arXiv: 28 Apr 2010

Will Multiple Probes of Dark Energy find Modified Gravity?

Authors: Charles Shapiro, Scott Dodelson, Ben Hoyle, Lado Samushia, Brenna Flaugher
http://arxiv.org/abs/1004.4810v1

One of the most pressing issues in cosmology is whether general relativity (GR) plus a dark sector is the underlying physical theory or whether a modified gravity model is needed. Upcoming dark energy experiments designed to probe dark energy with multiple methods can address this question by comparing the results of the different methods in constraining dark energy parameters. Disagreement would signal the breakdown of the assumed model (GR plus dark energy). We study the power of this consistency test by projecting constraints in the w_0-w_a plane from the four different techniques of the Dark Energy Survey in the event that the underlying true model is modified gravity. We find that the standard technique of looking for overlap has some shortcomings, propose as an alternative the Multi-dimensional Consistency Test, and introduce the methodology for projecting whether a given experiment will be able to distinguish a modified gravity model from GR.

 

 

 

arXiv: 27 Apr 2010

Gamma-Ray Bursts and Dark Energy - Dark Matter interaction

Authors: T. Barreiro, O. Bertolami, P. Torres

arXiv:1004.4562v1

In this work Gamma Ray Burst (GRB) data is used to place constraints on a putative coupling between dark energy and dark matter. Type Ia supernovae (SNe Ia) constraints from the Sloan Digital Sky Survey II (SDSS-II) first-year results, the cosmic microwave background radiation (CMBR) shift parameter from WMAP seven year results and the baryon acoustic oscillation (BAO) peak from the Sloan Digital Sky Survey (SDSS) are also discussed. The prospects for the field are assessed, as more GRB events become available.

Bouncing Palatini cosmologies and their perturbations

Authors: Tomi S. Koivisto

arXiv:1004.4298v1

Nonsingular cosmologies are investigated in the framework of f(R) gravity within the first order formalism. General conditions for bounces in isotropic and homogeneous cosmology are presented. It is shown that only a quadratic curvature correction is needed to predict a bounce in a flat or to describe cyclic evolution in a curved dust-filled universe. Formalism for perturbations in these models is set up. In the simplest cases, the perturbations diverge at the turnover. Conditions to obtain smooth evolution are derived.

 

 

arXiv: 26 Apr 2010

Cross-disciplinary research in astronomy

Authors: Eric D. Feigelson

arXiv:1004.4148v1

In the distant past, astronomy was often intertwined with religion into a unified cosmos. As science became a distinct cultural enterprise, astronomy has witnessed a variety of rich interactions with other fields. Mathematical statistics was stimulated in the 19th century by astronomical problems, and today astrostatistics is a small but growing cross-disciplinary field advancing methodology to address challenges in astronomical data analysis. Throughout the 20th century, astronomy became closely allied with physics such that astronomy and astrophysics are now profoundly intertwined. Physical chemistry played a major role in the identification of molecules in the Milky Way Galaxy, and astrochemistry is now an active subfield giving insights into cosmic molecular processes. The importance of cross-disciplinary interactions with engineering (for instrumentation), Earth sciences (for planetary studies), computer science (for astroinformatics) and life sciences (for astrobiology) is also growing. Cross-disciplinary research has been essential both for crucial discoveries in astronomy and for improving the quality of astronomical research. It should be fostered with increased flexibility in the training of young astronomers and with sufficient funding to nurture these fields.

Planet Migration

Authors: Stephen H. Lubow, Shigeru Ida

arXiv:1004.4137v1

Planet migration is the process by which a planet's orbital radius changes in time. The main agent for causing gas giant planet migration is the gravitational interaction of the young planet with the gaseous disk from which it forms. We describe the migration rates resulting from these interactions based on a simple model for disk properties. These migration rates are higher than is reasonable for planet survival. We discuss some proposed models for which the migration rates are lower. There are major uncertainties in migration rates due to a lack of knowledge about the detailed physical properties of disks. We also describe some additional forms of migration.

 

Neutrinos in Non-linear Structure Formation - The Effect on Halo Properties

Authors: Jacob Brandbyge, Steen Hannestad, Troels Haugboelle, Yvonne Y. Y. Wong

arXiv:1004.4105v1

We use N-body simulations to find the effect of neutrino masses on halo properties, and investigate how the density profiles of both the neutrino and the dark matter components change as a function of the neutrino mass. We compare our neutrino density profiles with results from the N-one-body method and find good agreement. We also show and explain why the Tremaine-Gunn bound for the neutrinos is not saturated. Finally we study how the halo mass function changes as a function of the neutrino mass and compare our results with the Sheth-Tormen semi-analytic formulae. Our results are important for surveys which aim at probing cosmological parameters using clusters, as well as future experiments aiming at measuring the cosmic neutrino background directly.

 

On Dark Matter, Spiral Galaxies, and the Axioms of General Relativity

Authors: Hubert L. Bray

arXiv:1004.4016v1

Beginning with a geometric motivation for dark matter going back to the axioms of general relativity, we show how scalar field dark matter, which naturally forms dark matter density waves due to its wave nature, may cause the observed barred spiral pattern density waves in many disk galaxies and triaxial shapes with plausible brightness profiles in many elliptical galaxies. If correct, this would provide a unified explanation for spirals and bars in spiral galaxies and for the brightness profiles of elliptical galaxies. We compare the results of preliminary computer simulations with photos of actual galaxies.

 

 

arXiv: 23 Apr 2010

Stability of multi-field cosmological solutions in the presence of a fluid
Authors: Jonathan Frazer, Andrew R Liddle

arXiv:1004.3888v1

We explore the stability properties of multi-field solutions in the presence of a perfect fluid, as appropriate to assisted quintessence scenarios. We show that the stability condition for multiple fields $\phi_i$ in identical potentials $V_i$ is simply $d^2V_i/d \phi_i^2 > 0$, exactly as in the absence of a fluid. A possible new instability associated with the fluid is shown not to arise in situations of cosmological interest.

Dark Matter in Modern Cosmology
Authors: S. Colafrancesco

The presence of Dark Matter (DM) is required in the universe regulated by the standard general relativistic theory of gravitation. The nature of DM is however still elusive to any experimental search. We discuss here the process of accumulation of evidence for the presence of DM in the universe, the astrophysical probes for the leading DM scenarios that can be obtained through a multi-frequency analysis of cosmic structures on large scales, and the strategies related to the multi-messenger and multi-experiment astrophysical search for the nature of the DM.

Missing Power vs low-l Alignments in the Cosmic Microwave Background: No Correlation in the Standard Cosmological Model
Authors: Devdeep Sarkar (Michigan), Dragan Huterer (Michigan), Craig J. Copi (CWRU), Glenn D. Starkman (CWRU), Dominik J. Schwarz (Bielefeld)

arXiv:1004.3784v1

On large angular scales (greater than about 60 degrees), the two-point angular correlation function of the temperature of the cosmic microwave background (CMB), as measured (outside of the plane of the Galaxy) by the Wilkinson Microwave Anisotropy Probe, shows significantly lower large-angle correlations than expected from the standard inflationary cosmological model. Furthermore, when derived from the full CMB sky, the two lowest cosmologically interesting multipoles, the quadrupole (l=2) and the octopole (l=3), are unexpectedly aligned with each other. Using randomly generated full-sky and cut-sky maps, we investigate whether these anomalies are correlated at a statistically significant level. We conclusively demonstrate that, assuming Gaussian random and statistically isotropic CMB anisotropies, there is no statistically significant correlation between the missing power on large angular scales in the CMB and the alignment of the l=2 and l=3 multipoles. The chance to measure the sky with both such a lack of large-angle correlation and such an alignment of the low multipoles is thus quantified to be below 10^{-6}.

Magnification as a Probe of Dark Matter Halos at high redshift
Authors: Ludovic Van Waerbeke, Hendrik Hildebrandt, Jes Ford, Martha Milkeraitis

arXiv:1004.3793v1
We propose a new approach for measuring the mass profile and shape of groups and clusters of galaxies, which uses lensing magnification of distant background galaxies. The main advantage of lensing magnification is that, unlike lensing shear, it relies on accurate photometric redshifts only and not galaxy shapes, thus enabling the study of the dark matter distribution with unresolved source galaxies. We present a feasibility study, using a real population of z > 2.5 Lyman Break Galaxies as source galaxies, and where, similar to galaxy-galaxy lensing, foreground lenses are stacked in order to increase the signal-to-noise. We find that there is an interesting new observational window for gravitational lensing as a probe of dark matter halos at high redshift, which does not require measurement of galaxy shapes.

--
********************************
Shant Baghram
Ph.D. Student
Cosmology group
Department of Physics
Sharif University of Technology
Tehran, Iran
E-mail: baghram@sharif.edu / shant.baghramian@gmail.com
Homepage:http://physics.sharif.edu/~baghram/
*************************************

arXiv: 22 Apr 2010

Systematic Effects on Determination of the Growth Factor from Redshift-space Distortions
Authors: Teppei Okumura, Y. P. Jing
arXiv:1004.3548v1
The linear growth factor of density perturbations is believed to be a powerful observable of future redshift surveys to probe physical properties of dark energy and to distinguish among various gravity theories. We investigate systematic effects on determination of the growth factor f from a measurement of redshift-space distortions. Using N-body simulations, we identify dark matter halos over a broad mass range. We compute the power spectra and correlation functions for the halos and then examine how well the redshift distortion parameter beta=f/b can be reconstructed as a function of halo mass, where b is the bias parameter. We find that beta measured for a fixed halo mass is generally a function of scale even on large scales both in Fourier and in configuration space, in contrast with the common expectation that beta approaches a constant described by Kaiser's formula on such scales. The scale dependence depends on the halo mass, being stronger for smaller halos. It also cannot be easily explained with the well-known distribution function of the halo peculiar velocities. Only for massive halos with b>1.5, beta approaches the linear theory prediction on scales of r or pi/k>30h^{-1}Mpc. Luminous red galaxies (LRG), targeted by the SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS), tend to reside in very massive halos. Our results indicate that if the central LRG sample is used for the measurement of redshift distortions, fortunately f can be measured unbiasedly. On the other hand, if one considers to use emission line galaxies, which are targeted by the BigBOSS survey and inhabited in halos of a broad mass range, the scale dependence of beta must be taken into account carefully; otherwise one might give incorrect constraints on dark energy or modified gravity theories. We also find that beta reconstructed in Fourier space behaves fairly better than that in configuration space.


Shear Power Spectrum Reconstruction using Pseudo-Spectrum Method
Authors: Chiaki Hikage, Masahiro Takada, Takashi Hamana, David Spergel
arXiv:1004.3542v1
This paper develops a pseudo power spectrum technique for measuring the lensing power spectrum from weak lensing surveys in both the full sky and flat sky limits. The power spectrum approaches have a number of advantages over the traditional correlation function approach. We test the pseudo spectrum method by using numerical simulations with square-shape boundary that include masked regions with complex configuration due to bright stars and saturated spikes. Even when 25% of total area of the survey is masked, the method recovers the E-mode power spectrum at a sub-percent precision over a wide range of multipoles 100


Dark energy from modified gravity with Lagrange multipliers
Authors: Salvatore Capozziello, Jiro Matsumoto, Shin'ichi Nojiri, Sergei D. Odintsov
arXiv:1004.3691v1
We study scalar-tensor theory, k-essence and modified gravity with Lagrange multiplier constraint which role is to reduce the number of degrees of freedom. Dark Energy cosmology of different types ($\Lambda$CDM, unified inflation with DE, smooth non-phantom/phantom transition epoch) is reconstructed in such models. It is shown that mathematical equivalence between scalar theory and $F(R)$ gravity is broken due to presence of constraint. The cosmological dynamics of $F(R)$ gravity is modified by the second $F_2(R)$ function dictated by the constraint. Dark Energy cosmology is defined by this function while standard $F_1(R)$ function is relevant for local tests (modification of newton regime). A general discussion on the role of Lagrange multipliers to make higher-derivative gravity canonical is developed.

arXiv: 21 Apr 2010

Structure formation in $f(R)$ gravity: A distinguishing probe between the dark energy and modified gravity
Authors: Shant Baghram, Sohrab Rahvar
arXiv:1004.3360v1
In this work, we study the large scale structure formation in the modified gravity in the framework of Palatini formalism and compare the results with the smooth dark energy models as a tool to distinguish between these models. Through the inverse method, we reconstruct the dynamics of universe, modified gravity action and the structure formation indicators like the screened mass function and gravitational slip parameter. Consequently, we extract the matter density power spectrum for these two models and show that the modified gravity and dark energy models predictions are slightly different from each other at large scales. It is also shown that the growth index in the modified gravity unlike to the dark energy models is a scale dependent parameter. The modification on the structure formation can change the CMB spectrum at large scales where due to the cosmic variance it is hard to detect this signature. We show that a large number of SNIa data in the order of 2000 will enable us to reconstruct the modified gravity action with suitable confidence level and test the cosmic acceleration models by the formation of the structures.


Cosmological Tests of Gravity
Authors: Bhuvnesh Jain, Justin Khoury (U. Penn)
arXiv:1004.3294v1
Modifications of general relativity provide an alternative explanation to dark energy for the observed acceleration of the universe. We review recent developments in modified gravity theories, focusing on higher dimensional approaches and chameleon/f(R) theories. We classify these models in terms of the screening mechanisms that enable such theories to approach general relativity on small scales (and thus satisfy solar system constraints). We describe general features of the modified Friedman equation in such theories.
The second half of this review describes experimental tests of gravity in light of the new theoretical approaches. We summarize the high precision tests of gravity on laboratory and solar system scales. We describe in some detail tests on astrophysical scales ranging from ~kpc (galaxy scales) to ~Gpc (large-scale structure). These tests rely on the growth and inter-relationship of perturbations in the metric potentials, density and velocity fields which can be measured using gravitational lensing, galaxy cluster abundances, galaxy clustering and the Integrated Sachs-Wolfe effect. A robust way to interpret observations is by constraining effective parameters, such as the ratio of the two metric potentials. Currently tests of gravity on astrophysical scales are in the early stages --- we summarize these tests and discuss the interesting prospects for new tests in the coming decade.


Fitting formulae of the reduced-shear power spectrum for weak lensing
Authors: Martin Kilbinger (Excellence Cluster Universe, USM, ShNU)
arXiv:1004.3493v1
Context. Weak gravitational lensing is a powerful probe of large-scale structure and cosmology. Most commonly, second-order correlations of observed galaxy ellipticities are expressed as a projection of the matter power spectrum, corresponding to the lowest-order approximation between the projected and 3d power spectrum.
Aims. The dominant lensing-only contribution beyond the zero-order approximation is the reduced shear, which takes into account not only lensing-induced distortions but also isotropic magnification of galaxy images. This involves an integral over the matter bispectrum. We provide a fast and general way to calculate this correction term.
Methods. Using a model for the matter bispectrum, we fit elementary functions to the reduced-shear contribution and its derivatives with respect to cosmological parameters. The dependence on cosmology is encompassed in a Taylor-expansion around a fiducial model.
Results. Within a region in parameter space comprising the WMAP7 68% error ellipsoid, the total reduced-shear power spectrum (shear plus fitted reduced-shear correction) is accurate to 1% (2%) for l<10^4>


Mass Function of Rich Galaxy Clusters and Its Constraint on $\sigma_8$
Authors: Z. L. Wen, J. L. Han, F. S. Liu
arXiv:1004.3337v1
The mass function of galaxy clusters is a powerful tool to constrain cosmological parameters, e.g., the mass fluctuation on the scale of 8 $h^{-1}$ Mpc, $\sigma_8$, and the abundance of total matter, $\Omega_m$. We first determine the scaling relations between cluster mass and cluster richness, summed $r$-band luminosity and the global galaxy number within a cluster radius. These relations are then used to two complete volume-limited rich cluster samples which we obtained from the Sloan Digital Sky Survey (SDSS). We estimate the masses of these clusters and determine the cluster mass function. Fitting the data with a theoretical expression, we get the cosmological parameter constraints in the form of $\sigma_8(\Omega_m/0.3)^{\alpha}=\beta$ and find out the parameters of $\alpha=$0.40--0.50 and $\beta=$0.8--0.9, so that $\sigma_8=$0.8--0.9 if $\Omega_m=0.3$. Our $\sigma_8$ value is slightly higher than recent estimates from the mass function of X-ray clusters and the Wilkinson Microwave Anisotropy Probe (WMAP) data, but consistent with the weak lensing statistics.




Cosmological constraints on generalized Chaplygin gas model: Markov Chain Monte Carlo approach
Authors: Lixin Xu, Jianbo Lu
arXiv:1004.3344v1
We use the Markov Chain Monte Carlo method to investigate a global constraints on the generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy from the latest observational data: the Constitution dataset of type supernovae Ia (SNIa), the observational Hubble data (OHD), the cluster X-ray gas mass fraction, the baryon acoustic oscillation (BAO), and the cosmic microwave background (CMB) data. In a non-flat universe, the constraint results for GCG model are, $\Omega_{b}h^{2}=0.0235^{+0.0021}_{-0.0018}$ ($1\sigma$) $^{+0.0028}_{-0.0022}$ $(2\sigma)$, $\Omega_{k}=0.0035^{+0.0172}_{-0.0182}$ ($1\sigma$) $^{+0.0226}_{-0.0204}$ $(2\sigma)$, $A_{s}=0.753^{+0.037}_{-0.035}$ ($1\sigma$) $^{+0.045}_{-0.044}$ $(2\sigma)$, $\alpha=0.043^{+0.102}_{-0.106}$ ($1\sigma$) $^{+0.134}_{-0.117}$ $(2\sigma)$, and $H_{0}=70.00^{+3.25}_{-2.92}$ ($1\sigma$) $^{+3.77}_{-3.67}$ $(2\sigma)$, which is more stringent than the previous results for constraint on GCG model parameters. Furthermore, according to the information criterion, it seems that the current observations much support $\Lambda$CDM model relative to the GCG model.


Reconstructing Dark Energy Potentials From Parameterized Deceleration Parameters
Authors: Yuting Wang, Lixin Xu, Jianbo Lu, Yuan-Xing Gui
arXiv:1004.3370v1
In this paper, the properties of dark energy are investigated according to the parameterized deceleration parameter $q(z)$, which is used to describe the extent of the accelerating expansion of the universe. The potential of dark energy $V(\phi)$ and the cosmological parameters, such as the dimensionless energy density $\Omega_{\phi}$, $\Omega_{m}$, and the state parameter $w_\phi$, are connected to it. Concretely, by giving two kinds of parameterized deceleration parameters $q(z)=a+\frac{bz}{1+z}$ and $q(z)=1/2+\frac{az+b}{(1+z)^2}$, the evolution of these parameters and the reconstructed potentials $V(\phi)$ are plotted and analyzed. It's found that the potentials run away with the evolution of universe.

arXiv: 20 Apr 2010

Constraint on the cosmological f(R) model from the multipole power spectrum of the SDSS luminous red galaxy sample and prospects for a future redshift survey
Authors: Kazuhiro Yamamoto (1), Gen Nakamura (1), Gert Huetsi (2), Tatsuya Narikawa (1), Takahiro Sato (1) ((1) Hiroshima U. (2)Tartu Observ.)
http://arxiv.org/abs/1004.3231v1
A constraint on the viable f(R) model is investigated by confronting theoretical predictions with the multipole power spectrum of the luminous red galaxy sample of the Sloan Digital Sky survey data release 7. We obtain a constraint on the Compton wavelength parameter of the f(R) model on the scales of cosmological large-scale structure. A prospect of constraining the Compton wavelength parameter with a future redshift survey is also investigated. The usefulness of the redshift-space distortion for testing the gravity theory on cosmological scales is demonstrated.

Constraints on growth index parameters from current and future observations
Authors: Jason Dossett, Mustapha Ishak, Jacob Moldenhauer, Yungui Gong, Anzhong Wang
arXiv:1004.3086v1
We use current and future simulated data of the growth rate of large scale structure in combination with data from supernova, BAO, and CMB surface measurements, in order to put constraints on the growth index parameters. We use a recently proposed parameterization of the growth index that interpolates between a constant value at high redshifts and a form that accounts for redshift dependencies at small redshifts. We also suggest here another exponential parameterization with a similar behaviour. The redshift dependent parametrizations provide a sub-percent precision level to the numerical growth function, for the full redshift range. Using these redshift parameterizations or a constant growth index, we find that current available data from galaxy redshift distortions and Lyman-alpha forests is unable to put significant constraints on any of the growth parameters. For example both $\Lambda$CDM and flat DGP are allowed by current growth data. We use an MCMC analysis to study constraints from future growth data, and simulate pessimistic and moderate scenarios for the uncertainties. In both scenarios, the redshift parameterizations discussed are able to provide significant constraints and rule out models when incorrectly assumed in the analysis. The values taken by the constant part of the parameterizations as well as the redshift slopes are all found to significantly rule out an incorrect background. We also find that, for our pessimistic scenario, an assumed constant growth index over the full redshift range is unable to rule out incorrect models in all cases. This is due to the fact that the slope acts as a second discriminator at smaller redshifts and therefore provide a significant test to identify the underlying gravity theory.


Boosting hierarchical structure formation with self-interacting dark matter
Authors: Wojciech A. Hellwing, Steffen R. Knollmann, Alexander Knebe
arXiv:1004.2929v1
We investigate the effect of long-range scalar interactions in dark matter (DM) models of cosmic structure formation with a particular focus on the formation times of haloes. Utilising $N$-body simulations with $512^3$ DM particles we show that in our models dark matter haloes form substantially earlier: tracing objects up to redshift $z\sim6$ we find that the formation time, as characterised by the redshift $z_{1/2}$ at which the halo has assembled half of its final mass, is gradually shifted from $z_{1/2}\approx 1.83$ in the fiducial \lcdm\ model to $z_{1/2}\approx 2.54$ in the most extreme self-interaction model. This is accompanied by a shift of the redshift that marks the transition between merger and steady accretion epochs from $z_{*}\approx 4.32$ in the \lcdm\ halos to $z_{*}\approx 6.39$ in our strongest interaction model. In other words, the self-interacting model employed in this work produces more structures at high redshifts, prolonging at the same time the steady accretion phases. These effects taken together can help the \lcdm\ model to account for a high redshift reionisation as indicated by the WMAP data and can alleviate issues related to the survival of the thin-disk dominated galaxies at low redshifts.


Lectures on LQG/LQC
Authors: Ghanashyam Date
arXiv:1004.2952v1
A School on Loop Quantum Gravity was held at the IMSc during Sept 8 -- 18, 2009. In the first week a basic introduction to LQG was provided while in the second week the focus was on the two main application, to cosmology (LQC) and to the black hole entropy. These notes are an expanded written account of the lectures that I gave. These are primarily meant for beginning researchers.

arXiv: 19 Apr 2010

Microlensing with advanced contour integration algorithm: Green's theorem to third order, error control, optimal sampling and limb darkening
Authors: V. Bozza

arXiv:1004.2796v1

Microlensing light curves are typically computed either by ray-shooting maps or by contour integration via Green's theorem. We present an improved version of the second method that includes a parabolic correction in Green's line integral. In addition, we present an accurate analytical estimate of the residual errors, which allows the implementation of an optimal strategy for the contour sampling. Finally, we give a prescription for dealing with limb-darkened sources reaching arbitrary accuracy. These optimizations lead to a substantial speed-up of contour integration codes along with a full mastery of the errors.



The stellar mass fraction and baryon content of galaxy clusters and groups
Authors: S. Andreon (INAF-OABrera)

arXiv:1004.2708v1

Abridged] The analysis of a sample of 52 clusters with precise and hypothesis-parsimonious measurements of mass shows that low mass clusters and groups are not simple scaled-down versions of their massive cousins in terms of stellar content: lighter clusters have more stars per unit cluster mass. The same analysis also shows that the stellar content of clusters and groups displays an intrinsic spread at a given cluster mass, i.e. clusters are not similar each other in the amount of stars they contain, not even at a fixed cluster mass. The stellar mass fraction depends on halo mass with (logarithmic) slope -0.55+/-0.08 and with 0.15+/-0.02 dex of intrinsic scatter at a fixed cluster mass. The intrinsic scatter at a fixed cluster mass we determine for gas mass fractions is smaller, 0.06+/-0.01 dex. The intrinsic scatter in both the stellar and gas mass fractions is a distinctive signature that the regions from which clusters and groups collected matter, a few tens of Mpc, are yet not representative, in terms of gas and baryon content, of the mean matter content of the Universe. The observed stellar mass fraction values are in marked disagreement with gasdynamics simulations with cooling and star formation of clusters and groups. We found the the baryon (gas+stellar) fraction is fairly constant for clusters and groups with 13.7



Constraints on Dark Energy Parameters from Correlations of CMB with LSS
Authors: Hong Li, Jun-Qing Xia

arXiv:1004.2774v1

In this paper, we combine the the latest observational data, including the WMAP five-year data (WMAP5), the baryon acoustic oscillations (BAO) and type Ia supernovae (SN) "union" compilation, and use the Markov Chain Monte Carlo method to determine the dark energy parameters. We pay particular attention to the Integrated Sache-Wolfe (ISW) data from the cross-correlations of cosmic microwave background (CMB) and large scale structure (LSS). In the \Lambda CDM model, we find that the ISW data, as a complement to the WMAP data, could significantly improve the constraint of curvature \Omega_k. We also check the improvement of constraints from the new prior on the Hubble constant and find this new prior could improve the constraint of \Omega_k by a factor of 2. Finally, we study the dynamical evolving EoS of dark energy from the current observational data. Based on the dynamical dark energy model, parameterizing as w(a)=w_0+w_a(1-a), we find that the \Lambda CDM model remains a good fit to the current data. When taking into account the ISW data, the error bars of w_0 and w_a could be shrunk slightly. Current constraints on the dynamical dark energy model are not conclusive. The future precision measurements are needed.



Mergers in Lambda-CDM: Uncertainties in Theoretical Predictions and Interpretations of the Merger Rate
Authors: Philip F. Hopkins, Darren Croton, Kevin Bundy, Sadegh Khochfar, Frank van den Bosch, Rachel S. Somerville, Andrew Wetzel, Dusan Keres, Lars Hernquist, Kyle Stewart, Joshua D. Younger, Shy Genel, Chung-Pei Ma

arXiv:1004.2708v1

Different methodologies lead to order-of-magnitude variations in predicted galaxy merger rates. We examine and quantify the dominant uncertainties. Different halo merger rates and subhalo 'destruction' rates agree to within a factor ~2 given proper care in definitions. If however (sub)halo masses are not appropriately defined or are under-resolved, the major merger rate can be dramatically suppressed. The dominant differences in galaxy merger rates owe to baryonic physics. Hydrodynamic simulations without feedback and older models that do not agree with the observed galaxy mass function propagate factor ~5 bias in the resulting merger rates. However, if the model matches the galaxy mass function, properties of central galaxies are sufficiently converged to give small differences in merger rates. But variations in baryonic physics of satellites have the most dramatic effect. The known problem of satellite 'over-quenching' in most semi-analytic models (SAMs), whereby SAM satellites are too efficiently stripped of gas, leads to order-of-magnitude under-estimates of the merger rate for low-mass/gas-rich/high-redshift galaxies. Fixing the satellite properties to observations avoids this and predicts higher merger rates, with residual factor ~2 uncertainties. Choice of mass ratio definition matters: at low masses, most true major mergers (in baryonic/dynamical galaxy mass) will appear to be minor mergers in their stellar or luminosity mass ratio. Observations and models using these criteria may underestimate major merger rates by factors ~5. Orbital parameters and gas fractions also introduce factor ~3 differences in amount of bulge formed by mergers, even for fixed mass ratio encounters.



Galileon gravity and its relevance to late time cosmic acceleration
Authors: Radouane Gannouji, M. Sami

We consider the covariant galileon gravity taking into account the third order and fourth order scalar field Lagrangians L_3(\pi) and L_4(\pi) consisting of three and four $\pi$'s with four and five derivatives acting on them respectively. The background dynamical equations are set up for the system under consideration and the stability of the self accelerating solution is demonstrated in general setting. We extended this study to the general case of the fifth order theory. For spherically symmetric static background, we spell out conditions for suppression of fifth force effects mediated by the galileon field $\pi$. We study the field perturbations in the fixed background and investigate conditions for their causal propagation. We also briefly discuss metric fluctuations and derive evolution equation for matter perturbations in galileon gravity.

arXiv: 16 Apr 2010

Disformal Scalar Fields and the Dark Sector of the Universe
Authors: M. Zumalacarregui, T. S. Koivisto, D. F. Mota, P. Ruiz-Lapuente
arXiv:1004.2684v1
Disformal transformations have proven to be very useful to devise models of the dark sector. In the present paper we apply such transformation to a single scalar field theory as a way to drive the field into a slow roll phase. The canonical scalar field Lagrangian, when coupled to a disformal metric, turns out to have relations to bimetric dark matter theories and to describe many specific dark energy models at various limits, thus providing a surprisingly simple parametrisation of a wide variety of models including tachyon, Chaplygin gas, K-essence and dilatonic ghost condensate. We investigate the evolution of the background and linear perturbations in disformal quintessence in order to perform a full comparison of the predictions with the cosmological data. The dynamics of the expansion, in particular the mechanism of the transition to accelerating phase, is described in detail. We then study the effects of disformal quintessence on cosmic microwave background (CMB) anisotropies and large scale structures (LSS). A likelihood analysis using the latest data on wide-ranging SNIa, CMB and LSS observations is performed allowing variations in six cosmological parameters and the two parameters specifying the model. We find that while a large region of parameter space remains compatible with observations, models featuring either too much early dark energy or too slow transition to acceleration are ruled out.

Cosmological constraints from Radial Baryon Acoustic Oscillation measurements and Observational Hubble data
Authors: Zhong-Xu Zhai, Hao-Yi Wan, Tong-Jie Zhang
arXiv:1004.2599v1
We use the Radial Baryon Acoustic Oscillation (RBAO) measurements, distant type Ia supernovae (SNe Ia), the observational $H(z)$ data (OHD) and the Cosmic Microwave Background (CMB) shift parameter data to constrain cosmological parameters of $\Lambda$CDM and XCDM cosmologies and further examine the role of OHD and SNe Ia data in cosmological constraints. We marginalize the likelihood function over $h$ by integrating the probability density $P\propto e^{-\chi^{2}/2}$ to obtain the best fitting results and the confidence regions in the $\Omega_{m}-\Omega_{\Lambda}$ plane.With the combination analysis for both of the {\rm $\Lambda$}CDM and XCDM models, we find that the confidence regions of 68.3%, 95.4% and 99.7% levels using OHD+RBAO+CMB data are in good agreement with that of SNe Ia+RBAO+CMB data which is consistent with the result of Lin et al's work. With more data of OHD, we can probably constrain the cosmological parameters using OHD data instead of SNe Ia data in the future.

Comment on "21-cm Radiation: A New Probe of Variation in the Fine-Structure Constant'
Authors: V.V. Flambaum, S.G. Porsev
arXiv:1004.2540v1
Khatri and Wandelt reported that change in the value of alpha by 1% changes the mean brightness temperature $T_b$ decrement of the CMB due to 21 cm absorption by 5% over the redshift range z $<$ 50. A drawback of their approach is that the dimensionful parameters are used. Changing of units leads to the change of the magnitude and even sign of the effect. Similar problems may be identified in a large number of other publications which consider limits on the variation of alpha using dimentionful parameters. We propose a method to obtain consistent results and provide an estimate of the effect.

Bayesian approach to the semi-analytic model of galaxy formation: methodology
Authors: Yu Lu, H.J. Mo, Martin D. Weinberg, Neal S. Katz (UMass, Amherst)
arXiv:1004.2518v1
We believe that a wide range of physical processes conspire to shape the observed galaxy population but we remain unsure of their detailed interactions. The semi-analytic model (SAM) of galaxy formation uses multi-dimensional parameterizations of the physical processes of galaxy formation and provides a tool to constrain these underlying physical interactions. Because of the high dimensionality, the parametric problem of galaxy formation may be profitably tackled with a Bayesian-inference based approach, which allows one to constrain theory with data in a statistically rigorous way. In this paper, we develop a generalized SAM using the framework of Bayesian inference. We show that, with a parallel implementation of an advanced Markov-Chain Monte-Carlo algorithm, it is now possible to rigorously sample the posterior distribution of the high-dimensional parameter space of typical SAMs. As an example, we characterize galaxy formation in the current $\Lambda$CDM cosmology using stellar mass function of galaxies as observational constraints. We find that the posterior probability distribution is both topologically complex and degenerate in some important model parameters. It is common practice to reduce the SAM dimensionality by fixing various parameters. However, this can lead to biased inferences and to incorrect interpretations of data owing to this parameter covariance. This suggests that some conclusions obtained from early SAMs may not be reliable. Using synthetic data to mimic systematic errors in the stellar mass function, we demonstrate that an accurate observational error model is essential to meaningful inference.

Gödel-type universes in Palatini f(R) gravity
Authors: J. Santos, M.J. Reboucas, T.B.R.F. Oliveira
arXiv:1004.2501v1
We examine the question as to whether the Palatini f(R) gravity theories permit space-times in which the causality is violated. We show that every perfect-fluid G\"{o}del-type solution of Palatini f(R) gravity with density $\rho$ and pressure $p$ that satisfy the weak energy condition $\rho+p \geq 0$ is necessarily isometric to the G\"odel geometry, demonstrating therefore that these theories present causal anomalies in the form of closed time-like curves. This result extends a theorem on G\"{o}del-type models to the framework of Palatini f(R) gravity theory. We concretely examine the G\"odel-type perfect-fluid solutions in specific $f(R) = R - \alpha/R^{n}$ Palatini gravity theory, where the free parameters $\alpha$ and $n$ have been recently constrained by a diverse set of observational data. We show that for positive matter density and for $\alpha$ and $n$ within the interval permitted by the observational data, this theory does not admit the G\"odel geometry as a perfect-fluid solution of its field equations. In this sense, this theory remedies the causal pathology in the form of closed time-like curves which is allowed in general relativity. We derive an expression for a critical radius $r_c$ (beyond which the causality is violated) for an arbitrary Palatini f(R) theory, thus making apparent that the violation of causality depends on the form of f(R) and on the matter content components. We also examine the violation of causality of G\"odel-type by considering a single scalar field as the matter content. For this source we show that Palatini f(R) gravity gives rise to a unique G\"odel-type solution with no violation of causality.

Cosmological Non-Linearities as an Effective Fluid
Authors: Daniel Baumann, Alberto Nicolis, Leonardo Senatore, Matias Zaldarriaga
arXiv:1004.2488v1
The universe is smooth on large scales but very inhomogeneous on small scales. Why is the spacetime on large scales modeled to a good approximation by the Friedmann equations? Are we sure that small-scale non-linearities do not induce a large backreaction? Related to this, what is the effective theory that describes the universe on large scales? In this paper we make progress in addressing these questions. We show that the effective theory for the long-wavelength universe behaves as a viscous fluid coupled to gravity: integrating out short-wavelength perturbations renormalizes the homogeneous background and introduces dissipative dynamics into the evolution of long-wavelength perturbations. The effective fluid has small perturbations and is characterized by a few parameters like an equation of state, a sound speed and a viscosity parameter. These parameters can be matched to numerical simulations or fitted from observations. We find that the backreaction of small-scale non-linearities is very small, being suppressed by the large hierarchy between the scale of non-linearities and the horizon scale. The effective pressure of the fluid is always positive and much too small to significantly affect the background evolution. Moreover, we prove that virialized scales decouple completely from the large-scale dynamics, at all orders in the post-Newtonian expansion. We propose that our effective theory be used to formulate a well-defined and controlled alternative to conventional perturbation theory, and we discuss possible observational applications. Finally, our way of reformulating results in second-order perturbation theory in terms of a long-wavelength effective fluid provides the opportunity to understand non-linear effects in a simple and physically intuitive way.

arXiv: 15 April 2010

Supernova, baryon acoustic oscillations, and CMB surface distance constraints on f(G) higher order gravity models
Authors: Jacob Moldenhauer, Mustapha Ishak, John Thompson, Damien A. Easson
arXiv:1004.2459v1
We consider recently proposed higher order gravity models where the action is built from the Einstein-Hilbert action plus a function f(G) of the Gauss-Bonnet invariant. The models were previously shown to pass physical acceptability conditions as well as solar system tests. In this paper, we compare the models to combined data sets of supernovae, baryon acoustic oscillations, and constraints from the CMB surface of last scattering. We find that the models provide fits to the data that are close to those of the LCDM concordance model. The results provide a pool of higher order gravity models that pass these tests and need to be compared to constraints from large scale structure and full CMB analysis.

Limits on the parameters of the equation of state for interacting dark energy
Authors: German Izquierdo, Diego Pavon
arXiv:1004.2360v1
Under the assumption that cold dark matter and dark energy interact with each other through a small coupling term, $Q$, we constrain the parameter space of the equation of state $w$ of those dark energy fields whose variation of the field since last scattering do not exceed Planck's mass. We use three parameterizations of $w$ and two different expressions for $Q$. Our work extends previous ones.

Future CMB cosmological constraints in a dark coupled universe
Authors: Matteo Martinelli, Laura Lopez Honorez, Alessandro Melchiorri, Olga Mena
arXiv:1004.2410v1
Cosmic Microwave Background satellite missions as the on-going Planck experiment are expected to provide the strongest constraints on a wide set of cosmological parameters. Those constraints, however, could be weakened when the assumption of a cosmological constant as the dark energy component is removed. Here we show that it will indeed be the case when there exists a coupling among the dark energy and the dark matter fluids. In particular, the expected errors on key parameters as the cold dark matter density and the angular diameter distance at decoupling are significantly larger when a dark coupling is introduced. We show that it will be the case also for future satellite missions as EPIC, unless CMB lensing extraction is performed.

arXiv: 14 Apr. 2010

Regarding the Line-of-Sight Baryonic Acoustic Feature in the Sloan Digital Sky Survey and Baryon Oscillation Spectroscopic Survey Luminous Red Galaxy Samples
Authors: Eyal A. Kazin, Michael R. Blanton, Roman Scoccimarro, Cameron K. McBride, Andreas A. Berlind
arXiv:1004.2244v1
We analyze the line-of-sight baryonic acoustic feature in the two-point correlation function {\xi} of the Sloan Digital Sky Survey (SDSS) luminous red galaxy (LRG) sample (0.16 <>

A more general interacting model of holographic dark energy
Authors: Fei Yu, Jingfei Zhang, Jianbo Lu, Wei Wang, Yuanxing Gui
arXiv:1004.2092v1
So far, there have been no theories or observational data that deny the presence of interaction between dark energy and dark matter. We extend naturally the holographic dark energy (HDE) model, proposed by Granda and Oliveros, in which the dark energy density includes not only the square of the Hubble scale, but also the time derivative of the Hubble scale to the case with interaction and the analytic forms for the cosmic parameters are obtained under the specific boundary conditions. The various behaviors concerning the cosmic expansion depend on the introduced numerical parameters which are also constrained. The more general interacting model inherits the features of the previous ones of HDE, keeping the consistency of the theory.

Enhanced Peculiar Velocities in Brane-Induced Gravity
Authors: Mark Wyman, Justin Khoury
arXiv:1004.2046v1
The mounting evidence for anomalously large peculiar velocities in our Universe presents a challenge for the LCDM paradigm. The recent estimates of the large scale bulk flow by Watkins et al. are inconsistent at the nearly 3 sigma level with LCDM predictions. Meanwhile, Lee and Komatsu have recently estimated that the occurrence of high-velocity merging systems such as the Bullet Cluster (1E0657-57) is unlikely at a 6.5-5.8 sigma level, with an estimated probability between 3.3x10^{-11} and 3.6x10^{-9} in LCDM cosmology. We show that these anomalies are alleviated in a broad class of infrared-modifed gravity theories, called brane-induced gravity, in which gravity becomes higher-dimensional at ultra large distances. These theories include additional scalar forces that enhance gravitational attraction and therefore speed up structure formation at late times and on sufficiently large scales. The peculiar velocities are enhanced by 24-34% compared to standard gravity, with the maximal enhancement nearly consistent at the 2 sigma level with bulk flow observations. The occurrence of the Bullet Cluster in these theories is 10^4 times more probable than in LCDM cosmology.

arXiv: 13 Apr 2010

Unmodified Gravity
Authors: Fergus Simpson, Brendan M. Jackson, John A. Peacock (Institute for Astronomy, University of Edinburgh)
arXiv:1004.1920v1
By relaxing the conventional assumption of a purely gravitational interaction between dark energy and dark matter, substantial alterations to the growth of cosmological structure can occur. In this work we focus on the homogeneous transfer of energy from a decaying form of dark energy. We present simple analytic solutions to the modified growth rates of matter fluctuations in these models, and demonstrate that neglecting physics within the dark sector may induce a significant bias in the inferred growth rate, potentially offering a false signature of modified gravity.



Evolution of proto-galaxy-clusters to their present form: theory and observations
Authors: Carl H. Gibson (Univ. of Cal. San Diego), Rudy E. Schild (Harvard)
arXiv:1004.2016v1
From hydro-gravitational-dynamics theory HGD, gravitational structure formation begins 30,000 years (10^12 s) after the turbulent big bang by viscous-gravitational fragmentation into super-cluster-voids and 10^46 kg proto-galaxy-super-clusters. Linear and spiral gas-proto-galaxies GPGs are the smallest fragments to emerge from the plasma epoch at decoupling at 10^13 s with Nomura turbulence morphology and length scale L_N ~ (خ³خ½/دپG)^1/2 ~10^20 m, determined by rate-of-strain خ³, photon viscosity خ½, and density دپ of the plasma fossilized at 10^12 s. GPGs fragment into 10^36 kg proto-globular-star-cluster PGC clumps of 10^24 kg primordial-fog-particle PFP dark matter planets. All stars form from planet mergers, with ~97% unmerged as galaxy baryonic-dark-matter BDM. The non-baryonic-dark-matter NBDM is so weakly collisional it diffuses to form galaxy cluster halos. It does not guide galaxy formation, contrary to conventional cold-dark-matter hierarchical clustering CDMHC theory (خ›=0). NBDM has ~97% of the mass of the universe. It binds rotating clusters of galaxies by gravitational forces. The galaxy rotational spin axis matches that for low wavenumber spherical harmonic components of CMB temperature anomalies and extends to 4.5x10^25 m (1.5 Gpc) in quasar polarization vectors, requiring a big bang turbulence origin. GPGs stick together by frictional processes of the frozen gas planets, just as PGCs have been meta-stable for the 13.7 Gyr age of the universe.



Cosmological correlation functions in scalar and vector inflationary models
Authors: Emanuela Dimastrogiovanni
arXiv:1004.1829v1
Ph.D thesis
This thesis is centered on three main subjects within the theory of inflation and cosmological perturbations: loop corrections to the power spectrum of curvature fluctuations generated during inflation; evolution of cosmological fluctuations in anisotropic pre-inflationary cosmologies; statistical anisotropy and non-Gaussianity predictions of models of inflation populated with vector fields. Currently, what makes even more interesting the study of 2-nd and higher order corrections to cosmological correlation functions as well as the computation of higher-than-two order correlators, is the almost unprecedented chance to confront theories with new and increasingly accurate experimental data that will shed more light in the physics of the early Universe. In the context of loop calculations, we have computed the corrections arising from scalar-tensor interactions in models of single-field inflation (both for the standard slow-roll model and for models described by Lagrangians with non-canonical kinetic terms). In the context of anisotropic cosmologies, also motivated by the observation of "anomalies" in the Cosmic Microwave Background (CMB) fluctuations, we have computed the bispectrum and the trispectrum of the curvature fluctuations in inflationary models with SU(2) vector fields, analyzing the statistical anisotropy features of the correlators in these models; finally, we have studied cosmological perturbations for a Universe with a Bianchi type-I background metric, with an energy density dominated by a pressureless fluid and in the presence of a cosmological constant.



The Hubble Constant
Authors: Wendy L. Freedman, Barry F. Madore
arXiv:1004.1856v1
Considerable progress has been made in determining the Hubble constant over the past two decades. We discuss the cosmological context and importance of an accurate measurement of the Hubble constant, and focus on six high-precision distance-determination methods: Cepheids, tip of the red giant branch, maser galaxies, surface brightness fluctuations, the Tully-Fisher relation and Type Ia supernovae. We discuss in detail known systematic errors in the measurement of galaxy distances and how to minimize them. Our best current estimate of the Hubble constant is 73 +/-2 (random) +/-4 (systematic) km/s/Mpc. The importance of improved accuracy in the Hubble constant will increase over the next decade with new missions and experiments designed to increase the precision in other cosmological parameters. We outline the steps that will be required to deliver a value of the Hubble constant to 2% systematic uncertainty and discuss the constraints on other cosmological parameters that will then be possible with such accuracy.



Dark Matter Subhalos In the Fermi First Source Catalog
Authors: Matthew R. Buckley, Dan Hooper
arXiv:1004.1644v1
The Milky Way's dark matter halo is thought to contain large numbers of smaller subhalos. These objects can contain very high densities of dark matter, and produce potentially observable fluxes of gamma rays. In this article, we study the gamma ray sources in the Fermi Gamma Ray Space Telescope's recently published First Source Catalog, and attempt to determine whether this catalog might contain a population of dark matter subhalos. We find that, while approximately 20-60 of the catalog's unidentified sources could plausibly be dark matter subhalos, such a population cannot be clearly identified as such at this time. From the properties of the sources in the First Source Catalog, we derive limits on the dark matter's annihilation cross section that are comparably stringent to those derived from recent observations of dwarf spheroidal galaxies.

On the 1/c Expansion of f(R) Gravity
Authors: Joachim Näf, Philippe Jetzer arXiv:1004.2014v1 We derive for applications to isolated systems - on the scale of the Solar System - the first relativistic terms in the $1/c$ expansion of the space time metric $g_{\mu\nu}$ for metric $f(R)$ gravity theories, where $f$ is assumed to be analytic at $R=0$. For our purpose it suffices to take into account up to quadratic terms in the expansion of $f(R)$, thus we can approximate $f(R) = R + aR^2$ with a positive dimensional parameter $a$. In the non-relativistic limit, we get an additional Yukawa correction with coupling strength $G/3$ and Compton wave length $\sqrt{6a}$ to the Newtonian potential, which is a known result in the literature. As an application, we derive to the same order the correction to the geodetic precession of a gyroscope in a gravitational field and the precession of binary pulsars. The result of the Gravity Probe B experiment yields the limit $a \lesssim 5 \times 10^{11} \, \mathrm{m}^2$, whereas for the pulsar B in the PSR J0737-3039 system we get a bound which is about $10^4$ times larger. On the other hand the E\"ot-Wash experiment provides the best laboratory bound $a \lesssim 10^{-10} \, \mathrm{m}^2$. Although the former bounds from geodesic precession are much larger than the laboratory ones, they are still meaningful in the case some type of chameleon effect is present and thus the effective values could be different at different length scales.

The Post-Newtonian Limit of f(R)-gravity in the Harmonic Gauge
Authors: A. Stabile arXiv:1004.1973v1 A general analytic procedure is developed for the post-Newtonian limit of f(R)-gravity in the harmonic gauge. The corrections to gravitational potential generated by an uniform mass ball-like source are calculated up to (v/c)^4 order in a pure perturbative framework. Considering the Taylor expansion of a generic function $f$ it is possible to obtain general solutions in term of the derivatives of $f$. Using the Green function method in the vacuum at Newtonian limit all more important topics about the Gauss and Birkhoff theorem are discussed. The spatial behavior of metric potentials and Ricci scalar are shown for typical distances Sun - Earth. A discussion about the theoretical constraints for free parameters in the theory complete this work.

arXiv: 12 Apr 2010

Dark energy: investigation and modeling
Authors: Shinji Tsujikawa
arXiv:1004.1493v1
Constantly accumulating observational data continue to confirm that about 70% of the energy density today consists of dark energy responsible for the accelerated expansion of the Universe. We present recent observational bounds on dark energy constrained by the type Ia supernovae, cosmic microwave background, and baryon acoustic oscillations. We review a number of theoretical approaches that have been adopted so far to explain the origin of dark energy. This includes the cosmological constant, modified matter models (such as quintessence, k-essence, coupled dark energy, unified models of dark energy and dark matter), modified gravity models (such as f(R) gravity, scalar-tensor theories, braneworlds), and inhomogeneous models. We also discuss observational and experimental constraints on those models and clarify which models are favored or ruled out in current observations.

Offset between dark matter and ordinary matter: evidence from a sample of 38 lensing clusters of galaxies
Authors: HuanYuan Shan, Bo Qin, Bernard Fort, Charling Tao, Xiang-Ping Wu, HongSheng Zhao
arXiv:1004.1475v1
We compile a sample of 38 galaxy clusters which have both X-ray and strong lensing observations, and study for each cluster the projected offset between the dominant component of baryonic matter center (measured by X-rays) and the gravitational center (measured by strong lensing). Among the total sample, 45% clusters have offsets >10". The >10" separations are significant, considering the arcsecond precision in the measurement of the lensing/X-ray centers. This suggests that it might be a common phenomenon in unrelaxed galaxy clusters that gravitational field is separated spatially from the dominant component of baryonic matter. It also has consequences for lensing models of unrelaxed clusters since the gas mass distribution may differ from the dark matter distribution and give perturbations to the modeling. Such offsets can be used as a statistical tool for comparison with the results of Lambda-CDM simulations and to test the modified dynamics.

On accretion of dark energy onto black- and worm-holes
Authors: José A. Jiménez Madrid, Prado Martín-Moruno
arXiv:1004.1428v1
We review some of the possible models that are able to describe the current Universe which point out the future singularities that could appear. We show that the study of the dark energy accretion onto black- and worm-holes phenomena in these models could lead to unexpected consequences, allowing even the avoidance of the considered singularities. We also review the debate about the approach used to study the accretion phenomenon which has appeared in literature to demonstrate the advantages and drawbacks of the different points of view. We finally suggest new lines of research to resolve the shortcomings of the different accretion methods. We then discuss future directions for new possible observations that could help choose the most accurate model.

Viscous Fluids and Gauss-Bonnet Modified Gravity
Authors: Olesya Gorbunova, Lorenzo Sebastiani
arXiv:1004.1505v1
We study effects of cosmic fluids on finite-time future singularities in modified $f(R,G)$-gravity, where $R$ and $G$ are the Ricci scalar and the Gauss-Bonnet invariant, respectively. We consider the fluid equation of state in the general form, $\omega=\omega(\rho)$, and we suppose the existence of a bulk viscosity. We investigate quintessence region ($\omega>-1$) and phantom region ($\omega<-1$) and the possibility to change or avoid the singularities in $f(R,G)$-gravity. Finally, we study the inclusion of quantum effects in large curvatures regime.

arXiv: 9 Apr 2010

Planet Formation: Statistics of spin rates and obliquities of extrasolar planets
Authors: Yamila Miguel, Adrián Brunini
arXiv:1004.1406v1
We develop a simple model of planetary formation, focusing our attention on those planets with masses less than 10 Earth masses and studying particularly the primordial spin parameters of planets resulting from the accretion of planetesimals and produced by the collisions between the embryos. As initial conditions, we adopt the oligarchic growth regime of protoplanets in a disc where several embryos are allowed to form. We take different initial planetary system parameters and for each initial condition, we consider an evolution of 20 millon of years of the system. We perform simulations for 1000 different discs, and from their results we derive the statistical properties of the assembled planets. We have taken special attention to the planetary obliquities and rotation periods, such as the information obtained from the mass and semi major axis diagram, which reflects the process of planetary formation. The distribution of obliquities was found to be isotropic, which means that planets can rotate in direct or indirect sense, regardless of their mass. Our results regarding the primordial rotation periods show that they are dependent on the region where the embryo was formed and evolved. According to our results, most of the planets have rotation periods between 10 and 10000 hours and there are also a large population of planets similar to terrestrial planets in the Solar System.

Solution to the Dark Energy Problem
Authors: Paul Howard Frampton
arXiv:1004.1285v1
I present a simple, and hopefully convincing, discussion of a solution to the dark energy problem, which arises because the visible universe is well approximated by a black hole.

Testing the Dark Matter Annihilation Model for the WMAP Haze
Authors: Matthew McQuinn, Matias Zaldarriaga
arXiv:1004.1189v1
Analyses have found a "haze" of anomalous microwave emission surrounding the Galactic Center in the WMAP sky maps. A recent study using Fermi data detected a similar haze in the gamma-ray. Several studies have modeled these hazes as radiation from the leptonic byproducts of dark matter annihilations, and arguably no convincing astrophysical alternative has been suggested. We discuss the characteristics of astrophysical cosmic ray sources that could potentially explain this microwave and gamma-ray emission. The most promising astrophysical scenarios involve cosmic ray sources that are clustered such that many fall within ~1 kpc of the Galactic Center. For example, we show that several hundred Galactic Center supernovae in the last million years plus a diffusion-hardened electron spectrum may be consistent with present constraints on this emission. Alternatively, it could be due to a burst of activity probably associated with Sagittarius A* occurring ~1 Myr ago and producing >10^51 erg in cosmic ray electrons. Different models predict different trends for the spectral index of the microwave and gamma-ray spectrum as a function of angle from the Galactic Center that should be robust to cosmic ray propagation uncertainties. In particular, if the haze is from dark matter annihilations, it should have a very hard microwave and gamma-ray spectrum for which the spectral shape does not change significantly with angle, which we argue would be difficult to achieve with any astrophysical mechanism. Observations with the Planck and Fermi satellites can distinguish between viable haze models using these signatures.

arXiv: 8 Apr 2010

Gravitational Lensing as Signal and Noise in Lyman-alpha Forest Measurements
Authors: Marilena LoVerde, Stefanos Marnerides, Lam Hui, Brice Menard, Adam Lidz
arXiv:1004.1165v1
In Lyman-alpha forest measurements it is generally assumed that quasars are mere background light sources which are uncorrelated with the forest. Gravitational lensing of the quasars violates this assumption. This effect leads to a measurement bias, but more interestingly it provides a valuable signal. The lensing signal can be extracted by correlating quasar magnitudes with the flux power spectrum and with the flux decrement. These correlations will be challenging to measure but their detection provides a direct measure of how features in the Lyman-alpha forest trace the underlying mass density field. Observing them will test the fundamental hypothesis that fluctuations in the forest are predominantly driven by fluctuations in mass, rather than in the ionizing background, helium reionization or winds. We discuss ways to disentangle the lensing signal from other sources of such correlations, including dust, continuum and background residuals. The lensing-induced measurement bias arises from sample selection: one preferentially collects spectra of magnified quasars which are behind overdense regions. This measurement bias is ~0.1-1% for the flux power spectrum, optical depth and the flux probability distribution. Since the effect is systematic, quantities such as the amplitude of the flux power spectrum averaged across scales should be interpreted with care.

Dwarf-Galaxy Cosmology
Authors: Regina Schulte-Ladbeck, Ulrich Hopp, Elias Brinks, Andrey Kravtsov
arXiv:1004.1139v1
Dwarf galaxies provide opportunities for drawing inferences about the processes in the early universe by observing our "cosmological backyard"-the Local Group and its vicinity. This special issue of the open-access journal Advances in Astronomy is a snapshot of the current state of the art of dwarf-galaxy cosmology.

The dark flow induced small scale kinetic Sunyaev Zel'dovich effect
Authors: Pengjie Zhang (SHAO)
arXiv:1004.0990v1
Recently Kashlinsky et al.(2008), Kashlinsky et al.(2010) discovered a $\sim 10^3$ km/$s$ bulk flow of the universe out to $z\simeq 0.3$, through the dark flow induced CMB dipole in directions of clusters. We point out that this dark flow also induces CMB temperature fluctuations at much smaller angular scales, through modulation of the inhomogeneous electron distribution on the uniform dark flow. This dark flow induced small scale kinetic Sunyaev Zel'dovich (SZ) effect is a non-negligible component of the CMB sky, only a factor of $\sim 2$ smaller than the conventional kinetic SZ effect at $\ell\sim 10^3-10^4$. Its existence worsens the low SZ problem recently found by the South Pole Telescope (Lueker et al. 2009), although no conclusive constraints can be drawn. It is also correlated with the large scale structure (LSS) and its correlation with 2MASS galaxy distribution reaches $0.3 \mu$K at $\ell=10^3$, under a directional dependent optimal weighting scheme. We estimate that, WMAP plus 2MASS is already able to detect this dark flow induced small scale kinetic SZ effect with $\sim 6\sigma$ confidence. Deeper galaxy surveys such as SDSS can further improve the measurement. Planck plus existing galaxy surveys can reach $\ga 14\sigma$ detection. Existing CMB-LSS cross correlation measurements shall be reanalyzed to probe the dark flow and to eliminate possible bias on the integrated Sachs-Wolfe effect measurement through the CMB-LSS cross correlation.

A model independent null test on the cosmological constant
Authors: Savvas Nesseris, Arman Shafieloo
arXiv:1004.0960v1
We use the Om statistic and the Genetic Algorithms (GA) in order to derive a null test on the spatially flat cosmological constant model $\Lambda$CDM. This is done in two steps: first, we apply the GA to the Constitution SNIa data in order to acquire a model independent reconstruction of the expansion history of the Universe $H(z)$ and second, we use the reconstructed $H(z)$ in conjunction with the Om statistic, which is constant only for the $\Lambda$CDM model, to derive our constraints. We find that while $\Lambda$CDM is consistent with the data at the $2\sigma$ level, some deviations from $\Lambda$CDM model at low redshifts seems to be mildly preferred.

arXiv: 7 Apr 2010

Dark matter distribution in galaxy groups from combined strong lensing and dynamics analysis
Authors: Karun Thanjavur (1,2,3), David Crampton (2,3), Jon Willis (2) ((1) CFHT, Hawaii, USA (2) UVic, Victoria, Canada (3) NRC-HIA, Victoria, Canada)
arXiv:1004.0699v1
Using a combined analysis of strong lensing and galaxy dynamics, we characterize the mass distributions and M/L ratios of galaxy groups, which form an important transition regime in Lambda-CDM cosmology. By mapping the underlying mass distribution, we test whether groups are dark matter dominated as hypothesized by the standard cosmogony, or isothermal as observed in baryon rich field galaxies. We present our lensing + galaxy dynamics formalism built around the dark matter dominant NFW and Hernquist distributions, compared against the Isothermal Sphere observed in galaxy scale objects. We show that mass measurement in the core of the group (r ~ 0.2 r_{vir}), determined jointly from a lens model and from differential velocity dispersion estimates, may effectively distinguish between these density distributions. We apply our method to MOS observations of two groups, SL2SJ1430+5546 and SL2SJ1431+5533, drawn from our CFHTLS lens catalog. With the measured lensing and dynamical masses, combined with a maximum likelihood estimator built around our model, we estimate the concentration index characterizing each density distribution and the corresponding virial mass of each group. Our results indicate that both groups are dark matter dominant, and reject the Isothermal distribution at >>3 sigma level. For both groups, the estimated i-band M/L ratios of ~260 Msun/Lsun, are similar to other published values for groups. The Gaussian distributions of the velocities of their member galaxies support a high degree of virialization. The differences in their virial masses, 2.8 and 1.6 x 10^14 Msun, and velocity dispersions, 720 and 560 km/s respectively, may indicate however that each group is at a different stage of transition to a cluster. We aim to populate this important transition regime with additional results from ongoing observations of the remaining lensing groups in our catalog.

A dynamical systems study of the inhomogeneous Lambda-CDM model
Authors: Roberto A. Sussman, German Izquierdo
arXiv:1004.0773v1
We consider spherically symmetric inhomogeneous dust models with a positive cosmological constant, $\Lambda$, given by the Lemaitre-Tolman-Bondi metric. These configurations provide a simple but useful generalization of the Lambda-CDM model describing cold dark matter (CDM) and a Lambda term, which seems to fit current cosmological observations. The dynamics of these models can be fully described by scalar evolution equations that can be given in the form of a proper dynamical system associated with a 4-dimensional phase space whose critical points and invariant subspaces are examined and classified. The phase space evolution of various configurations is studied in detail by means of two 2-dimensional subspaces: a projection into the invariant homogeneous subspace associated with Lambda-CDM solutions with FLRW metric, and a projection into a subspace generated by suitably defined fluctuations that convey the effects of inhomogeneity. We look at cases with perpetual expansion, bouncing and loitering behavior, as well as configurations with "mixed" kinematic patters, such as a collapsing region in an expanding background. In all cases, phase space trajectories emerge from and converge to stable past and future attractors in a qualitatively analogous way as in the case of the FLRW limit. However, we can identify in both projections of the phase space various qualitative features absent in the FLRW limit that can be useful in the construction of toy models of astrophysical and cosmological inhomogeneities.

Brane f(R) gravity cosmologies
Authors: Adam Balcerzak, Mariusz P. Dabrowski
arXiv:1004.0150v1
By the application of the generalized Israel junction conditions we derive cosmological equations for the fourth-order $f(R)$ brane gravity and study their cosmological solutions. We show that there exists a solution which describes a four-dimensional de-Sitter $(dS_4)$ brane embedded in a five-dimensional anti-de-Sitter $(AdS_5)$ bulk for a vanishing Weyl tensor contribution. On the other hand, for the case of a non-vanishing Weyl tensor contribution, there exists a static Einstein universe brane. We claim that in order to get some more general non-static $f(R)$ brane configurations, one needs to admit a dynamical matter energy-momentum tensor in the bulk rather than just a bulk cosmological constant.

Local non-Gaussianity from inflation
Authors: David Wands (ICG, Portsmouth, and YITP, Kyoto)
arXiv:1004.0818v1
The non-Gaussian distribution of primordial perturbations has the potential to reveal the physical processes at work in the very early Universe. Local models provide a well-defined class of non-Gaussian distributions that arise naturally from the non-linear evolution of density perturbations on super-Hubble scales starting from Gaussian field fluctuations during inflation. I describe the delta-N formalism used to calculate the primordial density perturbation on large scales and then review several models for the origin of local primordial non-Gaussianity, including the cuvaton, modulated reheating and ekpyrotic scenarios. I include an appendix with a table of sign conventions used in specific papers.

arXiv: 6 Apr 2010

Can Neutron stars constrain Dark Matter?
Authors: Chris Kouvaris, Peter Tinyakov
arXiv:1004.0586v1
We argue that observations of old neutron stars can impose constraints on dark matter candidates even with very small elastic or inelastic cross section, and self-annihilation cross section. We find that old neutron stars close to the galactic center or in globular clusters can maintain a surface temperature that could in principle be detected. Due to their compactness, neutron stars can acrete WIMPs efficiently even if the WIMP-to-nucleon cross section obeys the current limits from direct dark matter searches, and therefore they could constrain a wide range of dark matter candidates.

The Origin of Life from Primordial Planets
Authors: Carl H. Gibson (Univ. Cal. San Diego), Rudolph E. Schild (Harvard), N. C. Wickramasinghe (Cardiff Univ.)
arXiv:1004.0504v1
The origin of life and the origin of the universe represent two of the most important problems of science. Both are resolved by hydro-gravitational dynamics (HGD) cosmology (Gibson 1996, Schild 1996, Gibson 2009ab), which predicts frozen primordial hydrogen-helium gas planets in clumps as the dark matter of galaxies. Merging Earth-mass planets formed stars, moons and comets to incubate and cosmically seed the first life. Cometary panspermia (Hoyle and Wickramasinghe 1981, 1982; Wickramasinghe et al. 2009) occurs naturally by HGD mechanisms. Comets and moons are fragments from mergers of stardust covered frozen gas planets in their step-wise growth to star mass. Supernovae from stellar over-accretion of planets produce stardust (C, N, O, P etc.) chemical fertilizer. Planets collect this infected radioactive dust gravitationally, to provide liquid water domains in contact with life nutrients seeded with life prototypes. The first mutating, evolving, life from HGD likely occurred promptly, following the plasma to gas transition 300,000 years after the big bang when high densities of galaxies and a superabundance of hot primordial soup kitchens first overcame enormous odds against spontaneous creation (Wickramasinghe 2010, Joseph 2000). Images from optical, radio, and infrared space telescopes suggest life on Earth was neither first nor inevitable.

Cosmic Strings Collision in Cosmological Backgrounds
Authors: Hassan Firouzjahi, Salomeh Khoeini-Moghaddam, Shahram Khosravi
arXiv:1004.0068v1
The collisions of cosmic strings loops and the dynamics of junctions formations in expanding backgrounds are studied. The key parameter controlling the dynamics of junctions formation, the cosmic strings zipping and unzipping is the relative size of the loops compared to the Hubble expansion rate at the time of collision. We study analytically and numerically these processes for large super-horizon size loops, for small sub-horizon size loops as well as for loops with the radii comparable to the Hubble expansion rate at the time of collision.

Constraining the mass of the graviton using coalescing black-hole binaries
Authors: Drew Keppel, P. Ajith
arXiv:1004.0284v1
We study how well the mass of the graviton can be constrained from gravitational-wave (GW) observations of coalescing binary black holes. Whereas the previous investigations employed post-Newtonian (PN) templates describing only the inspiral part of the signal, the recent progress in analytical and numerical relativity has provided analytical waveform templates coherently describing the inspiral-merger-ringdown (IMR) signals. We show that a search for binary black holes employing IMR templates will be able to constrain the mass of the graviton much more accurately (about an order of magnitude) than a search employing PN templates. The best expected bound from GW observatories (lambda_g > 7.8 x 10^13 km from Adv. LIGO, lambda_g > 7.1 x 10^14 km from Einstein Telescope, and lambda_g > 5.9 x 10^17 km from LISA) are several orders-of-magnitude better than the best available model-independent bound (lambda_g > 2.8 x 10^12 km, from Solar system tests). Most importantly, GW observations will provide the first constraints from the highly dynamical, strong-field regime of gravity.

Revisiting the Cosmological Constraints on the Interacting Dark Energy Models
Authors: Hao Wei
arXiv:1004.0492v1
In this work, we consider the cosmological constraints on the interacting dark energy models. We generalize the models considered previously by Guo {\it et al.}, Costa and Alcaniz, and try to discuss two general types of models: type I models are characterized by $\rho_{_X}/\rho_m=f(a)$ and $f(a)$ can be any function of scale factor $a$, whereas type II models are characterized by $\rho_m=\rho_{m0}\,a^{-3+\epsilon(a)}$ and $\epsilon(a)$ can be any function of $a$. We obtain the cosmological constraints on the type I and II models with power-law, CPL-like, logarithmic $f(a)$ and $\epsilon(a)$ by using the latest observational data.

Galactic rotation curves in modified gravity with non-minimal coupling between matter and geometry
Authors: T. Harko
arXiv:1004.0576v1
We investigate the possibility that the behavior of the rotational velocities of test particles gravitating around galaxies can be explained in the framework of modified gravity models with non-minimal matter-geometry coupling. Generally, the dynamics of test particles around galaxies, as well as the corresponding mass deficit, is explained by postulating the existence of dark matter. The extra-terms in the gravitational field equations with geometry-matter coupling modify the equations of motion of test particles, and induce a supplementary gravitational interaction. Starting from the variational principle describing the particle motion in the presence of the non-minimal coupling, the expression of the tangential velocity of a test particle, moving in the vacuum on a stable circular orbit in a spherically symmetric geometry, is derived. The tangential velocity depends on the metric tensor components, as well as of the coupling function between matter and geometry. The Doppler velocity shifts are also obtained in terms of the coupling function. If the tangential velocity profile is known, the coupling term between matter and geometry can be obtained explicitly in an analytical form. The functional form of this function is obtained in two cases, for a constant tangential velocity, and for an empirical velocity profile obtained from astronomical observations, respectively. Therefore, these results open the possibility of directly testing the modified gravity models with non-minimal coupling between matter and geometry by using direct astronomical and astrophysical observations at the galactic or extra-galactic scale.

Dark Energy Stars and the CMB
Authors: George Chapline
arXiv:1004.0406v1
An initial state for the observable universe consisting of a finite region with a large vacuum energy will break-up due to near horizon quantum critical fluctuations. This will lead to a Freidman-like early universe consisting of an expanding cloud of dark energy stars and radiation. In this note we point out that this scenario provides a simple explanation for the present day density of dark matter as well as the level of CMB temperature flucuations. It is also predicted that all dark matter will be clumped on mass scales ~ 10E3 solar masses.

Galaxy Clusters as a probe of early dark energy
Authors: Ujjaini Alam, Zarija Lukić, Suman Bhattacharya
arXiv:1004.0437v1
We study a class of early dark energy (EDE) models, in which, unlike in standard dark energy models, a substantial amount of dark energy exists in the matter-dominated era. We self-consistently include dark energy perturbations, and show that these models may be successfully constrained using future observations of galaxy clusters, in particular the redshift abundance, and the Sunyaev-Zel'dovich (SZ) power spectrum. We make predictions for EDE models, as well as $\Lambda$CDM for incoming X-ray (eROSITA) and microwave (South Pole Telescope) observations. We show that galaxy clusters' mass function and the SZ power spectrum will put strong constraints both on the equation of state of dark energy today and the redshift at which EDE transits to present-day $\Lambda$CDM like behavior for these models, thus providing complementary information to the geometric probes of dark energy. Not including perturbations in EDE models leads to those models being practically indistinguishable from $\Lambda$CDM.