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

TL;DR

This paper explores unparticles as a novel form of energy with unique thermal properties, potentially contributing to dark matter, and provides calculations of their relic density and evolution in the universe.

Contribution

It introduces a new equation of state for unparticles and analyzes their thermal evolution, highlighting their potential role as dark matter.

Findings

Unparticles have an equation of state parameter $A9_ 0=1/(2d_ 0+1)

Unparticle energy density can be significant at present despite small initial values

Unparticles could serve as a dark matter candidate due to their relic density

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 $\omega_\U$, 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 $\rho_\U(T)$ evolves dramatically differently from that for photons. For $d_\U >1$, even if $\rho_\U(T_D)$ at a high decoupling temperature $T_D$ is very small, it is possible to have a large relic density $\rho_\U(T^0_\gamma)$ at present photon temperature $T^0_\gamma$, large enough to play the role of dark matter. We calculate $T_D$ and $\rho_\U(T^0_\gamma)$ using photon-unparticle interactions for illustration.