# From Cosmological Observations to Dark Energy and Modified Gravity

### Abstract:

In this course I will show how we go from cosmological observations to constraints on Dark Energy and Modified Gravity theories. Starting with an overview of the observational evidence for the Standard Lambda Cold Dark Matter Model, I focus on the Cosmological Constant Problem. I review the origin and evolution of cosmological structure, explain the physics behind the Cosmic Microwave Background, Galaxy Clustering and Weak Gravitational Lensing, and describe how cosmological information is extracted from their observations. Motivating the study of Dark Energy and Modified Gravity theories, I review some of the simplest models before describing the general scalar-tensor, Horndeski, Theory. Adopting an Effective Field Theory approach I will describe the cosmological properties of this model. The effect on cosmological background, perturbations and gravitational waves are considered, and I show how observations put strong constraints on this class of models. I review some of the cosmological surveys which are due to appear in the next few years, e.g., Euclid and LSST, and which are designed to test these models. Finally I look at possible outcomes and future directions.