Baojiu Li

Publications of Baojiu Li

• Redshift space distortions in f(R) gravity

  Authors: Elise Jennings, Carlton M. Baugh, Baojiu Li, Gong-Bo Zhao, Kazuya Koyama

  05/2012;

  We use large volume, high resolution N-body simulations to predict the clustering of dark matter in redshift space in f(R) modified gravity cosmologies. This is the first time that the nonlinearWe use large volume, high resolution N-body simulations to predict the clustering of dark matter in redshift space in f(R) modified gravity cosmologies. This is the first time that the nonlinear matter and velocity fields have been resolved to such a high level of accuracy over a broad range of scales in this class of models. We find significant deviations from the clustering signal in standard gravity, with an enhanced boost in power on large scales and stronger damping on small scales in the f(R) models compared to GR at redshifts z<1. We measure the velocity divergence (P_\theta \theta) and matter (P_\delta \delta) power spectra and find a large deviation in the ratios \sqrt{P_\theta \theta/P_\delta \delta} and P_\delta \theta/P_\delta\delta, between the f(R) models and GR for 0.03<k/(h/Mpc)<0.5. In linear theory these ratios equal the growth rate of structure on large scales. Our results show that the simulated ratios agree with the growth rate for each cosmology (which is scale dependent in the case of modified gravity) only for extremely large scales, k<0.06h/Mpc at z=0. The velocity power spectrum is substantially different in the f(R) models compared to GR, suggesting that this observable is a sensitive probe of modified gravity. We demonstrate how to extract the matter and velocity power spectra from the 2D redshift space power spectrum, P(k,\mu), and can recover the nonlinear matter power spectrum to within a few percent for k<0.1h/Mpc. The same model can match the monopole moment to within 3% for GR and 10% for the f(R) cosmology at k<0.2 h/Mpc at z=1. Our results suggest that the extraction of the velocity power spectrum from future galaxy surveys is a promising method to constrain deviations from GR.

• Modified Gravity Spins Up Galactic Halos

  Authors: Jounghun Lee, Gong-Bo Zhao, Baojiu Li, Kazuya Koyama

  04/2012;

  We investigate the effect of modified gravity on the specific angular momentum of galactic halos by analyzing the halo catalogs at z=0 from high-resolution N- body simulations for a f(R)-gravityWe investigate the effect of modified gravity on the specific angular momentum of galactic halos by analyzing the halo catalogs at z=0 from high-resolution N- body simulations for a f(R)-gravity model that meets the solar-system constraint. It is shown that the galactic halos in the $f(R)$ gravity model tend to acquire significantly higher specific angular momentum than those in the standard LCDM model. The largest difference in the specific angular momentum distribution between these two models occurs for the case of the isolated galactic halos with mass less than 10^{11}Msun/h, which are likely least shielded by the chameleon screening mechanism. As the specific angular momentum of galactic halos is rather insensitive to the other cosmological parameters, it can in principle be an independent discriminator of modified gravity. We speculate a possibility of using the relative abundance of the low surface brightness galaxies (LSBGs) as a test of GR given that the formation of the LSBGs befalls in the fast spinning dark halos.

• Chameleon f(R) gravity in the virialized cluster

  Authors: Lucas Lombriser, Kazuya Koyama, Gong-Bo Zhao, Baojiu Li

  03/2012;

  Current constraints on f(R) gravity from the large-scale structure are at the verge of penetrating into a region where the modified forces become nonlinearly suppressed. For a consistent treatment ofCurrent constraints on f(R) gravity from the large-scale structure are at the verge of penetrating into a region where the modified forces become nonlinearly suppressed. For a consistent treatment of observables at these scales, we study cluster quantities produced in chameleon and linearized Hu-Sawicki f(R) gravity dark matter N-body simulations. We find that the standard Navarro-Frenk-White halo density profile and the radial power law for the pseudo phase-space density provide equally good fits for f(R) clusters as they do in the Newtonian scenario. We give qualitative arguments for why this should be the case. For practical applications, we derive analytic relations, e.g., for the f(R) scalar field, the gravitational potential, and the velocity dispersion as seen within the virialized clusters. These functions are based on three degrees of freedom fitted to simulations, i.e., the characteristic density, scale, and velocity dispersion. We further analyze predictions for these fitting parameters from the gravitational collapse and the Jeans equation, which are found to agree well with the simulations. Our analytic results can be used to consistently constrain chameleon f(R) gravity with future observations on virialized cluster scales without the necessity of running a large number of simulations.

• A Unified Description of Screened Modified Gravity

  Authors: Philippe Brax, Anne-Christine Davis, Baojiu Li, Hans A. Winther

  03/2012;

  We consider modified gravity models driven by a scalar field whose effects are screened in high density regions due to the presence of non-linearities in its interaction potential and/or its couplingWe consider modified gravity models driven by a scalar field whose effects are screened in high density regions due to the presence of non-linearities in its interaction potential and/or its coupling to matter. Our approach covers chameleon, f(R) gravity, dilaton and symmetron models and allows a unified description of all these theories. We find that the dynamics of modified gravity are entirely captured by the time variation of the scalar field mass and its coupling to matter evaluated at the cosmological minimum of its effective potential, where the scalar field sits since an epoch prior to Big Bang Nucleosynthesis. This new parameterisation of modified gravity allows one to reconstruct the potential and coupling to matter and therefore to analyse the full dynamics of the models, from the scale dependent growth of structures at the linear level to non-linear effects requiring N-body simulations. This procedure is illustrated with explicit examples of reconstruction for chameleon, dilaton, f(R) and symmetron models.

• Modified Gravity Tomography

  Authors: Philippe Brax, Anne-Christine Davis, Baojiu Li

  11/2011;

  We consider the effect of a scalar field degree of freedom on the dynamics of gravity from small to large scales. We show that the effects of modified gravity can be completely captured by the timeWe consider the effect of a scalar field degree of freedom on the dynamics of gravity from small to large scales. We show that the effects of modified gravity can be completely captured by the time variations of the scalar field mass and its coupling to matter. This leads to a parameterisation of modified gravity where local constraints are easy to analyse and large scale scale structure effects apparent.

• Halos and Voids in f(R) Gravity

  Authors: Baojiu Li, Gong-Bo Zhao, Kazuya Koyama

  11/2011;

  In this paper, we study the distribution of dark matter halos and voids using high resolution simulations in f(R) gravity models with the chameleon mechanism to screen the fifth force in denseIn this paper, we study the distribution of dark matter halos and voids using high resolution simulations in f(R) gravity models with the chameleon mechanism to screen the fifth force in dense environment. For dark matter halos, we show that the semi-analytic thin shell condition, with a suitably-defined environment, provides a good approximation to describe the mass and environmental dependence of the screening of the fifth force in halos. Due to stronger gravity, there are far more massive halos and large voids in f(R) models compared with the \Lambda CDM model. The numbers of voids with an effective radius of 15Mpc/h are twice and four times as many as those in \Lambda CDM for f(R) models with |f_{R0}|=1e-5 and 1e-4 respectively. This provides a new means to test the models using the upcoming observational data. We also find that halos inside voids are all unscreened in our simulations, which are ideal objects for the gravity test.

• Environment Dependence of Dark Matter Halos in Symmetron Modified Gravity

  Authors: Hans A. Winther, David F. Mota, Baojiu Li

  10/2011;

  We investigate the environment dependence of dark matter halos in the symmetron modified gravity scenario. The symmetron is one of three known mechanisms for screening a fifth-force and therebyWe investigate the environment dependence of dark matter halos in the symmetron modified gravity scenario. The symmetron is one of three known mechanisms for screening a fifth-force and thereby recovering General Relativity in dense environments. The effectiveness of the screening depends on both the mass of the object and the environment it lies in. Using high-resolution N-body simulations we find a significant difference, which depends on the halos mass and environment, between the lensing and dynamical masses of dark matter halos similar to the f(R) modified gravity. The symmetron can however yield stronger signatures due to a freedom in the strength of the coupling to matter.

• ECOSMOG: An Efficient Code for Simulating Modified Gravity

  Authors: Baojiu Li, Gong-Bo Zhao, Romain Teyssier, Kazuya Koyama

  10/2011;

  We introduce a new code, ECOSMOG, to run N-body simulations for a wide class of modified gravity and dynamical dark energy theories. These theories generally have one or more new dynamical degrees ofWe introduce a new code, ECOSMOG, to run N-body simulations for a wide class of modified gravity and dynamical dark energy theories. These theories generally have one or more new dynamical degrees of freedom, the dynamics of which are governed by their (usually rather nonlinear) equations of motion. Solving these non-linear equations has been a great challenge in cosmology. Our code is based on the RAMSES code, which solves the Poisson equation on adaptively refined meshes to gain high resolutions in the high-density regions. We have added a solver for the extra degree(s) of freedom and performed numerous tests for the f(R) gravity model as an example to show its reliability. We find that much higher efficiency could be achieved compared with other existing mesh/grid-based codes thanks to two new features of the present code: (1) the efficient parallelisation and (2) the usage of the multigrid relaxation to solve the extra equation(s) on both the regular domain grid and refinements, giving much faster convergence even under much more stringent convergence criteria. This code is designed for performing high-accuracy, high-resolution and large-volume cosmological simulations for modified gravity and general dark energy theories, which can be utilised to test gravity and the dark energy hypothesis using the upcoming and future deep and high-resolution galaxy surveys.

• Structure Formation in the Symmetron Model

  Authors: Anne-Christine Davis, Baojiu Li, David F. Mota, Hans A. Winther

  08/2011;

  Scalar fields, strongly coupled to matter, can be present in nature and still be invisible to local experiments if they are subject to a screening mechanism. The symmetron is one such mechanism whichScalar fields, strongly coupled to matter, can be present in nature and still be invisible to local experiments if they are subject to a screening mechanism. The symmetron is one such mechanism which relies on restoration of a spontaneously broken symmetry in regions of high density to shield the scalar fifth force. We have investigated structure formation in the symmetron model by using N-body simulations and find strong observable signatures in both the linear and nonlinear matter power spectrum and on the halo mass function. The mechanism for suppressing the scalar fifth force in high density regions is also found to work very well.

• Linear Growth of Structure in the Symmetron Model

  Authors: Philippe Brax, Carsten van de Bruck, Anne-Christine Davis, Baojiu Li, Benoit Schmauch, Douglas J. Shaw

  08/2011;

  In the symmetron mechanism, the fifth force mediated by a coupled scalar field (the symmetron) is suppressed in high-density regions due to the restoration of symmetry in the symmetron potential. InIn the symmetron mechanism, the fifth force mediated by a coupled scalar field (the symmetron) is suppressed in high-density regions due to the restoration of symmetry in the symmetron potential. In this paper we study the background cosmology and large scale structure formation in the linear perturbation regime of the symmetron model. Analytic solutions to the symmetron in the cosmological background are found, which agree well with numerical results. We discuss the effect of the symmetron perturbation on the growth of matter perturbation, in particular the implications of the brief period of tachyonic instability caused by the negative mass squared of the symmetron during symmetry breaking. Our analysis and numerical results show that this instability has only very small effects on the growth of structures on sub-horizon scales, and even at horizon scales its influence is not as drastic as naively expected. The symmetron fifth force in the non-tachyonic regime does affect the formation of structure in a nontrivial way which could be cosmologically observable.

• Testing gravity using the environmental dependence of dark matter halos.

  Authors: Gong-Bo Zhao, Baojiu Li, Kazuya Koyama

  Physical review letters. 08/2011; 107(7):071303.

  In this Letter, we investigate the environmental dependence of dark matter halos in theories which attempt to explain the accelerated expansion of the Universe by modifying general relativity (GR).In this Letter, we investigate the environmental dependence of dark matter halos in theories which attempt to explain the accelerated expansion of the Universe by modifying general relativity (GR). Using high-resolution N-body simulations in f(R) gravity models which recover GR in dense environments by virtue of the chameleon mechanism, we find a significant difference, which depends on the environment, between the lensing and dynamical masses of dark matter halos. This environmental dependence of the halo properties can be used as a smoking gun to test GR observationally.

• Testing General Relativity using the Environmental Dependence of Dark Matter Halos

  Authors: Gong-Bo Zhao, Baojiu Li, Kazuya Koyama

  05/2011;

  In this Letter, we investigate the environmental dependence of dark matter halos in theories that attempt to explain the accelerated expansion of the Universe by modifying general relativity (GR).In this Letter, we investigate the environmental dependence of dark matter halos in theories that attempt to explain the accelerated expansion of the Universe by modifying general relativity (GR). Using high-resolution N-body simulations in f(R) gravity models which recover GR in dense environments by virtue of the chameleon mechanism, we find a strong environmentally-dependent difference between the lensing mass and dynamical mass estimates of dark matter halos. This environmental dependence of the halo properties can be used as a smoking gun to test GR observationally.

• Large-scale Structure in f(T) Gravity

  Authors: Baojiu Li, Thomas P. Sotiriou, John D. Barrow

  03/2011;

  In this work we study the cosmology of the general f(T) gravity theory. We express the modified Einstein equations using covariant quantities, and derive the gauge-invariant perturbation equations inIn this work we study the cosmology of the general f(T) gravity theory. We express the modified Einstein equations using covariant quantities, and derive the gauge-invariant perturbation equations in covariant form. We consider a specific choice of f(T), designed to explain the observed late-time accelerating cosmic expansion without including an exotic dark energy component. Our numerical solution shows that the extra degree of freedom of such f(T) gravity models generally decays as one goes to smaller scales, and consequently its effects on scales such as galaxies and galaxies clusters are small. But on large scales, this degree of freedom can produce large deviations from the standard LCDM scenario, leading to severe constraints on the f(T) gravity models as an explanation to the cosmic acceleration.

• Nonlinear Structure Formation with the Environmentally Dependent Dilaton

  Authors: Phil Brax, Carsten van de Bruck, Anne-C. Davis, Baojiu Li, Douglas J. Shaw

  02/2011;

  We have studied the nonlinear structure formation of the environmentally dependent dilaton model using $N$-body simulations. We find that the mechanism of suppressing the scalar fifth force inWe have studied the nonlinear structure formation of the environmentally dependent dilaton model using $N$-body simulations. We find that the mechanism of suppressing the scalar fifth force in high-density regions works very well. Within the parameter space allowed by the solar system tests, the dilaton model predicts small deviations of the matter power spectrum and the mass function from their $\Lambda$CDM counterparts. The importance of taking full account of the nonlinearity of the model is also emphasized.

• Generalizations of teleparallel gravity and local Lorentz symmetry

  Authors: Thomas P. Sotiriou, Baojiu Li, John D. Barrow

  12/2010;

  We analyze the relation between teleparallelism and local Lorentz invariance. We show that generic modifications of the teleparallel equivalent to general relativity will not respect local LorentzWe analyze the relation between teleparallelism and local Lorentz invariance. We show that generic modifications of the teleparallel equivalent to general relativity will not respect local Lorentz symmetry. We clarify the reasons for this and explain why the situation is different in general relativity. We give a prescription for constructing teleparallel equivalents for known theories. We also explicitly consider a recently proposed class of generalized teleparallel theories, called f(T) theories of gravity, and show why restoring local Lorentz symmetry in such theories cannot lead to sensible dynamics, even if one gives up teleparallelism.

• A Semi-analytic Ray-tracing Algorithm for Weak Lensing

  Authors: Baojiu Li, Lindsay J. King, Gong-Bo Zhao, HongSheng Zhao

  12/2010;

  We propose a new ray-tracing algorithm to measure the weak lensing shear and convergence fields directly from N-body simulations. We calculate the deflection of the light rays lensed by the 3-D massWe propose a new ray-tracing algorithm to measure the weak lensing shear and convergence fields directly from N-body simulations. We calculate the deflection of the light rays lensed by the 3-D mass density field or gravitational potential along the line of sight on a grid-by-grid basis, rather than using the projected 2-D lens planes. Our algorithm uses simple analytic formulae instead of numerical integrations in the computation of the projected density field along the line of sight, and so is computationally efficient, accurate and straightforward to implement. This will prove valuable in the interpretation of data from the next generation of surveys that will image many thousands of square degrees of sky. Comment: 13 pages, 5 figures

• N-body Simulations for f(R) Gravity using a Self-adaptive Particle-Mesh Code

  Authors: Gong-Bo Zhao, Baojiu Li, Kazuya Koyama

  11/2010;

  We perform high resolution N-body simulations for f(R) gravity based on a self-adaptive particle- mesh code MLAPM. The Chameleon mechanism that recovers General Relativity on small scales is fullyWe perform high resolution N-body simulations for f(R) gravity based on a self-adaptive particle- mesh code MLAPM. The Chameleon mechanism that recovers General Relativity on small scales is fully taken into account by self-consistently solving the non-linear equation for the scalar field. We independently confirm the previous simulation results, including the matter power spectrum, halo mass function and density profiles, obtained by Oyaizu et al. (Phys.Rev.D 78, 123524, 2008) and Schmidt et al. (Phys.Rev.D 79, 083518, 2009), and extend the resolution up to k~20 h/Mpc for the measurement of the matter power spectrum. Based on our simulation results, we discuss how the Chameleon mechanism affects the clustering of dark matter and halos on full non-linear scales.

• On the Effects of Coupled Scalar Fields on Structure Formation

  Authors: Baojiu Li, John D. Barrow

  10/2010;

  A coupling between a scalar field (representing the dark energy) and dark matter could produce rich phenomena in cosmology. It affects cosmic structure formation mainly through the fifth force, aA coupling between a scalar field (representing the dark energy) and dark matter could produce rich phenomena in cosmology. It affects cosmic structure formation mainly through the fifth force, a velocity-dependent force that acts parallel to particle's direction of motion and proportional to its speed, an effective rescaling of the particle masses, and a modified background expansion rate. In many cases these effects entangle and it is difficult to see which is the dominant one. Here we perform N-body simulations to study their qualitative behaviour and relative importance in affecting the key structure formation observables, for a model with exponential scalar field coupling. We find that the fifth force, a prominent example of the scalar-coupling effects, is far less important than the rescaling of particle mass or the modified expansion rate. In particular, the rescaling of particle masses is shown to be the key factor leading to less concentration of particles in halos than in LCDM, a pattern which is also observed in previous independent coupled scalar field simulations.

• f(T) Gravity and local Lorentz invariance

  Authors: Baojiu Li, Thomas P. Sotiriou, John D. Barrow

  10/2010;

  We show that in theories of generalised teleparallel gravity, whose Lagrangians are algebraic functions of the usual teleparallel Lagrangian, the action and the field equations are not invariantWe show that in theories of generalised teleparallel gravity, whose Lagrangians are algebraic functions of the usual teleparallel Lagrangian, the action and the field equations are not invariant under local Lorentz transformations. We also argue that these theories appear to have extra degrees of freedom with respect to general relativity. The usual teleparallel Lagrangian, which has been extensively studied and leads to a theory dynamically equivalent to general relativity, is an exception. Both of these facts appear to have been overlooked in the recent literature on f(T) gravity, but are crucial for assessing the viability of these theories as alternative explanations for the acceleration of the universe.

• Varying alpha from N-body Simulations

  Authors: Baojiu Li, David F. Mota, John D. Barrow

  09/2010;

  We have studied the Bekenstein-Sandvik-Barrow-Magueijo (BSBM) model for the spatial and temporal variations of the fine structure constant, alpha, with the aid of full N-body simulations whichWe have studied the Bekenstein-Sandvik-Barrow-Magueijo (BSBM) model for the spatial and temporal variations of the fine structure constant, alpha, with the aid of full N-body simulations which explicitly and self-consistently solve for the scalar field driving the alpha-evolution. We focus on the scalar field (or equivalently alpha) inside the dark matter halos and find that the profile of the scalar field is essentially independent of the BSBM model parameter. This means that given the density profile of an isolated halo and the background value of the scalar field, we can accurately determine the scalar field perturbation in that halo. We also derive an analytic expression for the scalar-field perturbation using the Navarro-Frenk-White halo profile, and show that it agrees well with numerical results, at least for isolated halos; for non-isolated halos this prediction differs from numerical result by a (nearly) constant offset which depends on the environment of the halo. Comment: 11 pages, 6 figures