A bound on the cosmic opacity of unparticles from the CMB temperature
  Unparticle cosmology gives an unconventional outlook on the dark sector of
cosmology, increasingly challenging $\Lambda\mbox{CDM}$ by $H_0$-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 $T_c\simeq
4\,T_{CMB}$ associated with $\Omega_{\cal U}\simeq 1$ ($\Omega_{\cal U} \simeq
10^4 \Omega_{CMB}$), where $T_{CMB}$ denotes the temperature of the Cosmic
Microwave Background (CMB). The CMB is hereby exposed to an enormous heat bath.
The age of the Universe constrained by the globular clusters, independently
puts a constraint on any heat exchange from unparticles to the CMB in
$\Lambda\mbox{CDM}$. Accordingly, we estimate the cross section of unparticles
interactions with CMB photons to be $\sigma_{\gamma \mathcal{U}} \lesssim
10^{-40}\ \mbox{m}^2=10^{-3}\ \mbox{nb}$. This bound is in the midway of
photon-neutrino and photon-photon cross sections. It puts unparticles in
late-time cosmology, if present, at the edge of the standard model.
