[AntonioLao]

One of the objectives of the mission of the satellite named WMAP/NASA launched in June 30, 2001 was to measure the mass contribution from positive curvature of space-time as well as the contribution from the negative curvature. See http://map.gsfc.nasa.gov/. What it found is that these contributions, within 2% accuracy of the measurements, exactly cancel each other. The clincher is that the universe is flat. From theoretical considerations it has been known for a long time that a particular solution of Einstein’s field equations of general relativity described a Euclidean four dimensional space-time flatness model. See http://en.wikipedia.org/wiki/Einstein_field_equations

Since the field equations are composed of 16 coupled hyperbolic-elliptic nonlinear partial differential equations, there exist an infinite number of solutions. There are models for varying degrees of positive curvature and negative curvature. However, the most interesting model is the one that describes a perfect balance between positive and negative yin-yang symmetry. Latest data gathered from WMAP/NASA measurements supported such a flat 4D universe. For some other contending models please consult: Weinberg, Steven, Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity, Wiley, New York, 1972, or see http://encarta.msn.com/media_461517872_761564398_-1_1/Models_of_the_Universe.html