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.

arXiv: 12 September 2011

A new approach to cosmological perturbations in f(R) models

Daniele Bertacca, Nicola Bartolo, Sabino Matarrese

arXiv:1109.2082v2

We propose an analytic procedure that allows to determine quantitatively the deviation in the behavior of cosmological perturbations between a given f(R) modified gravity model and a LCDM reference model. Our method allows to study structure formation in these models from the largest scales, of the order of the Hubble horizon, down to scales deeply inside the Hubble radius, without employing the so-called "quasi-static" approximation. Although we restrict our analysis here to linear perturbations, our technique is completely general and can be extended to any perturbative order.

The Complementarity of Redshift-space Distortions and the Integrated Sachs-Wolfe Effect

Charles Shapiro, Robert G. Crittenden, Will J. Percival (ICG, Portsmouth)

arXiv:1109.1981v1 

Assuming General Relativity is correct on large-scales, Redshift-Space Distortions (RSDs) and the Integrated Sachs-Wolfe effect (ISW) are both sensitive to the time derivative of the linear growth function. We investigate the extent to which these probes provide complementary or redundant information when they are combined to constrain the evolution of the linear velocity power spectrum, often quantified by the function f(z)\sigma_8(z), where f is the logarithmic derivative of \sigma_8 with respect to (1+z). Using a 3D spherical harmonic expansion, we compute the covariance matrices of the signals for a large galaxy redshift survey combined with a CMB survey like Planck. The spherical harmonic basis allows accurate ISW estimates by avoiding the plane-parallel approximation, and it retains RSD information that is otherwise lost when projecting angular clustering onto redshift shells. We find that the correlation between the ISW and RSD signals are low since the probes are sensitive to different modes. For our default surveys, on large scales (k<0.05 Mpc/h), the ISW can improve constraints on f\sigma_8 by more than 10% compared to using RSDs alone. In the future, when precision RSD measurements are available on smaller scales, the cosmological constraints from ISW measurements will not be competitive; however, they will remain a useful consistency test for possible systematic contamination and alternative models of gravity.

Distribution function approach to redshift space distortions

Uros Seljak, Patrick McDonald

arXiv:1109.1888v1 

We develop a phase space distribution function approach to redshift space distortions (RSD), in which the redshift space density can be written as a sum over velocity moments of the distribution function. These moments are density weighted and their lowest orders are density, momentum density, and stress energy density. The series expansion is convergent on large scales. We perform an expansion of these velocity moments into helicity modes, which are eigenmodes under rotation around the axis of Fourier mode direction, generalizing the scalar, vector, tensor decomposition of perturbations to an arbitrary order. We show that only equal helicity moments correlate and derive the angular dependence of the individual contributions to the redshift space power spectrum in terms of angle mu between wave vector and line of sight. We show that the dominant term of mu^2 dependence on large scales is the cross-correlation between the density and scalar part of momentum density, which can be related to the time derivative of the matter power spectrum. Additional terms contributing and dominating on small scales are the vector part of momentum density-momentum density correlations, the energy density-density correlations, and the scalar part of anisotropic stress density-density correlations. Similarly, we identify 7 terms contributing to mu^4 dependence. Some of the advantages of the distribution function approach are that the series expansion converges on large scales and remains valid in multi-stream situations. We finish with a brief discussion of implications for RSD in galaxies relative to dark matter, highlighting the issue of scale dependent bias of velocity moments correlators.

Testing Einstein Gravity with Cosmic Growth and Expansion

Gong-Bo Zhao, Hong Li, Eric V. Linder, Kazuya Koyama, David J. Bacon, Xinmin Zhang

arXiv:1109.1846v1

We test Einstein gravity using cosmological observations of both expansion and structure growth, including the latest data from supernovae (Union2.1), CMB (WMAP7), weak lensing (CFHTLS) and peculiar velocity of galaxies (SDSS DR7). We fit modified gravity parameters of the generalized Poisson equations simultaneously with the effective equation of state for the background evolution, exploring the covariances and model dependence. The results show that general relativity is a good fit to the combined data. Using a Pad{\'e} approximant form for the gravity deviations accurately captures the time and scale dependence for theories like $f(R)$ and DGP gravity, and weights high and low redshift probes fairly. For current observations, cosmic growth and expansion can be fit simultaneously with little degradation in accuracy, while removing the possibility of bias from holding one aspect fixed.

arXiv: 6 September 2011

Observables in gravity: a review

Johannes Tambornino

arXiv:1109.0740v1

We present an overview on observables in gravity mainly from a loop quantum gravity perspective. The gauge group of general relativity is the diffeomorphism group of the underlying manifold. Consequently, general relativity is a totally constrained theory with vanishing canonical Hamiltonian. This fact, often referred to as the problem of time, provides the main conceptual difficulty towards the construction of gauge-invariant local observables. Nevertheless, within the framework of complete observables, that encode relations between dynamical fields, remarkable progress has been made during the last 20 years. Although analytic control over observables for full gravity is still lacking, perturbative calculations have been performed and within de-parameterizable toy models it was possible for the first time to construct a full set of gauge invariant observables for a background independent field theory. We review these developments and comment on their implications for quantum gravity.

Constraining primordial non-Gaussianity with future galaxy surveys

Tommaso Giannantonio (EXC Munich), Cristiano Porciani (AIfA Bonn), Julien Carron (ETH Zurich), Adam Amara (ETH Zurich), Annalisa Pillepich (UC Santa Cruz)

arXiv:1109.0958v1

We study the constraining power on primordial non-Gaussianity of future surveys of the large-scale structure of the Universe for both near-term surveys (such as the Dark Energy Survey - DES) as well as longer term projects such as Euclid and WFIRST. Specifically we perform a Fisher matrix analysis forecast for such surveys, using DES-like and Euclid-like configurations as examples, and take account of any expected photometric and spectroscopic data. We focus on two-point statistics and we consider three observables: the 3D galaxy power spectrum in redshift space, the angular galaxy power spectrum, and the projected weak-lensing shear power spectrum. We study the effects of adding a few extra parameters to the basic LCDM set. We include the two standard parameters to model the current value for the dark energy equation of state and its time derivative, w_0, w_a, and we account for the possibility of primordial non-Gaussianity of the local, equilateral and orthogonal types, of parameter f_NL and, optionally, of spectral index n_fNL. We present forecasted constraints on these parameters using the different observational probes. We show that accounting for models that include primordial non-Gaussianity does not degrade the constraint on the standard LCDM set nor on the dark-energy equation of state. By combining the weak lensing data and the information on projected galaxy clustering, consistently including all two-point functions and their covariance, we find forecasted marginalised errors sigma (f_NL) ~ 3, sigma (n_fNL) ~ 0.12 for the local shape of primordial non-Gaussianity, while the orthogonal and equilateral constraints are weakened for the galaxy clustering case.

Model-Independent Reconstruction of the Expansion History of the Universe from Type Ia Supernovae

S. Benitez-Herrera, F. Roepke, W. Hillebrandt, C. Mignone, M. Bartelmann, J. Weller

arXiv:1109.0873v1

Based on the largest homogeneously reduced set of Type Ia supernova luminosity data currently available -- the Union2 sample -- we reconstruct the expansion history of the Universe in a model-independent approach. Our method tests the geometry of the Universe directly without reverting to any assumptions made on its energy content. This allows us to constrain Dark Energy models and non-standard cosmologies in a straightforward way. The applicability of the presented method is not restricted to testing cosmological models. It can be a valuable tool for pointing out systematic errors hidden in the supernova data and planning future Type Ia supernova cosmology campaigns.

Chameleon Gravity, Electrostatics, and Kinematics in the Outer Galaxy

R. Pourhasan, N. Afshordi, R.B. Mann, A.C. Davis

arXiv:1109.0538v1

Light scalar fields are expected to arise in theories of high energy physics (such as string theory), and find phenomenological motivations in dark energy, dark matter, or neutrino physics. However, the coupling of light scalar fields to ordinary (or dark) matter is strongly constrained from laboratory, solar system, and astrophysical tests of fifth force. One way to evade these constraints in dense environments is through the chameleon mechanism, where the field's mass steeply increases with ambient density. Consequently, the chameleonic force is only sourced by a thin shell near the surface of dense objects, which significantly reduces its magnitude. 
In this paper, we argue that thin-shell conditions are equivalent to "conducting" boundary conditions in electrostatics. As an application, we use the analogue of the method of images to calculate the back-reaction (or self-force) of an object around a spherical gravitational source. Using this method, we can explicitly compute the violation of equivalence principle in the outskirts of galactic haloes (assuming an NFW dark matter profile): Intermediate mass satellites can be slower than their larger/smaller counterparts by as much as 10% close to a thin shell.

arXiv: 5 September 2011

Neutral Hydrogen Tully Fisher Relation: The case for Newtonian Gravity

Sayan Chakraborti, Satej Khedekar

arXiv:1109.0529v1

Intrinsic luminosities are related to rotation velocities of disk galaxies by Tully Fisher (TF) relations. The Baryonic TF (BTF) relation has recently been explained with Dark Matter and Newtonian Gravity as well as Modified Newtonian Dynamics (MOND). However, recent work has pointed out that the currently used BTF relation ignores the contribution from hot gas and oversimplifies complex galaxy-scale physics. In this Letter, we advocate the use of the Neutral Hydrogen TF (HITF) relationship, which is free from dust obscuration and stellar evolution effects, as a clean probe of gravity and dynamics in the weak field regime. We incorporate the physics of hot gas from supernova feedback which drives the porosity of the Interstellar Medium (ISM). A simple model that includes supernovae feedback, is generalized to include a parametrized effective gravitational force law. We test our model against a catalogue of galaxies, spanning the full range of disks from dwarf galaxies to giant spirals, to demonstrate that a Kennicutt-Schmidt (KS) law for star formation and simple Newtonian gravity is adequate for explaining the observed HI scaling relations. The data rules out MOND-like theories, within the scope of this model.

Dimensionless cosmology

Ali Narimani, Adam Moss, Douglas Scott

arXiv:1109.0492v1

Although it is possible that some fundamental physical constants could vary in time, it is important to only consider dimensionless combinations, such as the fine structure constant or the equivalent coupling constant for gravity. Once all such dimensionless numbers have been given, then we can be sure that our cosmological picture is governed by the same physical laws as that of another civilization with an entirely different set of units. An additional feature of the standard model of cosmology raises an extra complication, namely that the epoch at which we live is a crucial part of the model. This can be defined by giving the value of any one of the evolving cosmological parameters. It takes some care to avoid inconsistent results for constraints on variable constants, which could be caused by effectively fixing more than one parameter today. We show examples of this effect by considering in some detail the physics of Big Bang nucleosynthesis, recombination and microwave background anisotropies, being careful to maintain dimensionlessness throughout. We also give a dimensionless version of the parameters of the standard cosmological model. Rigorously determining how to talk about the model in a way which avoids physical dimensions is a requirement for proceeding with a calculation to constrain time-varying fundamental constants.

Early-type galaxies at z~1.3. IV. Scaling relations in different environments

A. Raichoor, S. Mei, S.A. Stanford, B.P. Holden, F. Nakata, P. Rosati, F. Shankar, H. Ford, M. Huertas-Company, G. Illingworth, T. Kodama, M. Postman, A. Rettura, J.P. Blakeslee, R. Demarco, M.J. Jee, M. Tanaka, R.L. White

arXiv:1109.0284v1

We present the Kormendy and mass-size relations for early-type galaxies (ETGs) as a function of environment at z ~ 1.3. Our sample includes 76 ETGs in the Lynx supercluster and in the GOODS/CDF-S field, 31 ETGs in clusters, 18 in groups and 27 in the field, all with multi-wavelength photometry and HST/ACS observations and masses in the range 10^10 < M/Msun < 10^11.5. The Kormendy relation, in place at z ~ 1.3, does not depend on the environment. The mass-size relation reveals that ETGs overall appear to be more compact in denser environments: cluster ETGs have sizes on average twice as smaller than in the local universe, whereas field ETGs show a mass-size relation with a similar distribution than the local one. Our results imply that (1) the mass-size relation in the field did not evolve overall from z ~ 1.3 to present; (2) in denser environments, either ETGs sizes have evolved on average to twice their size, or more ETGs have been formed within the dense environment from not ETG progenitors or larger galaxies have been accreted to a pristine compact population to reproduce the mass-size relation observed in the local Universe. Our results are driven by galaxies with masses M < 2 \times 10^11 Msun and those with masses M ~ 10^11 Msun follow the same trends that the entire sample. Following Valentinuzzi et al. (2010a) definition of superdense ETGs, around 60-70% of our cluster sample is made of superdense ETGs.

Tilted Ekpyrosis

José Fonseca, David Wands (ICG, Portsmouth)

arXiv:1109.0448v2We consider a simple model of cosmological collapse driven by canonical fields with exponential potentials. We generalise the two-field ekpyrotic collapse to consider non-orthogonal or tilted potentials and give the general condition for isocurvature field fluctuations to have a scale-invariant spectrum in this model. In particular we show that tilted potentials allow for a slightly red spectrum of perturbations as required by current observations. However a red spectrum of fluctuations implies that the two-field ekpyrotic phase must have a finite duration and requires a preceding phase which sets the initial conditions for what otherwise appears to be a fine-tuned trajectory in the phase space.

handbook

arXiv: 1 September 2011

Extended Theories of Gravity

Salvatore Capozziello, Mariafelicia De Laurentis

arXiv:1108.6266v1

Extended Theories of Gravity can be considered a new paradigm to cure shortcomings of General Relativity at infrared and ultraviolet scales. They are an approach that, by preserving the undoubtedly positive results of Einstein's Theory, is aimed to address conceptual and experimental problems recently emerged in Astrophysics, Cosmology and High Energy Physics. In particular, the goal is to encompass, in a self-consistent scheme, problems like Inflation, Dark Energy, Dark Matter, Large Scale Structure and, first of all, to give at least an effective description of Quantum Gravity. We review the basic principles that any gravitational theory has to follow. The geometrical interpretation is discussed in a broad perspective in order to highlight the basic assumptions of General Relativity and its possible extensions in the general framework of gauge theories. Principles of such modifications are presented, focusing on specific classes of theories like f (R)-gravity and scalar-tensor gravity in the metric and Palatini approaches. The special role of torsion is also discussed. The conceptual features of these theories are fully explored and attention is payed to the issues of dynamical and conformal equivalence between them considering also the initial value problem. A number of viability criteria are presented considering the post-Newtonian and the post-Minkowskian limits. In particular, we discuss the problems of neutrino oscillations and gravitational waves in Extended Gravity. Finally, future perspectives of Extended Gravity are considered with possibility to go beyond a trial and error approach.

arXiv: 31 August 2011

What do gas-rich galaxies actually tell us about modified Newtonian dynamics?

Simon Foreman, Douglas Scott

arXiv:1108.5734v1

It has recently been claimed that measurements of the baryonic Tully-Fisher relation (BTFR), a power-law relationship between the observed baryonic masses and outer rotation velocities of galaxies, support the predictions of modified Newtonian dynamics for the slope and scatter in the relation, while challenging the cold dark matter (CDM) paradigm. We investigate these claims, and find that: 1) the scatter in the data used to determine the BTFR is in conflict with observational uncertainties on the data; 2) these data do not make strong distinctions regarding the best-fit BTFR parameters; 3) the literature contains a wide variety of measurements of the BTFR, many of which are discrepant with the recent results; and 4) the claimed CDM "prediction" for the BTFR is a gross oversimplification of the complex galaxy-scale physics involved. We conclude that the BTFR is currently untrustworthy as a test of CDM.

arXiv: 30 August 2011

Recovering MOND from extended metric theories of gravity

T. Bernal, S. Capozziello, J.C. Hidalgo, S. Mendoza

arXiv:1108.5588v1

We show that the Modified Newtonian Dynamics (MOND) regime can be fully recovered as the weak-field limit of a particular theory of gravity formulated in the metric approach. This is possible when Milgrom's acceleration constant is taken as a fundamental quantity which couples to the theory in a very consistent manner. As a consequence, the scale invariance of the gravitational interaction is naturally broken. In this sense, Newtonian gravity is the weak-field limit of general relativity and MOND is the weak-field limit of that particular extended theory of gravity. We also prove that a Noether's symmetry approach to the problem yields a conserved quantity coherent with this relativistic MONDian extension.

Cosmological parameter estimation using Particle Swarm Optimization (PSO)

Jayanti Prasad, Tarun Souradeep (Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune (India))

arXiv:1108.5600v1 

Obtaining the set of cosmological parameters consistent with observational data is an important exercise in current cosmological research. It involves finding the global maximum of the likelihood function in the multi-dimensional parameter space. Currently sampling based methods, which are in general stochastic in nature, like Markov-Chain Monte Carlo(MCMC), are being commonly used for parameter estimation. The beauty of stochastic methods is that the computational cost grows, at the most, linearly in place of exponentially (as in grid based approaches) with the dimensionality of the search space. MCMC methods sample the full joint probability distribution (posterior) from which one and two dimensional probability distributions, best fit (average) values of parameters and then error bars can be computed. In the present work we demonstrate the application of another stochastic method, named Particle Swarm Optimization (PSO), that is widely used in the field of engineering and artificial intelligence, for cosmological parameter estimation from WMAP seven years data. We find that there is a good agreement between the values of the best fit parameters obtained from PSO and publicly available code COSMOMC. However, there is a slight disagreement between error bars mainly due to the fact that errors are computed differently in PSO. Apart from presenting the results of our exercise, we also discuss the merits of PSO and explain its usefulness in more extensive search in higher dimensional parameter space.

A critical analysis of high-redshift, massive, X-ray selected galaxy clusters: I

Ben Hoyle, Raul Jimenez, Licia Verde, Shaun Hotchkiss

arXiv:1108.5458v1 

We critically investigate current statistical tests applied to high redshift clusters of galaxies in order to test the standard cosmological model and describe their range of validity. We carefully compare a sample of high-redshift, massive, X-ray selected galaxy clusters with realistic Poisson sample simulations of the theoretical mass function, which include the effect of marginalizing over cosmological parameters and assigning a mass error to the simulated clusters and re-sampling their masses. We compare the observations and simulations using the following statistical tests: the distributions of ensemble and individual existence probabilities (in the > M, > z sense), the redshift distributions, and the 2d Kolmogorov-Smirnov test. We find that if the observed survey geometry is 1.0<z<2.2 the clusters are not consistent with being the "Least Probable" simulated clusters at >95% confidence, after marginalizing over observational and theoretical uncertainties, and thus still suggest tension for the LCDM model. Although, we find that the clusters are less likely to be drawn from a purely random selection of re-sampled simulated clusters. We identify a possible, seemingly unphysical, solution by placing a hard redshift cut z<1.6 on the simulated clusters and repeating the above statistical tests, which reduces the tension to <95% when compared with the Least Probable re-sampled simulated clusters.

arXiv: 29 September 2011

Massive Cosmologies

G. D'Amico, C. de Rham, S. Dubovsky, G. Gabadadze, D. Pirtskhalava, A.J. Tolley

arXiv:1108.5231v1

We explore the cosmological solutions of a recently proposed extension of General Relativity with a Lorentz-invariant mass term. We show that the same constraint that removes the Boulware-Deser ghost in this theory also prohibits the existence of homogeneous and isotropic cosmological solutions. Nevertheless, within domains of the size of inverse graviton mass we find approximately homogeneous and isotropic solutions that can well describe the past and present of the Universe. At energy densities above a certain crossover value, these solutions approximate the standard FRW evolution with great accuracy. As the Universe evolves and density drops below the crossover value the inhomogeneities become more and more pronounced. In the low density regime each domain of the size of the inverse graviton mass has essentially non-FRW cosmology. This scenario imposes an upper bound on the graviton mass, which we roughly estimate to be an order of magnitude below the present-day value of the Hubble parameter. The bound becomes especially restrictive if one utilizes an exact self-accelerated solution that this theory offers. Although the above are robust predictions of massive gravity with an explicit mass term, we point out that if the mass parameter emerges from some additional scalar field condensation, the constraint no longer forbids the homogeneous and isotropic cosmologies. In the latter case, there will exist an extra light scalar field at cosmological scales, which is screened by the Vainshtein mechanism at shorter distances.

The Kolmogorov-Smirnov test for the CMB

Mona Frommert, Ruth Durrer, Jérôme Michaud

arXiv:1108.5354v1

We investigate the statistics of the cosmic microwave background using the Kolmogorov-Smirnov test. We show that, when we correctly de-correlate the data, the partition function of the Kolmogorov stochasticity parameter is compatible with the Kolmogorov distribution and, contrary to previous claims, the CMB data are compatible with Gaussian fluctuations with the correlation function given by standard Lambda-CDM. We then use the Kolmogorov-Smirnov test to derive upper bounds on residual point source power in the CMB, and indicate the promise of this statistics for further datasets, especially Planck, to search for deviations from Gaussianity or for point sources.

Dark Matter Seeding in Neutron Stars

M. A. Perez-Garcia, J. Silk, J. R. Stone

arXiv:1108.5206v1

We present a mechanism that may seed compact stellar objects with stable lumps of quark matter, or {\it strangelets}, through the self-annihilation of gravitationally accreted WIMPs. We show that dark matter particles with masses above a few GeV may provide enough energy in the nuclear medium for quark deconfinement and subsequent strangelet formation. If this happens this effect may then trigger a partial or full conversion of the star into a strange star. We set a new limit on the WIMP mass in the few-GeV range that seems to be consistent with recent indications in dark matter direct detection experiments.

Coarse-Grained Cosmological Perturbation Theory

Massimo Pietroni, Gianpiero Mangano, Ninetta Saviano, Matteo Viel

arXiv:1108.5203v1

Semi-analytical methods, based on Eulerian perturbation theory, are a promising tool to follow the time evolution of cosmological perturbations at small redshifts and at mildly nonlinear scales. All these schemes are based on two approximations: the existence of a smoothing scale and the single-stream approximation, where velocity dispersion of the dark matter fluid, as well as higher moments of the particle distributions, are neglected. Despite being widely recognized, these two assumptions are, in principle, incompatible, since any finite smoothing scale gives rise to velocity dispersion and higher moments at larger scales. We describe a new approach to perturbation theory, where the Vlasov and fluid equations are derived in presence of a finite coarse-graining scale: this allows a clear separation between long and short distance modes and leads to a hybrid approach where the former are treated perturbatively and the effect of the latter is encoded in external source terms for velocity, velocity dispersion, and all the higher order moments, which can be computed from N-body simulations. We apply the coarse-grained perturbation theory to the computation of the power spectrum and the cross-spectrum between density and velocity dispersion, and compare the results with N-body simulations, finding good agreement.

arXiv: 26 September 2011

The cosmic star formation rate: a theoretical approach

Luca Vincoletto

113 pages, 33 figures, 5 tables, Master degree thesis

arXiv:1108.5040v1

The cosmic star formation rate (CSFR), namely the star formation rate in a unitary comoving volume of the Universe, is a fundamental clue to investigate the history of the assembling and evolution of structures in the Universe. Here we develop a method to study the CSFR from a purely theoretical point of view. Starting from detailed models of chemical evolution, which best fit the properties of local galaxies, we obtain the histories of star formation of galaxies of different morphological types (ellipticals, spirals, irregulars). These histories are then used to determine the photometric evolution of galaxies by means of a spectro-photometric code. The next step in computing the CSFR is the calculation of the luminosity density for which we need to know how galaxies are distributed in the Universe at any redshift. This is possible thanks to the luminosity function (LF) of galaxies. At this point we calculate the CSFR under different evolutionary scenarios. First, we study the hypothesis of a pure luminosity evolution scenario, in which galaxies are supposed to form all at the same redshift and then evolve only in luminosity without any merging or interaction. In other words, we assume no number density evolution. Then we define scenarios in which the number density and the slope of the LF are assumed to vary with redshift. All our results have been compared with data available in literature. We conclude that a pure number density evolution does not provide a good fit to the data. On the other hand, a variation in the number density for ellipticals and spirals as a function of redshift can provide a better fit to the observed CSFR, although the data at very high redshift are still quite uncertain and this solution predicts that most of the metals in the Universe have been produced by spirals, a prediction which still needs to be tested.