# Publications by Tessa Baker

## Cosmology and fundamental physics with the Euclid satellite

arXiv (2012) 1206.1225

Euclid is a European Space Agency medium class mission selected for launch in 2019 within the Cosmic Vision 2015-2025 programme. The main goal of Euclid is to understand the origin of the accelerated expansion of the Universe. Euclid will explore the expansion history of the Universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

## The parameterized post-Friedmann framework for theories of modified gravity: Concepts, formalism, and examples.

Physical Review D: Particles, Fields, Gravitation and Cosmology **87** (2013) 024015

A unified framework for theories of modified gravity will be an essential tool for interpreting the forthcoming deluge of cosmological data. We present such a formalism, the parameterized post-Friedmann framework (PPF), which parameterizes the cosmological perturbation theory of a wide variety of modified gravity models. PPF is able to handle spin-0 degrees of freedom from new scalar, vector, and tensor fields, meaning that it is not restricted to simple models based solely on cosmological scalar fields. A direct correspondence is maintained between the parameterization and the underlying space of theories, which allows us to build up a “dictionary” of modified gravity theories and their PPF correspondences. In this paper we describe the construction of the parameterization and demonstrate its use through a number of worked examples relevant to the current literature. We indicate how the formalism will be implemented numerically, so that the dictionary of modified gravity can be pitted against forthcoming observations.

## Growth of perturbations in parametrized gravity for an Einstein-de Sitter universe

Physical Review D - Particles, Fields, Gravitation and Cosmology **85** (2012)

Parametrized frameworks for modified gravity are potentially useful tools for model-independent tests of general relativity on cosmological scales. The toy model of an Einstein-de Sitter universe provides a safe test bed in which to improve our understanding of their behavior. We implement a mathematically consistent parametrization at the level of the field equations, and use this to calculate the evolution of perturbations in an Einstein-de Sitter scenario. Our parametrization explicitly allows for new scalar degrees of freedom, and we compare this to theories in which the only degrees of freedom come from the metric and ordinary matter. The impact on the integrated Sachs-Wolfe effect and canonically conserved superhorizon perturbations is considered. © 2012 American Physical Society.

## Towards a Fully Consistent Parametrization of Modified Gravity

Physical Review D (Particles, Fields, Gravitation and Cosmology) **84** (0)

There is a distinct possibility that current and future cosmological data can be used to constrain Einstein’s theory of gravity on the very largest scales. To be able to do this in a model-independent way, it makes sense to work with a general parameterization of modified gravity. Such an approach would be analogous to the Parameterized Post-Newtonian (PPN) approach which is used on the scale of the Solar System. A few such parameterizations have been proposed and preliminary constraints have been obtained. We show that the majority of such parameterizations are only exactly applicable in the quasistatic regime. On larger scales they fail to encapsulate the full behavior of typical models currently under consideration. We suggest that it may be possible to capture the additions to the ‘Parameterized Post-Friedmann’ (PPF) formalism by treating them akin to fluid perturbations.

## Ambiguous Tests of General Relativity on Cosmological Scales

Journal of Cosmology and Astroparticle Physics (0)

There are a number of approaches to testing General Relativity (GR) on linear scales using Parameterized Post-Friedmann (PPF) methods. It is generally assumed that the details of any given parameterization are unimportant if one uses it as a diagnostic for deviations from GR. In this brief report we show that this is not so by taking two particular parameterizations and analyzing a subset of the current cosmological data. We argue that any PPF approach should always be accompanied by a characterization of the class of modified gravity models it is seeking to approximate.