Searching for modified growth patterns with tomographic surveys

Abstract

In alternative theories of gravity, designed to produce cosmic acceleration at the current epoch, the growth of large scale structure can be modified. We study the potential of upcoming and future tomographic surveys such as DES and LSST, with the aid of CMB and supernovae data, to detect departures from the growth of cosmic structure expected within General Relativity. We employ parametric forms to quantify the potential time-and scale-dependent variation of the effective gravitational constant, and the differences between the two Newtonian potentials. We then apply the Fisher matrix technique to forecast the errors on the modified growth parameters from galaxy clustering, weak lensing, CMB, and their cross-correlations across multiple photometric redshift bins. We find that even with conservative assumptions about the data, DES will produce nontrivial constraints on modified growth, and that LSST will do significantly better.

New constraints on parametrised modified gravity from correlations of the CMB with large scale structure

“…β 1 = 4/3 [46]. Moreover, f (R) models that closely mimic ΛCDM correspond to s ∼ 4 [18]. Therefore the number of free parameters in Eqs.…”

“…the mass of the scalar d.o.f. As shown in [18], in the case of scalar-tensor theories the parameters {β i , λ 2 i } are related in the following way…”

“…For the theories described by the action in Eq. (3), the functions µ and γ can be well represented by the following parametrisation introduced by [45] (and used in [18])…”

“…In order to evolve the potentials we employ the MG-CAMB code developed in [18] (and publicly available at http://www.sfu.ca/~gza5/MGCAMB.html) to evaluate the growth of perturbations in models of modified gravity. In this code, the energy-momentum equations remain the standard ones, and the modifications to the Poisson and anisotropy equations are encoded in two functions µ(a, k) and γ(a, k) defined by…”

“…This requirement is twofold: at the background level, the predicted expansion history has to be coherent with distance measurements such as those from Supernovae (SNe), the position of the CMB peaks, and baryon acoustic oscillations (BAO); on the other hand, the modified predictions for structure formation need to pass the test of comparison with the observed large scale structure (LSS) of the Universe. It has been shown for several cases -for instance for f (R) [14][15][16][17][18] and DGP [19][20][21][22][23][24] -that the structure formation test can dramatically reduce the parameter space left unconstrained from the background test, and it is therefore instrumental to the quest for a truly viable model.…”