Our next cosmology seminar takes place on **Friday 19th** of November,
seminar **room E349** at **2pm**. The speaker is **Danilo Artigas**
from the Institut d’Astrophysique Spatiale (Paris, France) and Quantum
Cosmos Lab (Cracow, Poland).

At 14:00, room E349, Danilo Artigas (IAS), will be talking about

### Hamiltonian formalism for cosmological perturbations: the separate-universe approach

*In order to describe inhomogeneities and anisotropies in the Universe,
one generally considers a Friedmann-Lemaître-Robertson-Walker
background and introduces perturbations on top of it. Such a method is
commonly used in Lagrangian formalism and is fundamental for example
in order to describe the formation of large-scale structures. On the
other hand, the so-called “separate-universe” approach aims to provide
an effective description of cosmological perturbations at large
scales. In this talk, we will formulate the perturbation theory and
the separate universe approach in the Hamiltonian language. We will
then compare both frameworks to determine the domain of validity of
the separate universe. As expected, both match at large scales since
anisotropic degrees of freedom decouple from homogeneous ones. We’ll
finish discussing the gauge matching between both approaches and the
construction of their gauge-invariant quantities.*