### Testing Modifications of General Relativity with Cosmological Observations

Lucas Lombriser

University of Zurich

ABSTRACT

Cosmic acceleration can either be explained by introducing large amounts
of dark energy or considering modifications to gravity. I present a Markov
chain Monte Carlo analysis of the self-accelerating and normal branch of
Dvali-Gabadadze-Porrati (DGP) braneworld gravity as well as of metric f(R)
gravity. Utilizing a variety of cosmological data sets, both DGP branches
require a brane tension or a cosmological constant at high significance
with no evidence for the unique DGP modifications. The f(R) gravity models
of interest here are designed to reproduce the LCDM expansion history with
modications to gravity described by a supplementary cosmological freedom,
the Compton wavelength parameter B0. Cosmological observations, primarily
from cluster abundance, yield tight constraints on the viable parameter
values of B0.