Exploring compensated isocurvature perturbations with CMB spectral distortion anisotropies

Understand

We develop a linear perturbation theory for the spectral $y$-distortions of the cosmic microwave background (CMB).

• The $y$-distortions generated during the recombination epoch are usually negligible because the energy transfer due to the Compton scattering is strongly suppressed at that time, but they can be significant if there is a considerable amount of compensated isocurvature perturbation (CIP), which is not tightly constrained from the present CMB observations.
• The linear $y$-distortions explicitly depend on the baryon density fluctuations, therefore $y$ anisotropies can completely resolve the degeneracy between the baryon isocurvature perturbations and the cold dark matter ones.
• This novel method is free from lensing contaminations that can affect the previous approach to the CIPs based on the nonlinear modulation of the CMB anisotropies.