© 2010, Birkhäuser, Springer Basel AG. As we have already mentioned in Chapter II, general relativity is a classic gravitation theory where the physical space is interpreted as a continuum, mathematically modeled by means of a time-oriented Lorentzian manifold. Einstein’s equation relating the gravitational field with the matter that creates this field is (Formula presented.) where (Formula presented.) is Einstein’s tensor of the Lorentzian metric (Formula presented.), T is the stress-energy tensor describing the matter, and ξ is a universal constant.