The future of cosmology and the role of non-linear perturbations
  Cosmological perturbation theory is a key tool to study the universe. The linear or first order theory is well understood, however, developing and applying the theory beyond linear order is at the cutting edge of current research in theoretical cosmology. In this article, I will describe some signatures of non-linear perturbation theory that do not exist at linear order, focusing on vorticity generation at second order. In doing so, we discuss why this, among other features such as induced gravitational waves and non-Gaussianities, shows that cosmological perturbation theory is crucial for testing models of the universe. 
Adam J. Christopherson
astro-ph.CO gr-qc