arXiv subjects

Claudio Llinares

Publications and source records attributed to Claudio Llinares.

At least 19 recordsLinked to original sources

arXiv2023

Extension of General Relativity with MOND limit predicts novel orbital structure in and around galaxies

Detailed knowledge of the different classes of stellar orbits that can be accommodated in a given galactic potential is a prerequisite when building self-consistent models using for instance the Schwazschild technique. Furthermore, observational properties of galaxies depend on what these classes of orbits are and on the presence of chaos in the systems. In the realistic case in which the starting point for modeling is not a gravitational potential, but an observed density distribution, we will require a gravitational theory to make the connection between the stars that we see and the movement these stars may be having. The argument can be turned upside down: understanding what orbits may be allowed by each gravitational theory may give us a greater insight on what these theories are and on how we can test them. Our aim is thus to understand novel properties of orbits that are predicted by the latest extension of the MOND phenomenology into the relativistic world. We integrated orbits numerically in a fixed density distribution. The potential required for such integration was obtained also numerically by assuming different gravitational models. We find that thanks to the presence of a mass term in the field equations, the theory can allocate new classes of orbits that do not exist in Newtonian gravity nor standard MOND. We discuss consequences that these new families of orbits can have in non-linear cosmological structure formation as well as explore a possible alternative model for galactic structure based on them.

astro-ph.GA↗

arXiv2021

Simulation techniques for modified gravity

The standard paradigm of cosmology assumes General Relativity (GR) is a valid theory for gravity at scales in which it has not been properly tested. Developing novel tests of GR and its alternatives is crucial if we want to give strength to the model or find departures from GR in the data. Since alternatives to GR are usually defined through nonlinear equations, designing new tests for these theories implies a jump in complexity and thus, a need for refining the simulation techniques. We summarize existing techniques for dealing with modified gravity (MG) in the context of cosmological simulations. $N$-body codes for MG are usually based on standard gravity codes. We describe the required extensions, classifying the models not according to their original motivation, but by the numerical challenges that must be faced by numericists. MG models usually give rise to elliptic equations, for which multigrid techniques are well suited. Thus, we devote a large fraction of this review to describing this particular technique. Contrary to other reviews on multigrid methods, we focus on the specific techniques that are required to solve MG equations and describe useful tricks. Finally, we describe extensions for going beyond the static approximation and dealing with baryons.

astro-ph.CO↗

arXiv2020

The Accuracy of Weak Lensing Simulations

We investigate the accuracy of weak lensing simulations by comparing the results of five independently developed lensing simulation codes run on the same input $N$-body simulation. Our comparison focuses on the lensing convergence maps produced by the codes, and in particular on the corresponding PDFs, power spectra and peak counts. We find that the convergence power spectra of the lensing codes agree to $\lesssim 2\%$ out to scales $\ell \approx 4000$. For lensing peak counts, the agreement is better than $5\%$ for peaks with signal-to-noise $\lesssim 6$. We also discuss the systematic errors due to the Born approximation, line-of-sight discretization, particle noise and smoothing. The lensing codes tested deal in markedly different ways with these effects, but they nonetheless display a satisfactory level of agreement. Our results thus suggest that systematic errors due to the operation of existing lensing codes should be small. Moreover their impact on the convergence power spectra for a lensing simulation can be predicted given its numerical details, which may then serve as a validation test.

astro-ph.CO↗

arXiv2019

Effects of chaos on the detectability of stellar streams

Observations show that stellar streams originating in satellite dwarf galaxies are frequent in the Universe. While such events are predicted by theory, it is not clear how many of the streams that are generated are washed out afterwards to the point in which it is imposible to detect them. Here we study how these diffusion times are affected by the fact that typical gravitational potentials of the host galaxies can sustain chaotic orbits. We do this by comparing the behaviour of simulated stellar streams that reside in chaotic or non-chaotic regions of the phase-space. We find that chaos does reduce the time interval in which streams can be detected. By analyzing detectability criteria in configuration and velocity space, we find that the impact of these results on the observations depends on the quality of both the data and the underlying stellar halo model. For all the stellar streams, we obtain a similar upper limit to the detectable mass.

astro-ph.GA↗

arXiv2019

Non-linear Phenomenology of Disformally Coupled Quintessence

The Quintessence model is one of the simplest and better known alternatives to Einstein's theory for gravity. The properties of the solutions have been studied in great detail in the background, linear and non-linear contexts in cosmology. Here we discuss new phenomenology that is induced by adding disformal terms to the interactions. Among other results, we show analytically and using cosmological simulations ran with the code \texttt{Isis} that the model posses a mechanism through which is it possible to obtain repulsive fifth forces, which are opposite to gravity. Although the equations are very complex, we also find that most of the new phenomenology can be explained by studying background quantities. We used our simulation data to test approximate relations that exist between the metric and scalar field perturbations as well as between the fifth force and gravity. Excellent agreement was found between exact and approximated solutions, which opens the way for running disformal gravity cosmological simulations using simply a Newtonian solver. These results could not only help us to find new ways of testing gravity, but also provide new motivations for building alternative models.

astro-ph.CO↗

arXiv2018

Detecting Domain Walls in Laboratory Experiments

The inherently unstable nature of domain walls makes their detection in laboratory experiments extremely challenging. We propose a method to stabilise domain walls in a particular modified gravity model inside a cavity. We suggest two ways in which the walls could be detected once stabilized: studying the trajectories of Ultra Cold Neutrons (UCN's) either via the deflection angle of a neutron beam induced by the attraction towards the wall or through the time difference of these particles passing through the wall. We give realistic estimates for these effects and expect that they should be detectable experimentally.

astro-ph.CO↗

arXiv2018

Cosmological Constraints from Fourier Phase Statistics

Most statistical inference from cosmic large-scale structure relies on two-point statistics, i.e.\ on the galaxy-galaxy correlation function (2PCF) or the power spectrum. These statistics capture the full information encoded in the Fourier amplitudes of the galaxy density field but do not describe the Fourier phases of the field. Here, we quantify the information contained in the line correlation function (LCF), a three-point Fourier phase correlation function. Using cosmological simulations, we estimate the Fisher information (at redshift $z=0$) of the 2PCF, LCF and their combination, regarding the cosmological parameters of the standard $Λ$CDM model, as well as a Warm Dark Matter (WDM) model and the $f(R)$ and Symmetron modified gravity models. The galaxy bias is accounted for at the level of a linear bias. The relative information of the 2PCF and the LCF depends on the survey volume, sampling density (shot noise) and the bias uncertainty. For a volume of $1h^{-3}\rm Gpc^3$, sampled with points of mean density $\bar{n} = 2\times10^{-3} h^{3}\ \rm Mpc^{-3}$ and a bias uncertainty of 13\%, the LCF improves the parameter constraints by about 20\% in the $Λ$CDM cosmology and potentially even more in alternative models. Finally, since a linear bias only affects the Fourier amplitudes (2PCF), but not the phases (LCF), the combination of the 2PCF and the LCF can be used to break the degeneracy between the linear bias and $σ_8$, present in 2-point statistics.

astro-ph.CO↗

arXiv2018

Testing modified gravity with globular clusters: the case of NGC 2419

The dynamics of globular clusters has been studied in great detail in the context of general relativity as well as with modifications of gravity that strongly depart from the standard paradigm such as MOND. However, at present there are no studies that aim to test the impact that less extreme modifications of gravity (e.g. models constructed as alternatives to dark energy) have on the behaviour of globular clusters. This Letter presents fits to the velocity dispersion profile of the cluster NGC 2419 under the symmetron modified gravity model. The data shows an increase in the velocity dispersion towards the centre of the cluster which could be difficult to explain within general relativity. By finding the best fitting solution associated with the symmetron model, we show that this tension does not exist in modified gravity. However, the best fitting parameters give a model that is inconsistent with the dynamics of the Solar System. Exploration of different screening mechanisms should give us the chance to understand if it is possible to maintain the appealing properties of the symmetron model when it comes to globular clusters and at the same time recover the Solar System dynamics properly.

astro-ph.CO↗

arXiv2017

The shrinking domain framework I: a new, faster, more efficient approach to cosmological simulations

The advent of the new generation of wide field galaxy surveys makes large N-body cosmological simulations a necessary evil. While the cosmological simulation codes have evolved a lot since the first calculations in the 80s, the computational requirements for generating data that is relevant for large surveys remain challenging. We propose an alternative approach that can speed up these simulations. The framework is based on the idea of reducing the size of the integration region following the lightcone of an observer at redshift zero and thus simulating only the parts of the Universe that can be observed. A possible implementation of this framework is presented, as well as tests of its accuracy and performance. These simple tests, based on conservative assumptions, show that the new framework gives a factor of three speed up with respect to the usual approach.

astro-ph.CO↗

arXiv2017

Weighted density fields as improved probes of modified gravity models

When it comes to searches for extensions to general relativity, large efforts are being dedicated to accurate predictions for the power spectrum of density perturbations. While this observable is known to be sensitive to the gravitational theory, its efficiency as a diagnostic for gravity is significantly reduced when Solar System constraints are strictly adhered to. We show that this problem can be overcome by studying weigthed density fields. We propose a transformation of the density field for which the impact of modified gravity on the power spectrum can be increased by more than a factor of three. The signal is not only amplified, but the modified gravity features are shifted to larger scales which are less affected by baryonic physics. Furthermore, the overall signal-to-noise increases, which in principle makes identifying signatures of modified gravity with future galaxy surveys more feasible. While our analysis is focused on modified gravity, the technique can be applied to other problems in cosmology, such as the detection of neutrinos, the effects of baryons or baryon acoustic oscillations.

astro-ph.CO↗

arXiv2017

Speeding up $N$-body simulations of modified gravity: Chameleon screening models

We describe and demonstrate the potential of a new and very efficient method for simulating certain classes of modified gravity theories, such as the widely studied $f(R)$ gravity models. High resolution simulations for such models are currently very slow due to the highly nonlinear partial differential equation that needs to be solved exactly to predict the modified gravitational force. This nonlinearity is partly inherent, but is also exacerbated by the specific numerical algorithm used, which employs a variable redefinition to prevent numerical instabilities. The standard Newton-Gauss-Seidel iterative method used to tackle this problem has a poor convergence rate. Our new method not only avoids this, but also allows the discretised equation to be written in a form that is analytically solvable. We show that this new method greatly improves the performance and efficiency of $f(R)$ simulations. For example, a test simulation with $512^3$ particles in a box of size $512 \, \mathrm{Mpc}/h$ is now 5 times faster than before, while a Millennium-resolution simulation for $f(R)$ gravity is estimated to be more than 20 times faster than with the old method. Our new implementation will be particularly useful for running very high resolution, large-sized simulations which, to date, are only possible for the standard model, and also makes it feasible to run large numbers of lower resolution simulations for covariance analyses. We hope that the method will bring us to a new era for precision cosmological tests of gravity.

astro-ph.CO↗

arXiv2017

Weak lensing by galaxy troughs with modified gravity

We study the imprints that theories of gravity beyond GR can leave on the lensing signal around line of sight directions that are predominantly halo-underdense (called troughs) and halo-overdense. To carry out our investigations, we consider the normal branch of DGP gravity, as well as a phenomenological variant thereof that directly modifies the lensing potential. The predictions of these models are obtained with N-body simulation and ray-tracing methods using the ECOSMOG and Ray-Ramses codes. We analyse the stacked lensing convergence profiles around the underdense and overdense lines of sight, which exhibit, respectively, a suppression and a boost w.r.t. the mean in the field of view. The modifications to gravity in these models strengthen the signal w.r.t. $Λ{\rm CDM}$ in a scale-independent way. We find that the size of this effect is the same for both underdense and overdense lines of sight, which implies that the density field along the overdense directions on the sky is not sufficiently evolved to trigger the suppression effects of the screening mechanism. These results are robust to variations in the minimum halo mass and redshift ranges used to identify the lines of sight, as well as to different line of sight aperture sizes and criteria for their underdensity and overdensity thresholds.

astro-ph.CO↗

arXiv2016

Distinguishing screening mechanisms with environment-dependent velocity statistics

Alternative theories of gravity typically invoke an environment-dependent screening mechanism to allow phenomenologically interesting deviations from general relativity (GR) to manifest on larger scales, while reducing to GR on small scales. The observation of the transition from screened to unscreened behavior would be compelling evidence for beyond-GR physics. We show that pairwise peculiar velocity statistics, in particular the relative radial velocity dispersion, $σ_\parallel$ , can be used to observe this transition when they are binned by some measure of halo environment. We established this by measuring the radial velocity dispersion between pairs of halos in N-body simulations for three $f(R)$ gravity and four symmetron models. We developed an estimator involving only line-of-sight velocities to show that this quantity is observable, and binned the results in halo mass, ambient density, and the isolatedness of halos. Ambient density is found to be the most relevant measure of environment; it is distinct from isolatedness, and correlates well with theoretical expectations for the symmetron model. By binning $σ_\parallel$ in ambient density, we find a strong environment-dependent signature for the symmetron models, with the velocities showing a clear transition from GR to non-GR behavior. No such transition is observed for $f(R)$, as the relevant scales are deep in the unscreened regime. Observations of the relative radial velocity dispersion in forthcoming peculiar velocity surveys, if binned appropriately by environment, therefore offer a valuable way of detecting the screening signature of modified gravity.

astro-ph.CO↗

arXiv2016

Modelling Void Abundance in Modified Gravity

We use a spherical model and an extended excursion set formalism with drifting diffusive barriers to predict the abundance of cosmic voids in the context of general relativity as well as f(R) and symmetron models of modified gravity. We detect spherical voids from a suite of N-body simulations of these gravity theories and compare the measured void abundance to theory predictions. We find that our model correctly describes the abundance of both dark matter and galaxy voids, providing a better fit than previous proposals in the literature based on static barriers. We use the simulation abundance results to fit for the abundance model free parameters as a function of modified gravity parameters, and show that counts of dark matter voids can provide interesting constraints on modified gravity. For galaxy voids, more closely related to optical observations, we find that constraining modified gravity from void abundance alone may be significantly more challenging. In the context of current and upcoming galaxy surveys, the combination of void and halo statistics including their abundances, profiles and correlations should be effective in distinguishing modified gravity models that display different screening mechanisms.

astro-ph.CO↗

arXiv2016

RAY-RAMSES: a code for ray tracing on the fly in N-body simulations

We present a ray tracing code to compute integrated cosmological observables on the fly in AMR N-body simulations. Unlike conventional ray tracing techniques, our code takes full advantage of the time and spatial resolution attained by the N-body simulation by computing the integrals along the line of sight on a cell-by-cell basis through the AMR simulation grid. Moroever, since it runs on the fly in the N-body run, our code can produce maps of the desired observables without storing large (or any) amounts of data for post-processing. We implemented our routines in the RAMSES N-body code and tested the implementation using an example of weak lensing simulation. We analyse basic statistics of lensing convergence maps and find good agreement with semi-analytical methods. The ray tracing methodology presented here can be used in several cosmological analysis such as Sunyaev-Zel'dovich and integrated Sachs-Wolfe effect studies as well as modified gravity. Our code can also be used in cross-checks of the more conventional methods, which can be important in tests of theory systematics in preparation for upcoming large scale structure surveys.

astro-ph.CO↗

arXiv2016

Probing scalar tensor theories for gravity in redshift space

We present measurements of the spatial clustering statistics in redshift space of various scalar field modified gravity simulations. We utilise the two-point and the three-point correlation functions to quantify the spatial distribution of dark matter halos within these simulations and thus discern between the models. We compare $Λ$CDM simulations to various modified gravity scenarios and find consistency with previous work in terms of 2-point statistics in real and redshift-space. However using higher order statistics such as the three-point correlation function in redshift space we find significant deviations from $Λ$CDM hinting that higher order statistics may prove to be a useful tool in the hunt for deviations from General Relativity.

astro-ph.CO↗

arXiv2016

Beyond $Λ$CDM: Problems, solutions, and the road ahead

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, $Λ$CDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of $Λ$CDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.

astro-ph.CO↗

arXiv2016

Wave Propagation in Modified Gravity

We investigate the propagation of scalar waves induced by matter sources in the context of scalar-tensor theories of gravity which include screening mechanisms for the scalar degree of freedom. The usual approach when studying these theories in the non-linear regime of cosmological perturbations is based on the assumption that scalar waves travel at the speed of light. Within General Relativity such approximation is good and leads to no loss of accuracy in the estimation of observables. We find, however, that mass terms and non-linearities in the equations of motion lead to propagation and dispersion velocities significantly different from the speed of light. As the group velocity is the one associated to the propagation of signals, a reduction of its value has direct impact on the behavior and dynamics of nonlinear structures within modified gravity theories with screening. For instance, the internal dynamics of galaxies and satellites submerged in large dark matter halos could be affected by the fact that the group velocity is smaller than the speed of light. It is therefore important, within such framework, to take into account the fact that different part of a galaxy will see changes in the environment at different times. A full non-static analysis may be necessary under those conditions.

gr-qc↗