Seljak is a cosmologist who is particularly well-known for his research on cosmic microwave background radiation, galaxy clustering and weak gravitational lensing, and the implications of these observations for the large scale structure of the universe.

Much of Seljak's recent work has been focused on how to extract fundamental properties of our universe from cosmological observations using analytical methods and numerical simulations. He has developed cosmological generative models of dark matter, stars and cosmic gas distributions, including differentiable FastPM code and its extensions.