We report the first measurements of band-gap energy versus applied hydrostatic pressure in the Si1-xGex system, a result achievable only with the recent availability of high-quality alloys. The Si/Si1-xGex (x=0.05-0.25) quantum wells used here were prepared by molecular beam epitaxy, and produced relatively intense photoluminescence (PL). Indeed, this PL emission from just two 25-Å-thick quantum wells was found to be comparable to that from high-optical quality, ultrahigh-purity bulk Si. A clear no-phonon emission line is found in the low-temperature (6 K) PL which displays a strictly linear PL energy dependence with pressure. The alloy and Si pressure dependencies (-1.50 and -1.52 meV/kbar) were found to be virtually identical thus proving the luminescence is from a "shallow" electronic state and associated with the X band gap of the pseudomorphically strained alloy.