Stringent bounds to spatial variations of the electron-to-proton mass ratio in the Milky Way

TL;DR

This study uses ammonia spectral lines in molecular clouds to set stringent bounds on spatial variations of the electron-to-proton mass ratio, achieving higher sensitivity than previous extragalactic constraints and suggesting potential new physics.

Contribution

First application of the ammonia method to dense molecular clouds in the Milky Way, providing the most sensitive bounds on spatial variations of the electron-to-proton mass ratio to date.

Findings

Bound on Delta_mu/mu < 1E-7 from molecular cloud observations

Potential positive offset indicating a possible variation of Delta_mu/mu around 4E-8

Results challenge existing laboratory constraints if interpreted as true variations

Abstract

The ammonia method to probe variations of the electron-to-proton mass ratio, Delta_mu/mu, is applied for the first time to dense prestellar molecular clouds in the Milky Way. Carefully selected sample of 21 NH_3/CCS pairs observed in the Perseus molecular cloud provide the offset Delta V (CCS-NH_3)= 36+/-7_{stat}+/-13.5_{sys} m/s . A similar offset of Delta V = 40.8 +/- 12.9_{stat} m/s between NH_3 (J,K) = (1,1) and N_2H+ J = 1-0 has been found in an isolated dense core L183 by Pagani et al. (2009). Overall these observations provide a safe bound of a maximum offset between ammonia and the other molecules at the level of Delta V < 100 m/s. This bound corresponds to Delta_mu/mu < 1E-7, which is an order of magnitude more sensitive than available extragalactic constraints. Taken at face value the measured Delta V shows positive shifts between the line centers of NH_3 and these two other molecules and suggest a real offset, which would imply a Delta_mu/mu about 4E-8. If Delta_mu/mu follows the gradient of the local gravitational potential, then the obtained results are in conflict with laboratory atomic clock experiments in the solar system by 5 orders of magnitude, thus requiring a chameleon-type scalar field model. New measurements involving other molecules and a wider range of objects along with verification of molecular rest frequencies are currently planned to confirm these first indications.