Simulating galaxy formation in f(R) modified gravity: Matter, halo, and galaxy-statistics

We present an analysis of the matter, halo and galaxy clustering in f(R)-gravity employing the SHYBONE full-physics hydrodynamical simulation suite. Our analysis focuses on the interplay between baryonic feedback and f(R)-gravity in the matter power spectrum, the matter and halo correlation functions, the halo and galaxy-host-halo mass function, the subhalo and satellite-galaxy count and the correlation function of the stars in our simulations. Our studies of the matter power spectrum in full physics simulations in f(R)-gravity show, that it will be very difficult to derive accurate fitting formulae for the power spectrum enhancement in f(R)-gravity which include baryonic effects. We find that the enhancement of the halo mass function due to f(R)-gravity and its suppression due to feedback effects do not show significant back-reaction effects and can thus be estimated from independent GR-hydro and f(R) dark matter only simulations. Our simulations furthermore show, that the number of subhaloes and satellite-galaxies per halo is not significantly affected by f(R)-gravity. Low mass haloes are nevertheless more likely to be populated by galaxies in f(R)-gravity. This suppresses the clustering of stars and the galaxy correlation function in the theory compared to standard cosmology

K-mouflage gravity models that pass Solar System and cosmological constraints

We show that Solar System tests can place very strong constraints on K-mouflage models of gravity, which are coupled scalar field models with nontrivial kinetic terms that screen the fifth force in regions of large gravitational acceleration. In particular, the bounds on the anomalous perihelion of the Moon imposes stringent restrictions on the K-mouflage Lagrangian density, which can be met when the contributions of higher-order operators in the static regime are sufficiently small. The bound on the rate of change of the gravitational strength in the Solar System constrains the coupling strength $\beta$ to be smaller than $0.1$. These two bounds impose tighter constraints than the results from the Cassini satellite and Big Bang Nucleosynthesis. Despite the Solar System restrictions, we show that it is possible to construct viable models with interesting cosmological predictions. In particular, relative to $\Lambda$-CDM, such models predict percent-level deviations for the clustering of matter and the number density of dark matter haloes. This makes these models predictive and testable by forthcoming observational missions.Comment: 15 page

New method for initial density reconstruction

A theoretically interesting and practically important question in cosmology is the reconstruction of the initial density distribution provided a late-time density field. This is a long-standing question with a revived interest recently, especially in the context of optimally extracting the baryonic acoustic oscillation (BAO) signals from observed galaxy distributions. We present a new efficient method to carry out this reconstruction, which is based on numerical solutions to the nonlinear partial differential equation that governs the mapping between the initial Lagrangian and final Eulerian coordinates of particles in evolved density fields. This is motivated by numerical simulations of the quartic Galileon gravity model, which has similar equations that can be solved effectively by multigrid Gauss-Seidel relaxation. The method is based on mass conservation, and does not assume any specific cosmological model. Our test shows that it has a performance comparable to that of state-of-the-art algorithms that were very recently put forward in the literature, with the reconstructed density field over ∼80% (50%) correlated with the initial condition at k ≲ 0.6 h=Mpc (1.0 h=Mpc). With an example, we demonstrate that this method can significantly improve the accuracy of BAO reconstruction

Weak lensing by voids in weak lensing maps

Cosmic voids are an important probe of large-scale structure that can constrain cosmological parameters and test cosmological models. We present a new paradigm for void studies: void detection in weak lensing convergence maps. This approach identifies objects that relate directly to our theoretical understanding of voids as underdensities in the total matter field and presents several advantages compared to the customary method of finding voids in the galaxy distribution. We exemplify this approach by identifying voids using the weak lensing peaks as tracers of the large-scale structure. We find self-similarity in the void abundance across a range of peak signal-to-noise selection thresholds. The voids obtained via this approach give a tangential shear signal up to ∼40 times larger than voids identified in the galaxy distribution

The e-MANTIS emulator: fast predictions of the non-linear matter power spectrum in f(R)f(R)CDM cosmology

In order to probe modifications of gravity at cosmological scales, one needs accurate theoretical predictions. N-body simulations are required to explore the non-linear regime of structure formation but are very time consuming. In this work, we release a new public emulator, dubbed e-MANTIS, that performs an accurate and fast interpolation between the predictions of $f(R)$ modified gravity cosmological simulations, run with ECOSMOG. We sample a wide 3D parameter space given by the current background scalar field value $10^{-7}<\left|f_{R_0}\right|<10^{-4}$, matter density $0.24<\Omega_\mathrm{m}<0.39$, and primordial power spectrum normalisation $0.6<\sigma_8<1.0$, with 110 points sampled from a Latin Hypercube. For each model we perform pairs of $f(R)$CDM and $\Lambda$CDM simulations covering an effective volume of $\left(560 \, h^{-1}\mathrm{Mpc}\right)^3$ with a mass resolution of $\sim 2 \times 10^{10} h^{-1} M_\odot$. We build an emulator for the matter power spectrum boost $B(k)=P_{f(R)}(k)/P_{\Lambda\mathrm{CDM}}(k)$ using a Gaussian Process Regression method. The boost is mostly independent of $h$, $n_{s}$, and $\Omega_{b}$, which reduces the dimensionality of the relevant cosmological parameter space. Additionally, it is more robust against statistical and systematic errors than the raw power spectrum, thus strongly reducing our computational needs. According to our dedicated study of numerical systematics, the resulting emulator has an estimated maximum error of $3\%$ across the whole cosmological parameter space, for scales $0.03 \ h\mathrm{Mpc}^{-1} < k < 7 \ h\mathrm{Mpc}^{-1}$, and redshifts$0 < z < 2$, while in most cases the accuracy is better than$1\%$. Such an emulator could be used to constrain$f(R)$ gravity with weak lensing analysesComment: 23 pages, 18 figures. Accepted for publication in MNRAS. Link to the emulator code: https://zenodo.org/doi/10.5281/zenodo.773836

The e-MANTIS emulator: fast predictions of the non-linear matter power spectrum in f(R)CDM cosmology

In order to probe modifications of gravity at cosmological scales, one needs accurate theoretical predictions. N-body simulationsare required to explore the non-linear regime of structure formation but are very time consuming. In this work, we release a newpublic emulator, dubbed E-MANTIS, that performs an accurate and fast interpolation between the predictions of f(R) modifiedgravity cosmological simulations, run with ECOSMOG. We sample a wide 3D parameter space given by the current backgroundscalar field value 10−7 < fR0 < 10−4, matter density 0.24 < m < 0.39, and primordial power spectrum normalization 0.6 < σ8 < 1.0, with 110 points sampled from a Latin hypercube. For each model we perform pairs of f(R)CDM and CDM simulations covering an effective volume of 560 h−1 Mpc3 with a mass resolution of ∼2 × 1010h−1M. We build an emulator for the matter power spectrum boost B(k) = Pf(R)(k)/PCDM(k) using a Gaussian process regression method. The boost is mostly independent of h, ns, and b, which reduces the dimensionality of the relevant cosmological parameter space. Additionally, it is more robust against statistical and systematic errors than the raw power spectrum, thus strongly reducing our computational needs. According to our dedicated study of numerical systematics, the resulting emulator has an estimated maximum error of 3 per cent across the whole cosmological parameter space, for scales 0.03 h Mpc−1 <k< 7 h Mpc−1, and redshifts 0 <z< 2, while in most cases the accuracy is better than 1 per cent. Such an emulator could be used to constrain f(R) gravity with weak lensing analyses

Dynamic task fusion for a block-structured finite volume solver over a dynamically adaptive mesh with local time stepping

Load balancing of generic wave equation solvers over dynamically adaptive meshes with local time stepping is dicult, as the load changes with every time step. Task-based programming promises to mitigate the load balancing problem. We study a Finite Volume code over dynamically adaptive block-structured meshes for two astrophysics simulations, where the patches (blocks) dene tasks. They are classied into urgent and low priority tasks. Urgent tasks are algorithmically latencysensitive. They are processed directly as part of our bulk-synchronous mesh traversals. Non-urgent tasks are held back in an additional task queue on top of the task runtime system. If they lack global side-eects, i.e. do not alter the global solver state, we can generate optimised compute kernels for these tasks. Furthermore, we propose to use the additional queue to merge tasks without side-eects into task assemblies, and to balance out imbalanced bulk synchronous processing phases

Can background cosmology hold the key for modified gravity tests?

Modified gravity theories are a popular alternative to dark energy as a possible explanation for the observed accelerating cosmic expansion, and their cosmological tests are currently an active research field. Studies in recent years have been increasingly focused on testing these theories in the nonlinear regime, which is computationally demanding. Here we show that, under certain circumstances, a whole class of theories can be ruled out by using background cosmology alone. This is possible because certain classes of models (i) are fundamentally incapable of producing specific background expansion histories, and (ii) said histories are incompatible with local gravity tests. As an example, we demonstrate that a popular class of models, f(R) gravity, would not be viable if observations suggest even a slight deviation of the background expansion history from that of the ΛCDM paradigm

Void Dynamics

Cosmic voids are becoming key players in testing the physics of our Universe. Here we concentrate on the abundances and the dynamics of voids as these are among the best candidates to provide information on cosmological parameters. Cai, Padilla \& Li (2014) use the abundance of voids to tell apart Hu \& Sawicki $f(R)$ models from General Relativity. An interesting result is that even though, as expected, voids in the dark matter field are emptier in $f(R)$ gravity due to the fifth force expelling away from the void centres, this result is reversed when haloes are used to find voids. The abundance of voids in this case becomes even lower in $f(R)$ compared to GR for large voids. Still, the differences are significant and this provides a way to tell apart these models. The velocity field differences between $f(R)$ and GR, on the other hand, are the same for halo voids and for dark matter voids. Paz et al. (2013), concentrate on the velocity profiles around voids. First they show the necessity of four parameters to describe the density profiles around voids given two distinct void populations, voids-in-voids and voids-in-clouds. This profile is used to predict peculiar velocities around voids, and the combination of the latter with void density profiles allows the construction of model void-galaxy cross-correlation functions with redshift space distortions. When these models are tuned to fit the measured correlation functions for voids and galaxies in the Sloan Digital Sky Survey, small voids are found to be of the void-in-cloud type, whereas larger ones are consistent with being void-in-void. This is a novel result that is obtained directly from redshift space data around voids. These profiles can be used to remove systematics on void-galaxy Alcock-Pacinsky tests coming from redshift-space distortions.Comment: 8 pages, 4 figures, to appear in the proceedings of IAU308 Symposium "The Zeldovich Universe

Subhalo abundance matching in f(R) gravity

Using the liminality N-body simulations of Shi et al., we present the first predictions for galaxy clustering in f(R) gravity using subhalo abundance matching. We find that, for a given galaxy density, even for an f(R) model with fR0=−10−6, for which the cold dark matter clustering is very similar to the cold dark matter model with a cosmological constant (ΛCDM), the predicted clustering of galaxies in the f(R) model is very different from ΛCDM. The deviation can be as large as 40% for samples with mean densities close to that of L∗ galaxies. This large deviation is testable given the accuracy that future large-scale galaxy surveys aim to achieve. Our result demonstrates that galaxy surveys can provide a stringent test of general relativity on cosmological scales, which is comparable to the tests from local astrophysical observations