arXiv subjects

David F. Mota

Publications and source records attributed to David F. Mota.

At least 109 records · Page 6Linked to original sources

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↗

arXiv2015

Cosmological simulations with hydrodynamics of screened scalar-tensor gravity with non-universal coupling

In this paper we study the effects of letting the dark matter and the gas in the Universe couple to the scalar field of the symmetron model, a modified gravity theory, with varying coupling strength. We also search for a way to distinguish between universal and non-universal couplings in observations. The research is performed utilising a series of hydrodynamic, cosmological N-Body simulations, studying the resulting power spectra and galaxy halo properties, such as the density and temperature profiles. Results show that in the cases of universal couplings, the deviations in the baryon fraction from $Λ$CDM are smaller than in the cases of non-universal couplings throughout the halos. The same is apparent in the power spectrum baryon bias, defined as the ratio of gas to dark matter power spectrum. Deviations of the density profiles and power spectra from the $Λ$CDM reference values can differ significantly between dark matter and gas because the dark matter deviations are mostly larger than the deviations in the gas.

astro-ph.CO↗

arXiv2015

Cosmology and fundamental physics with the Euclid satellite

Euclid is a European Space Agency medium class mission selected for launch in 2019 within the Cosmic Vision 2015-2025 programme. The main goal of Euclid is to understand the origin of the accelerated expansion of the Universe. Euclid will explore the expansion history of the Universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

astro-ph.CO↗

arXiv2015

Universal predictions of screened modified gravity on cluster scales

Modified gravity models require a screening mechanism to be able to evade the stringent constraints from local gravity experiments and, at the same time, give rise to observable astrophysical and cosmological signatures. Such screened modified gravity models necessarily have dynamics determined by complex nonlinear equations that usually need to be solved on a model-by-model basis to produce predictions. This makes testing them a cumbersome process. In this paper, we investigate whether there is a common signature for all the different models that is suitable to testing them on cluster scales. To do this we propose an observable related to the fifth force, which can be observationally related to the ratio of dynamical-to-lensing mass of a halo, and then show that the predictions for this observable can be rescaled to a near universal form for a large class of modified gravity models. We demonstrate this using the Hu-Sawicki $f(R)$, the Symmetron, the nDGP, and the Dilaton models, as well as unifying parametrizations. The universal form is determined by only three quantities: a strength, a mass, and a width parameter. We also show how these parameters can be derived from a specific theory. This self-similarity in the predictions can hopefully be used to search for signatures of modified gravity on cluster scales in a model-independent way.

astro-ph.CO↗

arXiv2015

Modified Gravity N-body Code Comparison Project

Self-consistent ${\it N}$-body simulations of modified gravity models are a key ingredient to obtain rigorous constraints on deviations from General Relativity using large-scale structure observations. This paper provides the first detailed comparison of the results of different ${\it N}$-body codes for the $f(R)$, DGP, and Symmetron models, starting from the same initial conditions. We find that the fractional deviation of the matter power spectrum from $Λ$CDM agrees to better than $1\%$ up to $k \sim 5-10~h/{\rm Mpc}$ between the different codes. These codes are thus able to meet the stringent accuracy requirements of upcoming observational surveys. All codes are also in good agreement in their results for the velocity divergence power spectrum, halo abundances and halo profiles. We also test the quasi-static limit, which is employed in most modified gravity ${\it N}$-body codes, for the Symmetron model for which the most significant non-static effects among the models considered are expected. We conclude that this limit is a very good approximation for all of the observables considered here.

astro-ph.CO↗

arXiv2015

Hydrodynamic Effects in the Symmetron and $f(R)$-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 $Λ$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 $Λ$CDM, that can be a factor of a few larger than the deviations found in density profiles and power spectra.

astro-ph.CO↗

arXiv2014

Halo velocity profiles in screened modified gravity theories

Screened modified gravity predicts potentially large signatures in the peculiar velocity field that makes it an interesting probe to test gravity on cosmological scales. We investigate the signatures induced by the Symmetron and a Chameleon $f(R)$ model in the peculiar velocity field using $N$-body simulations. By studying fifth force and halo velocity profiles we identify three general categories of effects found in screened modified gravity models: a fully screened regime where we recover $Λ$CDM to high precision, an unscreened regime where the fifth force is in full operation, and, a partially screened regime where screening occurs in the inner part of a halo, but the fifth force is active at larger radii. These three regimes can be pointed out very clearly by analyzing the deviation in the maximum cluster velocity. Observationally, the partially screened regime is of particular interest since an uniform increase of the gravitational force - as present in the unscreened regime - is degenerate with the (dynamical) halo mass estimate, and, thus, hard to detect.

astro-ph.CO↗

arXiv2014

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.

astro-ph.CO↗

arXiv2014

Cosmological simulations of screened modified gravity out of the static approximation: effects on matter distribution

In the context of scalar tensor theories for gravity, there is a universally adopted hypothesis when running N-body simulations that time derivatives in the equation of motion for the scalar field are negligible. In this work we propose to test this assumption for one specific scalar-tensor model with a gravity screening mechanism: the symmetron. To this end, we implemented the necessary modifications to include the non-static terms in the N-body code Ramses. We present test cases and results from cosmological simulations. Our main finding when comparing static vs. non-static simulations is that the global power spectrum is only slightly modified when taking into account the inclusion of non-static terms. On the contrary, we find that the calculation of the local power spectrum gives different measurements. Such results imply one must be careful when assuming the quasi-static approximation when investigating the environmental effects of modified gravity and screening mechanisms in structure formation of halos and voids distributions.

astro-ph.CO↗

arXiv2014

Shape of Clusters as a Probe of Screening Mechanisms in Modified Gravity

Scalar fields are crucial components in high energy physics and extensions of General Relativity. The fact they are not observed in the solar system may be due to a mechanism which screens their presence in high dense regions. We show how observations of the ellipticity of galaxy clusters can discriminate between models with and without scalar fields and even between different screening mechanisms. Using nowadays X-ray observations we put novel constraints on the different models.

astro-ph.CO↗

arXiv2014

CMB statistical anisotropy from noncommutative gravitational waves

Primordial statistical anisotropy is a key indicator to investigate early Universe models and has been probed by the cosmic microwave background (CMB) anisotropies. In this paper, we examine tensor-mode CMB fluctuations generated from anisotropic gravitational waves, parametrised by $P_h({\bf k}) = P_h^{(0)}(k) [ 1 + \sum_{LM} f_L(k) g_{LM} Y_{LM} (\hat{\bf k}) ]$, where $P_h^{(0)}(k)$ is the usual scale-invariant power spectrum. Such anisotropic tensor fluctuations may arise from an inflationary model with noncommutativity of fields. It is verified that in this model, an isotropic component and a quadrupole asymmetry with $f_0(k) = f_2(k) \propto k^{-2}$ are created and hence highly red-tilted off-diagonal components arise in the CMB power spectra, namely $\ell_2 = \ell_1 \pm 2$ in $TT$, $TE$, $EE$ and $BB$, and $\ell_2 = \ell_1 \pm 1$ in $TB$ and $EB$. We find that B-mode polarisation is more sensitive to such signals than temperature and E-mode polarisation due to the smallness of large-scale cosmic variance and we can potentially measure $g_{00} = 30$ and $g_{2M} = 58$ at 68% CL in a cosmic-variance-limited experiment. Such a level of signal may be measured in a PRISM like experiment, while the instrumental noise contaminates it in the $Planck$ experiment. These results imply that it is impossible to measure the noncommutative parameter if it is small enough for the perturbative treatment to be valid. Our formalism and methodology for dealing with the CMB tensor statistical anisotropy are general and straightforwardly applicable to other early Universe models.

astro-ph.CO↗

arXiv2014

Local Observables in a Landscape of Infrared Gauge Modes

Cosmological local observables are at best statistically determined by the fundamental theory describing inflation. When the scalar inflaton is coupled uniformly to a collection of subdominant massless gauge vectors, rotational invariance is obeyed locally. However, the statistical isotropy of fluctuations is spontaneously broken by gauge modes whose wavelength exceed our causal horizon. This leads to a landscape picture where primordial correlators depend on the position of the observer. We compute the stochastic corrections to the curvature power spectrum, show the existence of a new local observable (the shape of the quadrupole), and constrain the theory using Planck limits.

astro-ph.CO↗

arXiv2014

Statistics of Anisotropies in Inflation with Spectator Vector Fields

We study the statistics of the primordial power spectrum in models where massless gauge vectors are coupled to the inflaton, paying special attention to observational implications of having fundamental or effective horizons embedded in a bath of infrared fluctuations. As quantum infrared modes cross the horizon, they classicalize and build a background vector field. We find that the vector experiences a statistical precession phenomenon. Implications for primordial correlators and the interpretation thereof are considered. Firstly, we show how in general two, not only one, additional observables, a quadrupole amplitude and an intrinsic shape parameter, are necessary to fully describe the correction to the curvature power spectrum, and develop a unique parametrization for them. Secondly, we show that the observed anisotropic amplitude and the associated preferred direction depend on the volume of the patch being probed. We calculate non-zero priors for the expected deviations between detections based on microwave background data (which probes the entire Hubble patch) and large scale structure (which only probes a fraction of it).

astro-ph.CO↗

arXiv2013

Gravitational redshift profiles in the $f(R)$ and symmetron models

Aims. We investigate the gravitational redshift in clusters of galaxies in the symmetron and Hu-Sawicky $f(R)$ models. The characteristic feature of both models is the screening mechanism that hides the fifth force in dense environments recovering general relativity. Methods. We use $N$-body simulations that were run using the code \texttt{Isis}, which includes scalar fields, to analyse the deviation of observables in modified gravity models with respect to $Λ$CDM. Results. We find that due to the presence of the screening, the deviation is highly dependent on the halo mass. For instance, the $f(R)$ parameters $|f_{R0}|=10^{-5}$, $n=1$ cause an enhancement of the gravitational signal by up to 50% for halos with masses between $10^{13}\,M_\odot h^{-1}$ and $10^{14}\,M_\odot h^{-1}$. The characteristic mass range where the fifth force is most active varies with the model parameters. The usual assumption is that the presence of a fifth force leads to a deeper potential well and thus, a stronger gravitational redshift. However, we find that in cases in which only the central regions of the halos are screened, there could also be a weaker gravitational redshift.

astro-ph.CO↗

arXiv2013

Mass-varying neutrino in light of cosmic microwave background and weak lensing

We aim to constrain mass-varying neutrino models using large scale structure observations and produce forecast for the Euclid survey. We investigate two models with different scalar field potential and both positive and negative coupling parameters β. These parameters correspond to growing or decreasing neutrino mass, respectively. We explore couplings up to |β|<5. In the case of the exponential potential, we find an upper limit on $Ω_νh^2$<0.004 at 2-$σ$ level. In the case of the inverse power law potential the null coupling can be excluded with more than 2-σsignificance; the limits on the coupling are β>3 for the growing neutrino mass and β<-1.5 for the decreasing mass case. This is a clear sign for a preference of higher couplings. When including a prior on the present neutrino mass the upper limit on the coupling becomes |β|<3 at 2-$σ$ level for the exponential potential. Finally, we present a Fisher forecast using the tomographic weak lensing from an Euclid-like experiment and we also consider the combination with the cosmic microwave background (CMB) temperature and polarisation spectra from a Planck-like mission. If considered alone, lensing data is more efficient in constraining $Ω_ν$ with respect to CMB data alone. There is, however, a strong degeneracy in the β-$Ω_νh^2$ plane. When the two data sets are combined, the latter degeneracy remains, but the errors are reduced by a factor ~2 for both parameters.

astro-ph.CO↗

arXiv2013

Unifying static analysis of gravitational structures with a scale-dependent scalar field gravity as an alternative to dark matter

Aims. We investigated the gravitational effects of a scalar field within scalar-tensor gravity as an alternative to dark matter. Motivated by chameleon, symmetron and f(R)-gravity models, we studied a phenomenological scenario where the scalar field has both a mass (i.e. interaction length) and a coupling constant to the ordinary matter which scale with the local properties of the considered astrophysical system. Methods. We analysed the feasibility of this scenario using the modified gravitational potential obtained in its context and applied it to the galactic and hot gas/stellar dynamics in galaxy clusters and elliptical/spiral galaxies respectively. This is intended to be a first step in assessing the viability of this new approach in the context of "alternative gravity" models. Results. The main results are: 1. the velocity dispersion of elliptical galaxies can be fitted remarkably well by the suggested scalar field, with model significance similar to a classical Navarro-Frenk-White dark halo profile; 2. the analysis of the stellar dynamics and the gas equilibrium in elliptical galaxies has shown that the scalar field can couple with ordinary matter with different strengths (different coupling constants) producing and/or depending on the different clustering state of matter components; 3. elliptical and spiral galaxies, combined with clusters of galaxies, show evident correlations among theory parameters which suggest the general validity of our results at all scales and a way toward a possible unification of the theory for all types of gravitational systems we considered. All these results demonstrate that the proposed scalar field scenario can work fairly well as an alternative to dark matter.

astro-ph.CO↗

arXiv2013

Bimetric gravity doubly coupled to matter: theory and cosmological implications

A ghost-free theory of gravity with two dynamical metrics both coupled to matter is shown to be consistent and viable. Its cosmological implications are studied, and the models, in particular in the context of partially massless gravity, are found to explain the cosmic acceleration without resorting to dark energy.

hep-th↗

arXiv2013

Spatial variations of the fine-structure constant in symmetron models

We investigate the variation of the fine-structure constant, α, in symmetron models using N-body simulations in which the full spatial distribution of α 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 α 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 α. If the scalar-photon coupling strength βγ is of order unity we find that the variation of α 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 α 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.

astro-ph.CO↗