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

Shao-Long Chen; Xiao-Gang He; Xue-Peng Hu; Yi Liao

doi:10.1140/epjc/s10052-009-0888-3

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2024 Impact factor 4.8

Particles and Fields

Eur. Phys. J. C (2009) 60: 317-321
https://doi.org/10.1140/epjc/s10052-009-0888-3

Regular Article - Theoretical Physics

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

Shao-Long Chen¹^,2, Xiao-Gang He¹^,2, Xue-Peng Hu³ and Yi Liao³^*

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

^* e-mail: liaoy@nankai.edu.cn

Received: 13 November 2008
Revised: 28 December 2008
Published online: 13 February 2009

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 _γ ⁰ , 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.