2012A&A...540L...9L

We constrain a hypothetical variation in the fundamental physical constants over the course of cosmic time. We use unique observations of the CO(7-6) rotational line and the [CI] ³P₂-³P₁ fine-structure line towards a lensed galaxy at redshift z=5.2 to constrain temporal variations in the constant F=α²/µ, where µ is the electron-to-proton mass ratio and α is the fine-structure constant. The relative change in F between z=0 and z=5.2, ΔF/F=(F_obs-F_lab)/F_lab, is estimated from the radial velocity offset, ΔV=V_rot-V_fs, between the rotational transitions in carbon monoxide and the fine-structure transition in atomic carbon. We find a conservative value ΔV=(1±5)km/s (1σC.L.), which when interpreted in terms of ΔF/F gives ΔF/F<2x10^–5. Independent methods restrict the µ-variations at the level of Δµ/µ<1x10^–7 at z=0.7 (look-back time t_z0.7=6.4Gyr). Assuming that temporal variations in µ, if any, are linear, this leads to an upper limit on Δµ/µ<2x10^–7 at z=5.2 (t_z5.2=12.9Gyr). From both constraints on ΔF/F and Δµ/µ, one obtains for the relative change in α the estimate Δα/α<8x10^–6, which is at present the tightest limit on Δα/α at early cosmological epochs.