Thermal Unparticles: A New Form of Energy Density in the Universe

Abstract

Unparticle $\U$ with scaling dimension $d_\U$ has peculiar thermal properties due to its unique phase space structure. We find that the equation of state parameter $ω_\U$, the ratio of pressure to energy density, is given by $1/(2d_\U +1)$ providing a new form of energy in our universe. In an expanding universe, the unparticle energy density $ρ_\U(T)$ evolves dramatically differently from that for photons. For $d_\U >1$, even if $ρ_\U(T_D)$ at a high decoupling temperature $T_D$ is very small, it is possible to have a large relic density $ρ_\U(T^0_γ)$ at present photon temperature $T^0_γ$, large enough to play the role of dark matter. We calculate $T_D$ and $ρ_\U(T^0_γ)$ using photon-unparticle interactions for illustration.

5 pages; v3, journal version;