Discover Questions

Doesn't Temperature only applies to atomic structure ,how can they say space as a vacuum has a temperature of 2.7 kelvin? Did Einstein model his Field equation to an electric field equation in terms of volt per meter? Doesn't Newton gravity equation consists of four forces,why Einstein gravity equation only shows two forces? ∫du/√(a²-u²)? Is the Photon not a particle not a volume with mass not an entity, isn't only a mathematical construct and not a real substance?