Cosmic Microwave Background Spectrum Research Articles

Angular correlations of cosmic microwave background spectrum distortions from photon diffusion

ABSTRACT During cosmic recombination, charged particles bind into neutral atoms and the mean free path of photons rapidly increases, resulting in the familiar diffusion damping of primordial radiation temperature variations. An additional effect is a small photon spectrum distortion, because photons arriving from a particular sky direction were originally in thermal equilibrium at various spatial locations with different temperatures; the combination of these different blackbody temperature distributions results in a spectrum with a Compton y-distortion. Using the approximation that photons had zero mean free path prior to their second-to-last scattering, we derive an expression for the resulting y-distortion, and compute the angular correlation function of the diffusion y-distortion and its cross-correlation with the square of the photon temperature fluctuation. Detection of the cross-correlation is within reach of existing arcminute-resolution microwave background experiments such as the Atacama Cosmology Telescope and the South Pole Telescope.

Minkowski Functionals in 𝖲𝖮(3) for the spin-2 CMB polarisation field

The study of the angular power spectrum of Cosmic Microwave Background (CMB) anisotropies, both in intensity and in polarisation, has led to the tightest constraints on cosmological parameters. However, this statistical quantity is not sensitive to any deviation from Gaussianity and statistical isotropy in the CMB data. Minkowski Functionals (MFs) have been adopted as one of the most powerful statistical tools to study such deviations, since they characterise the topology and geometry of the field of interest. In this paper, we extend the application of MFs to CMB polarisation data by introducing a new formalism, where we lift the spin 2 polarisation field to a scalar function in a higher-dimensional manifold: the group of rotations of the sphere, SO(3). Such a function is defined as f = Q cos(2ζ) - U sin(2ζ). We analytically obtain the expected values for the MFs of f in the case of Gaussian isotropic polarisation maps. Furthermore, we present a new pipeline which estimates these MFs from input HEALPix polarisation maps. We apply it to CMB simulations in order to validate the theoretical results and the methodology. The pipeline is to be included in the publicly available Python package Pynkowski .

Warm Dark matter constraints from the joint analysis of CMB, Ly α, and global 21 cm data

ABSTRACT With the help of our previously built MCMC (Markov chain Monte Carlo)-based parameter estimation package cosmoreionmc, we investigate in detail the potential of 21 cm global signal, when combined with cosmic microwave background (CMB) and observations related to the Quasar (QSO) absorption spectra, to constraint the mass of warm dark matter (WDM) particle. For the first time, we simultaneously vary all the free parameters (mass of WDM particle, cosmological parameters, and astrophysical parameters) in a joint analysis with CMB, observations related to the QSO absorption spectra and 21 cm global signal, to address the long-overlooked issue of the possible degeneracies between the dark matter particle mass mX and cosmological/astrophysical parameters. From the existing CMB and QSO absorption spectra data, we can rule out mX < 2.8 keV at 95 per cent confidence level. Including a mock 21 cm global signal in the redshift range z = 25−5 expected to be observed with upcoming instruments designed for global signal, the forecasted constraint is found to be much tighter mX > 7.7 keV, assuming that the true dark matter model is the usual cold dark matter. In case the mock 21 cm signal is constructed for dark matter particles having mX = 7 keV, our forecasts indicate that (mX/keV)−1 is in the range [0.1, 0.2] (95 per cent confidence level). This implies that the future 21 cm data should allow detection of the WDM particle mass if mX ∼ 7 keV.

Ruling out strongly interacting dark matter–dark radiation models from joint observations of cosmic microwave background and quasar absorption spectra

ABSTRACT The cold dark matter paradigm provides a remarkably good description of the Universe’s large-scale structure. However, some discrepancies exist between its predictions and observations at very small sub-galactic scales. To address these issues, the consideration of a strong interaction between dark matter particles and dark radiation emerges as an intriguing alternative. In this study, we explore the constraints on those models using joint observations of cosmic microwave background and quasars absorption spectra with our previously built parameter estimation package cosmoreionmc. At 2σ confidence limits, this analysis rules out the strongly interacting dark matter–dark radiation models within the recently proposed effective theory of structure formation framework, representing the most stringent constraint on those models to the best of our knowledge. Future research using a 21-cm experiment holds the potential to reveal stronger constraints or uncover hidden interactions within the dark sector.

(Slow-)twisting inflationary attractors

We explore in detail the dynamics of multi-field inflationary models. We first revisit the two-field case and rederive the coordinate independent expression for the attractor solution with either small or large turn rate, emphasizing the role of isometries for the existence of rapid-turn solutions.Then, for three fields in the slow-twist regime we provide elegant expressions for the attractor solution for generic field-space geometries and potentials and study the behaviour of first order perturbations. For generic 𝒩-field models, our method quickly grows in algebraic complexity. We observe that field-space isometries are common in the literature and are able to obtain the attractor solutions and deduce stability for some isometry classes of 𝒩-field models. Finally, we apply our discussion to concrete supergravity models. These analyses conclusively demonstrate the existence of 𝒩 > 2 dynamical attractors distinct from the two-field case, and provide tools useful for future studies of their phenomenology in the cosmic microwave background and stochastic gravitational wave spectrum.

Back to the features: assessing the discriminating power of future CMB missions on inflationary models

Future Cosmic Microwave Background (CMB) experiments will deliver extremely accurate measurements of the E-modes pattern of the CMB polarization field. Given the sharpness of the E-modes transfer functions, such surveys make for a powerful detector of high-frequency signals from primordial features that may be lurking in current data sets. With a handful of toy models that increase the fit to the latest Planck data, but are of marginal statistical significance, we use a state-of-the-art forecast pipeline to illustrate the promising prospects to test primordial features in the next decade. Not only will future experiments allow us to detect such features in data, but they will also be able to discriminate between models and narrow down the physical mechanism originating them with high statistical significance. On the other hand, if the anomalies in the currently measured CMB spectra are just statistical fluctuations, all the current feature best fit candidates will be ruled out. Either way, our results show that primordial features are a clear target of forthcoming CMB surveys beyond the detection of tensor modes.

One μ to rule them all: CMB spectral distortions can probe domain walls, cosmic strings and low scale phase transitions

We present a new probe of purely gravitationally coupled sectors with large anisotropies. These anisotropies are damped via gravitational interactions with the baryon-photon fluid, which is heated up in the process. The injected heat causes measurable distortions of the cosmic microwave background spectrum. We give analytic estimates for the size of the distortions and outline how to calculate them from first principles. These methods are applied to anisotropies in the form of a domain wall/cosmic string network or caused by a first order phase transition or scalar field dynamics. We find that this method can potentially probe large regions of previously unconstrained parameter space and is very much complementary to up-coming searches of gravitational waves caused by such dark sectors.

Constraining First-Order Phase Transitions with Curvature Perturbations.

The randomness of the quantum tunneling process induces superhorizon curvature perturbations during cosmological first-order phase transitions. We for the first time utilize curvature perturbations to constrain the phase transition parameters, and find that the observations of the cosmic microwave background spectrum distortion and the ultracompact minihalo abundance can give strict constraints on the phase transitions below 100GeV, especially for the low-scale phase transitions and some electroweak phase transitions. The current constraints on the phase transition parameters are largely extended by the results of this work, therefore provide an novel approach to probe related new physics.

Level correlations of CMB temperature angular power spectrum

If the principle of statistical isotropy is valid, then the angular power spectrum (APS) of cosmic microwave background (CMB) radiation is uncorrelated between different multipoles. We propose a novel technique to analyse any possible correlations of the foreground-cleaned CMB temperature APS measures (\(C_{\ell }\) and \({\mathcal {D}}_\ell =({\ell (\ell +1)})/({2\pi })C_\ell \)). This is motivated by the behaviour of level spacings between random matrix eigenvalues. This method helps to distinguish uncorrelated statistically isotropic CMB APS from correlated APS, where the latter can arise due to breakdown of isotropy or presence of some residual systematics in the foreground cleaned CMB maps. Spacings of statistically isotropic CMB \(C_\ell \)’s and \({\mathcal {D}}_\ell \)’s are seen to closely obey Poisson statistics and introduction of correlations, changes the distribution to appropriate Wigner–Dyson statistics. For foreground-cleaned CMB, we employ the average spacing of consecutive multipole APS for multipoles \(\in [2,31]\). This estimator is sensitive to departures from the null hypothesis of zero correlations between the APS measures of statistically isotropic CMB. We study full sky WMAP 9-year ILC and 2018 Planck foreground-cleaned maps (Commander, NILC and SMICA). Sans parity distinctions, average spacings are in good agreement with theoretical expectation. With parity distinctions, even multipoles indicate unusually low average spacings for both \(C_\ell \)’s (at \(\ge \)98.86% C.L.) and \({\mathcal {D}}_\ell \)’s (at \({\ge }\)95.07% C.L.). We use an inpainting method based on constrained Gaussian realisations and show that for the Planck U73 and WMAP KQ75 masks, all the foreground-cleaned inpainted CMB maps robustly confirm the existence of such unusually low average spacings of even multipole APS. In addition, this signal is independent of the non-Gaussian cold spot.

Is there a Radio Excess from the Decoupling of Pre-recombination Bremsstrahlung?

Recently it has been suggested that thermal bremsstrahlung emission, when it decouples prior to recombination, creates an excess over the Planck cosmic microwave background spectrum at sub-GHz frequencies. Remarkable by itself, this would also explain a long-standing unexplained deficit in the predictions of the extragalactic radio background. In this brief note, I reiterate that no such non-thermal component can arise by itself when matter and radiation remain kinetically coupled.

Primordial non-gaussianities of inflationary step-like models

We use Minkowski Functionals (MF) to explore the possible presence of non-Gaussian signatures in the recently released cosmic microwave background (CMB) Planck maps. In particular, we investigate MF outcomes for two sets of simulated CMB maps: one seeded by the ΛCDM concordance model, another seeded by the ‘best-fit’ CMB angular power spectrum. By ‘best-fit’ we mean a simulated CMB spectrum seeded by an inflationary model that well reproduce the observed low-ℓ features of the CMB temperature power spectrum. In this way, we obtain the spurious signal introduced by the power spectra with different shapes, signal to be taken into account in the corresponding MF analyses. Interestingly, we investigate the response of the four MF (i.e., Area, Perimeter, Genus, Nclusters ) for several sets of simulated maps. Our analyses confirm that, as already reported in the literature, the Perimeter exhibits the best performance. Moreover, our results show that the Perimeter obtained from the Planck maps (2015) is better fitted by the simulated CMB maps seeded by the angular power spectrum with low-ℓ features with respect to the curves obtained from the CMB maps seeded by the ΛCDM model.

Novel cosmological bounds on thermally-produced axion-like particles

We constrain the coupling of thermally-produced axion-like particles (here axions) with photons and gluons, using data from the cosmic microwave background (CMB) spectra and baryon acoustic oscillations. The axion possesses an explicit soft breaking mass term and it is produced thermally in the early Universe from either axion-photon or axion-gluon processes, accounting for the recent progresses in the field. We derive the most stringent bounds on the axion-gluon coupling to date on the mass range considered 10-4 ≲ ma / eV ≲ 100, superseding the current bounds from SN1987A. The bounds on the axion-photon coupling are competitive with the results from the CAST collaboration for the axion mass ma ≳ 3eV. We comment on the forecast reaches that will be available given the sensitivity of future CMB-S4 missions.

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.

Inverse-Compton scattering of the cosmic infrared background

The thermal Sunyaev-Zel'dovich (tSZ) effect is the distortion generated in the cosmic microwave background (CMB) spectrum by the inverse-Compton scattering of CMB photons off free, energetic electrons, primarily located in the intracluster medium. Cosmic infrared background (CIB) photons from thermal dust emission in star-forming galaxies are expected to undergo the same process. In this work, we perform the first calculation of the resulting tSZ-like distortion in the CIB. Focusing on the CIB monopole, we use a halo model approach to calculate both the CIB signal and the Compton-$y$ field that generates the distortion. We self-consistently account for the redshift coevolution of the CIB and Compton-$y$ fields: they are (partially) sourced by the same dark matter halos, which introduces new aspects to the calculation as compared to the CMB case. We find that the inverse-Compton distortion to the CIB monopole spectrum has a positive (negative) peak amplitude of $\ensuremath{\approx}4\text{ }\text{ }\mathrm{Jy}/\mathrm{sr}$ ($\ensuremath{\approx}\ensuremath{-}5\text{ }\text{ }\mathrm{Jy}/\mathrm{sr}$) at 2260 GHz (940 GHz). In contrast to the usual tSZ effect, the distortion to the CIB spectrum has two null frequencies, at approximately 196 and 1490 GHz. We perform a Fisher matrix calculation to forecast the detectability of this new distortion signal by future experiments. PIXIE would have sufficient instrumental sensitivity to detect the signal at $4\ensuremath{\sigma}$, but foreground contamination reduces the projected signal-to-noise by a factor of $\ensuremath{\approx}70$. A future ESA Voyage 2050 spectrometer could detect the CIB distortion at $\ensuremath{\approx}5\ensuremath{\sigma}$ significance, even after marginalizing over foregrounds. A measurement of this signal would provide new information on the star formation history of the Universe, and the distortion anisotropies may be accessible by near-future ground-based experiments.

Scalar Quadratic Maximum-likelihood Estimators for the CMB Cross-power Spectrum

Estimating the cross-correlation power spectra of the cosmic microwave background, in particular, the TB and EB spectra, is important for testing parity symmetry in cosmology and diagnosing insidious instrumental systematics. The quadratic maximum-likelihood (QML) estimator provides optimal estimates of the power spectra, but it is computationally very expensive. The hybrid pseudo-C ℓ estimator is computationally fast but performs poorly on large scales. As a natural extension of previous work, in this article, we present a new unbiased estimator based on the Smith–Zaldarriaga (SZ) approach of E–B separation and the scalar QML approach to reconstruct the cross-correlation power spectrum, called the QML-SZ estimator. Our new estimator relies on the ability to construct scalar maps, which allows us to use a scalar QML estimator to obtain the cross-correlation power spectrum. By reducing the pixel number and algorithm complexity, the computational cost is nearly one order of magnitude smaller and the running time is nearly two orders of magnitude faster in the test situations.

Uncovering the history of cosmic inflation from anomalies in cosmic microwave background spectra

We propose an inflationary primordial feature model that can explain both the large and small-scale anomalies in the currently measured cosmic microwave background anisotropy spectra, revealing a clip of adventurous history of the Universe during its primordial epoch. Although the model is currently statistically indistinguishable from the Standard Model, we show that planned observations such as the Simons Observatory, LiteBIRD and CMB-S4 will complement each other in distinguishing the model differences due to their accurate E-mode polarization measurements, offering very optimistic prospects for a detection or exclusion. The model predicts a signal of classical primordial standard clock, which can also be used to distinguish the inflation and alternative scenarios in a model-independent fashion.

The benefits of CMB delensing

The effects of gravitational lensing of the cosmic microwave background (CMB) have been measured at high significance with existing data and will be measured even more precisely in future surveys. Reversing the effects of lensing on the observed CMB temperature and polarization maps provides a variety of benefits. Delensed CMB spectra have sharper acoustic peaks and more prominent damping tails, allowing for improved inferences of cosmological parameters that impact those features. Delensing reduces B-mode power, aiding the search for primordial gravitational waves and allowing for lower variance reconstruction of lensing and other sources of secondary CMB anisotropies. Lensing-induced power spectrum covariances are reduced by delensing, simplifying analyses and improving constraints on primordial non-Gaussianities. Biases that result from incorrectly modeling nonlinear and baryonic feedback effects on the lensing power spectrum are mitigated by delensing. All of these benefits are possible without any changes to experimental or survey design. We develop a self-consistent, iterative, all-orders treatment of CMB delensing on the curved sky and demonstrate the impact that delensing will have with future surveys.

Modified dispersion relations and a potential explanation of the EDGES anomaly

The Experiment to Detect the Global Epoch of Reionisation Signature (EDGES) collaboration has recently reported an important result related to the absorption signal in the Cosmic Microwave Background radiation spectrum. This signal corresponds to the red-shifted 21-cm line at z simeq 17.2, whose amplitude is about twice the expected value. This represents a deviation of approximately 3.8sigma from the predictions of the standard model of cosmology, i.e. the Lambda CDM model. This opens a window for testing new physics beyond both the standard model of particle physics and the Lambda CDM model. In this work, we explore the possibility of explaining the EDGES anomaly in terms of modified dispersion relations. The latter are typically induced in unified theories and theories of quantum gravity, such as String/M-theories and Loop Quantum Gravity. These modified dispersion relations affect the density of states per unit volume and thus the thermal spectrum of the Cosmic Microwave Background photons. The temperature of the 21-cm brightness temperature is modified accordingly giving a potential explanation of the EDGES anomaly.

Cosmology with the SZ spectrum: Measuring the Universe’s temperature with galaxy clusters

The hot gas in clusters of galaxies creates a distinctive spectral distortion in the cosmic microwave background (CMB) via the Sunyaev-Zel’dovich (SZ) effect. The spectral signature of the SZ can be used to measure the CMB temperature at cluster redshift (TCMB(z)) and to constrain the monopole of the y-type spectral distortion of the CMB spectrum. In this work, we start showing the measurements of TCMB(z) for a sample extracted from the Second Catalog of galaxy clusters produced by Planck (PSZ2) and containing 75 clusters selected from CHEX-MATE. Then we show the forecasts for future CMB experiments about the constraints on the monopole of the y-type spectral distortion of the CMB spectrum via the spectrum of the SZ effect.

CMB lensing power spectrum estimation without instrument noise bias

The power spectrum of cosmic microwave background (CMB) lensing will be measured to sub-percent precision with upcoming surveys, enabling tight constraints on the sum of neutrino masses and other cosmological parameters. Measuring the lensing power spectrum involves the estimation of the connected trispectrum of the four-point function of the CMB map, which requires the subtraction of a large Gaussian disconnected noise bias. This reconstruction noise bias receives contributions both from CMB and foreground fluctuations as well as instrument noise (both detector and atmospheric noise for ground-based surveys). The debiasing procedure therefore relies on the quality of simulations of the instrument noise which may be expensive or inaccurate. We propose a new estimator that makes use of at least four splits of the CMB maps with independent instrument noise. This estimator makes the CMB lensing power spectrum completely insensitive to any assumptions made in modeling or simulating the instrument noise. We show that this estimator, in many practical situations, leads to no substantial loss in signal-to-noise. We provide an efficient algorithm for its computation that scales with the number of splits m as \U0001d4aa(m2) as opposed to a naive \U0001d4aa(m4) expectation.