What is space-time?
A. Space-time is the union of the ordinary three-dimensional space together with time, that represents the fourth dimension of the physical world. Space-time is like an immense roll of ribbon, that at the moment of the creation of the universe was all wound. During the evolution of the universe, the ribbon,that represents the space,unrolled in the past,is unrolling in the present and will unroll in the future,until the universe will exist. Even if the ribbon has only two dimensions, length and breadth, this model gives us well the image of the time that passes, while the ribbon is unrolling. To represent more clearly the evolving of the universe in space-time, we can think to a sphere of an elastic material, that, being point-like at the moment of the creation (the big bang) of the universe, expanded uniformly, until it represents the universe as we see it today, with some hundreds of billions of galaxies,whose distances are increasing continously, making the universe always less dense
