Thursday, January 26, 2012

arXiv: 26 January 2012

 Hubble flow variance and the cosmic rest frame
 We characterize the radial and angular variance of the Hubble flow in the COMPOSITE sample of 4534 galaxy distances. Independent of any cosmological assumptions other than the existence of a suitably averaged linear Hubble law, we find with decisive Bayesian evidence (ln B >> 5) that the Hubble constant averaged in spherical radial shells is closer to its global value when referred to the rest frame of the Local Group rather than to the standard rest frame of the Cosmic Microwave Background (CMB) radiation. Angular averages reveal a dipole structure in the Hubble flow variance, correlated with structures within a sphere of radius 30/h - 60/h Mpc. Furthermore, the angular map of Hubble flow variance is found to coincide with the angular map of the residual CMB temperature dipole in the Local Group rest frame, with correlation coefficient -0.92. This suggests a new mechanism for the origin of the CMB dipole: in addition to a local boost it is generated by differences in the distance to the surface of last scattering, of a maximum +/- 0.35/h Mpc, which arise from foreground structures within 60/h Mpc, a 0.6% effect. The dipole feature is accounted for by our position in a filamentary sheet between Local Voids and Local Walls, producing a foreground density gradient on scales up to 60/h Mpc on opposite sides of the sky. This result potentially eliminates problems of interpretation of "bulk flows". Furthermore, anomalies associated with large angles in the CMB anisotropy spectrum, and also the dark flow inferred from the kinetic Sunyaev-Zel'dovich effect on small angular scales, need to be critically re-examined.
 
 The Direction of Gravity
 Gravity directs the paths of light rays and the growth of structure. Moreover, gravity on cosmological scales does not simply point down: it accelerates the universal expansion by pulling outward, either due to a highly negative pressure dark energy or an extension of general relativity. We examine methods to test the properties of gravity through cosmological measurements. We then consider specific possibilities for a sound gravitational theory based on the Galileon shift symmetry. The evolution of the laws of gravity from the early universe to the present acceleration to the future fate -- the paths of gravity -- carries rich information on this fundamental force of physics and on the mystery of dark energy.
 
 
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arXiv: 25 January 2012

Measuring cosmological distances by coalescing binaries
 Gravitational waves detected from well-localized inspiraling binaries would allow us to determine, directly and independently, binary luminosity and redshift. In this case, such systems could behave as "standard candles" providing an excellent probe of cosmic distances up to z <0.1 and complementing other indicators of cosmological distance ladder.
 
 
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arXiv: 24 January 2012

 Evidence for Quadratic Tidal Tensor Bias from the Halo Bispectrum
The relation between the clustering properties of luminous matter in the form of galaxies and the underlying dark matter distribution is of fundamental importance for the interpretation of ongoing and upcoming galaxy surveys. The so called local bias model, where galaxy density is a function of local matter density, is frequently discussed as a means to infer the matter power spectrum or correlation function from the measured galaxy correlation. However, gravitational evolution generates a term quadratic in the tidal tensor and thus non-local in the density field, even if this term is absent in the initial conditions (Lagrangian space). Because the term is quadratic, it contributes as a loop correction to the power spectrum, so the standard linear bias picture still applies on large scales, however, it contributes at leading order to the bispectrum for which it is significant on all scales. Such a term could also be present in Lagrangian space if halo formation were influenced by the tidal field. We measure the corresponding coupling strengths from the matter-matter-halo bispectrum in numerical simulations and find a non-vanishing coefficient for the tidal tensor term. We find no scale dependence of the bias parameters up to k=0.1 h/Mpc and that the tidal effect is increasing with halo mass. While the Lagrangian bias picture is a better description of our results than the Eulerian bias picture, our results suggest that there might be a tidal tensor bias already in the initial conditions. We also find that the coefficients of the quadratic density term deviate quite strongly from the theoretical predictions based on the spherical collapse model and a universal mass function. Both quadratic density and tidal tensor bias terms must be included in the modeling of galaxy clustering of current and future surveys if one wants to achieve the high precision cosmology promise of these datasets.
 
Singularity phenomena in viable f(R) gravity
 The curvature singularity in viable f(R) gravity models is examined when the background density is dense. This singularity could be eliminated by adding the $R^{2}$ term in the Lagrangian. Some of cosmological consequences, in particular the source for the scalar mode of gravitational waves, are discussed.
 
 
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arXiv: 23 Januaru 2012

The hybrid inflation waterfall and the primordial curvature perturbation
Authors: David H. Lyth
Without demanding a specific form for the inflaton potential, we obtain an estimate of the contribution to the curvature perturbation generated during the linear era of the hybrid inflation waterfall. The spectrum of this contribution peaks at some wavenumber $k=k_*$, and goes like $k^3$ for $k\ll k_*$, making it typically negligible on cosmological scales. The scale $k_*$ can be outside the horizon at the end of inflation, in which case $\zeta=- (g^2 - \vev{g^2})$ with $g$ gaussian. Taking this into account, the cosmological bound on the abundance of black holes is likely to be satisfied if the curvaton mass $m$ much bigger than the Hubble parameter $H$, but is likely to be violated if $m\lsim H$. Coming to the contribution to $\zeta$ from the rest of the waterfall, we are led to consider the use of the `end-of-inflation' formula, giving the contribution to $\zeta$ generated during a sufficiently sharp transition from nearly-exponential inflation to non-inflation, and we state for the first time the criterion for the transition to be sufficiently sharp. Our formulas are applied to supersymmetric GUT inflation and to supernatural/running-mass inflation
 
 
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arXiv: 20 January 2012

 Testing gravity with halo density profiles observed through gravitational lensing
We present a new test of the modified gravity endowed with the Vainshtein mechanism with the density profile of a galaxy cluster halo observed through gravitational lensing. A scalar degree of freedom in the galileon modified gravity is screened by the Vainshtein mechanism to recover Newtonian gravity on high-density regions, however it might not be completely hidden on the outer side of a cluster of galaxies. Then the modified gravity might yield an observational signature in a surface mass density of a cluster of galaxies measured through gravitational lensing, since the scalar field could contribute to the lensing potential. We investigate how the breaking of the Vainshtein mechanism affects the surface mass density observed through gravitational lensing, assuming that the density profile of a cluster of galaxies follows the original Navarro-Frenk-White (NFW) profile, the generalized NFW profile and the Einasto profile. We compare the theoretical predictions with observational results of the surface mass density reported recently by other researchers. We obtain constraints on the amplitude and the typical scale of the breaking of the Vainshtein mechanism in a subclass of the generalized galileon model.
 

 

Title: The density profiles of Dark Matter halos in Spiral Galaxies

Authors: Gianluca Castignani, Noemi Frusciante, Daniele Vernieri, Paolo Salucci

 In spiral galaxies, we explain their non-Keplerian rotation curves (RCs) by means of a non-luminous component embedding the stellar-gaseous disks. Understanding the detailed properties of this component (labelled Dark Matter, DM) is one of the most pressing issues of Cosmology. We investigate the recent relationship (claimed by Walker et al. 2010) between $r $, the galaxy radial coordinate, and the dark halo contribution to the circular velocity at $r$, {\it a}) in the framework of the Universal Rotation Curve (URC) paradigm and directly {\it b}) by means of the kinematics of a large sample of Dark matter dominated spirals. We find a general agreement between the W+10 claim, the distribution of DM emerging from URC and that inferred in the (low luminosity) objects. We show that the emerging phenomenology, linking the spiral's luminosity, radii and circular velocities, implies an evident inconsistency with (naive) predictions in the $\Lambda$CDM scenario.
 
Dark Matter: A Brief Review
From astronomical observations, we know that dark matter exists, makes up 23% of the mass budget of the Universe, clusters strongly to form the load-bearing frame of structure for galaxy formation, and hardly interacts with ordinary matter except gravitationally. However, this information is not enough to identify the particle specie(s) that make up dark matter. As such, the problem of determining the identity of dark matter has largely shifted to the fields of astroparticle and particle physics. In this talk, I will review the current status of the search for the nature of dark matter. I will provide an introduction to possible particle candidates for dark matter and highlight recent experimental astroparticle- and particle-physics results that constrain the properties of those candidates. Given the absence of detections in those experiments, I will advocate a return of the problem of dark-matter identification to astronomy, and show what kinds of theoretical and observational work might be used to pin down the nature of dark matter once and for all. This talk is intended for a broad astronomy audience.
 
 A closer look at CMB constraints on WIMP dark matter
We use Cosmic Microwave Background data from the WMAP, SPT, BICEP, and QUaD experiments to obtain constraints on the dark matter particle mass $m_\chi$, and show that the combined data requires $m_\chi > 7.6$ GeV at the 95% confidence level for the $\chi \chi \rightarrow b \bar b$ channel. We examine whether the bound on $m_\chi$ is sensitive to $\sigma_8$ measurements made by galaxy cluster observations. The large uncertainty in $\sigma_8$ and the degeneracy with $\Omega_{\rm m}$ allow only small improvements in the dark matter mass bound. Increasing the number of effective neutrino-like degrees of freedom to $N_{\rm eff} = 3.85$ improves the mass bound to $m_\chi > 8.6$ GeV at 95% confidence, for the $\chi \chi \rightarrow b \bar b$ channel. We also study models in which dark matter halos at $z<60$ reionize the Universe. We compute the Ostriker-Vishniac power resulting from partial reionization at intermediate redshifts $10<z<60$, but find the effect to be small. We discuss the importance of the large angle polarization as a complementary probe of dark matter annihilation. By performing Monte Carlo simulations, we show that future experiments that measure the $EE$ power spectrum from $20 < l < 50$ can exclude $m_\chi \sim$ 10 GeV at the 2 (3) $\sigma$ level provided the error bars are smaller than 4 (3) $\times$ cosmic variance.
 
 
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Monday, January 2, 2012

arXiv: 30 December 2011

 Viability of the cluster mass function formalism in parametrised modified gravity
 Model-independent parametrisations for examining departures from General Relativity have been increasingly studied over the past few years. Various observables have been used to constrain the parameters and forecasts for future surveys have been carried out. In one such forecast, galaxy cluster counts were used to constrain the parameters. Here, we carry out a limited set of $N$-body simulations, with a modified Poisson equation, to examine the accuracy of existing mass functions for modified gravity cosmologies. As well as altering the gravitational calculation, we include the effect of a screening scale to ensure consistency of the theory with solar system tests. Our results suggest that if a screening scale exists its effect can be taken into account in the cluster count calculation through its effect on the linear matter power spectrum. If this is done, the accuracy of the standard mass function formalism in modified gravity theories with reasonably small departures from General Relativity, as tested in this work, is comparable to the standard case.
 
 Discrimination between Lambda-CDM, quintessence, and modified gravity models using wide area surveys
 In the past decade or so observations of supernovae, Large Scale Structures (LSS), and Cosmic Microwave Background (CMB) have confirmed the presence of what is called dark energy, and measured its density as well as the value of other cosmological parameters according to concordance - Lambda-CDM model with few percent uncertainties. Next generation of surveys will allow to distinguish between a Lambda-CDM and alternative models such as modified gravity and (interacting)-quintessence models. In this work we parametrize homogeneous and anisotropic components of matter density in the context of interacting dark energy models with the goal discriminating between f(R) modified gravity and its generalization, and interacting dark energy models, for which we also propose a phenomenological description of energy-momentum conservation equations inspired by particle physics. It is based on the fact that the simplest interactions between particles/fields are elastic scattering and decay. The parametrization of growth rate proposed here is nonetheless general and can be used to constrain other interactions. We also present a crude estimation of the accuracy of the measurement of these parameters using Euclid and Planck surveys data.
 
 Measuring CMB non-Gaussianity as a probe of Inflation and Cosmic Strings
Authors: D.M. Regan
 The leading candidate for the very early universe is described by a period of rapid expansion known as inflation. While the standard paradigm invokes a single slow-rolling field, many different models may be constructed which fit the current observational evidence. In this work we outline theoretical and observational studies of non-Gaussian fluctuations produced by models of inflation and by cosmic strings - topological defects that may be generated in the very early universe during a phase transition. In particular, we consider the imprint of cosmic strings on the cosmic microwave background (CMB) and describe a formalism for the measurement of general four-point correlation functions, or trispectra, using the CMB. In addition we describe the application of our methodology to non-Gaussian signals imprinted in the large scale structure of the universe. Such deviations from Gaussianity are generally expressed in terms of the so-called bispectrum and trispectrum.
 
 
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