Lecture 37 - The Accelerating Universe
In the late 1990s, observation of distant supernovae established the fact that the Universe is, contrary to expectations, actually accelerating. Measurements of cosmic microwave background also strongly indicate that the Universe does not have large-scale curvature.
The obvious conclusion drawn from these is that only about 30% of the energy density of the Universe comes from baryonic matter that we are familiar with. The rest, that is 70% of the energy density of the Universe, must come from vacuum energy, metaphorically called as the `dark energy'.
A surprising and startling discovery was made around the year 2,000 – the universe is not only expanding, but the expansion is accelerating! In the first half of this lecture, I describe how this great discovery was made. If the expansion is ‘accelerating’ then it implies two things, (i) There must be a cosmic repulsion, and (ii) This cosmic repulsion must be dominating over the attractive gravity in the present epoch. What could this ‘cosmic repulsion’ or ‘repulsive gravity’ be due to? In the second part of this lecture, I describe the present state of our understanding of this deep and intriguing question.