arXiv subjects

Baojiu Li

Publications and source records attributed to Baojiu Li.

At least 163 records · Page 9Linked to original sources

arXiv2014

The observational status of Galileon gravity after Planck

We use the latest CMB data from Planck, together with BAO measurements, to constrain the full parameter space of Galileon gravity. We constrain separately the three main branches of the theory known as the Cubic, Quartic and Quintic models, and find that all yield a very good fit to these data. Unlike in $Λ{\rm CDM}$, the Galileon model constraints are compatible with local determinations of the Hubble parameter and predict nonzero neutrino masses at over $5σ$ significance. We also identify that the low-$l$ part of the CMB lensing spectrum may be able to distinguish between $Λ{\rm CDM}$ and Galileon models. In the Cubic model, the lensing potential deepens at late times on sub-horizon scales, which is at odds with the current observational suggestion of a positive ISW effect. Compared to $Λ$CDM, the Quartic and Quintic models predict less ISW power in the low-$l$ region of the CMB temperature spectrum, and as such are slightly preferred by the Planck data. We illustrate that residual local modifications to gravity in the Quartic and Quintic models may render the Cubic model as the only branch of Galileon gravity that passes Solar System tests.

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arXiv2014

A clear and measurable signature of modified gravity in the galaxy velocity field

The velocity field of dark matter and galaxies reflects the continued action of gravity throughout cosmic history. We show that the low-order moments of the pairwise velocity distribution, $v_{12}$, are a powerful diagnostic of the laws of gravity on cosmological scales. In particular, the projected line-of-sight galaxy pairwise velocity dispersion, $σ_{12}(r)$, is very sensitive to the presence of modified gravity. Using a set of high-resolution N-body simulations we compute the pairwise velocity distribution and its projected line-of-sight dispersion for a class of modified gravity theories: the chameleon \fR gravity and Galileon gravity (cubic and quartic). The velocities of dark matter halos with a wide range of masses would exhibit deviations from General Relativity at the $(5-10)σ$ level. We examine strategies for detecting these deviations in galaxy redshift and peculiar velocity surveys. If detected, this signature would be a "smoking gun" for modified gravity.

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arXiv2014

Halo model and halo properties in Galileon gravity cosmologies

We investigate the performance of semi-analytical modelling of large-scale structure in Galileon gravity cosmologies using results from N-body simulations. We focus on the Cubic and Quartic Galileon models that provide a reasonable fit to CMB, SNIa and BAO data. We demonstrate that the Sheth-Tormen mass function and linear halo bias can be calibrated to provide a very good fit to our simulation results. We also find that the halo concentration-mass relation is well fitted by a power law. The nonlinear matter power spectrum computed in the halo model approach is found to be inaccurate in the mildly nonlinear regime, but captures reasonably well the effects of the Vainshtein screening mechanism on small scales. In the Cubic model, the screening mechanism hides essentially all of the effects of the fifth force inside haloes. In the case of the Quartic model, the screening mechanism leaves behind residual modifications to gravity, which make the effective gravitational strength time-varying and smaller than the standard value. Compared to normal gravity, this causes a deficiency of massive haloes and leads to a weaker matter clustering on small scales. For both models, we show that there are realistic halo occupation distributions of Luminous Red Galaxies that can match both the observed large-scale clustering amplitude and the number density of these galaxies.

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arXiv2014

Spherical collapse in Galileon gravity: fifth force solutions, halo mass function and halo bias

We study spherical collapse in the Quartic and Quintic Covariant Galileon gravity models within the framework of the excursion set formalism. We derive the nonlinear spherically symmetric equations in the quasi-static and weak-field limits, focusing on model parameters that fit current CMB, SNIa and BAO data. We demonstrate that the equations of the Quintic model do not admit physical solutions of the fifth force in high density regions, which prevents the study of structure formation in this model. For the Quartic model, we show that the effective gravitational strength deviates from unity at late times ($z \lesssim 1$), becoming larger if the density is low, but smaller if the density is high. This shows that the Vainshtein mechanism at high densities is not enough to screen all of the modifications of gravity. This makes halos that collapse at $z \lesssim 1$ feel an overall weaker gravity, which suppresses halo formation. However, the matter density in the Quartic model is higher than in standard $Λ$CDM, which boosts structure formation and dominates over the effect of the weaker gravity. In the Quartic model there is a significant overabundance of high-mass halos relative to $Λ$CDM. Dark matter halos are also less biased than in $Λ$CDM, with the difference increasing appreciably with halo mass. However, our results suggest that the bias may not be small enough to fully reconcile the predicted matter power spectrum with LRG clustering data.

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arXiv2014

The Integrated Sachs-Wolfe effect in $f(R)$ gravity

We study the late-time Integrated Sachs-Wolfe (ISW) effect in $f(R)$ gravity using N-body simulations. In the $f(R)$ model under study, the linear growth rate is larger than that in general relativity (GR). This slows down the decay of the cosmic potential and induces a smaller ISW effect on large scales. Therefore, the $\dotΦ$ (time derivative of the potential) power spectrum at $k<0.1h$/Mpc is suppressed relative to that in GR. In the non-linear regime, relatively rapid structure formation in $f(R)$ gravity boosts the non-linear ISW effect relative to GR, and the $\dotΦ$ power spectrum at $k>0.1h$/Mpc is increased (100$\%$ greater on small scales at $z=0$). We explore the detectability of the ISW signal via stacking supercluster and supervoids. The differences in the corresponding ISW cold or hot spots are $\sim 20\%$ for structures of $\sim 100$Mpc/$h$. Such differences are greater for smaller structures, but the amplitude of the signal is lower. The high amplitude of ISW signal detected by Granett et al. can not explained in the $f(R)$ model. We find relatively big differences between $f(R)$ and GR in the transverse bulk motion of matter, and discuss its detectability via the relative frequency shifts of photons from multiple lensed images.

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arXiv2014

Massive Gravity wrapped in the Cosmic Web

We study how the filamentary pattern of the cosmic web changes if the true gravity deviates from the general relativity (GR) on the large scale. The f(R) gravity whose strength is controlled to satisfy the current observational constraints on the cluster scale is adopted as our fiducial model and a large N-body simulation of high-resolution is utilized for this study. By applying the minimal spanning tree algorithm to the halo catalogs from the simulation at various epochs, we identify the main stems of the rich superclusters located in the most prominent filamentary section of the cosmic web and determine their spatial extents per member cluster as the degree of their straightness. It is found that the $f(R)$ gravity has an effect of significantly bending the superclusters and that the effect becomes stronger as the universe evolves. Even in the case where the deviation from GR is too small to be detectable by any other observables, the degree of the supercluster straightness exhibits conspicuous difference between the $f(R)$ and the GR models. Our results also imply that the supercluster straightness could be a useful discriminator of f(R) gravity from the coupled dark energy since it is shown to evolve differently between the two models. As a final conclusion, the degree of the straightness of the rich superclusters should provide a powerful cosmological test of large scale gravity.

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arXiv2013

Chameleon f(R) gravity in the virialized cluster

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 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.

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arXiv2013

Revisiting the matter power spectra in $f(R)$ gravity

In this paper, we study the non-linear matter power spectrum in a specific family of $f(R)$ models that can reproduce the $Λ$CDM background expansion history, using high resolution $N$-body simulations based on the {\sc ecosmog} code. We measure the matter power spectrum in the range of $0.05h{\rm Mpc}^{-1} 10^{-3}$, we find no chameleon screening in dense regions at late times ($z<3$), which means that those models could be ruled out due to the factor-of-$1/3$ enhancement to the strength of Newtonian gravity. We also give the best-fit parameters for a generalised PPF fitting formula which works well for the models studied here.

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arXiv2013

Simulating the quartic Galileon gravity model on adaptively refined meshes

We develop a numerical algorithm to solve the high-order nonlinear derivative-coupling equation associated with the quartic Galileon model, and implement it in a modified version of the RAMSES N-body code to study the effect of the Galileon field on the large-scale matter clustering. The algorithm is tested for several matter field configurations with different symmetries, and works very well. This enables us to perform the first simulations for a quartic Galileon model which provides a good fit to the cosmic microwave background (CMB) anisotropy, supernovae and baryonic acoustic oscillations (BAO) data. Our result shows that the Vainshtein mechanism in this model is very efficient in suppressing the spatial variations of the scalar field. However, the time variation of the effective Newtonian constant caused by the curvature coupling of the Galileon field cannot be suppressed by the Vainshtein mechanism. This leads to a significant weakening of the strength of gravity in high-density regions at late times, and therefore a weaker matter clustering on small scales. We also find that without the Vainshtein mechanism the model would have behaved in a completely different way, which shows the crucial role played by nonlinearities in modified gravity theories and the importance of performing self-consistent N-body simulations for these theories.

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arXiv2013

Modeling halo mass functions in chameleon f(R) gravity

On cosmological scales, observations of the cluster abundance currently place the strongest constraints on f(R) gravity. These constraints lie in the large-field limit, where the modifications of general relativity can correctly be modeled by setting the Compton wavelength of the scalar field to its background value. These bounds are, however, at the verge of penetrating into a regime, where the modifications become nonlinearly suppressed due to the chameleon mechanism and cannot be described by this linearized approximation. For future constraints based on observations subjected to cluster abundance, it is therefore essential to consistently model the chameleon effect. We analyze descriptions of the halo mass function in chameleon f(R) gravity using a mass- and environment-dependent spherical collapse model in combination with excursion set theory and phenomenological fits to N-body simulations in the LCDM and f(R) gravity scenarios. Our halo mass functions consistently incorporate the chameleon suppression and cosmological parameter dependencies, improving upon previous formalisms and providing an important extension to N-body simulations for the application in consistent tests of gravity with observables sensitive to the abundance of clusters.

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arXiv2013

Galaxy Infall Kinematics as a Test of Modified Gravity

Infrared modifications of General Relativity (GR) can be revealed by comparing the mass of galaxy clusters estimated from weak lensing to that from infall kinematics. We measure the 2D galaxy velocity distribution in the cluster infall region by applying the galaxy infall kinematics (GIK) model developed by Zu and Weinberg (2013) to two suites of f(R) and Galileon modified gravity simulations. Despite having distinct screening mechanisms, namely, the Chameleon and the Vainshtein effects, the f(R) and Galileon clusters exhibit very similar deviations in their GIK profiles from GR, with ~ 100-200 k/s enhancement in the characteristic infall velocity at r=5 Mpc/h and 50-100 km/s broadening in the radial and tangential velocity dispersions across the entire infall region, for clusters with mass ~ 10^{14} Msol/h at z=0.25. These deviations are detectable via the GIK reconstruction of the redshift--space cluster-galaxy cross-correlation function, xi_cg^s(r_p,r_π), which shows ~ 1-2 Mpc/h increase in the characteristic line-of-sight distance r_π^c at r_p<6 Mpc/h from GR predictions. With overlapping deep imaging and large redshift surveys in the future, we expect that the GIK modelling of xi_cg^s, in combination with the stacked weak lensing measurements, will provide powerful diagnostics of modified gravity theories and the origin of cosmic acceleration.

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arXiv2013

The parameter space in Galileon gravity models

We present the first constraints on the full parameter space of the Galileon modified gravity model, considering both the cosmological parameters and the coefficients which specify the additional terms in the Lagrangian due to the Galileon field, which we call the Galileon parameters. We use the latest cosmic microwave background measurements, along with distance measurements from supernovae and baryonic acoustic oscillations, performing a Monte Carlo Markov Chain exploration of the 9-dimensional parameter space. The integrated Sachs-Wolfe signal can be very different in Galileon models compared to standard gravity, making it essential to use the full CMB data rather than the CMB distance priors. We demonstrate that meaningful constraints are only possible in the Galileon parameter space after taking advantage of a scaling degeneracy. We find that the Galileon model can fit the WMAP 9-year results better than the standard Λ-Cold Dark Matter model, but gives a slightly worse fit overall once lower redshift distance measurements are included. The best-fitting cosmological parameters (e.g. matter density, scalar spectral index, fluctuation amplitude) can differ by more than 2σ in the Galileon model compared with ΛCDM. We highlight other potential constraints of the Galileon model using galaxy clustering and weak lensing measurements.

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arXiv2013

Novel Probes of Gravity and Dark Energy

The discovery of cosmic acceleration has stimulated theorists to consider dark energy or modifications to Einstein's General Relativity as possible explanations. The last decade has seen advances in theories that go beyond smooth dark energy -- modified gravity and interactions of dark energy. While the theoretical terrain is being actively explored, the generic presence of fifth forces and dark sector couplings suggests a set of distinct observational signatures. This report focuses on observations that differ from the conventional probes that map the expansion history or large-scale structure. Examples of such novel probes are: detection of scalar fields via lab experiments, tests of modified gravity using stars and galaxies in the nearby universe, comparison of lensing and dynamical masses of galaxies and clusters, and the measurements of fundamental constants at high redshift. The observational expertise involved is very broad as it spans laboratory experiments, high resolution astronomical imaging and spectroscopy and radio observations. In the coming decade, searches for these effects have the potential for discovering fundamental new physics. We discuss how the searches can be carried out using experiments that are already under way or with modest adaptations of existing telescopes or planned experiments. The accompanying paper on the Growth of Cosmic Structure describes complementary tests of gravity with observations of large-scale structure.

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arXiv2013

Gravity theories, Transverse Doppler and Gravitational Redshifts in Galaxy Clusters

There is growing interest in testing alternative gravity theories using the subtle gravitational redshifts in clusters of galaxies. However, current models all neglect a transverse Doppler redshift of similar magnitude, and some models are not self-consistent. An equilibrium model would fix the gravitational and transverse Doppler velocity shifts to be about 6sigma^2/c and 3sigma^2/2c in order to fit the observed velocity dispersion sigma self-consistently. This result comes from the Virial Theorem for a spherical isotropic cluster, and is insensitive to the theory of gravity. A gravitational redshift signal also does not directly distinguish between the Einsteinian and f(R) gravity theories, because each theory requires different dark halo mass function to keep the clusters in equilibrium. When this constraint is imposed, the gravitational redshift has no sensitivity to theory. Indeed our N-body simulations show that the halo mass function differs in f(R), and that the transverse Doppler effect is stronger than analytically predicted due to non-equilibrium.

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arXiv2013

Hierarchical clustering in chameleon $f(R)$ gravity

We use a suite of high resolution state-of-the-art N-body Dark Matter simulations of chameleon $f(R)$ gravity to study the higher order volume averaged correlation functions $\bar{ξ_n}$ together with the hierarchical $n$-th order correlation amplitudes $S_n=\barξ_n/\barξ_2^{n-1}$ and density distribution functions (PDF). We show that under the non-linear modifications of gravity the hierarchical scaling of the reduced cumulants is preserved. This is however characterised by significant changes of both the $\bar{ξ_n}$ and $S_n$'s values and their scale dependence with respect to General Relativity gravity (GR). In addition, we measure a significant increase of the non linear $σ_8$ parameter reaching $14, 5$ and 0.5% in excess of the GR value for the three flavours of our $f(R)$ models. We further note that the values of the reduced cumulants up to order $n=9$ are significantly increased in $f(R)$ gravity for all our models at small scales $R\simlt 30\hmpc$. In contrast the values of the hierarchical amplitudes, $S_n$'s, are smaller in $f(R)$ indicating that the modified gravity density distribution functions are deviating from the GR case. Furthermore we find that the redshift evolution of relative deviations of the $f(R)$ hierarchical correlation amplitudes is fastest at high and moderate redshifts $1\leq z \leq4$. The growth of these deviations significantly slows down in the low redshift universe. We also compute the PDFs and show that for scales below $\sim 20\hmpc$ they are significantly shifted in $f(R)$ gravity towards the low densities. Finally we discuss the implications of our theoretical predictions for measurements of the hierarchical clustering in galaxy redshifts surveys, including the important problems of the galaxy biasing and redshifts space distortions.

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arXiv2013

Exploring Vainshtein mechanism on adaptively refined meshes

There has been a lot of research interest in modified gravity theories which utilise the Vainshtein mechanism to recover standard general relativity in regions with high matter density, such as the Dvali-Gabadadze-Porrati and Galileon models. The strong nonlinearity in the field equations of these theories implies that accurate theoretical predictions could only be made using high-resolution cosmological simulations. Previously, such simulations were usually done on regular meshes, which limits both their performance and the accuracy. In this paper, we report the development of a new algorithm and code, based on ECOSMOG, that uses adaptive mesh refinements to improve the efficiency and precision in simulating the models with Vainshtein mechanism. We have made various code tests against the numerical reliability, and found consistency with previous simulations. We also studied the velocity field in the self-accelerating branch of the DGP model. The code, parallelised using MPI, is suitable for large cosmological simulations of Galileon-type modified gravity theories.

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arXiv2013

Systematic simulations of modified gravity: chameleon models

In this work we systematically study the linear and nonlinear structure formation in chameleon theories of modified gravity, using a generic parameterisation which describes a large class of models using only 4 parameters. For this we have modified the N-body simulation code ECOSMOG to perform a total of 65 simulations for different models and parameter values, including the default LCDM. These simulations enable us to explore a significant portion of the parameter space. We have studied the effects of modified gravity on the matter power spectrum and mass function, and found a rich and interesting phenomenology where the difference with the LCDM paradigm cannot be reproduced by a linear analysis even on scales as large as k~0.05h/Mpc, since the latter incorrectly assumes that the modification of gravity depends only on the background matter density. Our results show that the chameleon screening mechanism is significantly more efficient than other mechanisms such as the dilaton and symmetron, especially in high-density regions and at early times, and can serve as a guidance to determine the parts of the chameleon parameter space which are cosmologically interesting and thus merit further studies in the future.

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arXiv2013

Voids in Modified Gravity: Excursion Set Predictions

We investigate the behavior of the fifth force in voids in chameleon models using the spherical collapse method. Contrary to Newtonian gravity, we find the fifth force is repulsive in voids. The strength of the fifth force depends on the density inside and outside the void region as well as its radius. It can be many times larger than the Newtonian force and their ratio is in principle unbound. This is very different from the case in halos, where the fifth force is no more than 1/3 of gravity. The evolution of voids is governed by the Newtonian gravity, the effective dark energy force and the fifth force. While the first two forces are common in both LCDM and chameleon universes, the fifth force is unique to the latter. Driven by the outward-pointing fifth force, individual voids in chameleon models expand faster and grow larger than in a LCDM universe. The expansion velocity of the void shell can be 20% to 30% larger for voids of a few Mpc/h in radius, while their sizes can be larger by ~10%. These differences are smaller for larger voids of the same density. We compare void statistics using excursion set theory; for voids of the same size, their number density is larger in chameleon models. The fractional difference increases with void size. The chance of having voids of radius ~25 Mpc/h can be 2.5 times larger. This difference is about 10 times larger than that in the halo mass function. We find strong environmental dependence of void properties in chameleon models. The differences in size and expansion velocity with GR are both larger for small voids in high density regions. In general, the difference between chameleon models and LCDM in void properties (size, expansion velocity and distribution function) are larger than the corresponding quantities for halos. This suggests that voids might be better candidates than halos for testing gravity.

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