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Thermal unparticles: a new form of energy density in the universe

Authors:	Shao-Long Chen Xiao-Gang He Xue-Peng Hu and Yi Liao

Institution:	(1) Department of Physics and Center for Theoretical Sciences, National Taiwan University, Taipei, Taiwan;(2) Leung Center for Cosmology and Particle Astrophysics, National Taiwan University, Taipei, Taiwan;(3) Department of Physics, Nankai University, Tianjin, 300071, China

Abstract:	An unparticle ${\mathcal {U}}$ with scaling dimension $d_{{\mathcal {U}}}$ has peculiar thermal properties due to its unique phase space structure. We find that the equation of state parameter $\omega_{{\mathcal {U}}}$ , the ratio of pressure to energy density, is given by $1/(2d_{{\mathcal {U}}}+1),$ providing a new form of energy in our universe. In an expanding universe, the unparticle energy density $\rho_{{\mathcal {U}}}(T)$ evolves dramatically differently from that for photons. For $d_{{\mathcal {U}}}>1$ , even if $\rho_{{\mathcal {U}}}(T_{\mathrm{D}})$ at a high decoupling temperature T _D is very small, it is possible to have a large relic density $\rho_{{\mathcal {U}}}(T^{0}_{\gamma})$ at present photon temperature T _γ⁰, large enough to play the role of dark matter. We calculate T _D and $\rho_{{\mathcal {U}}}(T^{0}_{\gamma})$ using photon–unparticle interactions for illustration.

Keywords:	PACS" target="_blank">PACS 98 80 Cq 11 15 Tk 11 25 Hf 14 80 -j
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