Ratra has worked in a number of areas of cosmology and astroparticle and early universe physics.

In 1988, Ratra and Jim Peebles of Princeton University proposed the first dynamical dark energy scalar field, or quintessence, model.

[2][3] Dark energy is the leading candidate for the mechanism that is responsible for causing the observed accelerated cosmological expansion.

[8][9][10] Ratra's early universe research includes the first consistent semi-classical computation of the spectrum of energy density perturbations from inflation.

[11][12] He has also computed the power spectrum of energy density perturbations in non-spatially-flat inflation models.