[leorocky]

Stupid question maybe, but if you can create mass from photons, wouldn't that start to affect the shape of the universe if it happened enough? Could you use this, in theory, to turn an ever expanding universe into one that contracts? Just keep turning energy into mass?

[oddthink]

Nope. In GR, energy has as much "gravity" as the equivalent mass, so it won't affect anything. (Energy is more tied to the expanding universe, though, as the wavelength increases too, so it's an important distinction in early-universe cosmology.)