Spectral Distortion of the CMB by the Cumulative CO Emission from Galaxies throughout Cosmic History

We show that the cumulative CO emission from galaxies throughout cosmic history distorts the spectrum of the cosmic microwave background (CMB) at a level that is well above the detection limit of future instruments, such as the Primordial Inflation Explorer (PIXIE). The modeled CO signal has a prominent bump in the frequency interval 100-200 GHz, with a characteristic peak intensity of ~ 2$\times$10${}^{-23}$ W m${}^{-2}$ Hz${}^{-1}$ sr${}^{-1}$. Most of the CO foreground originates from modest redshifts, z ~ 2-5, and needs to be efficiently removed for more subtle distortions from the earlier universe to be detected.