Probing the electron-to-proton mass ratio gradient in the Milky Way with Class I methanol masers
In one sentence
By comparing two methanol-maser frequencies whose ratio depends sensitively on the electron-to-proton mass ratio, we placed one of the tightest constraints to date on whether that fundamental constant of nature varies across our own Galaxy.
What’s the question?
Some theories beyond the Standard Model predict that the values of fundamental dimensionless constants — like the electron-to-proton mass ratio μ = me / mp — are not the same everywhere, but vary subtly with environment, with depth in a gravitational potential, or simply with location in the Universe. A spatial gradient of μ across the Milky Way would show up as a systematic frequency shift in any line whose frequency depends sensitively on μ.
What did we do?
We used Class I methanol masers — common, narrow, and bright — observed at two transitions (44 and 95 GHz) whose frequency ratio is sensitive to μ. By comparing line centroids of the two transitions toward many sources spread across galactocentric distances of 4–12 kpc, we placed one of the most stringent constraints to date on the spatial gradient, |kμ| < 2 × 10⁻⁹ kpc⁻¹.
Why does it matter?
Constraints on the spatial variation of fundamental constants test whole classes of physics-beyond-the-Standard-Model. Astronomical environments give us the largest possible “lab”; methanol masers give us the precision needed to make the test count.
My role
Co-author. Contributed the underlying molecular physics — particularly the hyperfine structure of methanol — that connects the observed line frequencies to μ.