Beyond the Cosmological Standard Model
  After a decade and a half of research motivated by the accelerating universe,
theory and experiment have a reached a certain level of maturity. The
development of theoretical models beyond \Lambda, or smooth dark energy, often
called modified gravity, has led to broader insights into a path forward, and a
host of observational and experimental tests have been developed. In this
review we present the current state of the field and describe a framework for
anticipating developments in the next decade. We identify the guiding
principles for rigorous and consistent modifications of the standard model, and
discuss the prospects for empirical tests. We begin by reviewing attempts to
consistently modify Einstein gravity in the infrared, focusing on the notion
that additional degrees of freedom introduced by the modification must screen
themselves from local tests of gravity. We categorize screening mechanisms into
three broad classes: mechanisms which become active in regions of high
Newtonian potential, those in which first derivatives become important, and
those for which second derivatives are important. Examples of the first class,
such as f(R) gravity, employ the familiar chameleon or symmetron mechanisms,
whereas examples of the last class are galileon and massive gravity theories,
employing the Vainshtein mechanism. In each case, we describe the theories as
effective theories. We describe experimental tests, summarizing laboratory and
solar system tests and describing in some detail astrophysical and cosmological
tests. We discuss future tests which will be sensitive to different signatures
of new physics in the gravitational sector. Parts that are more relevant to
theorists vs. observers/experimentalists are clearly indicated, in the hope
that this will serve as a useful reference for both audiences, as well as
helping those interested in bridging the gap between them.
