Lectures at YITP

Prof Rajesh Dubey was invited at Yukawa Institute of Theoretical Physics, Japan for his lectures in Gravitational Waves and its role in Universe acceleration since Big Bang.

The accelerating expansion of the universe is the observation that the expansion of the universe is such that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time.

The accelerated expansion was discovered during 1998, by two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team, which both used distant type 1a supernovae to measure the acceleration. The idea was that as type 1a supernovae have almost the same intrinsic brightness (a standard candle), and since objects that are further away appear dimmer, we can use the observed brightness of these supernovae to measure the distance to them. The distance can then be compared to the supernovae's cosmological redshift, which measures how much the universe has expanded since the supernova occurred. The unexpected result was that objects in the universe are moving away from another at an accelerated rate. Cosmologists at the time expected that recession velocity would always be decelerating, due to the gravitational attraction of the matter in the universe.

The accelerated expansion of the universe is thought to have begun since the universe entered its dark energy dominated era roughly 5 billion years ago. Within the framework of general relativity, an accelerated expansion can be accounted for by a positive value of the cosmological constant Λ, equivalent to the presence of a positive vaccum energy, dubbed "dark energy". While there are alternative possible explanations, the description assuming dark energy (positive Λ) is used in the current standard model of cosmology, which also includes cold dark matter (CDM) and is known as the Lambda-CDM model.

The discovery of the accelerating expansion of the Universe arguably provides the most profound puzzle in contemporary physics. It implies that either the Universe is dominated by a component, dubbed “Dark Energy”, that turns gravity's pull into a push, or that General Relativity, our understanding of gravity itself, breaks down on cosmological scales. Either way, it deeply challenges our knowledge of the basic laws of Nature. Characterizing and understanding the origin of the accelerated expansion is a truly global and interdisciplinary enterprise.