A constraint on a varying proton--electron mass ratio 1.5 billion years
  after the Big Bang
  A molecular hydrogen absorber at a lookback time of 12.4 billion years,
corresponding to 10$\%$ of the age of the universe today, is analyzed to put a
constraint on a varying proton--electron mass ratio, $\mu$. A high resolution
spectrum of the J1443$+$2724 quasar, which was observed with the Very Large
Telescope, is used to create an accurate model of 89 Lyman and Werner band
transitions whose relative frequencies are sensitive to $\mu$, yielding a limit
on the relative deviation from the current laboratory value of
$\Delta\mu/\mu=(-9.5\pm5.4_{\textrm{stat}} \pm 5.3_{\textrm{sys}})\times
10^{-6}$.
