119117607
Simulations of Galileon modified gravity: Clustering statistics in real and redshift space
We use N-body simulations to study the statistics of massive halos and redshift space distortions for theories with a standard \Lambda CDM expansion history and a galileon-type scalar field. The extra scalar field increases the gravitational force, leading to enhanced structure formation. We compare our measurements of the real space matter power spectrum and halo properties with fitting formula for estimating these quantities analytically. We find that a model for power spectrum, halo mass-function and halo bias, derived from \Lambda CDM simulations can fit the results from our simulations of modified gravity when \sigma_8 is appropriately adjusted. We also study the redshift space distortions in the two point correlation function measured from these simulations, finding a difference in the ratio of the redshift space to real space clustering amplitude relative to standard gravity on all scales. We find enhanced clustering on scales r>10 Mpc/h and increased damping of the correlation function for scales r<9 Mpc/h. The boost in the clustering on large scales due to the enhanced gravitational forces cannot be mimicked in a standard gravity model by simply changing \sigma_8. This result illustrates the usefulness of redshift space distortion measurements as a probe of modifications to General Relativity.
56400065
Testing phenomenological and theoretical models of dark matter density profiles with galaxy clusters
We use the stacked gravitational lensing mass profile of four high-mass (M& 10 15 M⊙) galaxy clusters around z ≈ 0.3 from Umetsu et al. (2011) to fit density profiles of phenomeno logical (NFW, Einasto, Sersic, Stadel and Hernquist) and theoretical (non-singular Isothermal Sphere, DARKexp and Kang & He) models of the dark matter distribution. We account for largescale structure effects, including a 2-halo term in the anal ysis. We find that the Stadel model provides the best fit to the data as measured by the reduced χ 2 . It is followed by the generalized NFW profile with a free inner slope and by the Einasto profile. T he NFW model provides the best fit if we neglect the 2-halo term, in agreement with resul ts from Umetsu et al. (2011). Among the theoretical profiles, the DARKexp model with a sing le form parameter has the best performance, almost identical to that of the Stadel pro file. This may indicate a connection between this theoretical model and the phenomenology of dark matter halos, shedding light on the dynamical basis of empirical profiles which emerge fro m numerical simulations.
14272822
Nonlinear evolution of f(R) cosmologies. III. Halo statistics
The statistical properties of dark matter halos, the building blocks of cosmological observables associated with structure in the Universe, offer many opportunities to test models for cosmic acceleration, especially those that seek to modify gravitational forces. We study the abundance, bias, and profiles of halos in cosmological simulations for one such model: the modified action f(R) theory. The effects of f(R) modified gravity can be separated into a large- and small-field limit. In the large-field limit, which is accessible to current observations, enhanced gravitational forces raise the abundance of rare massive halos and decrease their bias but leave their (lensing) mass profiles largely unchanged. This regime is well described by scaling relations based on a modification of spherical collapse calculations. In the small-field limit, the enhancement of the gravitational force is suppressed inside halos and the effects on halo properties are substantially reduced for the most massive halos. Nonetheless, the scaling relations still retain limited applicability for the purpose of establishing conservative upper limits on the modification to gravity.
197515616
Estimating the Redshift Distribution of Faint Galaxy Samples
We present an empirical method for estimating the underlying redshift distribution N(z) of galaxy photometric samples from photometric observables. The method does not rely on photometric redshift (photo-z) estimates for individual galaxies, which typically suffer from biases. Instead, it assigns weights to galaxies in a spectroscopic subsample such that the weighted distributions of photometric observables (e.g., multiband magnitudes) match the corresponding distributions for the photometric sample. The weights are estimated using a nearest-neighbor technique that ensures stability in sparsely populated regions of color-magnitude space. The derived weights are then summed in redshift bins to create the redshift distribution. We apply this weighting technique to data from the Sloan Digital Sky Survey as well as to mock catalogs for the Dark Energy Survey, and compare the results to those from the estimation of photo-z’s derived by a neural network algorithm. We find that the weighting method accurately recovers the underlying redshift distribution, typically better than the photo-z reconstruction, provided the spectroscopic subsample spans the range of photometric observables covered by the photometric sample.
16918450
Lensing magnification: implications for counts of submillimetre galaxies and SZ clusters
We study lensing magnification of source galaxies by intervening galaxy groups and clusters using a halo model. Haloes are modelled with truncated NFW profiles with ellipticity added to their lensing potential and propagated to observable lensing statistics. We present the formalism to calculate observable effects due to a distribution of haloes of different masses at different redshifts along the line of sight. We calculate the effects of magnification on the number counts of high-redshift galaxies. Using Balloon-borne Large Aperture Submillimetre Telescope (BLAST) survey data for submillimetre galaxies (SMGs), we find that magnification affects the steep, high flux part of the counts by about 60 per cent. The effect becomes much stronger if the intrinsic distribution is significantly steeper than observed. We also consider the effect of this high-redshift galaxy population on contaminating the Sunyaev-Zel'dovich (SZ) signal of massive clusters using the halo model approach. We find that for the majority of clusters expected to be detected with ongoing SZ surveys, there is significant contamination from the Poisson noise due to background SMGs. This contribution can be comparable to the SZ increment for typical clusters and can also contaminate the SZ decrement of low-mass clusters. Thus, SZ observations, especially for the increment part of the SZ spectrum, need to include careful modelling of this irreducible contamination for mass estimation. Lensing further enhances the contamination, especially close to the cores of massive clusters and for very disturbed clusters with large magnification cross-section.
16976292
Self-calibration of cluster dark energy studies: Observable-mass distribution
The exponential sensitivity of cluster number counts to the properties of the dark energy implies a comparable sensitivity to not only the mean but also the actual distribution of an observable-mass proxy given the true cluster mass. For example a 25% scatter in mass can provide a $\ensuremath{\sim}50%$ change in the number counts at $z\ensuremath{\sim}2$ for the upcoming SPT survey. Uncertainty in the scatter of this amount would degrade dark energy constraints to uninteresting levels. Given the shape of the actual mass function, the properties of the distribution may be internally monitored by the shape of the observable mass function. As a proof of principle, for a simple mass-independent Gaussian distribution the scatter may be self-calibrated to allow a measurement of the dark energy equation of state of $\ensuremath{\sigma}(w)\ensuremath{\sim}0.1$. External constraints on the mass variance of the distribution that are more accurate than $\ensuremath{\Delta}{\ensuremath{\sigma}}_{\mathrm{ln}﻿M}^{2}l0.01$ at $z\ensuremath{\sim}1$ can further improve constraints by up to a factor of 2. More generally, cluster counts and their sample variance measured as a function of the observable provide internal consistency checks on the assumed form of the observable-mass distribution that will protect against misinterpretation of the dark energy constraints.
118374217
Energy conditions bounds and supernovae data
The energy conditions play an important role in the description of some important properties of the Universe, including the current accelerating expansion phase and the possible recent phase of super-acceleration. In a recent work we have provided a detailed study of the energy conditions for the recent past by deriving bounds from energy conditions and by making the confrontation of the bounds with supernovae data. Here, we extend and update these results in two different ways. First, by carrying out a new statistical analysis for q(z) estimates needed for the confrontation between the bounds and supernovae data. Second, by providing a new picture of the energy conditions fulfillment and violation in the light of the recently compiled Union set of 307 type Ia supernovae and by using two different statistical approaches.
118387259
Energy conditions bounds and their confrontation with supernovae data
The energy conditions play an important role in the understanding of several properties of the Universe, including the current accelerating expansion phase and the possible existence of the so-called phantom fields. We show that the integrated bounds provided by the energy conditions on cosmological observables such as the distance modulus {mu}(z) and the lookback time t{sub L}(z) are not sufficient (or necessary) to ensure the local fulfillment of the energy conditions, making explicit the limitation of these bounds in the confrontation with observational data. We recast the energy conditions as bounds on the deceleration and normalized Hubble parameters, obtaining new bounds which are necessary and sufficient for the local fulfillment of the energy conditions. A statistical confrontation, with 1{sigma}-3{sigma} confidence levels, between our bounds and supernovae data from the gold and combined samples is made for the recent past. Our analyses indicate, with 3{sigma} confidence levels, the fulfillment of both the weak energy condition (WEC) and dominant energy condition (DEC) for z{<=}1 and z < or approx. 0.8, respectively. In addition, they suggest a possible recent violation of the null energy condition (NEC) with 3{sigma}, i.e. a very recent phase of superacceleration. Our analyses also show the possibility of violationmore » of the strong energy condition (SEC) with 3{sigma} in the recent past (z{<=}1), but interestingly the q(z)-best-fit curve crosses the SEC--fulfillment divider at z{approx_equal}0.67, which is a value very close to the beginning of the epoch of cosmic acceleration predicted by the standard concordance flat {lambda}CDM scenario.« less
119272920
REVISITING THE CONFRONTATION OF THE ENERGY CONDITIONS WITH SUPERNOVAE DATA
In the standard Friedmann–Lemaˆotre–Robertson–Walker (FLRW) approach to model the Universe the violation of the so-called energy conditions is related to some important properties of the Universe as, for example, the current and the inflationary accelerating expansion phases. The energy conditions are also necessary in the formulation and proofs of Hawking-Penrose singularity theorems. In two recent articles we have derived bounds from energy conditions and made confrontations of these bounds with supernovae data. Here, we extend these results in following way: first, by using our most recent statistical procedure for calculating new q(z) estimates from the gold and combined type Ia supernovae samples; second, we use these estimates to obtain a new picture of the energy conditions fulfillment and violation for the recent past (z ≤ 1) in the context of the standard cosmology.
119201134
DARK ENERGY EQUATION OF STATE AND COSMIC TOPOLOGY
The immediate observational consequence of a non-trivial spatial topology of the Universe is that an observer could potentially detect multiple images of radiating sources. In particular, a non-trivial topology will generate pairs of correlated circles of temperature fluctuations in the anisotropies maps of the cosmic microwave background (CMB), the so-called circles-in-the-sky. In this way, a detectable non-trivial spatial topology may be seen as an observable attribute, which can be probed through the circles-in-the-sky for all locally homogeneous and isotropic universes with no assumptions on the cosmological dark energy (DE) equation of state (EOS) parameters. We show that the knowledge of the spatial topology through the circles-in-the-sky offers an effective way of reducing the degeneracies in the DE EOS parameters. We concretely illustrate the topological role by assuming, as an exanple, a Poincar\'{e} dodecahedral space topology and reanalyzing the constraints on the parameters of a specific EOS which arise from the supernovae type Ia, baryon acoustic oscillations and the CMB plus the statistical topological contribution.