Structure Formation in Modified Gravity Cosmologies

Abstract

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source• a link is made to the metadata record in Durham E-Theses• the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. AbstractWe study linear and nonlinear structure formation in cosmologies where the accelerated expansion is driven by modifications to general relativity (GR). We focus on Galileon andNonlocal gravity, which are two classes of models that have been attracting much attention.We derive the linearly perturbed model equations and solve them with suitably modified versions of Einstein-Boltzmann codes. We also derive the perturbed equations keeping the relevant nonlinear terms for small scale structure formation, which we solve using Nbody codes and semi-analytical techniques that were developed for these models. Using CMB, SNIa and BAO data we find strong evidence for nonzero active neutrino masses (Σm ν ≈ 0.6 eV) in all three main branches of covariant Galileon cosmologies, known as the Cubic, Quartic and Quintic models. However, in all branches, the lensing potential does not decay at late times on sub-horizon scales, which contradicts the measured positive sign of the ISW effect, thereby ruling out the Galileon model. The Nonlocal model we study should be able to fit the CMB with similar parameter values as ΛCDM. The N-body simulation results show that the covariant Galileon model admits realistic halo occupation distributions of luminous red galaxies, even for model parameters whose linear growth is noticieably enhanced (σ 8 ≈ 1) relative to ΛCDM. In the Cubic Galileon model the screening mechanism is very efficient on scales 1Mpc, but in the Quartic and Quintic sectors, as well as in the Nonlocal model, we identify potential tensions with Solar System bounds.We illustrate that, despite the direct modifications to the lensing potential in the CubicGalileon and Nonlocal models, cluster masses estimated from lensing remain the same as in GR. The lensing effects produced by cosmic voids found in the simulations of the Cubic Galileon are significanly boosted (≈ 100%) compared to GR, which strongly motivates using voids in tests of gravity. The combination of linear and nonlinear theory results presented here for Galileon and Nonlocal gravity is an example of what it could be done for any serious alternative models to ΛCDM, which will be tested by future experiments.

Stealth black holes in shift symmetric kinetic gravity braiding

“…A GW detection with an optical counterpart in LISA (0.1 mHz -0.1 Hz) will put a stronger conclusion to the GW speed. On the other hand, several works have shown that the cubic Galileon is disfavored by cosmic microwave background, baryon acoustic oscillations, integrated Sachs-Wolfe, and weak lensing [77][78][79][80] and, recently, it has been claimed that cosmological data favor the Galileon ghost condensate over ΛCDM [81]. Also, several recent works have shown that cosmological data cannot rule out KGB [82][83][84] and that the braiding might even be the inevitable limit of scalar-tensor theories should all future cosmological data continue to remain consistent with GR [90][91][92].…”

“…We shall mostly focus our attention on background solutions in the subset of Horndeski theories called kinetic gravity braiding [38,39]. KGB has become popular lately because of its consistency with the GW170817 bound on the gravitational wave speed [72][73][74][75][76] and a number of cosmological constraints [77][78][79][80][81][82][83][84]. Many important theories-quintessence, k-essence, cubic Galileon, Galileon ghost condensate, and GR, to name a few-are special cases of KGB (see Eq.…”

Hair-dressing Horndeski: an approach to hairy solutions in cubic Horndeski gravity

“…) is statistically ruled out by cosmological data including weak lensing [57][58][59][60]. The rest of the quadratic and cubic sectors remain phenomenologically interesting when cosmological data is taken into consideration [61][62][63].…”

“…Apart from this, the cubic sector is quite appealing since its static and spherically symmetric solutions are known to be free from ghost and gradient instabilities under perturbations of the metric and the scalar field [55,56]. It is worth mentioning that the cubic sector escapes the latest observational bounds on the speed of gravitational waves [2,26,27], though some works also claim that its cubic galileon limit (i.e, for which G 3 ∼ X) is inconsistent with the cosmic microwave background, baryon acoustic oscillations, and weak lensing [57][58][59][60]. Recent work has also shown that Horndeski theory remains phenomenologically viable even when cosmological observations are taken into consideration [61][62][63].…”

Cosmological perturbations in f(G) gravity

“…However, the standard model of cosmology (ΛCDM ) has suffered with the explanation of the cause of the acceleration of the expansion of the universe at least without inclusion of cosmological constant. Moreover, there are important open questions in active theoretical and observational cosmology such as the horizon problem, the flatness problem, the monopole problem, the origin and fate of the universe, the formation of the primordial universe, the inhomogeneity and anisotropy of the universe [10,11,12,13], the way cosmological perturbations and primordial fluctuations of the early universe produced the large scale structures, and how the astropysical objects such as stars, galaxies were formed and how they have evolved [14,15,16,17,18].…”