions from the scalar modes significantly dominate those arising from the tensors. This holds true also for an earlier version of the model based on a U(1) gauge field. The scalar-induced distortions might be observed in future experiments, and the current COBE/FIRAS constraints already limit the parameter space of these models. Additionally, we find that delaying the onset of fast roll in the SU(2) scenario (to enhance the modes at the scales relevant for spectral distortions, while respecting the CMB constraints at larger scales) poses a greater challenge compared to the U(1) case. We propose a way to control the axion speed by varying the size of its coupling to the gauge fields.

THEMIS 2.0: A self-consistent model for dust extinction, emission, and polarisation

Context. Recent observational constraints in emission, extinction, and polarisation have at least partially invalidated most of the astronomical standard grain models for the diffuse interstellar medium. Moreover, laboratory measurements on interstellar silicate analogues have shown quite significant differences with the optical properties used in these standard models. Aims. To address these issues, our objective is twofold: (i) to update the optical properties of silicates and (ii) to develop The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS) to allow the calculation of polarised extinction and emission. Methods. Based on optical constants measured in the laboratory from 5 µm to 1 mm for amorphous silicates and on observational constraints in mid-IR extinction and X-ray scattering, we defined new optical constants for the THEMIS silicates. Absorption and scattering efficiencies for spheroidal grains using these properties were subsequently derived with the discrete dipole approximation. Results. These new optical properties make it possible to explain the dust emission and extinction, both total and polarised. It is noteworthy that the model is not yet pushed to its limits since it does not require the perfect alignment of all grains to explain the observations and it therefore has the potential to accommodate the highest polarisation levels inferred from extinction measurements. Moreover, the dispersion of the optical properties of the different silicates measured in the laboratory naturally explain the variations in both the total and polarised emission and extinction observed in the diffuse interstellar medium. Conclusions. A single, invariant model calibrated on one single set of observations is obsolete for explaining contemporary observations. We are proposing a completely flexible dust model based entirely on laboratory measurements that has the potential to make major advances in understanding the exact nature of interstellar grains and how they evolve as a function of their radiative and dynamic environment. Even if challenging, this is also relevant for future cosmic microwave background (CMB) missions that will aim to perform precise measurements of the CMB spectral distortions and polarisation.

Towards measurements of CMB spectral distortions

High-precision spectroscopy of the Cosmic Microwave Background (CMB) is one of the three themes that have been selected by the ESA Voyage 2050 programme. Several space missions dedicated to the measurement of the CMB spectral distortions such as PIXIE, PRISTINE, and FOSSIL have been proposed since 2011. Additionally, balloon-borne projects, such as BISOU, have also been considered as a pathfinder for a future space mission. This paper presents an effort undertaken to define future missions and instruments dedicated to the measurement of the CMB spectral distortions. For this purpose, models allowing to optimize both instrument concept and mission parameters (mission duration, scanning strategy, …) have been developed. It has been applied to two specific cases, FOSSIL, a space mission proposed to answer the ESA M7 call in 2022 , and BISOU, a CNES Phase 0 study for a balloon-borne mission.

Frequency–Redshift Relation of the Cosmic Microwave Background

We point out that a modified temperature–redshift relation (T-z relation) of the cosmic microwave background (CMB) cannot be deduced by any observational method that appeals to an a priori thermalisation to the CMB temperature T of the excited states in a probe environment of independently determined redshift z. For example, this applies to quasar-light absorption by a damped Lyman-alpha system due to atomic as well as ionic fine-splitting transitions or molecular rotational bands. Similarly, the thermal Sunyaev-Zel’dovich (thSZ) effect cannot be used to extract the CMB’s T-z relation. This is because the relative line strengths between ground and excited states in the former and the CMB spectral distortion in the latter case both depend, apart from environment-specific normalisations, solely on the dimensionless spectral variable x=hνkBT. Since the literature on extractions of the CMB’s T-z relation always assumes (i) ν(z)=(1+z)ν(z=0), where ν(z=0) is the observed frequency in the heliocentric rest frame, the finding (ii) T(z)=(1+z)T(z=0) just confirms the expected blackbody nature of the interacting CMB at z>0. In contrast to the emission of isolated, directed radiation, whose frequency–redshift relation (ν-z relation) is subject to (i), a non-conventional ν-z relation ν(z)=f(z)ν(z=0) of pure, isotropic blackbody radiation, subject to adiabatically slow cosmic expansion, necessarily has to follow that of the T-z relation T(z)=f(z)T(z=0) and vice versa. In general, the function f(z) is determined by the energy conservation of the CMB fluid in a Friedmann–Lemaitre–Robertson–Walker universe. If the pure CMB is subject to an SU(2) rather than a U(1) gauge principle, then f(z)=1/41/3(1+z) for z≫1, and f(z) is non-linear for z∼1.

Meso-inflationary QCD axion

We study the possibility that the axion Peccei-Quinn symmetry is spontaneously broken after the beginning of inflation. This scenario interpolates between preinflationary and postinflationary axion cosmology with significant phenomenological differences from both. Since the axion is not present at the early stages of inflation large inflationary fluctuations are produced only at scales not constrained by cosmic microwave background (CMB), avoiding the strongest isocurvature constraints. The energy density in isocurvature perturbations at short scales however can be comparable with the adiabatic contribution from misalignment. These large overdensities can lead to the formation of axion miniclusters and also to constraints from Lyman-$\ensuremath{\alpha}$ forest and future CMB spectral distortion measurements. If Peccei-Quinn symmetry is broken during the first O(25) $e$-foldings of inflation no axions are produced from the string network but contributions from the annihilation of domain walls can further boost the abundance. This scenario is minimally realized if the Hubble scale during inflation drops below the Peccei-Quinn symmetry breaking scale but other realizations are possible.

A closer look at dark photon explanations of the excess radio background

ABSTRACT The observed excess radio background has remained a puzzle for over a decade. A recent new physics solution involves dark matter that decays into dark photons in the presence of a thermal dark photon background. The produced non-thermal dark photon spectrum then converts into standard photons around the reionization era, yielding an approximate power-law radio excess with brightness temperature T(ν) ≃ ν−2.5 over a wide range of frequencies, ν. This simple power-law model comes intriguingly close to the current data, even if several ingredients are required to make it work. In this paper, we investigate some of the details of this model, showcasing the importance of individual effects. In particular, significant deviation from a power law is present at $\nu \lesssim 100\, {\rm MHz}$ and $\nu \gtrsim 1\, {\rm GHz}$. These effects result in improving the fit to data compared to a power-law spectrum, and may become testable in future observations. We also highlight independent signatures that can be tested with future cosmic microwave background spectral distortion experiments such as PIXIE. However, there are challenges for the model from the observed radio background anisotropies, as discussed here. We furthermore highlight a possible runaway process due to the finite width of the dark matter decay profile, which suggests that additional work might be required to obtain a viable model.

Cosmological constraints on decaying axion-like particles: a global analysis

Axion-like particles (ALPs) decaying into photons are known to affect a wide range of astrophysical and cosmological observables. In this study we focus on ALPs with masses in the keV–MeV range and lifetimes between 104 and 1013 seconds, corresponding to decays between the end of Big Bang Nucleosynthesis and the formation of the Cosmic Microwave Background (CMB). Using the CosmoBit module of the global fitting framework GAMBIT, we combine state-of-the-art calculations of the irreducible ALP freeze-in abundance, primordial element abundances (including photodisintegration through ALP decays), CMB spectral distortions and anisotropies, and constraints from supernovae and stellar cooling. This approach makes it possible for the first time to perform a global analysis of the ALP parameter space while varying the parameters of ΛCDM as well as several nuisance parameters. We find a lower bound on the ALP mass of around ma > 300 keV, which can only be evaded if ALPs are stable on cosmological timescales. Future observations of CMB spectral distortions with a PIXIE-like mission are expected to improve this bound by two orders of magnitude.

Cosmic microwave background spectral distortions from Rayleigh scattering at second order

Cosmic microwave background (CMB) spectral distortion from Rayleigh scattering is calculated for the first time in rigorous second-order cosmological perturbation theory. The new spectral distortion is sensitive to acoustic dissipation at $10^{-2}<k{\rm Mpc}/h<1$, which slightly extends the scale constrained by the CMB anisotropies. The spectral shape is different from either temperature perturbations or any other traditional spectral distortions from Compton scattering, such as $y$ and $\mu$. The new spectral distortion is not formed in the late Universe, unlike the thermal Sunyaev-Zel'dovich effect degenerated with the primordial $y$ distortions since photons must be hot for Rayleigh scattering. Therefore, ideal measurements can distinguish the signal from the other effects and extract new information during recombination. Assuming cosmological parameters consistent with the recent CMB anisotropy measurements, we find the new spectral distortion is $6.5\times 10^{-3}$Jy/str, which is one order of magnitude smaller than the currently proposed target sensitivity range of voyage 2050.

Measuring CMB Spectral Distortions from Antarctica with COSMO: Blackbody Calibrator Design and Performance Forecast

COSMO is a ground-based instrument to measure the spectral distortions (SD) of the Cosmic Microwave Background (CMB). In this paper, we present preliminary results of electromagnetic simulations of its reference blackbody calibrator. HFSS simulations provide a calibrator reflection coefficient of Rsim 10^{-6}, corresponding to an emissivity epsilon =1-R=0.999999. We also provide a forecast for the instrument performance by using an ILC-based simulation. We show that COSMO can extract the isotropic Comptonization parameter (modeled as |y|=1.77 cdot 10^{-6}) as |y|=(1.79pm 0.19)cdot 10^{-6}, in the presence of the main Galactic foreground (thermal dust) and of CMB anisotropies, and assuming perfect atmospheric emission removal.

CMB spectral distortions revisited: A new take on μ distortions and primordial non-Gaussianities from FIRAS data

Deviations from the blackbody spectral energy distribution of the CMB are a precise probe of physical processes active both in the early universe (such as those connected to particle decays and inflation) and at later times (e.g. reionization and astrophysical emissions). Limited progress has been made in the characterization of these spectral distortions after the pioneering measurements of the FIRAS instrument on the COBE satellite in the early 1990s, which mainly targeted the measurement of their average amplitude across the sky. Since at present no follow-up mission is scheduled to update the FIRAS measurement, in this work we re-analyze the FIRAS data and produce a map of $\mu$-type spectral distortion across the sky. We provide an updated constraint on the $\mu$ distortion monopole $|\langle\mu\rangle|<47\times 10^{-6}$ at 95\% confidence level that sharpens the previous FIRAS estimate by a factor of $\sim 2$. We also constrain primordial non-Gaussianities of curvature perturbations on scales $10\lesssim k\lesssim 5\times 10^4$ through the cross-correlation of $\mu$ distortion anisotropies with CMB temperature and, for the first time, the full set of polarization anisotropies from the Planck satellite. We obtain upper limits on $f_{\rm NL}\lesssim 3.6 \times 10^6$ and on its running $n_{\rm NL}\lesssim 1.4$ that are limited by the FIRAS sensitivity but robust against galactic and extragalactic foreground contaminations. We revisit previous similar analyses based on data of the Planck satellite and show that, despite their significantly lower noise, they yield similar or worse results to ours once all the instrumental and astrophysical uncertainties are properly accounted for. Our work is the first to self-consistently analyze data from a spectrometer and demonstrate the power of such instrument to carry out this kind of science case with reduced systematic uncertainties.

A New Readout Electronic for Kinetic Inductance Detectors

With this contribution we show the readout electronics for kinetic inductance detectors (KIDs) that we are developing based on commercial IQ transceivers from National Instruments and using a Virtex 5 class FPGA. It will be the readout electronics of the COSmic Monopole Observer (COSMO) experiment, a ground based cryogenic Martin–Puplett Interferometer searching for the cosmic microwave background spectral distortions. The readout electronics require a sampling rate in the range of tens of kHz, which is both due to a fast rotating mirror modulating the signal and the time constant of the COSMO KIDs. In this contribution we show the capabilities of our readout electronics using Niobium KIDs developed by Paris Observatory for our 5 K cryogenic system. In particular, we demonstrate the capability to detect 23 resonators from frequency sweeps and to readout the state of each resonator with a sampling rate of about 8 kHz. The readout is based on a finite-state machine where the first two states look for the resonances and generate the comb of tones, while the third one performs the acquisition of phase and amplitude of each detector in free running. Our electronics are based on commercial modules, which brings two key advantages: they can be acquired easily and it is relative simple to write and modify the firmware within the LabView environment in order to meet the needs of the experiment.

CMB spectral distortions from continuous large energy release

ABSTRACT Accurate computations of spectral distortions of the cosmic microwave background (CMB) are required for constraining energy release scenarios at redshifts z ≳ 103. The existing literature focuses on distortions that are small perturbations to the background blackbody spectrum. At high redshifts (z ≳ 106), this assumption can be violated, and the CMB spectrum can be significantly distorted at least during part of its cosmic evolution. In this paper, we carry out accurate thermalization computations, evolving the distorted CMB spectrum in a general, fully non-linear way, consistently accounting for the time-dependence of the injection process, modifications to the Hubble expansion rate and relativistic Compton scattering. Specifically, we study single energy injection and decaying particle scenarios, discussing constraints on these cases. We solve the thermalization problem using two independent numerical approaches that are now available in CosmoTherm as dedicated setups for computing CMB spectral distortions in the large distortion regime. New non-linear effects at low frequencies are furthermore highlighted, showing that these warrant a more rigorous study. This work eliminates one of the long-standing simplifications in CMB spectral distortion computations, which also opens the way to more rigorous treatments of distortions induced by high-energy particle cascade, soft photon injection, and in the vicinity of primordial black holes.

The QCD phase transition behind a PBH origin of LIGO/Virgo events?

The best-motivated scenario for a sizable primordial black hole (PBH) contribution to the LIGO/Virgo binary black hole mergers invokes the QCD phase transition, which naturally enhances the probability to form PBH with masses of stellar scale. We reconsider the expected mass function associated not only to the QCD phase transition proper, but also the following particle antiparticle annihilation processes, and analyse the constraints on this scenario from a number of observations: the specific pattern in cosmic microwave background (CMB) anisotropies induced by accretion onto PBHs, CMB spectral distortions, gravitational wave searches, and direct counts of supermassive black holes (SMBHs) at high redshift. We find that the scenario is not viable, unless an ad hoc mass evolution for the PBH mass function and a cutoff in power-spectrum very close to the QCD scale are introduced by hand. Despite these negative results, we note that a future detection of coalescing binaries involving sub-solar PBHs has the potential to check the cosmological origin of SMBHs at the e ± annihilation epoch, if indeed the PBH mass function is shaped by the changes to the equation of state driven by the thermal history of the universe.

Unlocking the synergy between CMB spectral distortions and anisotropies

Measurements of the cosmic microwave background (CMB) spectral distortions (SDs) will open a new window on the very early universe, providing new information complementary to that gathered from CMB temperature and polarization anisotropies. In this paper, we study their synergy as a function of the characteristics of the considered experiments. In particular, we examine a wide range of sensitivities for possible SD measurements, spanning from FIRAS up to noise levels 1000 times better than PIXIE, and study their constraining power when combined with current or future CMB anisotropy experiments such as Planck or LiteBIRD plus CMB-S4. We consider a number of different cosmological models such as the ΛCDM, as well as its extensions with the running of the scalar spectral index, the decay or the annihilation of dark matter (DM) particles. While upcoming CMB anisotropy experiments will be able to decrease the uncertainties on inflationary parameters such as As and ns by about a factor 2 in the ΛCDM case, we find that an SD experiment 100 times more sensitive than PIXIE (comparable to the proposed Super-PIXIE satellite) could potentially further contribute to constrain these parameters. This is even more significant in the case of the running of the scalar spectral index. Furthermore, as expected, constraints on DM particles decaying at redshifts probed by SDs will improve by orders of magnitude even with an experiment 10 times worse than PIXIE as compared to CMB anisotropies or Big Bang Nucleosynthesis bounds. On the contrary, DM annihilation constraints will not significantly improve over CMB anisotropy measurements. Finally, we forecast the constraints obtainable with sensitivities achievable either from the ground or from a balloon.

Searching for the Radiative Decay of the Cosmic Neutrino Background with Line-Intensity Mapping

We study the possibility to use line-intensity mapping (LIM) to seek photons from the radiative decay of neutrinos in the cosmic neutrino background. The Standard Model prediction for the rate for these decays is extremely small, but it can be enhanced if new physics increases the neutrino electromagnetic moments. The decay photons will appear as an interloper of astrophysical spectral lines. We propose that the neutrino-decay line can be identified with anisotropies in LIM clustering and also with the voxel intensity distribution. Ongoing and future LIM experiments will have-depending on the neutrino hierarchy, transition, and experiment considered-a sensitivity to an effective electromagnetic transition moment ∼10^{-12}-10^{-8}(m_{i}c^{2}/0.1 eV)^{3/2}μ_{B}, where m_{i} is the mass of the decaying neutrino and μ_{B} is the Bohr magneton. This will be significantly more sensitive than cosmic microwave background spectral distortions, and it will be competitive with stellar cooling studies. As a by-product, we also report an analytic form of the one-point probability distribution function for neutrino-density fluctuations, obtained from the quijote simulations using symbolic regression.

Comparison of numerical methods for computing the repeated Compton scattering of photons in isotropic media

ABSTRACT Repeated Compton scattering of photons with thermal electrons is one of the fundamental processes at work in many astrophysical plasma. Solving the exact evolution equations is hard and one common simplification is based on Fokker–Planck (FP) approximations of the Compton collision term. Here we carry out a detailed numerical comparison of several FP approaches with the exact scattering kernel solution for a range of test problems assuming isotropic media and thermal electrons at various temperatures. The Kompaneets equation, being one of the most widely used FP approximations, fails to account for Klein–Nishina corrections and enhanced Doppler boosts and recoil at high energies. These can be accounted for with an alternative FP approach based on the exact first and second moments of the scattering kernel. As demonstrated here, the latter approach works very well in dilute media, but inherently fails to reproduce the correct equilibrium solution in the limit of many scattering. Conditions for the applicability of the FP approximations are clarified, overall showing that the Kompaneets equation provides the most robust approximation to the full problem, even if inaccurate in many cases. We close our numerical analysis by briefly illustrating the solutions for the spectral distortions of the cosmic microwave background (CMB) after photon injection at redshift z ≲ 105, when double Compton and Bremsstrahlung emission can be omitted. We demonstrate that the exact treatment using the scattering kernel computed with cspack is often needed. This work should provide an important step towards accurate computations of the CMB spectral distortions from high-energy particle cascades.

Updated cosmological constraints on Macroscopic Dark Matter

We revise the cosmological phenomenology of Macroscopic Dark Matter (MDM) candidates, also commonly dubbed as Macros. A possible signature of MDM is the capture of baryons from the cosmological plasma in the pre-recombination epoch, with the consequent injection of high-energy photons in the baryon-photon plasma. By keeping a phenomenological approach, we consider two broad classes of MDM in which Macros are composed either of ordinary matter or antimatter. In both scenarios, we also analyze the impact of a non-vanishing electric charge carried by Macros. We derive constraints on the Macro parameter space from three cosmological processes: the change in the baryon density between the end of the Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB) decoupling, the production of spectral distortions in the CMB and the kinetic coupling between charged MDM and baryons at the time of recombination. In the case of neutral Macros we find that the tightest constraints are set by the baryon density condition in most of the parameter space. For Macros composed of ordinary matter and with binding energy I, this leads to the following bound on the reduced cross-section: σX/M X ≲ 6.8 · 10-7(I/MeV)-1.56 cm2 g-1. Charged Macros with surface potential V X, instead, are mainly constrained by the tight coupling with baryons, resulting in σX/MX ≲ 2 · 10-11(|V X|/MeV)-2cm2 g-1. Finally, we show that future CMB spectral distortions experiments, like PIXIE and SuperPIXIE, would have the sensitivity to probe larger regions of the parameter space: this would allow either for a possible evidence or for an improvement of the current bounds on Macros as dark matter candidates.

Impact of Dust on Spectral Distortion Measurements of the Cosmic Microwave Background

Abstract Spectral distortions of the cosmic microwave background (CMB) are sensitive to energy injection by exotic physics in the early universe. The proposed Primordial Inflation Explorer (PIXIE) mission has the raw sensitivity to provide meaningful limits on new physics, but only if foreground emission can be adequately modeled. We quantify the impact of interstellar dust on Compton y and μ measurements by considering a range of grain size distributions and compositions constrained by theoretical and observational priors. We find that PIXIE can marginalize over a modest number of dust parameters and still recover y and μ estimates, though with increased uncertainty. As more foreground components are included (synchrotron, free–free), estimates of y degrade, and measurement of μ in the range sometimes considered for the standard ΛCDM of 2 × 10−8 becomes infeasible without ancillary low-frequency foreground information. An additional concern is dust absorption of the CMB monopole, a subtle effect that must be included. We quantify one form of model discrepancy error, finding that the error introduced by fitting our interstellar medium dust model with a modified blackbody is too large for CMB spectral distortions to be detectable. The greatest challenge may be the cosmic infrared background (CIB). We find that μ and y are extremely sensitive to modeling choices for the CIB, and quantify biases expected for a range of assumptions.

New horizons in cosmology with spectral distortions of the cosmic microwave background

This Voyage 2050 paper highlights the unique science opportunities using spectral distortions of the cosmic microwave background (CMB). CMB spectral distortions probe many processes throughout the history of the Universe, delivering novel information that complements past, present and future efforts with CMB anisotropy and large-scale structure studies. Precision spectroscopy, possible with existing technology, would not only provide key tests for processes expected within the cosmological standard model but also open an enormous discovery space to new physics. This offers unique scientific opportunities for furthering our understanding of inflation, recombination, reionization and structure formation as well as dark matter and particle physics. A dedicated experimental approach could open this new window to the early Universe in the decades to come, allowing us to turn the long-standing upper distortion limits obtained with COBE/FIRAS some 25 years ago into clear detections of the expected standard distortion signals and also challenge our current understanding of the laws of nature.