Cosmological constraints on radion evolution in the universal extra dimension model

The constraints on the radion evolution in the universal extra dimension (UED) model from cosmic microwave background (CMB) and Type Ia supernovae (SNe Ia) data are studied. In the UED model, where both the gravity and standard model fields can propagate in the extra dimensions, the evolution of the extra-dimensional volume, the radion, induces variation of fundamental constants. We discuss the effects of variation of the relevant constants in the context of UED for the CMB power spectrum and SNe Ia data. We then use the three-year WMAP data to constrain the radion evolution at z∼1100, and the 2σ constraint on ρ˙/ρ₀ (ρ is a function of the radion, to be defined in the text) is [-8.8,6.6]×10^-13  yr^-1. The SNe Ia gold sample yields a constraint on ρ˙/ρ₀, for redshift between 0 and 1, to be [-4.7,14]×10^-13  yr^-1. Furthermore, the constraints from SNe Ia can be interpreted as bounds on the evolution QCD scale parameter, Λ˙_QCD/Λ_QCD,0, [-1.4,2.8]×10^-11  yr^-1, without reference to the UED model.