Baryonic matter consists ofquarks and particles made from quarks, likeprotons andneutrons. Free neutrons have ahalf-life of 613.9 seconds. Electrons and protons appear to be stable, to the best of current knowledge. (Theories ofproton decay predict that the proton has a half life on the order of at least 1032 years. To date, there is no experimental evidence of proton decay.);
Because they are stable, are components of all normal atoms, and determine their chemical properties, theproton is the most prevalentbaryon, while theelectron is the most prevalentlepton;
Theproton massmp is composed primarily ofgluons, and of thequarks (theup quark anddown quark) making up the proton. Hencemp, and therefore the ratioμ, are easily measurable consequences of thestrong force. In fact, in thechiral limit,mp is proportional to theQCD energy scale, ΛQCD. At a given energy scale, thestrongcoupling constantαs is related to the QCD scale (and thusμ) as
whereβ0 = −11 + 2n/3, withn being the number offlavors ofquarks.
Astrophysicists have tried to find evidence thatμ has changed over the history of the universe. (The same question has also been asked of thefine-structure constant.) One interesting cause of such change would be change over time in the strength of thestrong force.
Ifμ were to change, then the change in the wavelengthλi of eachrest framewavelength can be parameterised as:
where Δμ/μ is the proportional change inμ andKi is a constant which must be calculated within a theoretical (or semi-empirical) framework.
Reinhold et al. (2006) reported a potential 4standard deviation variation inμ by analysing the molecular hydrogenabsorption spectra ofquasars Q0405-443 and Q0347-373. They found thatΔμ/μ = (2.4 ± 0.6)×10−5. King et al. (2008) reanalysed the spectral data of Reinhold et al. and collected new data on another quasar, Q0528-250. They estimated thatΔμ/μ = (2.6 ± 3.0)×10−6, different from the estimates of Reinhold et al. (2006).
Murphy et al. (2008) used the inversion transition of ammonia to conclude that|Δμ/μ| <1.8×10−6 at redshiftz = 0.68. Kanekar (2011) used deeper observations of the inversion transitions of ammonia in the same system atz = 0.68 towards 0218+357 to obtain|Δμ/μ| <3×10−7.
Bagdonaite et al. (2013) usedmethanol transitions in the spirallensinggalaxyPKS 1830-211 to find∆μ/μ = (0.0 ± 1.0) × 10−7 atz = 0.89.[2][3]Kanekar et al. (2015) used near-simultaneous observations of multiple methanol transitions in the same lens, to find∆μ/μ < 1.1 × 10−7 atz = 0.89. Using three methanol lines with similar frequencies to reduce systematic effects, Kanekar et al. (2015) obtained∆μ/μ < 4 × 10−7.
Note that any comparison between values of Δμ/μ at substantially different redshifts will need a particular model to govern the evolution of Δμ/μ. That is, results consistent with zero change at lower redshifts do not rule out significant change at higher redshifts.
Barrow, John D. (2003).The Constants of Nature: From Alpha to Omega – the Numbers That Encode the Deepest Secrets of the Universe. London: Vintage.ISBN0-09-928647-5.
