Measurements made by the COBE FIRAS satellite (shown above) showed that the cosmic microwave background (CMB) is a blackbody to exquisite () precision. It is known, however, that deviations from this perfect blackbody must exist, at the level of (), due to the redshifting of electrons in the expanding universe (and the subsequent energy loss by photons through Compton scattering) and the diffusive damping of acoustic waves in the primordial plasma. These processes inject energy at redshifts (), when the processes keeping the plasma in perfect equilibrium have decoupled. Detecting CMB spectral distortions would thus directly probe empirically unexplored epochs of cosmic time. The technology now exists to detect these small spectral distortions, and there is excitement in the community for a satellite concept called PIXIE, which would target primordial spectral distortions, signatures of reionization, and large-scale foreground-subtracted measurements of primordial B-mode polarization.

In my own work (described below), I have identified 3 new sources of CMB spectral distortions, and on the advocacy side, was an organizer of the workshop on CMB spectral distortions held at the KICP last year. In the future, I’d like to develop techniques to remove foreground contaminants from spectral distortion measurements, explore the possibility of robust inflationary spectral distortion predictions using effective field theory,  better understand the interplay between the radiation field and recombining atoms, and explore the impact of early cosmic structure formation on small scales on the distortion signal.

Probing early-universe phase transitions with CMB spectral distortions

If a GUT-scale phase transition occured after inflation, large domains in different vacuum states could be produced via the Kibble mechanism. The resulting network of scaling seeds supports horizon-scale density perturbations at all time (as shown in the below cartoon), stirring the baryon-photon plasma, which then generates CMB spectral distortions as the perturbations diffusively damp. […]

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Spectral distortions from the dissipation of tensor perturbations

Acoustic waves generate CMB spectral distortions because the linear superposition of blackbodies seen by electrons as they rescatter CMB photons is not itself a blackbody (see right panel of above figure). Gravitational waves similarly produce CMB spectral distortions due to the induced quadrupole seen by electrons at the surface of last scattering (see left panel […]

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Spectral distortions from isocurvature perturbations

Isocurvature perturbations are fluctuations in the relative number densities of particles at constant total density. Like adiabatic (or total density) fluctuations, primordial isocurvature perturbations will excite acoustic waves in the baryon-photon plasma. Jens Chluba and I computed the spectral distortions (deviations of the spatially averaged CMB from a perfect blackbody) generated when these acoustic waves […]

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