I understand that a material can’t typically reach absolute zero through radiative cooling because the cosmic background radiation will always warm the material to around 2.7 K. But suppose you have a crystal of material that doesn’t have any phonons that would be accessible with energy on the order of 1.9 mm (the CMBR wavelength). Wouldn’t the material then be transparent to the CMBR, and therefore not subject to heating by it? What’s to stop such a material from simply radiatively cooling until its entire lattice is at absolute zero?