arXiv: 30 July 2010

Parameterizing and Measuring Dark Energy Trajectories from Late-Inflatons

Authors: Zhiqi Huang, J. Richard Bond, Lev Kofman
http://arxiv.org/abs/1007.5297v1

Bulk dark energy properties are determined by the redshift evolution of its pressure-to-density ratio, $w_{de}(z)$. An experimental goal is to decide if the dark energy is dynamical, as in the quintessence (and phantom) models treated here. We show that a three-parameter approximation $w_{de}(z; \epsilon_s, \epsilon_{\phi\infty}, \zeta_s)$ fits well the ensemble of trajectories for a wide class of late-inflaton potentials $V(\phi)$. Markov Chain Monte Carlo probability calculations are used to confront our $w_{de}(z)$ trajectories with current observational information on Type Ia supernova, Cosmic Microwave Background, galaxy power spectra, weak lensing and the Lyman-${\alpha}$ forest. We find the best constrained parameter is a low redshift slope parameter, $\epsilon_s \propto (\partial \ln V / \partial \phi)^2$ when the dark energy and matter have equal energy densities. A tracking parameter $\epsilon_{\phi\infty}$ defining the high-redshift attractor of $1+w_{de}$ is marginally constrained. Poorly determined is $\zeta_s$, characterizing the evolution of $\epsilon_s$, and a measure of $\partial^2 \ln V / \partial \phi^2$ . The constraints we find already rule out some popular quintessence and phantom models, or restrict their potential parameters. We also forecast how the next generation of cosmological observations improve the constraints: by a factor of about five on $\epsilon_s$ and $\epsilon_{\phi\infty}$, but with $\zeta_s$ remaining unconstrained (unless the true model significantly deviates from $\Lambda$CDM). Thus potential reconstruction beyond an overall height and a gradient is not feasible for the large space of late-inflaton models considered here.

 

Dark Energy and Extending the Geodesic Equations of Motion: Connecting the Galactic and Cosmological Length Scales

Authors: Achilles D. Speliotopoulos

http://arxiv.org/abs/1007.5266v1

Recently, an extension of the geodesic equations of motion using the Dark Energy length scale was proposed. Here, we apply this extension to the analyzing the motion of test particles at the galactic scale and longer. A cosmological check of the extension is made using the observed rotational velocity curves and core sizes of 1393 spiral galaxies. We derive the density profile of a model galaxy using this extension, and with it, we calculate $\sigma_8$ to be $0.73_{\pm 0.12}$; this is within experimental error of the WMAP value of $0.761_{-0.048}^{+0.049}$. We then calculate $R_{200}$ to be $206_{\pm 53}$ kpc, which is in reasonable agreement with observations.

 

Non-minimally coupled f(R) Cosmology

Authors: Shruti Thakur, Anjan A Sen, T.R. Seshadri

http://arxiv.org/abs/1007.5250v1

We investigate the consequences of non-minimal gravitational coupling to matter and study how it differs from the case of minimal coupling by choosing certain simple forms for the nature of coupling, The values of the parameters are specified at $z=0$ (present epoch) and the equations are evolved backwards to calculate the evolution of cosmological parameters. We find that the Hubble parameter evolves more slowly in non-minimal coupling case as compared to the minimal coupling case. In both the cases, the universe accelerates around present time, and enters the decelerating regime in the past. Using the latest Union2 dataset for supernova Type Ia observations as well as the data for baryon acoustic oscillation (BAO) from SDSS observations, we constraint the parameters of Linder exponential model in the two different approaches. We find that there is a upper bound on model parameter in minimal coupling. But for non-minimal coupling case, there is range of allowed values for the model parameter.

 

Scale-dependence of Non-Gaussianity in the Curvaton Model

Authors: Christian T. Byrnes, Kari Enqvist, Tomo Takahashi

http://arxiv.org/abs/1007.5148v1

We investigate the scale-dependence of f_NL in the self-interacting curvaton model. We show that the scale-dependence, encoded in the spectral index n_{f_NL}, can be observable by future cosmic microwave background observations, such as CMBpol, in a significant part of the parameter space of the model. We point out that together with information about the trispectrum g_NL, the self-interacting curvaton model parameters could be completely fixed by observations. We also discuss the scale-dependence of g_NL and its implications for the curvaton model, arguing that it could provide a complementary probe in cases where the theoretical value of n_{f_NL} is below observational sensitivity.

 

Horava-Lifshitz Cosmology: A Review

Authors: Shinji Mukohyama

http://arxiv.org/abs/1007.5199v1

This article reviews basic construction and cosmological implications of a power-counting renormalizable theory of gravitation recently proposed by Horava. We explain that (i) at low energy this theory does not exactly recover general relativity but instead mimic general relativity plus dark matter; that (ii) higher spatial curvature terms allow bouncing and cyclic universes as regular solutions; and that (iii) the anisotropic scaling with the dynamical critical exponent z=3 solves the horizon problem and leads to scale-invariant cosmological perturbations even without inflation. We also comment on issues related to an extra scalar degree of freedom called scalar graviton. In particular, for spherically-symmetric, static, vacuum configurations we prove non-perturbative continuity of the lambda->1+0 limit, where lambda is a parameter in the kinetic action and general relativity has the value lambda=1. We also derive the condition under which linear instability of the scalar graviton does not show up.

 

Dark Energy and Extending the Geodesic Equations of Motion: Its Construction and Experimental Constraints

Authors: Achilles D. Speliotopoulos

http://arxiv.org/abs/1007.5270v1

With the discovery of Dark Energy, $\Lambda_{DE}$, there is now a universal length scale, $\ell_{DE}=c/(\Lambda_{DE} G)^{1/2}$, associated with the universe that allows for an extension of the geodesic equations of motion. In this paper, we will study a specific class of such extensions, and show that contrary to expectations, they are not automatically ruled out by either theoretical considerations or experimental constraints. In particular, we show that while these extensions affect the motion of massive particles, the motion of massless particles are not changed; such phenomena as gravitational lensing remain unchanged. We also show that these extensions do not violate the equivalence principal, and that because $\ell_{DE}=14010^{800}_{820}$ Mpc, a specific choice of this extension can be made so that effects of this extension are not be measurable either from terrestrial experiments, or through observations of the motion of solar system bodies. A lower bound for the only parameter used in this extension is set.

 

Finite entanglement entropy from the zero-point-area of spacetime

Authors: T. Padmanabhan

http://arxiv.org/abs/1007.5066v1

The calculation of entanglement entropy S of quantum fields in spacetimes with horizon shows that, quite generically, S (a) is proportional to the area A of the horizon and (b) is divergent. I argue that this divergence, which arises even in the case of Rindler horizon in flat spacetime, is yet another indication of a deep connection between horizon thermodynamics and gravitational dynamics. In an emergent perspective of gravity, which accommodates this connection, the fluctuations around the equipartition value in the area elements will lead to a minimal quantum of area, of the order of L_P^2, which will act as a regulator for this divergence. In a particular prescription for incorporating L_P^2 as zero-point-area of spacetime, this does happen and the divergence in entanglement entropy is regularized, leading to S proportional to (A/L_P^2) in Einstein gravity. In more general models of gravity, the surface density of microscopic degrees of freedom is different which leads to a modified regularisation procedure and the possibility that the entanglement entropy - when appropriately regularised - matches the Wald entropy.

 

arXiv: 29 July 2010

Dark Energy from Quantum Matter

Authors: Claudio Dappiaggi, Thomas-Paul Hack, Jan Möller, Nicola Pinamonti
http://arxiv.org/abs/1007.5009v1

We study the backreaction of free quantum fields on a flat Robertson-Walker spacetime. Apart from renormalization freedom, the vacuum energy receives contributions from both the trace anomaly and the thermal nature of the quantum state. The former represents a dynamical realisation of dark energy, while the latter mimics an effective dark matter component. The semiclassical dynamics yield two classes of asymptotically stable solutions. The first reproduces the concordance model in a suitable regime. The second lacks a classical counterpart, but is in excellent agreement with recent observations.

 

Key questions about Galactic Center dynamics

Authors: Tal Alexander (Weizmann Institute of Science)

http://arxiv.org/abs/1007.4991v1

I discuss four key questions about Galactic Center dynamics, their implications for understanding both the environment of the Galactic MBH and galactic nuclei in general, and the progress made in addressing them. The questions are (1) Is the stellar system around the MBH relaxed? (2) Is there a "dark cusp" around the MBH? (3) What is the origin of the stellar disk(s)?, and (4) What is the origin of the S-stars?

 

No Indications of Axion-Like Particles From Fermi

Authors: Alexander V. Belikov, Lisa Goodenough, Dan Hooper

http://arxiv.org/abs/1007.4862v1

As very high energy (~100 GeV) gamma rays travel over cosmological distances, their flux is attenuated through interactions with the extragalactic background light. Observations of distant gamma ray sources at energies between ~200 GeV and a few TeV by ground-based gamma ray telescopes such as HESS, however, suggest that the universe is more transparent to very high energy photons than had been anticipated. One possible explanation for this is the existence of axion-like-particles (ALPs) which gamma rays can efficiently oscillate into, enabling them to travel cosmological distances without attenuation. In this article, we use data from the Fermi Gamma Ray Space Telescope to calculate the spectra at 1-100 GeV of two gamma ray sources, 1ES1101-232 at redshift z=0.186 and H 2356-309 at z=0.165, and use this in conjunction with the measurements of ground-based telescopes to test the ALP hypothesis. We find that the observations can be well-fit by an intrinsic power-law source spectrum with indices of -2.01 and -2.77 for 1ES1101-232 and H 2356-309, respectively, and that no ALPs or other exotic physics is necessary to explain the observed degree of attenuation.

 

Cosmological density perturbations from conformal scalar field: infrared properties and statistical anisotropy

Authors: M. Libanov, V. Rubakov

http://arxiv.org/abs/1007.4949v1

We consider a scenario in which primordial scalar perturbations are generated when complex conformal scalar field rolls down its negative quartic potential. Initially, these are the perturbations of the phase of this field; they are converted into the adiabatic perturbations at a later stage. A potentially dangerous feature of this scenario is the existence of perturbations in the radial field direction, which have red power spectrum. We show, however, that the infrared effects are harmless, as they can be absorbed into field redefinition. We also evaluate the statistical anisotropy inherent in the model due to the existence of the long-ranged radial perturbations.

 

Multimetric extension of the PPN formalism: experimental consistency of repulsive gravity

Authors: Manuel Hohmann, Mattias N. R. Wohlfarth

http://arxiv.org/abs/1007.4945v1

Recently we discussed a multimetric gravity theory containing several copies of standard model matter each of which couples to its own metric tensor. This construction contained dark matter sectors interacting repulsively with the visible matter sector, and was shown to lead to cosmological late-time acceleration. In order to test the theory with high-precision experiments within the solar system we here construct a simple extension of the parametrized post-Newtonian (PPN) formalism for multimetric gravitational backgrounds. We show that a simplified version of this extended formalism allows the computation of a subset of the PPN parameters from the linearized field equations. Applying the simplified formalism we find that the PPN parameters of our theory do not agree with the observed values, but we are able to improve the theory so that it becomes consistent with experiments of post-Newtonian gravity and still features its promising cosmological properties.

 

 

arXiv: 28 July 2010

Testing the Consistence of Gamma Ray Burst Data-set and Supernovae Union2

Authors: Lixin Xu, Yuting Wang
http://arxiv.org/abs/1007.4734v1

In this paper, we test the consistence of Gamma Ray Bursts (GRBs) Data-set and Supernovae Union2 (SNU2) via the so-called {\it multi-dimensional consistence test} under the assumption that $\Lambda$CDM model is a potentially correct cosmological model. We find that the probes are inconsistent with $1.456\sigma$.

 

Numerical Modelling of the Vertical Structure and Dark Halo Parameters in Disc Galaxies

Authors: A. Khoperskov (1), D. Bizyaev (2 and 3), N. Tiurina (2), M. Butenko (1) ((1) Volgograde State University, Russia (2) Sternberg Astronomical Institute, Russia (3) Apache Point Obsrvatory and NMSU, USA)

http://arxiv.org/abs/1007.4693v1

The non-linear dynamics of bending instability and vertical structure of a galactic stellar disc embedded into a spherical halo are studied with N-body numerical modelling. Development of the bending instability in stellar galactic disc is considered as the main factor that increases the disc thickness. Correlation between the disc vertical scale height and the halo-to-disc mass ratio is predicted from the simulations. The method of assessment of the spherical-to-disc mass ratio for edge-on spiral galaxies with small bulges is considered. Modelling of eight edge-on galaxies: NGC 891, NGC 4738, NGC 5170, UGC 6080, UGC 7321, UGC 8286, UGC 9422 and UGC 9556 is performed. Parameters of stellar discs, dark haloes and bulges are estimated. The lower limit of the dark-to-luminous mass ratio in our galaxies is of the order of 1 within the limits of their stellar discs. The dark haloes dominate by mass in the galaxies with very thin stellar discs (NGC 5170, UGC 7321 and UGC 8286).

 

Cell Count Moments in the Halo Model

Authors: J. N. Fry, Stephane Colombi, Pablo Fosalba, Anand Balaraman, Istvan Szapudi, Romain Teyssier

http://arxiv.org/abs/1007.4667v1

We study cell count moments up to fifth order of the distributions of haloes, of halo substructures as a proxy for galaxies, and of mass in the context of the halo model and compare theoretical predictions to the results of numerical simulations. On scales larger than the size of the largest cluster, we present a simple point cluster model in which results depend only on cluster-cluster correlations and on the distribution of the number of objects within a cluster, or cluster occupancy. The point cluster model leads to expressions for moments of galaxy counts in which the volume-averaged moments on large scales approach those of the halo distribution and on smaller scales exhibit hierarchical clustering with amplitudes $S_k$ determined by moments of the occupancy distribution. In this limit, the halo model predictions are purely combinatoric, and have no dependence on halo profile, concentration parameter, or potential asphericity. The full halo model introduces only two additional effects: on large scales, haloes of different mass have different clustering strengths, introducing relative bias parameters; and on the smallest scales, halo structure is resolved and details of the halo profile become important, introducing shape-dependent form factors. Because of differences between discrete and continuous statistics, the hierarchical amplitudes for galaxies and for mass behave differently on small scales even if galaxy number is exactly proportional to mass, a difference that is not necessarily well described in terms of bias.

 

On The Existence Of Anisotropic Cosmological Models In Higher-Order Theories Of Gravity

Authors: Jonathan Middleton

http://arxiv.org/abs/1007.4669v1

We investigate the behaviour on approach to the initial singularity in higher-order extensions of general relativity by finding exact cosmological solutions for a wide class of models in which the Lagrangian is allowed to depend nonlinearly upon the three possible linear and quadratic scalars built from the Riemann tensor; $R$, $R_{ab}R^{ab}$ and $R_{abcd}R^{abcd}$. We present new anisotropic vacuum solutions analagous to the Kasner solutions of general relativity and extend previous results to a much wider range of fourth order theories of gravity. We discuss the implications of these results for the behaviour of the more general anisotropic Bianchi type VIII and IX cosmologies as the initial singularity is approached. Furthermore, we also consider the existence conditions for some other simple anisotropic Bianchi I vacuum solutions in which the expansion in each direction is of exponential, rather than power-law behaviour and their relevance for cosmic ``no-hair'' theorems.

 

General Relativity As an Aether Theory

Authors: Maurice J. Dupre, Frank J. Tipler

http://arxiv.org/abs/1007.4572v1

Most early twentieth century relativists --- Lorentz, Einstein, Eddington, for examples --- claimed that general relativity was merely a theory of the aether. We shall confirm this claim by deriving the Einstein equations using aether theory. We shall use a combination of Lorentz's and Kelvin's conception of the aether. Our derivation of the Einstein equations will not use the vanishing of the covariant divergence of the stress-energy tensor, but instead equate the Ricci tensor to the sum of the usual stress-energy tensor and a stress-energy tensor for the aether, a tensor based on Kelvin's aether theory. A crucial first step is generalizing the Cartan formalism of Newtonian gravity to allow spatial curvature, as conjectured by Gauss and Riemann.

 

 

 

 

arXiv: 27 July 2010

Amino Acids in Comets and Meteorites: Stability under Gamma Radiation and Preservation of Chirality

Authors: Susana Iglesias-Groth, Franco Cataldo, Ornella Ursini, Arturo Manchado

http://arxiv.org/abs/1007.4529v1

Amino acids in solar system bodies may have played a key role in the chemistry that led to the origin of life on Earth. We present laboratory studies testing the stability of amino acids against gamma radiation photolysis. All the 20 chiral amino acids in the levo form used in the proteins of the current terrestrial biochemistry have been irradiated in the solid state with gamma radiation to a dose of 3.2 MGy which is the dose equivalent to that derived by radionuclide decay in comets and asteroids in 1.05x109 years. For each amino acid the radiolysis degree and the radioracemization degree was measured by differential scanning calorimetry (DSC) and by optical rotatory dispersion (ORD) spectroscopy. From these measurements a radiolysis rate constant kdsc and a radioracemization rate constant krac have been determined for each amino acid and extrapolated to a dose of 14 MGy which corresponds to the expected total dose delivered by the natural radionuclides decay to all the organic molecules present in comets and asteroids in 4.6x109 years, the age of the Solar System. It is shown that all the amino acids studied can survive a radiation dose of 14 MGy in significant quantity although part of them are lost in radiolytic processes. Similarly, also the radioracemization process accompanying the radiolysis does not extinguish the chirality. The knowledge of the radiolysis and radioracemization rate constants may permit the calculation of the original concentration of the amino acids at the times of the formation of the Solar System starting from the concentration found today in carbonaceous chondrites. For some amino acids the concentration in the presolar nebula could have been up to 6 times higher than currently observed in meteorites.

 

Searching for a Cosmological Preferred Axis: Union2 Data Analysis and Comparison with Other Probes

Authors: I. Antoniou, L. Perivolaropoulos

http://arxiv.org/abs/1007.4347v1

We review compare and extend recent studies searching for evidence for a preferred cosmological axis. We start from the Union2 SnIa dataset and use the hemisphere comparison method to search for a preferred axis in the data. We find that the hemisphere of maximum accelerating expansion rate is in the direction (l,b)=(306^\circ, 15^\circ) (\Omega_m=0.19) while the hemisphere of minimum acceleration is in the opposite direction (l,b)=(126^\circ, -15^\circ) (\Omega_m=0.30). The level of anisotropy is described by the normalized difference of the best fit values of \Omega_m between the two hemispheres in the context of \lcdm fits. We find a maximum anisotropy level in the Union2 data of \frac{\Delta \Omega_m_max}{\Omega_m}=0.42. This level does not necessarily correspond to statistically significant anisotropy because it is reproduced by about 30% of simulated isotropic data mimicking the best fit Union2 dataset. However, when combined with the axes directions of other cosmological observations (bulk velocity flow axis, three axes of CMB low multipole moments and quasar optical polarization alignment axis), the statistical evidence for a cosmological anisotropy increases dramatically. We estimate the probability that the above independent six axes directions would be so close in the sky to be less than 1%. Thus either the relative coincidence of these six axes is a very large statistical fluctuation or there is an underlying physical or systematic reason that leads to their correlation.

 

Baryon Asymmetry of Universe as Manifestation of Quantum Phase Transition

Authors: V.R. Shaginyan, G.S. Japaridze, M.Ya. Amusia, A.Z. Msezane, K.G. Popov

http://arxiv.org/abs/1007.4317v1

A novel mechanism for explaining the matter-antimatter asymmetry of the universe is considered. The mechanism does not require the baryon number violating interactions or CP violation at a microscopic level. Our analysis of the matter-antimatter asymmetry is in the context of salient experimental results obtained in the condensed matter physics.

 

Generalized Schwarzschild's method

Authors: Mir Abbas Jalali, Scott Tremaine

http://arxiv.org/abs/1007.4298v1

We describe a new finite element method (FEM) to construct continuous equilibrium distribution functions of stellar systems. The method is a generalization of Schwarzschild's orbit superposition method from the space of discrete functions to continuous ones. In contrast to Schwarzschild's method, FEM produces a continuous distribution function (DF) and satisfies the intra element continuity and Jeans equations. The method employs two finite-element meshes, one in configuration space and one in action space. The DF is represented by its values at the nodes of the action-space mesh and by interpolating functions inside the elements. The Galerkin projection of all equations that involve the DF leads to a linear system of equations, which can be solved for the nodal values of the DF using linear or quadratic programming, or other optimization methods. We illustrate the superior performance of FEM by constructing ergodic and anisotropic equilibrium DFs for spherical stellar systems (Hernquist models). We also show that explicitly constraining the DF by the Jeans equations leads to smoother and/or more accurate solutions with both Schwarzschild's method and FEM.

 

Scale-dependent non-Gaussianity probes inflationary physics

Authors: Christian T. Byrnes, Mischa Gerstenlauer, Sami Nurmi, Gianmassimo Tasinato, David Wands

http://arxiv.org/abs/1007.4277v1

We calculate the scale dependence of the bispectrum and trispectrum in (quasi) local models of non-Gaussian primordial density perturbations, and characterize this scale dependence in terms of new observable parameters. They can help to discriminate between models of inflation, since they are sensitive to properties of the inflationary physics that are not probed by the standard observables. We find consistency relations between these parameters in certain classes of models. We apply our results to a scenario of modulated reheating, showing that the scale dependence of non-Gaussianity can be significant. We also discuss the scale dependence of the bispectrum and trispectrum, in cases where one varies the shape as well as the overall scale of the figure under consideration. We conclude providing a formulation of the curvature perturbation in real space, which generalises the standard local form by dropping the assumption that f_NL and g_NL are constants.

 

Analytic Description of DGP Perturbations on All Scales

Authors: Sanjeev S. Seahra, Wayne Hu

http://arxiv.org/abs/1007.4242v1

We develop analytic solutions for the linear evolution of metric perturbations in the DGP braneworld modified gravity scenario including near-horizon and superhorizon modes where solutions in the bulk are required. These solutions apply to both the self-accelerating and normal branch and elucidate the nature of coordinate singularities and initial data in the bulk as well as their effect on perturbation evolution on the brane. Even on superhorizon scales, the evolution of metric perturbations is no longer necessarily scale free due to multiple resonances in the bulk. Based on these analytic solutions, we devise convenient fitting functions for the evolution that bridge the various spatial and temporal regimes. Compared with a direct numerical integration of the bulk equations, the fits are accurate at the percent level and are sufficient for current and upcoming observational tests

 

Astrometric Microlensing by Local Dark Matter Subhalos

Authors: Adrienne L. Erickcek (CITA/Perimeter Institute), Nicholas M. Law (Dunlap Institute)

http://arxiv.org/abs/1007.4228v1

High-resolution N-body simulations of dark matter halos indicate that the Milky Way contains numerous subhalos. When a dark matter subhalo passes in front of a star, the light from that star will be deflected by gravitational lensing, leading to a small change in the star's apparent position. This astrometric microlensing signal depends on the inner density profile of the subhalo and can be greater than a few microarcseconds for an intermediate-mass subhalo (Mvir > 10000 solar masses) passing within arcseconds of a star. Current and near-future instruments could detect this signal, and we evaluate SIM's, Gaia's, and ground-based telescopes' potential as subhalo detectors. We develop a general formalism to calculate a subhalo's astrometric lensing cross section over a wide range of masses and density profiles, and we calculate the lensing event rate by extrapolating the subhalo mass function predicted by simulations down to the subhalo masses potentially detectable with this technique. We find that, although the detectable event rates are predicted to be low on the basis of current simulations, lensing events may be observed if the central regions of dark matter subhalos are more dense than current models predict (>1 solar mass within 0.1 pc of the subhalo center). Furthermore, targeted astrometric observations can be used to confirm the presence of a nearby subhalo detected by gamma-ray emission. We show that, for sufficiently steep density profiles, ground-based adaptive optics astrometric techniques could be capable of detecting intermediate-mass subhalos at distances of hundreds of parsecs, while SIM could detect smaller and more distant subhalos.

 

Bayesian Analysis of Inflation I: Parameter Estimation for Single Field Models

Authors: Michael J. Mortonson (CCAPP/Ohio State), Hiranya V. Peiris (Cambridge, UCL), Richard Easther (Yale)

http://arxiv.org/abs/1007.4205v1

Future astrophysical datasets promise to strengthen constraints on models of inflation, and extracting these constraints requires methods and tools commensurate with the quality of the data. In this paper we describe ModeCode, a new, publicly available code that computes the primordial scalar and tensor power spectra for single field inflationary models. ModeCode solves the inflationary mode equations numerically, avoiding the slow roll approximation. It is interfaced with CAMB and CosmoMC to compute cosmic microwave background angular power spectra and perform likelihood analysis and parameter estimation. ModeCode is easily extendable to additional models of inflation, and future updates will include Bayesian model comparison. Errors from ModeCode contribute negligibly to the error budget for analyses of data from Planck or other next generation experiments. We constrain representative single field models (phi^n with n=2/3, 1, 2, and 4, natural inflation, and "hilltop" inflation) using current data, and provide forecasts for Planck. From current data, we obtain weak but nontrivial limits on the post-inflationary physics, which is a significant source of uncertainty in the predictions of inflationary models, while we find that Planck will dramatically improve these constraints. In particular, Planck will link the inflationary dynamics with the post-inflationary growth of the horizon, and thus begin to probe the "primordial dark ages" between TeV and GUT scale energies.

 

The Bias and Mass Function of Dark Matter Halos in Non-Markovian Extension of the Excursion Set Theory

Authors: Chung-Pei Ma (UC Berkeley), Michele Maggiore (Univ of Geneva), Antonio Riotto (CERN, INFN), Jun Zhang (UT Austin)

http://arxiv.org/abs/1007.4201v1

The excursion set theory based on spherical or ellipsoidal gravitational collapse provides an elegant analytic framework for calculating the mass function and the large-scale bias of dark matter haloes. This theory assumes that the perturbed density field evolves stochastically with the smoothing scale and exhibits Markovian random walks in the presence of a density barrier. Here we derive an analytic expression for the halo bias in a new theoretical model that incorporates non-Markovian extension of the excursion set theory with a stochastic barrier. This model allows us to handle non-Markovian random walks and to calculate perturbativly these corrections to the standard Markovian predictions for the halo mass function and halo bias. Our model contains only two parameters: kappa, which parameterizes the degree of non-Markovianity and whose exact value depends on the shape of the filter function used to smooth the density field, and a, which parameterizes the degree of stochasticity of the barrier. Appropriate choices of kappa and a in our new model can lead to a closer match to both the halo mass function and halo bias in the latest N-body simulations than the standard excursion set theory.

 

Virialization of high redshift dark matter haloes

Authors: Andrew Davis, Anson D'Aloisio, Priyamvada Natarajan

http://arxiv.org/abs/1007.4196v1

We present results of a study of the virial state of high redshift dark matter haloes in an N-body simulation. We find that the majority of collapsed, bound haloes are not virialized at any redshift slice in our study ($z=15-6$) and have excess kinetic energy. At these redshifts, merging is still rampant and the haloes cannot strictly be treated as isolated systems. To assess if this excess kinetic energy arises from the environment, we include the surface pressure term in the virial equation explicitly and relax the assumption that the density at the halo boundary is zero. Upon inclusion of the surface term, we find that the haloes are much closer to virialization, however, they still have some excess kinetic energy. We report trends of the virial ratio including the extra surface term with three key halo properties: spin, environment, and concentration. We find that haloes with closer neighbors depart more from virialization, and that haloes with larger spin parameters do as well. We conclude that except at the lowest masses ($M < 10^6 \Msun$), dark matter haloes at high redshift are not fully virialized. This finding has interesting implications for galaxy formation at these high redshifts, as the excess kinetic energy will impact the subsequent collapse of baryons and the formation of the first disks.

 

Nonspherical similarity solutions for dark halo formation

Authors: Mark Vogelsberger (1), Roya Mohayaee (2), Simon D.M. White (3) ((1) Harvard/CfA, (2) IAP Paris, (3) MPA)
http://arxiv.org/abs/1007.4195v1

We carry out fully 3-dimensional simulations of evolution from self-similar, spherically symmetric linear perturbations of a Cold Dark Matter dominated Einstein-de Sitter universe. As a result of the radial orbit instability, the haloes which grow from such initial conditions are triaxial with major-to-minor axis ratios of order 3:1. They nevertheless grow approximately self-similarly in time. In all cases they have power-law density profiles and near-constant velocity anisotropy in their inner regions. Both the power-law index and the value of the velocity anisotropy depend on the similarity index of the initial conditions, the former as expected from simple scaling arguments. Halo structure is thus not "universal" but remembers the initial conditions. On larger scales the density and anisotropy profiles show two characteristic scales, corresponding to particles at first pericentre and at first apocentre after infall. They are well approximated by the NFW model only for one value of the similarity index. In contrast, at all radii within the outer caustic the pseudo phase-space density can be fit by a single power law with an index which depends only very weakly on the similarity index of the initial conditions. This behaviour is very similar to that found for haloes formed from LCDM initial conditions and so can be considered approximately universal.

 

The recovery of General Relativity in massive gravity via the Vainshtein mechanism

Authors: E. Babichev, C. Deffayet, R. Ziour

http://arxiv.org/abs/1007.4506v1

We study in detail static spherically symmetric solutions of non linear Pauli-Fierz theory. We obtain a numerical solution with a constant density source. This solution shows a recovery of the corresponding solution of General Relativity via the Vainshtein mechanism. This result has already been presented by us in a recent letter, and we give here more detailed information on it as well as on the procedure used to obtain it. We give new analytic insights upon this problem, in particular for what concerns the question of the number of solutions at infinity. We also present a weak field limit which allows to capture all the salient features of the numerical solution, including the Vainshtein crossover and the Yukawa decay.

 

Palatini formulation of modified gravity with a nonminimal curvature-matter coupling

Authors: Tiberiu Harko, Francisco S. N. Lobo

http://arxiv.org/abs/1007.4415v1

We derive the field equations and the equations of motion for massive test particles in modified theories of gravity with an arbitrary coupling between geometry and matter by using the Palatini formalism. We show that the independent connection can be expressed as the Levi-Civita connection of an auxiliary, matter Lagrangian dependent metric, which is related with the physical metric by means of a conformal transformation. Similarly to the metric case, the field equations impose the non-conservation of the energy-momentum tensor. We derive the explicit form of the equations of motion for massive test particles in the case of a perfect fluid, and the expression of the extra-force is obtained in terms of the matter-geometry coupling functions and of their derivatives. Generally, the motion is non-geodesic, and the extra force is orthogonal to the four-velocity.

 

 

arXiv: 26 July 2010

Trumpeting the Vuvuzela: UltraDeep HI observations with MeerKAT

Authors: B. W. Holwerda, S-L. Blyth

http://arxiv.org/abs/1007.4101v1

The MeerKAT UltraDeep HI Survey aims to observe the 21 cm emission line of neutral hydrogen gas out to a redshift of z=1 and beyond. From both direct detections and stacked signal, we will address the HI mass function, the cosmic neutral gas density of the Universe (Omega_HI) and their evolution over cosmic times, as well as galaxy evolution via e.g., the Tully-Fisher relation, the relation between HI mass and Hubble Type or stellar mass, and the Schmidt-Kennicutt star-formation law. We propose to observe two fields, the COSMOS and Chandra Deep Field South (CDF-S) for 1000 hours each, adding an additional 4000 hours to one of these fields in 2015 when the full instantaneous bandwidth of MeerKAT (0.58-2.5 GHz) will be realised.

 

It's Hard to Learn How Gravity and Electromagnetism Couple

Authors: Yi-Zen Chu, David M. Jacobs, Yifung Ng, Glenn D. Starkman

http://arxiv.org/abs/1007.3992v1

We construct the most general effective Lagrangian coupling gravity and electromagnetism up to mass dimension 6 by enumerating all possible non-minimal coupling terms respecting both diffeomorphism and gauge invariance. In all, there are only two unique terms after field re-definitions; one is known to arise from loop effects in QED while the other is a parity violating term which may be generated by weak interactions within the standard model of particle physics. We show that neither the cosmological propagation of light nor, contrary to earlier claims, solar system tests of General Relativity are useful probes of these terms. These non-minimal couplings of gravity and electromagnetism may remain a mystery for the foreseeable future.

 

TASI Lectures on Holographic Space-Time, SUSY and Gravitational Effective Field Theory

Authors: Tom Banks

http://arxiv.org/abs/1007.4001v1

I argue that the conventional field theoretic notion of vacuum state is not valid in quantum gravity. The arguments use gravitational effective field theory, as well as results from string theory, particularly the AdS/CFT correspondence. Different solutions of the same low energy gravitational field equations correspond to different quantum systems, rather than different states in the same system. I then introduce {\it holographic space-time} a quasi-local quantum mechanical construction based on the holographic principle. I argue that models of quantum gravity in asymptotically flat space-time will be exactly super-Poincare invariant, because the natural variables of holographic space-time for such a system, are the degrees of freedom of massless superparticles. The formalism leads to a non-singular quantum Big Bang cosmology, in which the asymptotic future is required to be a de Sitter space, with cosmological constant (c.c.) determined by cosmological initial conditions. It is also approximately SUSic in the future, with the gravitino mass $K \Lambda^{1/4}$.

 

 

arXive: 24 July 2010

Threshold Clustering Functions and Thermal Inhomogeneities in the Lyman-Alpha Forest

Authors: Khee-Gan Lee, David N. Spergel
http://arxiv.org/abs/1007.3734v1

We introduce to astrophysics the threshold clustering function S2 first derived by Torquato et al. (1988), which effectively samples the flux probability distribution (PDF) of the Ly-alpha forest at different spatial scales. These statistics are tested on mock Ly-alpha forest spectra based on various toy models for He II reionization, with homogeneous models with various temperature-density relations as well as models with temperature inhomogeneities. These mock samples have systematics and noise added to simulate the latest Sloan Digital Sky Survey Data Release 7 (SDSS DR7) data. We find that the flux PDF from SDSS DR7 can be used to constrain the temperature-density relation gamma (where T \propto (1 + Delta)^{gamma -1}) of the intergalactic medium (IGM) at z=2.5 to a precision of Delta(gamma) = 0.2 at ~4-sigma confidence. The flux PDF is degenerate to temperature inhomogeneities in the IGM arising from He II reionization, but we find S2 can detect these inhomogeneities at ~5-sigma, with the assumption that the flux continuum of the Ly-alpha forest can be determined to 9% accuracy, approximately the error from current fitting methods. If the flux continuum can be determined to 3% accuracy, then S2 is capable of constraining the characteristic scale of temperature inhomogeneities, with ~4-sigma differentiation between toy models with hot bubble radii of 50 Mpc/h and 100 Mpc/h.

 

Dynamics of anisotropic f(R) cosmology

Authors: Genly Leon, Emmanuel N. Saridakis

http://arxiv.org/abs/1007.3956v1

We construct general anisotropic cosmological scenarios governed by an f(R) gravitational sector. Focusing then on Kantowski-Sachs geometries in the case of $R^n$-gravity we perform a detailed phase-space analysis. We find that at late times the universe can result to a state of accelerating expansion, and additionally, for a particular n-range (2<n<3) it exhibits phantom behavior. Furthermore, isotropization has been achieved independently of the initial anisotropy degree. Moreover, contracting solutions have also a large probability to be the late-time states of the universe. Finally, we can also obtain the realization of the cosmological bounce and turnaround, as well as of cyclic cosmology. These features indicate that anisotropic geometries in modified gravitational frameworks present radically different cosmological behaviors comparing to the simple isotropic scenarios.

 

The dynamics of generalized Palatini Theories of Gravity

Authors: Vincenzo Vitagliano, Thomas P. Sotiriou, Stefano Liberati

http://arxiv.org/abs/1007.3937v1

It is known that in f(R) theories of gravity with an independent connection which can be both non-metric and non symmetric, this connection can always be algebraically eliminated in favour of the metric and the matter fields, so long as it is not coupled to the matter explicitly. We show here that this is a special characteristic of f(R) actions, and it is not true for actions that include other curvature invariants. This contradicts some recent claims in the literature. We clarify the reasons of this contradiction.

 

 

arXiv: 22 July 2010

Precision Cosmology Defeats Void Models for Acceleration

Authors: Adam Moss, James P. Zibin, Douglas Scott
http://arxiv.org/abs/1007.3725v1

The suggestion that we occupy a privileged position near the centre of a large, nonlinear, and nearly spherical void has recently attracted much attention as an alternative to dark energy. Putting aside the philosophical problems with this scenario, we perform the most complete and up-to-date comparison with cosmological data. We use supernovae and the full cosmic microwave background spectrum as the basis of our analysis. We also include constraints from radial baryonic acoustic oscillations, the local Hubble rate, age, big bang nucleosynthesis, the Compton y-distortion, and for the first time include the local amplitude of matter fluctuations, \sigma_8. These all paint a consistent picture in which voids are in severe tension with the data. In particular, void models predict a very low local Hubble rate, suffer from an "old age problem", and predict much less local structure than is observed.

 

Hydrogen 21-cm Intensity Mapping at redshift 0.8

Authors: Tzu-Ching Chang, Ue-Li Pen, Kevin Bandura, Jeffrey B. Peterson

http://arxiv.org/abs/1007.3709v1

Observations of 21-cm radio emission by neutral hydrogen at redshifts z ~ 0.5 to ~ 2.5 are expected to provide a sensitive probe of cosmic dark energy. This is particularly true around the onset of acceleration at z ~ 1, where traditional optical cosmology becomes very difficult because of the infrared opacity of the atmosphere. Hitherto, 21-cm emission has been detected only to z=0.24. More distant galaxies generally are too faint for individual detections but it is possible to measure the aggregate emission from many unresolved galaxies in the 'cosmic web'. Here we report a three dimensional 21-cm intensity field at z=0.53 to 1.12. We then co-add HI emission from the volumes surrounding about ten thousand galaxies (from the DEEP2 optical galaxy redshift survey. We detect the aggregate 21-cm glow at a significance of ~ 4 sigma.

 

Probing fundamental physics with pulsars

Authors: D.R. Lorimer, M.A. McLaughlin

http://arxiv.org/abs/1007.3545v1

Pulsars provide a wealth of information about General Relativity, the equation of state of superdense matter, relativistic particle acceleration in high magnetic fields, the Galaxy's interstellar medium and magnetic field, stellar and binary evolution, celestial mechanics, planetary physics and even cosmology. The wide variety of physical applications currently being investigated through studies of radio pulsars rely on: (i) finding interesting objects to study via large-scale and targeted surveys; (ii) high-precision timing measurements which exploit their remarkable clock-like stability. We review current surveys and the principles of pulsar timing and highlight progress made in the rotating radio transients, intermittent pulsars, tests of relativity, understanding pulsar evolution, measuring neutron star masses and the pulsar timing array.

 

 

arXiv: 21 July 2010

Dark Matter decay and annihilation in the Local Universe: CLUES from Fermi

Authors: A.J. Cuesta (IAA-CSIC), T.E. Jeltema (UCO/Lick), F. Zandanel (IAA-CSIC), S. Profumo (UCSC), F. Prada (IAA-CSIC), G. Yepes (UAM), A. Klypin (NMSU), Y. Hoffman (HU), S. Gottloeber (AIP), J. Primack (UCSC), M.A. Sanchez-Conde (IAC), C. Pfrommer (CITA)

http://arxiv.org/abs/1007.3469v1

We present all-sky simulated Fermi maps of gamma-rays from dark matter decay and annihilation in the Local Universe. The dark matter distribution is obtained from a constrained cosmological simulation of the neighboring large-scale structure provided by the CLUES project. The dark matter fields of density and density squared are then taken as an input for the Fermi observation simulation tool to predict the gamma-ray photon counts that Fermi would detect in 5 years of all-sky survey for given dark matter models. Signal-to-noise sky maps have also been obtained by adopting the current Galactic and isotropic diffuse background models released by the Fermi collaboration. We point out the possibility for Fermi to detect a dark matter gamma-ray signal in extragalactic structures. In particular, we conclude here that Fermi observations of nearby clusters (e.g. Virgo and Coma) and filaments are expected to give stronger constraints on decaying dark matter compared to previous studies, especially for dark matter decay models fitting the positron excess as measured by PAMELA. This is the first time that dark matter filaments are shown to be promising targets for indirect detection of dark matter. We make the dark matter density and density squared maps available online at

Observable circles-in-the-sky in flat universes

Authors: B. Mota, M.J. Reboucas, R. Tavakol

http://arxiv.org/abs/1007.3466v1

An important, and potentially detectable, signature of a non-trivial topology for the universe is the presence of so called circles-in-the-sky in the cosmic microwave background (CMB). Recent searches, confined to antipodal and nearly antipodal circles, have however failed to detect any. This outcome, coupled with recent theoretical results concerning the detectability of very nearly flat universes, is sufficient to exclude a detectable non-trivial cosmic topology for most observers in the inflationary limit ($0< |\Omega_{tot}-1| \lesssim 10^{-5}$). In a recent paper we have studied the consequences of these searches for circles if the Universe turns out to be exactly flat ($\Omega_{tot} = 1 $) as is often assumed. More specifically, we have derived the maximum angles of deviation possible from antipodicity of pairs of matching circles associated with the shortest closed geodesic for all multiply-connected flat orientable $3$-manifolds. These upper bounds on the deviation from antipodicity demonstrate that in a flat universe for some classes of topology there remains a substantial fraction of observers for whom the deviation from antipodicity of the matching circles is considerably larger than zero, which implies that the searches for circles-in-the-sky undertaken so far are not enough to exclude the possibility of a detectable non-trivial flat topology. Here we briefly review these results and discuss their consequences in the search for circles-in-the-sky in a flat universes.

 

The Cosmic Microwave Background in an Inhomogeneous Universe

Authors: Chris Clarkson, Marco Regis (Cape Town)

http://arxiv.org/abs/1007.3443v1

The dimming of Type Ia supernovae could be the result of Hubble-scale inhomogeneity in the matter and spatial curvature, rather than signaling the presence of a dark energy component. A key challenge for such models is to fit the detailed spectrum of the cosmic microwave background (CMB). We present a detailed discussion of the small-scale CMB in an inhomogeneous universe, focusing on spherically symmetric `void' models. We include the dynamical effects of radiation while analysing the problem, in contrast to previous work which treated it as a homogeneous test field. This is a surprisingly important effect and we reach substantially different conclusions. Models which are open at CMB distances fit the CMB power spectrum without fine tuning; these models also fit the supernovae and local Hubble rate data. Asymptotically flat models may fit the CMB, but require some extra assumptions. We argue that a full treatment of the radiation in these models is necessary if we are to understand the full constraints from the CMB, as well as other observations which rely on it, such as spectral distortions of the black body spectrum, the kinematic Sunyaev-Zeldovich effect or the Baryon Acoustic Oscillations.

 

Determining Habitability: Which exoEarths should we search for life?

Authors: J. Horner, B. W. Jones

http://arxiv.org/abs/1007.3413v1

Within the next few years, the first Earth-mass planets will be discovered around other stars. Some of those worlds will certainly lie within the classical "habitable zone" of their parent stars, and we will quickly move from knowing of no exoEarths to knowing many. For the first time, we will be in a position to carry out a detailed search for the first evidence of life beyond our Solar System. However, such observations will be hugely taxing and time consuming to perform, and it is almost certain that far more potentially habitable worlds will be known than it is possible to study. It is therefore important to catalogue and consider the various effects which make a promising planet more or less suitable for the development of life. In this work, we review the various planetary, dynamical and stellar influences that could influence the habitability of exoEarths. The various influences must be taken in concert when we attempt to decide where to focus our first detailed search for life. While there is no guarantee that any given planet will be inhabited, it is vitally important to ensure that we focus our time and effort on those planets most likely to yield a positive result.

 

 

 

arXiv: 20 July 2010

Testing the Void against Cosmological data: fitting CMB, BAO, SN and H0

Authors: Tirthabir Biswas, Alessio Notari, Wessel Valkenburg

http://arxiv.org/abs/1007.3065v1

In this paper, instead of invoking Dark Energy, we try and fit various cosmological observations with a large Gpc scale under-dense region (Void) which is modeled by a Lemaitre-Tolman-Bondi metric that at large distances becomes a homogeneous FLRW metric. We improve on previous analyses by allowing for nonzero overall curvature, accurately computing the distance to the last-scattering surface and the observed scale of the Baryon Acoustic peaks, and investigating important effects that could arise from having nontrivial Void density profiles. We mainly focus on the WMAP 7-yr data (TT and TE), Supernova data (SDSS SN), Hubble constant measurements (HST) and Baryon Acoustic Oscillation data (SDSS and LRG). We find that the inclusion of a nonzero overall curvature drastically improves the goodness of fit of the Void model, bringing it very close to that of a homogeneous universe containing Dark Energy, while by varying the profile one can increase the value of the local Hubble parameter which has been a challenge for these models. We also try to gauge how well our model can fit the large-scale-structure data, but a comprehensive analysis will require the knowledge of perturbations on LTB metrics. The model is consistent with the CMB dipole if the observer is about 15 Mpc off the centre of the Void. Remarkably, such an off-center position may be able to account for the recent anomalous measurements of a large bulk flow from kSZ data. Finally we provide several analytical approximations in different regimes for the LTB metric, and a numerical module for CosmoMC, thus allowing for a MCMC exploration of the full parameter space.

 

On the role of shear in cosmological averaging

Authors: Maria Mattsson, Teppo Mattsson

http://arxiv.org/abs/1007.2939v1

Using the spherically symmetric inhomogeneous Lemaitre-Tolman-Bondi dust solution, we study how the shear and the backreaction depend on the sharpness of the spatial transition between voids and walls and on the size of the voids. The voids considered here are regions with matter density Omega ~ 0 and expansion rate Ht ~ 1, while the walls are regions with matter density Omega ~ 1 and expansion rate Ht ~ 2/3. The results indicate that both the volume-average shear and the variance of the expansion rate grow proportional to the sharpness of the transition and diverge in the limit of a step function, but, for realistic-sized voids, are virtually independent of the size of the void. However, the backreaction, given by the difference of the variance and the shear, has a finite value in the step-function limit. By comparing the exact result for the backreaction to the case where the shear is neglected by treating the voids and walls as separate Friedmann-Robertson-Walker models, we find that the shear suppresses the backreaction by a factor of (r_0/t_0)^2, the squared ratio of the void size to the horizon size. This exemplifies the importance of using the exact solution for the interface between the regions of different expansion rates and densities. The suppression is justified to hold also for a network of compensated voids, but may not hold if the universe is dominated by uncompensated voids.

 

A New Solution of The Cosmological Constant Problems

Authors: John D. Barrow, Douglas J. Shaw

http://arxiv.org/abs/1007.3086v1

We extend the usual gravitational action principle by promoting the bare cosmological constant (CC) from a parameter to a field which can take many possible values. Variation leads to a new integral constraint equation which determines the classical value of the effective CC that dominates the wave function of the universe. In a Friedmann background cosmology with observed matter and radiation content the expected value of the effective CC, is calculated from measurable quantities to be O(1/t_U^2)~ 10^(-122) (in natural units), as observed, where t_U is the present age of the universe. Any application of our model produces a falsifiable prediction for Lambda in terms of other measurable quantities. This leads to a specific prediction for the observed spatial curvature parameter of Omega_k0 =5.2 10^(-5), which is of the magnitude expected if inhomogeneities have an inflationary origin. This explanation of the CC requires no fine tunings, extra dark energy fields, or Bayesian selection in a multiverse.

 

Phenomenology of $\Lambda$-CDM model: a possibility of accelerating Universe with positive pressure

Authors: Saibal Ray, Maxim Khlopov, Utpal Mukhopadhyay, Partha Pratim Ghosh

http://arxiv.org/abs/0711.0686v2

Among various phenomenological $\Lambda$ models, a time-dependent model $\dot \Lambda\sim H^3$ is selected here to investigate the $\Lambda$-CDM cosmology. Using this model the expressions for the time-dependent equation of state parameter $\omega$ and other physical parameters are derived. It is shown that in $H^3$ model accelerated expansion of the Universe takes place at negative energy density, but with a positive pressure. It has also been possible to obtain the change of sign of the deceleration parameter $q$ during cosmic evolution.

 

 

arXiv: 19 July 2010

Cosmology of a covariant Galileon field

Authors: Antonio De Felice, Shinji Tsujikawa

We study the cosmology of a covariant scalar field respecting a Galilean symmetry in flat space-time. We show the existence of a tracker solution that finally approaches a de Sitter fixed point responsible for cosmic acceleration today. The viable region of model parameters is clarified by deriving conditions under which ghosts and Laplacian instabilities of scalar and tensor perturbations are absent. The field equation of state exhibits a peculiar phantom-like behavior along the tracker, which allows a possibility to observationally distinguish the Galileon gravity from the Lambda-CDM model.

 

Where is everybody? -- Wait a moment ... New approach to the Fermi paradox

Authors: I.Bezsudnov, А.Snarskii

http://arxiv.org/abs/1007.2774v1

The Fermi Paradox is the apparent contradiction between the high probability extraterrestrial civilizations' existence and the lack of contact with such civilizations. In general, solutions to Fermi's paradox come down to either estimation of Drake equation parameters i.e. our guesses about the potential number of extraterrestrial civilizations or simulation of civilizations development in the universe. We consider a new type of cellular automata, that allows to analyze Fermi paradox. We introduce bonus stimulation model (BS-model) of development in cellular space (Universe) of objects (Civilizations). When civilizations get in touch they stimulate development each other, increasing their life time. We discovered nonlinear threshold behaviour of total volume of civilizations in universe and on the basis of our model we built analogue of Drake equation.

 

 

arXiv: 16 July 2010

Filtering out the cosmological constant in the Palatini formalism of modified gravity

Authors: Florian Bauer

http://arxiv.org/abs/1007.2546v1

According to theoretical physics the cosmological constant (CC) is expected to be much larger in magnitude than other energy densities in the universe, which is in stark contrast to the observed Big Bang evolution. We address this old CC problem not by introducing an extremely fine-tuned counterterm, but in the context of modified gravity in the Palatini formalism. In our model the large CC term is filtered out, and it does not prevent a standard cosmological evolution. We discuss the filter effect in the epochs of radiation and matter domination as well as in the asymptotic de Sitter future. The final expansion rate can be much lower than inferred from the large CC without using a fine-tuned counterterm. Finally, we show that the CC filter works also in the Kottler (Schwarzschild-de Sitter) metric describing a black hole environment with a CC compatible to the future de Sitter cosmos.

Reconstructing the interaction term between dark matter and dark energy

Authors: Freddy Cueva, Ulises Nucamendi

http://arxiv.org/abs/1007.2459v1

We apply a parametric reconstruction method to a homogeneous, isotropic and spatially flat Friedmann-Robertson-Walker (FRW) cosmological model filled of a fluid of dark energy (DE) with constant equation of state parameter interacting with dark matter (DM). The reconstruction method is based on expansions of the general interaction term and the relevant cosmological variables in terms of Chebyshev polynomials which form a complete set orthonormal functions. This interaction term describes an exchange of energy flow between the DE and DM within dark sector. To show how the method works we do the reconstruction of the interaction function expanding it in terms of only the first three Chebyshev polynomials and obtain the best estimation for the coefficients of the expansion as well as for the DE equation of the state constant parameter w using the type Ia Supernova SCP Union data set (307 SNe-Ia). The preliminary reconstruction shows that in the best scenario there is an energy transfer from DM to DE which worsen the problem of the cosmic coincidence in comparison with the LCDM model. We conclude that this fact is an indication of a serious drawback for the existence of such interaction between dark components.

 

Observational Constraints on Visser's Cosmological Model

Authors: Marcio E.S. Alves, Fabio C. Carvalho, Jose C.N. de Araujo, Oswaldo D. Miranda, Carlos A. Wuensche, Edivaldo M. Santos

http://arxiv.org/abs/1007.2554v1

Theories of gravity for which gravitons can be treated as massive particles have presently been studied as realistic modifications of General Relativity, and can be tested with cosmological observations. In this work, we study the ability of a recently proposed theory with massive gravitons, the so-called Visser theory, to explain the measurements of luminosity distance from the Union2 compilation, the most recent Type-Ia Supernovae (SNe Ia) dataset, adopting the current ratio of the total density of non-relativistic matter to the critical density ($\Omega_m$) as a free parameter. We also combine the SNe Ia data with constraints from Baryon Acoustic Oscillations (BAO) and CMB measurements. We find that, for the allowed interval of values for $\Omega_m$, a model based on Visser's theory can produce an accelerated expansion period without any dark energy component, but the combined analysis (SNe Ia + BAO + CMB) shows that the model is disfavored when compared with $\Lambda$CDM model.

 

Pressure Support in Galaxy Disks: Impact on Rotation Curves and Dark Matter Density Profiles

Authors: J. J. Dalcanton, A. Stilp

http://arxiv.org/abs/1007.2535v1

Rotation curves constrain a galaxy's underlying mass density profile, under the assumption that the observed rotation produces a centripetal force that exactly balances the inward force of gravity. However, most rotation curves are measured using emission lines from gas, which can experience additional forces due to pressure. In realistic galaxy disks, the gas pressure declines with radius, providing additional radial support to the disk. The measured tangential rotation speed will therefore tend to lag the true circular velocity of a test particle. The gas pressure is dominated by turbulence, and we evaluate its likely amplitude from recent estimates of the gas velocity dispersion and surface density. We show that where the amplitude of the rotation curve is comparable to the characteristic velocities of the interstellar turbulence, pressure support may lead to underestimates of the mass density of the underlying dark matter halo and the inner slope of its density profile. These effects may be significant for galaxies with rotation speeds <75km/s, but are unlikely to be significant in higher mass galaxies. We find that pressure support can be sustained over long timescales, because any reduction in support due to the conversion of gas into stars is compensated for by an inward flow of gas. However, we point to many uncertainties in assessing the importance of pressure support in galaxies. Thus, while pressure support may alleviate possible tensions between rotation curve observations and LambdaCDM on kiloparsec scales, it should not be viewed as a definitive solution at this time.

 

Three-Point Correlation Functions of SDSS Galaxies: Luminosity and Color Dependence in Redshift and Projected Space

Authors: Cameron K. McBride, Andrew J. Connolly, Jeffrey P. Gardner, Ryan Scranton, Jeffrey A. Newman, Roman Scoccimarro, Idit Zehavi, Donald P. Schneider

http://arxiv.org/abs/1007.2414v1

The three-point correlation function (3PCF) provides an important view into the clustering of galaxies that is not available to its lower order cousin, the two-point correlation function (2PCF). Higher order statistics, such as the 3PCF, are necessary to probe the non-Gaussian structure and shape information expected in these distributions. We measure the clustering of spectroscopic galaxies in the Main Galaxy Sample of the Sloan Digital Sky Survey (SDSS), focusing on the shape or configuration dependence of the reduced 3PCF in both redshift and projected space. This work constitutes the largest number of galaxies ever used to investigate the reduced 3PCF, using over 220,000 galaxies in three volume-limited samples. We find significant configuration dependence of the reduced 3PCF at 3-27 Mpc/h, in agreement with LCDM predictions and in disagreement with the hierarchical ansatz. Below 6 Mpc/h, the redshift space reduced 3PCF shows a smaller amplitude and weak configuration dependence in comparison with projected measurements suggesting that redshift distortions, and not galaxy bias, can make the reduced 3PCF appear consistent with the hierarchical ansatz. The reduced 3PCF shows a weaker dependence on luminosity than the 2PCF, with no significant dependence on scales above 9 Mpc/h. On scales less than 9 Mpc/h, the reduced 3PCF appears more affected by galaxy color than luminosty. We demonstrate the extreme sensitivity of the 3PCF to systematic effects such as sky completeness and binning scheme, along with the difficulty of resolving the errors. Some comparable analyses make assumptions that do not consistently account for these effects.

 

 

 

arXiv: 15 July 2010

Constraints on standard and non-standard early Universe models from CMB B-mode polarization

Authors: Yin-Zhe Ma, Wen Zhao, Michael L. Brown

http://arxiv.org/abs/1007.2396v1

We investigate the observational signatures of three models of the early Universe in the $B$-mode polarization of the Cosmic Microwave Background (CMB) radiation. In addition to the standard single field inflationary model, we also consider the constraints obtainable on the loop quantum cosmology model (from Loop Quantum Gravity) and on cosmic strings, expected to be copiously produced during the latter stages of Brane inflation. We first examine the observational features of the three models, and then use current $B$-mode polarization data from the BICEP and QUaD experiments to constrain their parameters. We also examine the detectability of the primordial $B$-mode signal predicted by these models and forecast the parameter constraints achievable with future CMB polarization experiments. We find that: (a) since $B$-mode polarization measurements are mostly unaffected by parameter degeneracies, they provide the cleanest probe of these early Universe models; (b) using the BICEP and QUaD data we obtain the following parameter constraints: $r=0.02^{+0.31}_{-0.26}$ ($1\sigma$ for the tensor-to-scalar ratio in the single field inflationary model); $m < 1.36\times 10^{-8} \text{M}_{\text{pl}}$ and $k_{*} < 2.43 \times 10^{-4} \text{Mpc}^{-1}$ ($1\sigma$ for the mass and scale parameters in the loop quantum cosmology model); and $G\mu < 5.77 \times 10^{-7}$ ($1\sigma$ for the cosmic string tension); (c) although it is not statistically significant, there is weak evidence for both the pre-inflationary bounce of the loop quantum cosmology model and for a non-zero cosmic string tension; (d) future CMB observations (both satellite missions and forthcoming sub-orbital experiments) will provide much more rigorous tests of these early Universe models.

 

The dynamical behavior of $f(T)$ theory

Authors: Puxun Wu, Hongwei Yu

http://arxiv.org/abs/1007.2348v1

Recently, a new model obtained from generalizing teleparallel gravity, named $f(T)$ theory, is proposed to explain the present cosmic accelerating expansion with no need of dark energy. In this paper, we analyze the dynamical property of this theory. For a concrete power law model, we obtain that the dynamical system has a stable de Sitter phase along with an unstable radiation dominated phase and an unstable matter dominated one. We show that the Universe can evolve from a radiation dominated era to a matter dominated one, and finally enter an exponential expansion phase.

 

Limits on decaying dark energy density models from the CMB temperature-redshift relation

Authors: Philippe Jetzer, Denis Puy, Monique Signore, Crescenzo Tortora

http://arxiv.org/abs/1007.2325v1

The nature of the dark energy is still a mystery and several models have been proposed to explain it. Here we consider a phenomenological model for dark energy decay into photons and particles as proposed by Lima (J. Lima, Phys. Rev. D 54, 2571 (1996)). He studied the thermodynamic aspects of decaying dark energy models in particular in the case of a continuous photon creation and/or disruption. Following his approach, we derive a temperature redshift relation for the CMB which depends on the effective equation of state $w_{eff}$ and on the "adiabatic index" $\gamma$. Comparing our relation with the data on the CMB temperature as a function of the redshift obtained from Sunyaev-Zel'dovich observations and at higher redshift from quasar absorption line spectra, we find $w_{eff}=-0.97 \pm 0.034$, adopting for the adiabatic index $\gamma=4/3$, in good agreement with current estimates and still compatible with $w_{eff}=-1$, implying that the dark energy content being constant in time.

 

Fingerprinting Dark Energy II: weak lensing and galaxy clustering tests

Authors: Domenico Sapone, Martin Kunz, Luca Amendola

http://arxiv.org/abs/1007.2188v1

The characterization of dark energy is a central task of cosmology. To go beyond a cosmological constant, we need to introduce at least an equation of state and a sound speed and consider observational tests that involve perturbations. If dark energy is not completely homogeneous on observable scales then the Poisson equation is modified and dark matter clustering is directly affected. One can then search for observational effects of dark energy clustering using dark matter as a probe. In this paper we exploit an analytical approximate solution of the perturbation equations in a general dark energy cosmology to analyze the performance of next-decade large scale surveys in constraining equation of state and sound speed. We find that tomographic weak lensing and galaxy redshift surveys can constrain the sound speed of the dark energy only if the latter is small, of the order of $c_{s}\lesssim0.01$ (in units of $c$). For larger sound speeds the error grows to 100% and more. We conclude that large scale structure observations contain very little information about the perturbations in canonical scalar field models with a sound speed of unity. Nevertheless, they are able to detect the presence of "cold" dark energy, i.e. a dark energy with non-relativistic speed of sound

 

 

arXiv: 14 July 2010

Understanding the shape of the galaxy two-point correlation function at z~1 in the COSMOS field

Authors: S. de la Torre, L. Guzzo, K. Kovac, C. Porciani, U. Abbas, B. Meneux, C.M. Carollo, T. Contini, J.-P. Kneib, O. Le Fevre, S.J. Lilly, V. Mainieri, A. Renzini, D. Sanders, M. Scodeggio, N. Scoville, G. Zamorani, S. Bardelli, M. Bolzonella, A. Bongiorno, K. Caputi, G. Coppa, O. Cucciati, L. de Ravel, P. Franzetti, B. Garilli, A. Iovino, P. Kampczyk, C. Knobel, A.M. Koekemoer, F. Lamareille, J.-F. Le Borgne, V. Le Brun, C. Maier, M. Mignoli, R. Pello, Y. Peng, E. Perez-Montero, E. Ricciardelli, J. Silverman, M. Tanaka, L. Tasca, L. Tresse, D. Vergani, N. Welikala, E. Zucca, D. Bottini, A. Cappi, P. Cassata, A. Cimatti, M. Fumana, O. Ilbert, A. Leauthaud, D. Maccagni, C. Marinoni, H.J. McCracken, P. Memeo, P. Nair, P. Oesch, L. Pozzetti, V. Presotto, R. Scaramella
http://arxiv.org/abs/1007.1984v1

We investigate how the shape of the galaxy two-point correlation function as measured in the zCOSMOS survey depends on local environment, quantified in terms of the density contrast on scales of 5 Mpc/h. We show that the flat shape previously observed at redshifts between z=0.6 and z=1 can be explained by this volume being simply 10% over-abundant in high-density environments, with respect to a Universal density probability distribution function. When galaxies corresponding to the top 10% tail of the distribution are excluded, the measured w_p(r_p) steepens and becomes indistinguishable from LCDM predictions on all scales. This is the same effect recognised by Abbas & Sheth in the SDSS data at z~0 and explained as a natural consequence of halo-environment correlations in a hierarchical scenario. Galaxies living in high-density regions trace dark matter halos with typically higher masses, which are more correlated. If the density probability distribution function of the sample is particularly rich in high-density regions because of the variance introduced by its finite size, this produces a distorted two-point correlation function. We argue that this is the dominant effect responsible for the observed "peculiar" clustering in the COSMOS field.

 

Tidal Imprints of a Dark Sub-Halo on the Outskirts of the Milky Way II. Perturber Azimuth

Authors: Sukanya Chakrabarti, Leo Blitz

http://arxiv.org/abs/1007.1982v1

We extend our analysis of the observed disturbances on the outskirts of the HI disk of the Milky Way. We employ the additional constraints of the phase of the modes of the observed HI image and asymmetry in the radial velocity field to derive the azimuth of the perturber inferred to be responsible for the disturbances in the HI disk. We carry out a modal analysis of the phase of the disturbances in the HI image and in SPH simulations of a Milky Way-like galaxy tidally interacting with dark perturbers, the relative offset of which we utilize to derive the perturber azimuth. To make a direct connection with observations, we express our results in sun-centered coordinates, predicting that the perturber responsible for the observed disturbances is between $-50 \la l \la -10$. We show explicitly that the phase of the disturbances in the outskirts of simulated galaxies at the time that best fits the Fourier amplitudes, our primary metric for the azimuth determination, is relatively insensitive to the equation of state. Our calculations here represent our continuing efforts to develop the "Tidal Analysis" method of Chakrabarti \& Blitz (2009; CB09). CB09 employed SPH simulations to examine tidal interactions between perturbing dark sub-halos and the Milky Way. They found that the amplitudes of the Fourier modes of the observed planar disturbances are best-fit by a perturbing dark sub-halo with mass one-hundredth that of the Milky Way, and a pericentric approach distance of $\sim 5-10~\rm kpc$. The overarching goal of this work is to attempt to outline an alternate procedure to optical studies for characterizing and potentially discovering dwarf galaxies -- whereby one can approximately infer the azimuthal location of a perturber, its mass and pericentric distance (CB09) from analysis of its tidal gravitational imprints on the HI disk of the primary galaxy

 

Constraining Primordial Non-Gaussianity with High-Redshift Probes

Authors: Jun-Qing Xia, Anna Bonaldi, Carlo Baccigalupi, Gianfranco De Zotti, Sabino Matarrese, Licia Verde, Matteo Viel

http://arxiv.org/abs/1007.1969v1

We present an analysis of the constraints on the amplitude of primordial non-Gaussianity of local type described by the dimensionless parameter $f_{\rm NL}$. These constraints are set by the auto-correlation functions (ACFs) of two large scale structure probes, the radio sources from NRAO VLA Sky Survey (NVSS) and the quasar catalogue of Sloan Digital Sky Survey Release Six (SDSS DR6 QSOs), as well as by their cross-correlation functions (CCFs) with the cosmic microwave background (CMB) temperature map (Integrated Sachs-Wolfe effect). Several systematic effects that may affect the observational estimates of the ACFs and of the CCFs are investigated and conservatively accounted for. Our approach exploits the large-scale scale-dependence of the non-Gaussian halo bias. The derived constraints on {$f_{\rm NL}$} coming from the NVSS CCF and from the QSO ACF and CCF are weaker than those previously obtained from the NVSS ACF, but still consistent with them. Finally, we obtain the constraints on $f_{\rm NL}=53\pm25$ ($1\,\sigma$) and $f_{\rm NL}=47\pm21$ ($1\,\sigma$) from NVSS data and SDSS DR6 QSO data, respectively.

 

The cosmic microwave background bispectrum from the non-linear evolution of the cosmological perturbations

Authors: Cyril Pitrou, Jean-Philippe Uzan, Francis Bernardeau

http://arxiv.org/abs/1003.0481v2

This article presents the first computation of the complete bispectrum of the cosmic microwave background temperature anisotropies arising from the evolution of all cosmic fluids up to second order, including neutrinos. Gravitational couplings, electron density fluctuations and the second order Boltzmann equation are fully taken into account. Comparison to limiting cases that appeared previously in the literature are provided. These are regimes for which analytical insights can be given. The final results are expressed in terms of equivalent fNL for different configurations. It is found that for moments up to lmax=2000, the signal generated by non-linear effects is equivalent to fNL~5 for both local-type and equilateral-type primordial non-Gaussianity.