arXiv: 30 September 2011

Gravitational redshift of galaxies in clusters as predicted by general relativity

Radoslaw Wojtak, Steen H. Hansen, Jens Hjorth

arXiv:1109.6571v1

The theoretical framework of cosmology is mainly defined by gravity, of which general relativity is the current model. Recent tests of general relativity within the \Lambda Cold Dark Matter (CDM) model have found a concordance between predictions and the observations of the growth rate and clustering of the cosmic web. General relativity has not hitherto been tested on cosmological scales independent of the assumptions of the \Lambda CDM model. Here we report observation of the gravitational redshift of light coming from galaxies in clusters at the 99 per cent confidence level, based upon archival data. The measurement agrees with the predictions of general relativity and its modification created to explain cosmic acceleration without the need for dark energy (f(R) theory), but is inconsistent with alternative models designed to avoid the presence of dark matter.

arXiv: 29 September 2011

Interacting Dark Energy -- constraints and degeneracies

Timothy Clemson, Kazuya Koyama, Gong-Bo Zhao, Roy Maartens, Jussi Väliviita

arXiv:1109.6234v1

In standard cosmologies, dark energy interacts only gravitationally with dark matter. There could be a non-gravitational interaction in the dark sector, leading to changes in the effective DE equation of state, in the redshift dependence of the DM density and in structure formation. We use CMB, BAO and SNIa data to constrain a model where the energy transfer in the dark sector is proportional to the DE density. There are two subclasses, defined by the vanishing of momentum transfer either in the DM or the DE frame. We conduct a Markov-Chain Monte-Carlo analysis to obtain best-fit parameters. The background evolution allows large interaction strengths, and the constraints from CMB anisotropies are weak. The growth of DM density perturbations is much more sensitive to the interaction, and can deviate strongly from the standard case. However, the deviations are degenerate with galaxy bias and thus more difficult to constrain. Interestingly, the ISW signature is suppressed since the non-standard background evolution can compensate for high growth rates. We also discuss the partial degeneracy between interacting DE and modified gravity, and how this can be broken.

The Halo Model of Large Scale Structure for Warm Dark Matter

Robyn M. Dunstan, Kevork N. Abazajian, Emil Polisensky, Massimo Ricotti

arXiv:1109.6291v1

We present a comprehensive analysis of the halo model of cosmological large to small-scale structure statistics in the case of warm dark matter (WDM) structure formation scenarios. We include the effects of WDM on the linear matter power spectrum, halo density profile, halo concentration relation, halo mass function, subhalo density profile, subhalo mass function and biasing of the smooth dark matter component. As expected, we find large differences at the smallest physical scales in the nonlinear matter power spectrum predicted in the halo model between WDM and cold dark matter even for reasonably high-scale WDM particle masses. We find that significant effects are contributed from the alteration of the halo density profile and concentration, as well as the halo mass function. We further find that the effects of WDM on the subhalo population are important but sub-dominant. Clustering effects of the biasing of the smooth component in WDM is not largely significant.

arXiv: 28 September 2011

Modified Gravity and the CMB

Philippe Brax, Anne-Christine Davis

arXiv:1109.5862v1

We consider the effect of modified gravity on the peak structure of the Cosmic Microwave Background (CMB) spectrum. We focus on simple models of modified gravity mediated by a massive scalar field coupled to both baryons and cold dark matter. This captures the features of chameleon, symmetron, dilaton and $f(R)$ models. We find that the CMB peaks can be affected in three independent ways provided the Compton radius of the massive scalar is not far-off the sound horizon at last scattering. When the coupling of the massive scalar to Cold Dark Matter (CDM) is large, the anomalous growth of the CDM perturbation inside the Compton radius induces a change in the peak amplitudes. When the coupling to baryons is moderately large, the speed of sound is modified and the peaks shifted to higher momenta. Finally when both couplings are non-vanishing, a new contribution proportional to the Newton potential appears in the Sachs-Wolfe temperature and increases the peak amplitudes. We also show how, given any temporal evolution of the scalar field mass, one can engineer a corresponding modified gravity model of the chameleon type. This opens up the possibility of having independent constraints on modified gravity from the CMB peaks and large scale structures at low redshifts.

arXiv: 27 September 2011

Dark Energy from the log-transformed convergence field

Hee-Jong Seo, Masanori Sato, Masahiro Takada, Scott Dodelson

arXiv:1109.5639v1

A logarithmic transform of the convergence field improves `the information content', ie., the overall precision associated with the measurement of the amplitude of the convergence power spectrum by improving the covariance matrix properties. The translation of this improvement in the information content to that in cosmological parameters, such as those associated with dark energy, requires knowing the sensitivity of the log-transformed field to those cosmological parameters. In this paper we use N-body simulations with ray tracing to generate convergence fields at multiple source redshifts as a function of cosmology. The gain in information associated with the log-transformed field does lead to tighter constraints on dark energy parameters, but only if shape noise is neglected. The presence of shape noise quickly diminishes the advantage of the log mapping, more quickly than we would expect based on the information content. With or without shape noise, using a larger pixel size allows for a more efficient log-transformation.

Hybrid Inflation: Multi-field Dynamics and Cosmological Constraints

Sébastien Clesse

arXiv:1109.5575v1

The dynamics of hybrid models is usually approximated by the evolution of a scalar field slowly rolling along a nearly flat valley. Inflation ends with a waterfall phase, due to a tachyonic instability. This final phase is usually assumed to be nearly instantaneous. In this thesis, we go beyond these approximations and analyze the exact 2-field dynamics of hybrid models. Several effects are put in evidence: 1) the possible slow-roll violations along the valley induce the non existence of inflation at small field values. Provided super-planckian fields, the scalar spectrum of the original model is red, in agreement with observations. 2) The initial field values are not fine-tuned along the valley but also occupy a considerable part of the field space exterior to it. They form a structure with fractal boundaries. Using bayesian methods, their distribution in the whole parameter space is studied. Natural bounds on the potential parameters are derived. 3) For the original model, inflation is found to continue for more than 60 e-folds along waterfall trajectories in some part of the parameter space. The scalar power spectrum of adiabatic perturbations is modified and is generically red, possibly in agreement with CMB observations. Topological defects are conveniently stretched outside the observable Universe. 4) The analysis of the initial conditions is extended to the case of a closed Universe, in which the initial singularity is replaced by a classical bounce. In the third part of the thesis, we study how the present CMB constraints on the cosmological parameters could be ameliorated with the observation of the 21cm cosmic background, by future giant radio-telescopes. Forecasts are determined for a characteristic Fast Fourier Transform Telescope, by using both Fisher matrix and MCMC methods.

arXiv: 26 September 2011

Microlensing of Kepler Stars as a Method of Detecting Primordial Black Hole Dark Matter

Kim Griest, Matthew J. Lehner, Agnieszka M. Cieplak,Bhuvnesh Jain

arXiv:1109.4975v1

If the Dark Matter consists of primordial black holes (PBHs), we show that gravitational lensing of stars being monitored by NASA's Kepler search for extra-solar planets can cause significant numbers of detectable microlensing events. A search through the roughly 150,000 lightcurves would result in large numbers of detectable events for PBHs in the mass range $5 \ten{-10}\msun$ to $\aten{-4}\msun$. Non-detection of these events would close almost two orders of magnitude of the mass window for PBH dark matter. The microlensing rate is higher than previously noticed due to a combination of the exceptional photometric precision of the Kepler mission and the increase in cross section due to the large angular sizes of the relatively nearby Kepler field stars. We also present a new formalism for calculating optical depth and microlensing rates in the presence of large finite-source effects.

Cosmic String constraints from WMAP and SPT

Cora Dvorkin (1,2), Mark Wyman (1), Wayne Hu (1) ((1) KICP, University of Chicago, (2) Institute for Advanced Study, Princeton)

arXiv:1109.4947v1

The predictions of the inflationary LCDM paradigm match today's high-precision measurements of the cosmic microwave background anisotropy with remarkable precision. The same data put tight limits on other sources of anisotropy. Cosmic strings are a particularly interesting alternate source to constrain. Strings are topological defects, remnants of inflationary-era physics that persist after the big bang. They are formed in a variety of models of inflation, including popular string theory models such as brane inflation. In this paper, we show that measurements of temperature anisotropy by the South Pole Telescope break a parameter degeneracy in the WMAP data, permitting us to place a strong upper limit on the possible string contribution to the CMB anisotropy: the power sourced by zero-width strings must be <1.75% (95% CL) of the total. In the model we use, this translates to an upper limit on the string tension of Gmu < 1.7x10^{-7}. These limits imply that the best hope for detecting strings in the CMB will come from B-mode polarization measurements at arcminute scales rather than the degree scales pursued for gravitational wave detection.

The Mildly Non-Linear Regime of Structure Formation

Svetlin Tassev, Matias Zaldarriaga

arXiv:1109.4939v1

We present a simple physically motivated picture for the mildly non-linear regime of structure formation, which captures the effects of the bulk flows. We apply this picture to develop a method to significantly reduce the sample variance in cosmological N-body simulations at the scales relevant to the Baryon Acoustic Oscillations (BAO). The results presented in this paper will allow for a speed-up of an order of magnitude (or more) in the scanning of the cosmological parameter space using N-body simulations for studies which require a good handle of the mildly non-linear regime, such as those targeting the BAO. Using this physical picture we develop a simple model, which allows for the rapid calculation of the mildly non-linear matter power spectrum to percent level accuracy, and for robust estimation of the BAO scale.

arXiv: 23 September 2011

Self-Calibration Technique for 3-point Intrinsic Alignment Correlations in Weak Lensing Surveys

M. A. Troxel, Mustapha Ishak (The University of Texas at Dallas)

arXiv:1109.4896v1

The intrinsic alignment (IA) of galaxies has been shown to be a significant barrier to precision cosmic shear measurements. Zhang [P. Zhang, Astrophys. J. 720, 1090 (2010)] proposed a self-calibration technique for the power spectrum to calculate the induced gravitational shear-galaxy intrinsic ellipticity correlation (GI) in weak lensing surveys with photo-z measurements which is expected to reduce the IA contamination by at least a factor of 10 for currently proposed surveys. We confirm this using an independent analysis and propose an expansion to the self-calibration technique for the bispectrum in order to calculate the dominant IA gravitational shear-gravitational shear-intrinsic ellipticity correlation (GGI) contamination. We first establish an estimator to extract the galaxy density-density-intrinsic ellipticity (ggI) correlation from the galaxy ellipticity-density-density measurement for a photo-z galaxy sample. We then develop a relation between the GGI and ggI bispectra, which allows for the estimation and removal of the GGI correlation from the cosmic shear signal. We explore the performance of these two methods, compare to other possible sources of error, and show that the GGI self-calibration technique can potentially reduce the IA contamination by up to a factor of 5-10 for all but a few bin choices, thus reducing the contamination to the percent level. The self-calibration is less accurate for adjacent bins, but still allows for a factor of three reduction in the IA contamination. The self-calibration thus promises to be an efficient technique to isolate both the 2-point and 3-point intrinsic alignment signals from weak lensing measurements.

Future constraints on the Hu-Sawicki modified gravity scenario

Matteo Martinelli, Alessandro Melchiorri, Olga Mena, Valentina Salvatelli, Zahara Girones

We present current and future constraints on the Hu and Sawicki modified gravity scenario. This model can reproduce a late time accelerated universe and evade solar system constraints. While current cosmological data still allows for distinctive deviations from the cosmological constant picture, future measurements of the growth of structure combined with Supernova Ia luminosity distance data will greatly improve present constraints.

Cosmic Acceleration from Causal Backreaction in a Smoothly Inhomogeneous Universe

Brett Bochner

arXiv:1109.4686v1

A phenomenological formalism is presented in which the apparent acceleration of the universe is generated by large-scale structure formation, thus eliminating the coincidence and magnitude fine-tuning problems of the Cosmological Constant in the Concordance Model, as well as potential instability issues with dynamical Dark Energy. The observed acceleration results from the combined effect of innumerable local perturbations, due to individually virialized systems, overlapping together in a smoothly-inhomogeneous adjustment of the FRW metric, in a process governed by the causal flow of inhomogeneity information outward from each clumped system. We discuss several arguments from the literature claiming to place sharp limits upon the strength of backreaction-related effects, and show why such arguments are not applicable in a physically realistic cosmological analysis. A selection of simply-parameterized models are presented, including several which are capable of fitting the luminosity distance data from Type Ia supernovae essentially as well as the best-fit flat \Lambda CDM model, without resort to Dark Energy, any modification to gravity, or a local void. Simultaneously, these models can reproduce measured cosmological parameters such as the age of the universe, the matter density required for spatial flatness, the present-day deceleration parameter, and the angular scale of the Cosmic Microwave Background to within a reasonable proximity of their Concordance values. We conclude by considering potential observational signatures for distinguishing this cosmological formalism from \Lambda CDM or Dark Energy, as well as the possible long-term fate of such a universe with ever-spreading spheres of influence for its increasingly superposed perturbations.

Moving mesh cosmology: characteristics of galaxies and haloes

Dusan Keres (1), Mark Vogelsberger (2), Debora Sijacki (2), Volker Springel (3), Lars Hernquist (2) ((1) UC Berkeley, (2) Harvard/CfA, (3) HITS)

arXiv:1109.4638v1

We discuss cosmological hydrodynamic simulations of galaxy formation performed with the new moving-mesh code AREPO, which promises higher accuracy compared with the traditional SPH technique that has been widely employed for this problem. We use an identical set of physics in corresponding simulations carried out with the well-tested SPH code GADGET, adopting also the same high-resolution gravity solver. We are thus able to compare both simulation sets on an object-by-object basis, allowing us to cleanly isolate the impact of different hydrodynamical methods on galaxy and halo properties. In accompanying papers, we focus on an analysis of the global baryonic statistics predicted by the simulation codes (Vogelsberger et al. (2011)), and complementary idealized simulations that highlight the differences between the hydrodynamical schemes (Sijacki et al. (2011)). Here we investigate their influence on the baryonic properties of simulated galaxies and their surrounding haloes. We find that AREPO leads to significantly higher star formation rates for galaxies in massive haloes and to more extended gaseous disks in galaxies, which also feature a thinner and smoother morphology than their GADGET counterparts. Consequently, galaxies formed in AREPO have larger sizes and higher specific angular momentum than their SPH correspondents. We show that these differences persist as a function of numerical resolution. While both codes agree to acceptable accuracy on a number of baryonic properties of cosmic structures, our results thus clearly demonstrate that galaxy formation simulations greatly benefit from the use of more accurate hydrodynamical techniques such as AREPO and call into question the reliability of galaxy formation studies in a cosmological context using traditional formulations of SPH. [Abridged]

arXiv: 21 September 2011

Dark matter halo occupation: environment and clustering

Rupert Croft (CMU), Tiziana Di Matteo (CMU), Nishikanta Khandai (CMU), Volker Springel (HITS), Anirban Jana (PSC), Jeffrey Gardner (UW)

arXiv:1109.4169v1

We use a large dark matter simulation of a LambdaCDM model to investigate the clustering and environmental dependence of the number of substructures in a halo. Focusing on redshift z=1, we find that the halo occupation distribution is sensitive at the tens of percent level to the surrounding density and to a lesser extent to asymmetry of the surrounding density distribution. We compute the autocorrelation function of halos as a function of occupation, building on the finding of Wechsler et al. (2006) and Gao and White (2007) that halos (at fixed mass) with more substructure are more clustered. We compute the relative bias as a function of occupation number at fixed mass, finding a strong relationship. At fixed mass, halos in the top 5% of occupation can have an autocorrelation function ~ 1.5-2 times higher than the mean. We also compute the bias as a function of halo mass, for fixed halo occupation. We find that for group and cluster sized halos, when the number of subhalos is held fixed, there is a strong anticorrelation between bias and halo mass. Such a relationship represents an additional challenge to the halo model.

Galileons on Cosmological Backgrounds

Garrett Goon, Kurt Hinterbichler, Mark Trodden

arXiv:1109.3450v1

We construct four-dimensional effective field theories of a generalized DBI galileon field, the dynamics of which naturally take place on a Friedmann-Robertson-Walker spacetime. The theories are invariant under non-linear symmetry transformations, which can be thought of as being inherited from five-dimensional bulk Killing symmetries via the probe brane technique through which they are constructed. The resulting model provides a framework in which to explore the cosmological role that galileons may play as the universe evolves.

arXiv: 20 September 2011

The integrated Sachs-Wolfe imprints of cosmic superstructures: a problem for $Λ$CDM

Seshadri Nadathur, Shaun Hotchkiss, Subir Sarkar

arXiv:1109.4126v1 

A crucial diagnostic of the $\Lambda$CDM cosmological model is the integrated Sachs-Wolfe (ISW) effect of large-scale structure on the cosmic microwave background (CMB). The ISW imprint of superstructures of size $\sim100\;h^{-1}$Mpc at redshift $z\sim0.5$ has been detected with $>4\sigma$ significance, however it has been noted that the signal is much larger than expected. We revisit the calculation using linear theory predictions in $\Lambda$CDM cosmology for the number density of superstructures and their radial density profile, and take possible selection effects into account. While our expected signal is larger than previous estimates, it is still inconsistent by $>3\sigma$ with the observation. If the observed signal is indeed due to the ISW effect then huge, extremely underdense voids are far more common in the observed universe than predicted by $\Lambda$CDM.

The Matter Power Spectrum of Dark Energy Models and the Harrison-Zel'dovich Prescription

Iván Durán (Barcelona, Spain), Fernando Atrio-Barandela (Salamanca, Spain), Diego Pavón (Barcelona, Spain)

arXiv:1109.4038v1

The Harrison-Zel'dovich ansatz assumes that the amplitude of matter density perturbations at horizon crossing is the same at all scales. Using this prescription, we show how to construct the matter power spectrum of generic dark energy models that evolve similarly to the standard $\Lambda$CDM cosmology prior to matter radiation equality. Our approach allows to test models using data on the normalization of matter and radiation power spectra without fully solving the dynamical equations that describe the evolution of energy density perturbations.

Some aspects of field equations in generalised theories of gravity

T. Padmanabhan

arXiv:1109.3846v1

A class of theories of gravity based on a Lagrangian which depends on the curvature and metric - but not on the derivatives of the curvature tensor - is of interest in several contexts including in the development of the paradigm that treats gravity as an emergent phenomenon. This class of models contains, as an important subset, all Lanczos-Lovelock models of gravity. I derive several identities and properties which are useful in the study of these models and clarify some of the issues that seem to have received insufficient attention in the past literature.

arXiv: 19 September 2011

Observing the Multiverse with Cosmic Wakes

Matthew Kleban (NYU), Thomas S. Levi (UBC), Kris Sigurdson (UBC)

arXiv:1109.3473v1

Current theories of the origin of the Universe, including string theory, predict the existence of a multiverse containing many bubble universes. These bubble universes will generically collide, and collisions with ours produce cosmic wakes that enter our Hubble volume, appear as unusually symmetric disks in the cosmic microwave background (CMB) and disturb large scale structure (LSS). There is preliminary observational evidence consistent with one or more of these disturbances on our sky. However, other sources can produce similar features in the CMB temperature map and so additional signals are needed to verify their extra-universal origin. Here we find, for the first time, the detailed three-dimensional shape and CMB temperature and polarization signals of the cosmic wake of a bubble collision in the early universe consistent with current observations. The predicted polarization pattern has distinctive features that when correlated with the corresponding temperature pattern are a unique and striking signal of a bubble collision. These features represent the first verifiable prediction of the multiverse paradigm and might be detected by current experiments such as Planck and future CMB polarization missions. A detection of a bubble collision would confirm the existence of the Multiverse, provide compelling evidence for the string theory landscape, and sharpen our picture of the Universe and its origins.

arXiv: 16 September 2011

Properties of Galaxies and Groups: Nature versus Nurture

Sami-Matias Niemi

PHD theisi

arXiv:1109.3426v1

Due to the inherently nonlinear nature of gravity cosmological N-body simulations have become an invaluable tool when the growth of structure is being studied and modelled closer to the present epoch. Large simulations with high dynamical range have made it possible to model the formation and growth of cosmic structure with unprecedented accuracy. Moreover, galaxies, the basic building blocks of the Universe, can also be modelled in cosmological context. However, despite all the simulations and successes in recent decades, there are still many unanswered questions in the field of galaxy formation and evolution. One of the longest standing issue being the significance of the formation place and thus initial conditions to a galaxy's evolution in respect to environment, often formulated simply as "nature versus nurture" like in human development and psychology. Unfortunately, our understanding of galaxy evolution in different environments is still limited, albeit, for example, the morphology-density relation has shown that the density of the galaxy's local environment can affect its properties. Consequently, the environment should play a role in galaxy evolution, however despite the efforts, the exact role of the galaxy's local environment to its evolution remains open. This thesis introduction discusses briefly the background cosmology, cosmological N-body simulations and semi-analytical models. The second part is reserved for groups of galaxies, whether they are gravitationally bound, and what this may imply for galaxy evolution. The third part of the thesis concentrates on describing results of a case study of isolated field elliptical galaxies. The final chapter discusses another case study of luminous infra-red galaxies.

Second-order weak lensing from modified gravity

R. Ali Vanderveld, Robert R. Caldwell, Jason Rhodes

arXiv:1109.3189v1

We explore the sensitivity of weak gravitational lensing to second-order corrections to the spacetime metric within a cosmological adaptation of the parameterized post-Newtonian framework. Whereas one might expect nonlinearities of the gravitational field to introduce non-Gaussianity into the statistics of the lensing convergence field, we show that such corrections are actually always small within a broad class of scalar-tensor theories of gravity. We show this by first computing the weak lensing convergence within our parameterized framework to second order in the gravitational potential, and then computing the relevant post-Newtonian parameters for scalar-tensor gravity theories. In doing so we show that this potential systematic factor is generically negligible, thus clearing the way for weak lensing to provide a direct tracer of mass on cosmological scales despite uncertainties in possible departures from general relativity.

Cosmic acceleration from modified gravity with Palatini formalism

A. Borowiec, M. Kamionka, A. Kurek, M. Szydlowski

arXiv:1109.3420v1

We study new FRW type cosmological models of modified gravity treated on the background of Palatini approach. These models are generalization of Einstein gravity by the presence of a scalar field non-minimally coupled to the curvature. The models employ Starobinsky's term in the Lagrangian and dust matter. Therefore, as a by-product, an exhausted cosmological analysis of quadratic gravity is presented. We investigate dynamics of our models, confront them with the currently available astrophysical data as well as against $\Lambda$CDM model. We have used the dynamical system methods to investigate dynamics of the models. It reveals the presence of final sudden singularity. Fitting free parameters we have demonstrated that this class of models is in very good agreement with the data (including CMB measurements) in a sense that their Hubble diagrams are practically indistinguishable from the standard $\Lambda$CDM model. Therefore Bayesian methods of model selection have been employed in order to indicate preferred model. Only in the light of CMB data the concordance model remains invincible.

arXiv: 15 September 2011

Feeding your Inflaton: Non-Gaussian Signatures of Interaction Structure

Neil Barnaby, Sarah Shandera

arXiv:1109.2985v1

Primordial non-Gaussianity is generated by interactions of the inflaton field, either self-interactions or couplings to other sectors. These two physically different mechanisms can lead to nearly indistinguishable bispectra of the equilateral type, but generate distinct patterns in the relative scaling of higher order moments. We illustrate these classes in a simple effective field theory framework where the flatness of the inflaton potential is protected by a softly broken shift symmetry. Since the distinctive difference between the two classes of interactions is the scaling of the moments, we investigate the implications for observables that depend on the series of moments. We obtain analytic expressions for the Minkowski functionals and the halo mass function for an arbitrary structure of moments, and use these to demonstrate how different classes of interactions might be distinguished observationally. Our analysis casts light on a number of theoretical issues, in particular we clarify the difference between the physics that keeps the distribution of fluctuations nearly Gaussian, and the physics that keeps the calculation under control.

The Sagittarius impact as an architect of spirality and outer rings in the Milky Way

Chris W. Purcell, James S. Bullock, Erik Tollerud, Miguel Rocha, Sukanya Chakrabarti

arXiv:1109.2918v1

Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand the Galaxy's morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Center in our model that are reminiscent of the low- latitude arcs observed in the same area of the Milky Way. Previous models have focused on Sgr itself to reproduce the dwarf's orbital history and place associated constraints on the shape of the Milky Way gravitational potential, treating the Sgr impact event as a trivial influence on the Galactic disk. Our results show that the Milky Way's morphology is not purely secular in origin and that low-mass minor mergers predicted to be common throughout the Universe probably have a similarly important role in shaping galactic structure.

arXiv: 14 September 2011

Testing the cosmic distance duality with X-ray gas mass fraction and supernovae data

R. S. Goncalves, R. F. L. Holanda, J. S. Alcaniz

arXiv:1109.2790v1

In this paper we discuss a new cosmological model-independent test for the cosmic distance duality relation (CDDR), $\eta = D_{L}(L)(1+z)^{-2}/D_{A}(z)=1$, where $D_{A}(z)$ and $D_{L}(z)$ are the angular and luminosity distances, respectively. Using the general expression for X-ray gas mass fraction ($f_{gas}$) of galaxy clusters, $f_{gas} \propto D_L{D_A}^{1/2}$, we show that $f_{gas}$ observations jointly with type Ia supernovae (SNe Ia) data furnish a validity test for the CDDR. To perform our analysis we use 38 $f_{gas}$ measurements recently studied by two groups considering different assumptions to describe the clusters (La Roque {\it{et al.}} 2006 and Ettori {\it{et al.}} 2009) and two subsamples of SNe Ia distance lumonosity extracted from the Union2 compilation. In our test we consider the $\eta$ parameter as a function of the redshift parameterized by two different functional forms. It is found that the La Roque {\it{et al.}} (2006) sample is in perfect agreement with the duality relation ($\eta = 1$) whereas the Ettori {\it{et al.}} (2009) sample presents a significant conflict.

The Dynamical State of Dark Matter Haloes in Cosmological Simulations I: Correlations with Mass Assembly History

Chris Power, Alexander Knebe, Steffen R. Knollmann

arXiv:1109.2671v1

Using a statistical sample of dark matter haloes drawn from a suite of cosmological N-body simulations of the Cold Dark Matter (CDM) model, we quantify the impact of a simulated halo's mass accretion and merging history on two commonly used measures of its dynamical state, the virial ratio eta and the centre of mass offset Delta r. Quantifying this relationship is important because the degree to which a halo is dynamically equilibrated will influence the reliability with which we can measure characteristic equilibrium properties of the structure and kinematics of a population of haloes. We begin by verifying that a halo's formation redshift zform correlates with its virial mass Mvir and we show that the fraction of its recently accreted mass and the likelihood of it having experienced a recent major merger increases with increasing Mvir and decreasing zform. We then show that both eta and Delta r increase with increasing Mvir and decreasing zform, which implies that massive recently formed haloes are more likely to be dynamically unrelaxed than their less massive and older counterparts. Our analysis shows that both eta and Delta r are good indicators of a halo's dynamical state, showing strong positive correlations with recent mass accretion and merging activity, but we argue that Delta r provides a more robust and better defined measure of dynamical state for use in cosmological N-body simulations at z~0. We find that Delta r < 0.04 is sufficient to pick out dynamically relaxed haloes at z=0. Finally, we assess our results in the context of previous studies, and consider their observational implications.

What determines the fraction of elliptical galaxies in clusters?

Gabriella De Lucia, Fabio Fontanot, Dave Wilman

arXiv:1109.2599v1

We study the correlation between the morphological mix of cluster galaxies and the assembly history of the parent cluster by taking advantage of two independently developed semi-analytic models for galaxy formation and evolution. In our models, both the number of cluster members and that of elliptical members increase as a function of cluster mass, in such a way that the resulting elliptical fractions are approximately independent of cluster mass. The population of cluster ellipticals exhibit a marked bimodal distribution as a function of galaxy stellar mass, with a dip at masses $\sim 10^{10}\,{\rm M}_{\odot}$. In the framework of our models, this bimodality originates from the combination of a strongly decreasing number of galaxies with increasing stellar mass, and a correspondingly increasing probability of experiencing major mergers. We show that the correlation between the measured elliptical fraction and the assembly history of the parent cluster is weak, and that it becomes stronger in models that adopt longer galaxy merger times. We argue that this results from the combined effect of a decreasing bulge production due to a reduced number of mergers, and an increasing survival probability of pre-existing ellipticals, with the latter process being more important than the former.

Lensing Time Delays and Cosmological Complementarity

Eric V. Linder

arXiv:1109.2592v1

Time delays in strong gravitational lensing systems possess significant complementarity with distance measurements to determine the dark energy equation of state, as well as the matter density and Hubble constant. Time delays are most useful when observations permit detailed lens modeling and variability studies, requiring high resolution imaging, long time monitoring, and rapid cadence. We quantify the constraints possible between a sample of 150 such time delay lenses and a near term supernova program, such as might become available from an Antarctic telescope such as KDUST and the Dark Energy Survey. Adding time delay data to supernovae plus cosmic microwave background information can improve the dark energy figure of merit by almost a factor 5 and determine the matter density \Omega_m to 0.004, Hubble constant h to 0.7%, and dark energy equation of state time variation w_a to 0.26, systematics permitting.

arXiv: 13 September 2011

On the "The Kolmogorov-Smirnov test for the CMB" by M.Frommert, R.Durrer and J.Michaud

V.G.Gurzadyan, A.A.Kocharyan

arXiv:1109.2529v1

In arxiv:1108.5354 the Kolmogorov-Smirnov (K-S) test and Kolmogorov stochasticity parameter (KSP) is applied to CMB data. Their interpretation of the KSP method, however, lacks essential elements. In addition, their main result on the Gaussianity of CMB was not a matter of debate in previous KSP-CMB studies which also included predictions on cold spots, point sources.

Interpreting supernovae observations in a lumpy universe

Chris Clarkson, George Ellis, Andreas Faltenbacher, Roy Maartens, Obinna Umeh, Jean-Philippe Uzan

arXiv:1109.2484v1

Light from `point sources' such as supernovae is observed with a beam width of order of the sources' size -- typically less than 1 AU. Such a beam probes matter and curvature distributions that are very different from coarse-grained representations in N-body simulations or perturbation theory, which are smoothed on scales much larger than 1 pc. The beam typically travels through unclustered dark matter and hydrogen with a mean density much less than the cosmic mean, and through dark matter mini-halos and hydrogen clouds. Large dark matter halos are rarely encountered directly and so are mainly experienced through their Weyl (tidal) curvature. How observations of many such beams averages this Weyl curvature into the Ricci curvature of the background is not understood. If modelled incorrectly this can lead to significant changes to the inferred background cosmology. Standard analyses predict a huge variance for such tiny beam sizes, and non-linear corrections appear to be non-trivial. By considering different reasonable approximations which yield very different cosmologies we argue that modelling ultra-narrow beams accurately is a critical problem for precision cosmology.

Does the growth of structure affect our dynamical models of the universe? The averaging, backreaction and fitting problems in cosmology

Chris Clarkson, George Ellis, Julien Larena, Obinna Umeh

arXiv:1109.2314v1

Structure occurs over a vast range of scales in the universe. Our large-scale cosmological models are coarse-grained representations of what exists, which have much less structure than there really is. An important problem for cosmology is determining the influence the small-scale structure in the universe has on its large-scale dynamics and observations. Is there a significant, general relativistic, backreaction effect from averaging over structure? One issue is whether the process of smoothing over structure can contribute to an acceleration term and so alter the apparent value of the cosmological constant. If this is not the case, are there other aspects of concordance cosmology that are affected by backreaction effects? Despite much progress, this 'averaging problem' is still unanswered, but it cannot be ignored in an era of precision cosmology.

Smoothed Particle Hydrodynamics in Astrophysics

Volker Springel

arXiv:1109.2219v1

This review discusses Smoothed Particle Hydrodynamics (SPH) in the astrophysical context, with a focus on inviscid gas dynamics. The particle-based SPH technique allows an intuitive and simple formulation of hydrodynamics that has excellent conservation properties and can be coupled to self-gravity easily and highly accurately. The Lagrangian character of SPH allows it to automatically adjust its resolution to the clumping of matter, a property that makes the scheme ideal for many applications in astrophysics, where often a large dynamic range in density is encountered. We discuss the derivation of the basic SPH equations in their modern formulation, and give an overview about extensions of SPH developed to treat physics such as radiative transfer, thermal conduction, relativistic dynamics or magnetic fields. We also briefly describe some of the most important applications areas of SPH in astrophysical research. Finally, we provide a critical discussion of the accuracy of SPH for different hydrodynamical problems, including measurements of its convergence rate for important classes of problems.

The Bispectrum of f(R) Cosmologies

Héctor Gil-Marín, Fabian Schmidt, Wayne Hu, Raul Jimenez, Licia Verde

arXiv:1109.2115v1

In this paper we analyze a suite of cosmological simulations of modified gravitational action f(R) models, where cosmic acceleration is induced by a scalar field that acts as a fifth force on all forms of matter. In particular, we focus on the bispectrum of the dark matter density field on mildly non-linear scales. For models with the same initial power spectrum, the dark matter bispectrum shows significant differences for cases where the final dark matter power spectrum also differs. Given the different dependence on bias of the galaxy power spectrum and bispectrum, bispectrum measurements can close the loophole of galaxy bias hiding differences in the power spectrum. Alternatively, changes in the initial power spectrum can also hide differences. By constructing LCDM models with very similar final non-linear power spectra, we show that the differences in the bispectrum are reduced (<4%) and are comparable with differences in the imperfectly matched power spectra. These results indicate that the bispectrum depends mainly on the power spectrum and less sensitively on the gravitational signatures of the f(R) model. This weak dependence of the matter bispectrum on gravity makes it useful for breaking degeneracies associated with galaxy bias, even for models beyond general relativity.

Hydrodynamic simulations on a moving Voronoi mesh

Volker Springel

arXiv:1109.2218v1

At the heart of any method for computational fluid dynamics lies the question of how the simulated fluid should be discretized. Traditionally, a fixed Eulerian mesh is often employed for this purpose, which in modern schemes may also be adaptively refined during a calculation. Particle-based methods on the other hand discretize the mass instead of the volume, yielding an approximately Lagrangian approach. It is also possible to achieve Lagrangian behavior in mesh-based methods if the mesh is allowed to move with the flow. However, such approaches have often been fraught with substantial problems related to the development of irregularity in the mesh topology. Here we describe a novel scheme that eliminates these weaknesses. It is based on a moving unstructured mesh defined by the Voronoi tessellation of a set of discrete points. The mesh is used to solve the hyperbolic conservation laws of ideal hydrodynamics with a finite volume approach, based on a second-order Godunov scheme with an exact Riemann solver. A particularly powerful feature of the approach is that the mesh-generating points can in principle be moved arbitrarily. If they are given the velocity of the local flow, a highly accurate Lagrangian formulation of continuum hydrodynamics is obtained that is free of mesh distortion problems, while it is at the same time fully Galilean-invariant, unlike ordinary Eulerian codes. We describe the formulation and implementation of our new Voronoi-based hydrodynamics, and we discuss a number of illustrative test problems that highlight its performance in practical applications.

Luminosity distance in Swiss cheese cosmology with randomized voids. II. Magnification probability distributions

Éanna É. Flanagan, Naresh Kumar, Ira Wasserman, R. Ali Vanderveld

arXiv:1109.1873v1

We study the fluctuations in luminosity distances due to gravitational lensing by large scale (> 35 Mpc) structures, specifically voids and sheets. We use a simplified "Swiss cheese" model consisting of a \Lambda -CDM Friedman-Robertson-Walker background in which a number of randomly distributed non-overlapping spherical regions are replaced by mass compensating comoving voids, each with a uniform density interior and a thin shell of matter on the surface. We compute the distribution of magnitude shifts using a variant of the method of Holz & Wald (1998), which includes the effect of lensing shear. The standard deviation of this distribution is ~ 0.027 magnitudes and the mean is ~ 0.003 magnitudes for voids of radius 35 Mpc, sources at redshift z_s=1.0, with the voids chosen so that 90% of the mass is on the shell today. The standard deviation varies from 0.005 to 0.06 magnitudes as we vary the void size, source redshift, and fraction of mass on the shells today. If the shell walls are given a finite thickness of ~ 1 Mpc, the standard deviation is reduced to ~ 0.013 magnitudes. This standard deviation due to voids is a factor ~ 3 smaller than that due to galaxy scale structures. We summarize our results in terms of a fitting formula that is accurate to ~ 20%, and also build a simplified analytic model that reproduces our results to within ~ 30%. Our model also allows us to explore the domain of validity of weak lensing theory for voids. We find that for 35 Mpc voids, corrections to the dispersion due to lens-lens coupling are of order ~ 4%, and corrections to due shear are ~ 3%. Finally, we estimate the bias due to source-lens clustering in our model to be negligible.