Prof. Sahni works mainly in cosmology. His work on inflation includes a formulation of the cosmic no-hair theorem. He is also interested in the relic gravity wave background from inflation and its effects on the CMB. He also works on higher-dimensional braneworld cosmology, assessing its effects both on the early universe and at later times, as a candidate for dark energy. Prof. Sahni has developed a model-independent methodology for reconstructing properties of dark energy from observations. He is deeply interested in gravitational clustering and in the morphology of the supercluster-void network, having developed with collaborators a powerful diagnostic called the Shapefinder to determine the topological and geometric properties of the cosmic web.