Abstract: The discovery of the accelerated expansion of the Universe remains one of the most profound puzzles in modern science. Within the framework of Einstein’s General Relativity, this accelerated expansion implies the existence of "Dark Energy"—a gravitationally repulsive component that makes up roughly 68% of the Universe's energy density. When combined with Dark Matter (~27%) and ordinary matter (~5%), the standard model of the Universe is consistent with observations. However, the physical nature of these dark sectors remains unknown.

In this talk, I will review the observational evidence for cosmic acceleration and the theoretical models of Dark Energy. I will then explore the alternative: that the acceleration is not caused by exotic dark energy, but by a breakdown of General Relativity on cosmological scales. Focusing on the Scalar-Tensor theory of gravity, I will discuss how a dynamical scalar field coupled to curvature can mimic the effects of both Dark Matter and Dark Energy, providing a unified perspective on the dark sector of the Universe.

About the Speaker: Dr. Sanil Unnikrishnan is an Associate Professor in the Department of Physics at St Stephen’s College, University of Delhi. He is also Visiting Associate at IUCAA, Pune. Sanil was Post Doctoral Fellow at Max Planck Partner Group on Cosmology and Gravity, IISER, Trivandrum from 2012 to 2014 and Post Doctoral Fellow at IUCAA, Pune from 2009 to 2012. His research interests are in Cosmological Perturbation Theory, Inflation and Early Universe Cosmology, Dark Energy and Dark Matter and Modified theory of gravity as an alternative to dark energy.

We look forward to your active participation.