Growth of cosmic structure: Probing dark energy beyond expansion

Abstract

The quantity and quality of cosmic structure observations have greatly accelerated in recent years, and further leaps forward will be facilitated by imminent projects. These will enable us to map the evolution of dark and baryonic matter density fluctuations over cosmic history. The way that these fluctuations vary over space and time is sensitive to several pieces of fundamental physics: the primordial perturbations generated by GUT-scale physics; neutrino masses and interactions; the nature of dark matter and dark energy. We focus on the last of these here: the ways that combining probes of growth with those of the cosmic expansion such as distance-redshift relations will pin down the mechanism driving the acceleration of the Universe.One way to explain the acceleration of the Universe is invoke dark energy parameterized by an equation of state w. Distance measurements provide one set of constraints on w, but dark energy also affects how rapidly structure grows; the greater the acceleration, the more suppressed the growth of structure. Upcoming surveys are therefore designed to probe w with direct observations of the distance scale and the growth of structure, each complementing the other on systematic errors and constraints on dark energy. A consistent set of results will greatly increase the reliability of the final answer.Another possibility is that there is no dark energy, but that General Relativity does not describe the laws of physics accurately on large scales. While the properties of gravity have been measured with exquisite precision at stellar system scales and densities, within our solar system and by binary pulsar systems, its properties in different environments are poorly constrained. To fully understand if General Relativity is the complete theory of gravity we must test gravity across a spectrum of scales and densities. Rapid developments in gravitational wave astronomy and numerical relativity are directed at testing gravity in the high curvature, high density regime. Cosmological evolution provides a polar opposite test bed, probing how gravity behaves in the lowest curvature, low density environments.There are a number of different implementations of astrophysically relevant modifications of gravity. Generically, the models are able to reproduce the distance measurements but at the cost of altering the growth of structure. In particular, as detailed below, the Poisson equation relating over-densities to gravitational potentials is altered, and the potential that determines the geodesics of relativistic particles (such as photons) arXiv:1309.5385v2 [astro-ph.CO]

Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe

“…In the context of cosmic acceleration, clustering measurements can distinguish between models for acceleration that rely on dark energy and those that require modified gravity (Huterer et al 2015). This measurement yields f 8 s , where f measures the growth rate and 8 s measures the amplitude of matter fluctuations.…”

A 2500 deg2 CMB Lensing Map from Combined South Pole Telescope and Planck Data

“…The fluctuation amplitude at redshift z∼1100 (roughly 400,000 years after inflation) is already well constrained by measurements of the primary cosmic microwave background (CMB) temperature and polarization anisotropy power spectra (e.g., Planck Collaboration et al 2015d). Measurements of structure from z∼1100 to today primarily constrain the efficiency with which fluctuations have grown as a function of wavelength and redshift, i.e., the cosmic growth function (e.g., Huterer et al 2015). Among the most interesting and important applications of measuring the growth function are measuring the masses of the neutrinos by detecting their influence on growth as a function of physical wavelength (e.g., Abazajian et al 2015) and distinguishing between dark energy and modified gravity as the cause of cosmic acceleration (e.g., Weinberg et al 2013;Huterer et al 2015).…”

Reconstructing thawing quintessence with multiple datasets

“…The quintessence field affects both the background expansion history of the Universe, and the rate at which matter over-densities grow. A key area in current and future dark energy (DE) observations is the combination of data on the background expansion history and the structure-growth history [4]. The relation between these two histories is sensitive to the properties of the dark energy mechanism, and could allow stronger model constraints than either method separately.…”

“…Some of the prospects for constraining quintessence models with these are discussed in Refs. [1,4,21].…”