# Cosmological probes of modified gravity

The most dramatic discovery in science in the past few years has been the revelation that the expansion of the Universe is actually accelerating, apparently driven by an unknown ‘dark energy’ which dominates the energy density of the Universe. Predictions based on General Relativity plus the Standard Model of particle physics fail to explain this. The nature of dark energy is then one of the biggest mysteries of cosmology, and it raises some of the most outstanding questions in physics.

The objective of this proposal is to investigate the so called Modified Gravity approach. This hypotheses assumes that there is no dark energy field, but the acceleration of the Universe occurs via ``dark gravity", i.e. a weakening of gravity on the largest scales, due to a modification of General Relativity itself. Our programme consists on computing astrophysical predictions of Modified Gravity theories and confront them against observational data.

## About the project

One possible explanation for dark energy may be Einstein’s famous cosmological constant. Another possibility is that dark energy may be an exotic form of matter called quintessence. Alternatively, instead of attributing the accelerated expansion to our lack of understanding of the constituents of the Universe, one can attribute it to our lack of understanding of gravity on cosmological scales. In particular, one may consider that such measurements indicate the break down of General Relativity at astrophysical scales. For example, an effective weakening of gravity on the largest scales could explain the accelerated expansion and other cosmological observations without invoking an additional smooth ``dark'' component.

Seen from a fundamental physics point of view, the two approaches of General Relativity coupled with a smooth Dark Energy (GR+DE) and Modified Gravity are quite distinct: while General Relativity does not make use of any gravitating fields, besides a rank-two tensor (the metric), almost any consistent gravitational theory that one can think of (from Newtonian gravity to models with extra dimensions, such as string theory) will do so. In fact, within novel high energy physics models the gravity interaction is not only carried by the two-tensor, but also by a scalar-field (e.g. Scalar-Tensor theories) or by a vector field (e.g. Vector-Tensor theories). In fact, many of these new gravitating fields are unavoidable (and indeed ubiquitous) in theories which have additional space-time dimensions, including string theory and the so-called Brane world scenarios.

Present and Future surveys, combining galaxy number counts and weak lensing measurements, will map the evolution of matter perturbations and gravitational potentials from the matter dominated epoch until today. This will tighten the constraints on allowed expansion histories, and test the relationships between matter overdensities, local curvature, and the Newtonian potential. These relationships, given by Einstein's equations of General Relativity, can be modified in alternative gravity theories. The combination of all these measurements will then provide a powerful probe on the properties and evolution of dark energy and Modified Gravity.

## Objectives

The main objective of this proposal is to shed some light into the question: Could the acceleration of the Universe be a sign of the break down of General Relativity at large scales?

In order to achieve such objective we aim to confront predictions from theoretical models of alternative gravity theories against astronomical data.

## Outcomes

The intensive investigation of Modified Gravity models that we are proposing in this project will leave an important legacy:

- in a deeper understanding of the interplay between gravity and expansion history and structure;
- the relation between cosmological and local observational constraints;
- the special properties of General Relativity itself;
- the techniques needed to distinguish different candidate models, and the limitations and degeneracies within those techniques;
- the development of tests that can probe the validity of General Relativity itself on cosmological scales, independent of any particular alternative model.

## Background

Time-frame: 2010-2013

## Sub-projects

## Financing

The Research Council of Norway - FRINAT grant: 197251/V30