We measure the temperature dependence of the indirect band gap of isotopically purified 28Si:P in the regime from 0.1 K to 3 K by high-resolution absorption spectroscopy of the donor bound exciton transition. The measurements increase the up-to-date precision of the temperature-dependent band gap change by more than one order of magnitude and reveal a T4 dependence which is about a factor of two less than observed in previous measurements. Such a T4 dependence is predicted by theory, but the absolute values differ between our experiment and the most up-to-date calculations by a factor of 30, corroborating that the electron-phonon interaction at low temperatures is still not correctly included into theory. What is more, the ability of such very high-precision band-gap measurements facilitates the use of time- and spatially resolved 28Si:P absorption as a contactless, local thermometer and electric field sensor with a demonstrated time resolution of milliseconds.