Far-infrared fine-structure transitions of [C I] and [C II] and rotational transitions of CO are used to probe hypothetical variations of the electron-to-proton mass ratio μ = m_e/m_p at the epoch of reionization (z > 6). A constraint on Δμ/μ = (μ_obs - μ_lab)/μ_lab = (0.7 ± 1.2) × 10^-5 (1σ) obtained at z=6.31 is the most stringent up-to-date limit on the variation of μ at such high redshift. For all available estimates of Δμ/μ ranging between z = 0 and z ∼ 1100, the epoch of recombination, a regression curve Δμ/μ = k_μ(1 + z)^p, with k_μ = (1.6 ± 0.3) в 10^-8 and p = 2.00 ± 0.03, is deduced. If confirmed, this would imply a dynamical nature of dark matter/dark energy.

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