2011ApJ...726...65K

Quantum-mechanical tunneling inversion transition in ammonia (NH₃) is actively used as a sensitive tool to study possible variations of the electron-to-proton mass ratio, µ = m_e/m_p. The molecule H₃O⁺ has the inversion barrier significantly lower than that of NH₃. Consequently, its tunneling transition occurs in the far-infrared (FIR) region and mixes with rotational transitions. Several such FIR and submillimeter transitions are observed from the interstellar medium in the Milky Way and in nearby galaxies. We show that the rest-frame frequencies of these transitions are very sensitive to the variation of µ, and that their sensitivity coefficients have different signs. Thus, H₃O⁺ can be used as an independent target to test hypothetical changes in µ measured at different ambient conditions of high (terrestrial) and low (interstellar medium) matter densities. The environmental dependence of µ and coupling constants is suggested in a class of chameleon-type scalar field models–candidates to dark energy carrier.