ISIS: a new N-body cosmological code with scalar fields based on RAMSES. Code presentation and application to the shapes of clusters

Several extensions of the standard cosmological model include scalar fields as new degrees of freedom in the underlying gravitational theory. A particular class of these scalar field theories include screening mechanisms intended to hide the scalar field below observational limits in the solar system, but not on galactic scales, where data still gives freedom to find possible signatures of their presence. In order to make predictions to compare with observations coming from galactic and clusters scales (i.e. in the non-linear regime of cosmological evolution), cosmological N-body simulations are needed, for which codes that can solve for the scalar field must be developed. We present a new implementation of scalar-tensor theories of gravity which include screening mechanisms. The code is based in the already existing code RAMSES, to which we have added a non-linear multigrid solver that can treat a large class of scalar tensor theories of modified gravity. We present details of the implementation and the tests that we made to it. As application of the new code, we have studied the influence that two particular modified gravity theories, the symmetron and $f(R)$ gravity, have on the shape of cluster sized dark matter halos and found consistent results with previous estimations made with a static analysis.Comment: 13 pages, 6 figures, matches version accepted for publication in A&

Hydrodynamic Effects in the Symmetron and f(R)f(R)-gravity Models

In this paper we present the first results from implementing two scalar-tensor modified gravity theories, the symmetron and the Hu-Sawicki $f(R)$-gravity model, into a hydrodynamic N-body code with dark matter particles and a baryonic ideal gas. The study is a continuation of previous work where the symmetron and $f(R)$ have been successfully implemented in the RAMSES code, but for dark matter only. By running simulations, we show that the deviation from $\Lambda$CDM in these models for the gas density profiles are significantly lower than the dark matter equivalents. When it comes to the matter power-spectrum we find that hydrodynamic simulations agree very well with dark matter only simulations as long as we consider scales larger than $k\sim 0.5$ h/Mpc. In general the effects of modified gravity on the baryonic gas is found to not always mirror the effects it has on the dark matter. The largest signature is found when considering temperature profiles. We find that the gas temperatures in the modified gravity model studied here show deviations, when compared to $\Lambda$CDM, that can be a factor of a few larger than the deviations found in density profiles and power spectra.Comment: 11 pages, 10 figures, submitted to MNRA

Spatial variations of the fine-structure constant in symmetron models

We investigate the variation of the fine-structure constant, {\alpha}, in symmetron models using N-body simulations in which the full spatial distribution of {\alpha} at different redshifts has been calculated. In particular, we obtain simulated sky maps for this variation, and determine its power spectrum. We find that in high-density regions of space (such as deep inside dark matter halos) the value of {\alpha} approaches the value measured on Earth. In the low-density outskirts of halos the scalar field value can approach the symmetry breaking value and leads to significantly different values of {\alpha}. If the scalar-photon coupling strength {\beta}{\gamma} is of order unity we find that the variation of {\alpha} inside dark matter halos can be of the same magnitude as the recent claims by Webb et al. of a dipole variation. Importantly, our results also show that with low-redshift symmetry breaking these models exhibit some dependence of {\alpha} on lookback time (as opposed to a pure spatial dipole) which could in principle be detected by sufficiently accurate spectroscopic measurements, such as those of ALMA and the ELT-HIRES.Comment: 11 pages, 9 figure

Dark matter haloes in modified gravity and dark energy: interaction rate, small-, and large-scale alignment

We study the properties of dark matter haloes in a wide range of modified gravity models, namely, $f(R)$, DGP, and interacting dark energy models. We study the effects of modified gravity and dark energy on the internal properties of haloes, such as the spin and the structural parameters. We find that $f(R)$ gravity enhance the median value of the Bullock spin parameter, but could not detect such effects for DGP and coupled dark energy. $f(R)$ also yields a lower median sphericity and oblateness, while coupled dark energy has the opposite effect. However, these effects are very small. We then study the interaction rate of haloes in different gravity, and find that only strongly coupled dark energy models enhance the interaction rate. We then quantify the enhancement of the alignment of the spins of interacting halo pairs by modified gravity. Finally, we study the alignment of the major axes of haloes with the large-scale structures. The alignment of the spins of interacting pairs of haloes in DGP and coupled dark energy models show no discrepancy with GR, while $f(R)$ shows a weaker alignment. Strongly coupled dark energy shows a stronger alignment of the halo shape with the large-scale structures.Comment: 11 pages, 6 figures, MNRAS Accepte

Chameleon dark energy models with characteristic signatures

In chameleon dark energy models, local gravity constraints tend to rule out parameters in which observable cosmological signatures can be found. We study viable chameleon potentials consistent with a number of recent observational and experimental bounds. A novel chameleon field potential, motivated by f(R) gravity, is constructed where observable cosmological signatures are present both at the background evolution and in the growth-rate of the perturbations. We study the evolution of matter density perturbations on low redshifts for this potential and show that the growth index today gamma_0 can have significant dispersion on scales relevant for large scale structures. The values of gamma_0 can be even smaller than 0.2 with large variations of gamma on very low redshifts for the model parameters constrained by local gravity tests. This gives a possibility to clearly distinguish these chameleon models from the Lambda-Cold-Dark-Matter model in future high-precision observations.Comment: 16 pages, 8 figure

Equivalence of the long-wavelength approximation and the truncated Taylor expansion in relativistic Coulomb excitation

The long-wavelength approximation and the truncated Taylor expansion are frequently used in the theory of relativistic Coulomb excitation to obtain multipole expansions of the interaction. It is shown in this note that these two approximations are exactly equivalent.Comment: 5 page

Chameleons with Field Dependent Couplings

Certain scalar-tensor theories exhibit the so-called chameleon mechanism, whereby observational signatures of scalar fields are hidden by a combination of self-interactions and interactions with ambient matter. Not all scalar-tensor theories exhibit such a chameleon mechanism, which has been originally found in models with inverse power run-away potentials and field independent couplings to matter. In this paper we investigate field-theories with field-dependent couplings and a power-law potential for the scalar field. We show that the theory indeed is a chameleon field theory. We find the thin-shell solution for a spherical body and investigate the consequences for E\"ot-Wash experiments, fifth-force searches and Casimir force experiments. Requiring that the scalar-field evades gravitational tests, we find that the coupling is sensitive to a mass-scale which is of order of the Hubble scale today.Comment: 17 pages, 20 figure

SCALAR: an AMR code to simulate axion-like dark matter models

We present a new code, SCALAR, based on the high-resolution hydrodynamics and N-body code RAMSES, to solve the Schr\"odinger equation on adaptive refined meshes. The code is intended to be used to simulate axion or fuzzy dark matter models where the evolution of the dark matter component is determined by a coupled Schr\"odinger-Poisson equation, but it can also be used as a standalone solver for both linear and non-linear Schr\"odinger equations with any given external potential. This paper describes the numerical implementation of our solver and presents tests to demonstrate how accurately it operates.Comment: 17 pages, 11 figure

Solitons in the dark: non-linear structure formation with fuzzy dark matter

We present the results of a full cosmological simulation with the new code SCALAR, where dark matter is in form of fuzzy dark matter, described by a light scalar field with a mass of $m_{\rm B} = 2.5 \times 10^{-22}$ eV and evolving according to the Schr\"{o}dinger-Poisson system of equations. In comoving units, the simulation volume is $2.5 ~ h^{-1} {\rm Mpc}$ on a side, with a resolution of $20~h^{-1}{\rm pc}$ at the finest refinement level. We analyse the formation and the evolution of central solitonic cores, which are found to leave their imprints on dark matter density profiles, resulting in shallower central densities, and on rotation curves, producing an additional circular velocity peak at small radii from the center. We find that the suppression of structures due to the quantum nature of the scalar field results in an shallower halo mass function in the low-mass end compared to the case of a $\Lambda$CDM simulation, in which dark matter is expected to cluster at all mass scales even if evolved with the same initial conditions used for fuzzy dark matter. Furthermore, we verify the scaling relations characterising the solution to the Schr\"{o}dinger-Poisson system, for both isolated and merging halos, and we find that they are preserved by merging processes. We characterise each fuzzy dark matter halo in terms of the dimensionless quantity $\Xi \propto \left | E_{\rm halo} \right |/M_{\rm halo}^3$ and we show that the core mass is tightly linked to the halo mass by the core-halo mass relation $M_{\rm core}/M_{\rm halo} \propto \Xi^{1/3}$. We also show that the core surface density of the simulated fuzzy dark matter halos does not follow the scaling with the core radius as observed for dwarf galaxies, representing a big challenge for the fuzzy dark matter model as the sole explanation of core formation.Comment: 15 pages, 12 figure