A bound on the cosmic opacity of unparticles from the CMB temperature

Abstract

Unparticle cosmology gives an unconventional outlook on the dark sector of cosmology, increasingly challenging ΛCDM by H0-tension. This model derives from a finite temperature broken conformal symmetry of radiation, described by a non-radiative correction with unknown sign in energy density. This symmetry breaking has a sign ambiguity, in which corrections about the IR fixed point are normal or tachyonic. While the first has been ruled out in a recent study, the latter possesses a late-time temperature Tc≃4TCMB associated with ΩU≃1 (ΩU≃104ΩCMB), where TCMB denotes the temperature of the Cosmic Microwave Background (CMB). Thus exposed to the enormous heat bath of unparticles, TCMB is constrained by the astronomical age of the Universe Tu,0. The relative temperature shift ΔT/T in any heat exchange to the CMB is hereby limited to the uncertainty of a few percent in Tu,GC. This bounds the effective cross-section of unparticles interactions with CMB photons to σγU≲10−40m2=10−3nb. Assuming a standard cross-section across the broadband mass and energy spectrum of unparticles, a more stringent bound σγU≲10−3pb derives from COBE-FIRAS constraints on spectral distortions. The first is midway of photon-neutrino σγν and photon–photon cross-sections σγγ, the second is on par with σγν. These constraints put late-time unparticle cosmology, if present, at the edge of the standard model.