Title: Unification, Cosmology and all that

Abstract: Twentieth century opened up two new fronts of research for Physics (i) the Nuclear front (ii) the Astrophysical front. We can say that within a century both theory and experiments on the two fronts reasonably well "finished off" the corresponding subjects. We begin by summarising Einstein's General Relativity which put forth the notion of "local symmetry". On the nuclear front also, we have a complete theory. Electromagnetism, and strong and weak nuclear forces need separate coupling constants, but surprisingly they have exactly the same structure repeated, that of local gauge invariance, resembling Einstein's general covariance. This raised the hopes for a Unified Theory. Expansion of the Universe was another great discovery. We discuss the success of General Relativity in explaining this phenomenon. But as we enter the 21st century, there is good news for young physicists, because the theories refuse to unify despite the development of very elegant and powerful mathematical frameworks, and the problems of Inflationary Universe, Dark Energy as well the Higgs boson seem to suggest that we do not understand what is "the vacuum" in Physics.