  Albert Einstein visited Spain only once, precisely one hundred years ago. The
circumstances, of a very different kind, of this visit will be explained here.
In special, some important events happened to Einstein during that period,
which, eventually, were key for converting modern cosmology into a genuine
physical theory. Among them is the famous Einstein-Friedmann controversy,
first, on the mathematical validity of Friedmann's equations and, later, their
possible usefulness as a reliable tool to describe the real world. A summary of
the deepest ideas underlying Einstein's contributions to the theory of
relativity, which he had already completed before his visit, will precede the
discussion, also supplemented with a description, in very simple terms, of the
three main relativistic theories, namely Galileo's one, and Einstein's special
and general theory. They pave the way towards a definitive theory of total
relativity, so far unattainable. It will be recalled that the most general
relativity principle, faithfully reflecting Ernst Mach's far-reaching ideas,
might have much to do with the symmetry-breaking paradigm, a most crucial tool
in quantum field theory and high energy physics.
