CMB spectral distortions
The CMB remains the most perfect blackbody radiation ever measured in nature (Fixsen et al, ApJ 1996). Tiny departures from this ideal blackbody shape, known as CMB spectral distortions (e.g. Chluba & Sunyaev, MNRAS 2012), provide a new observational window into the physics of the early Universe beyond the information encoded in CMB anisotropies, probing redshifts up to about two million, well before the last-scattering surface, when the Universe was opaque. Such distortions arise when energy is injected into the photon-baryon plasma, altering the spectrum through well-understood processes such as Compton scattering, bremsstrahlung, and double Compton emission. Depending on the epoch and mechanism of energy release, these distortions manifest as different characteristic signals, most notably μ-type and y-type distortions. While CMB anisotropies probe conditions at the surface of last scattering, spectral distortions trace energy release across the entire cosmic history—from inflationary reheating and particle decays, to reionization and large-scale structure formation—thus providing a powerful and complementary probe of fundamental physics and cosmology.
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Extracting foreground-obscured μ-distortion anisotropies to constrain primordial non-Gaussianity
Remazeilles & Chluba, MNRAS 478, 807 (2018)
The damping of small-scale primordial density perturbations at wavenumbers 50 ≲ k ≲ 10⁴ Mpc⁻¹ (Silk damping) dissipates acoustic energy into the photon-baryon plasma, transferring it to the CMB photons and producing μ-type spectral distortions of the CMB blackbody spectrum. Aside from average distortion of the CMB spectrum, anisotropies of μ-type distortions can arise from anisotropic heating mechanisms. In particular, if primordial perturbations are non-Gaussian at such scales—as predicted by multi-field inflation models or by a non-Bunch-Davies vacuum in single-field inflation (Ganc & Komatsu, PRD 2012)—mode couplings between short and long wavelengths modulate the damping process, generating anisotropic μ-distortions correlated with CMB temperature and polarization anisotropies (Pajer & Zaldarriaga, PRL 2012). A measurable μT cross-correlation between CMB temperature and μ-distortion anisotropies with an imager would thus provide the first indirect detection of pre-recombination μ-distortions in the absence of a dedicated spectrometer, while simultaneously ruling out standard single-field inflation models. In this paper, we developed a component separation method that exploits the spectral signature of μ-type distortions to suppress Galactic foregrounds and deproject CMB anisotropies, producing clean maps of CMB temperature and μ-distortion anisotropies with zero reciprocal contamination. This enables unbiased reconstruction of the μT cross-power spectrum without residual TT leakage. Applying this technique to simulations of future CMB missions (LiteBIRD, CORE, PIXIE, PICO), we demonstrated that next-generation imagers could detect μ-distortion anisotropies through cross-correlation with CMB temperature and polarization, thereby probing primordial fluctuations and non-Gaussianity on scales far smaller than those accessible to current CMB anisotropy or large-scale structure surveys.
Our technique was adopted by the CMB-S4 Collaboration for μ-distortion forecasts (Zegeye et al., PRD 2023) and later applied in Zegeye et al., PRD (2025) to show how the low-frequency SKA survey could enhance the detection of CMB spectral distortion anisotropies. I co-authored this latter work, which was featured in Nature Astronomy’s research highlights in April 2025.
Remazeilles, Ravenni, Chluba, MNRAS 512, 455 (2022)
This paper extended our previous work by leveraging CMB polarization to enhance the detection of μ-distortion anisotropies. Multi-field inflation models and non-Bunch-Davies vacuum initial conditions predict sizeable primordial non-Gaussian primordial perturbations and anisotropic μ-type spectral distortions of the CMB blackbody. Whereas CMB anisotropies probe non-Gaussianity (fNL parameter) at wavenumbers k ≃ 0.05 Mpc⁻¹, μ-distortion anisotropies are sensitive to much smaller scales, k ≃ 740 Mpc⁻¹. Cross-correlating CMB anisotropies with μ-distortion anisotropies can therefore shed light on the aforementioned inflation models. In this work, we investigated the potential of a future CMB satellite like LiteBIRD to measure the μT and μE cross-power spectra—between anisotropic μ-distortions and CMB temperature and E-mode polarization anisotropies—in the presence of foregrounds, and presented LiteBIRD forecasts on fNL(k ≃ 740 Mpc⁻¹). We showed that μE cross-correlations with CMB polarization provide more constraining power on fNL than μT cross-correlations in the presence of foregrounds, owing to their higher correlation, and that the joint combination of μT and μE observables offers additional leverage for detecting small-scale primordial non-Gaussianity with future CMB imagers.