Beyond the cosmological standard model

Abstract

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 Λ 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 recent 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 of the field become important, and those for which second derivatives of the field 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 and discuss prospects for completion in a more fundamental theory. We describe experimental tests of each class of theories, summarizing laboratory and solar system tests and describing in some detail astrophysical and cosmological tests. Finally, we discuss prospects for future tests which will be sensitive to different signatures of new physics in the gravitational sector.The review is structured so that those 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.

BeyondΛCDM: Problems, solutions, and the road ahead

“…Ref. [75], instead), although some of them will be discussed briefly in some of the following sections. Such proposals cover a vast spectrum of possibilities, ranging from: denial, with the hope that the problem is fully harmless in all areas of interest in particle physics and cosmology; to anthropic arguments, where multiverse explanations are employed for why naturalness might not be needed; and modified gravity proposals, where one tries to, for example, screen the CC in a degravitation framework using (e.g.)…”

“…One approach is to attempt to constrain the parameters and functions appearing in parameterized cosmological frameworks [155,214,215,163], while another is to study specific models that are theoretically well-motivated, or exhibit screening mechanisms [75,216].…”

Exploring the liminality: properties of haloes and subhaloes in borderlinef(R) gravity

“…The second class proposes new physics in the gravity sector, so that the standard theory of gravity, Einstein's General Relativity (GR), is modified on cosmological scales to accommodate the accelerated expansion. This latter class of theories are commonly referred to as modified gravity (Clifton et al 2013; E-mail: difu.shi@durham.ac.uk Joyce et al 2015) which is increasingly becoming an active research area.…”

Halo velocity profiles in screened modified gravity theories

“…where γ de-E-mail: maxbg@astro.uio.no pends on the local environment) are dubbed screened modified gravity models since the fifth force is screened in highdensity environments. Five types of screening mechanisms are currently discussed in the literature (see, e.g., Khoury 2010;Joyce et al 2014):…”