arXiv: 8 December 2010

Reconstructing Dark Energy : A Comparison of Cosmological Parameters

http://arxiv.org/abs/1012.1591v1
Authors: Alexander V. Pan, Ujjaini Alam
A large number of cosmological parameters have been suggested for obtaining information on the nature of dark energy. In this work, we study the efficacy of these different parameters in discriminating theoretical models of dark energy, using both currently available supernova (SNe) data, and simulations of future observations. We find that the current data does not put strong constraints on the nature of dark energy, irrespective of the cosmological parameter used. For future data, we find that the although deceleration parameter can accurately reconstruct some dark energy models, it is unable to discriminate between different models of dark energy, therefore limiting its usefulness. Physical parameters such as the equation of state of dark energy, or the dark energy density do a good job of both reconstruction and discrimination if the matter density is known to high accuracy. However, uncertainty in matter density reduces the efficacy of these parameters. A recently proposed parameter, Om(z), constructed from the first derivative of the SNe data, works very well in discriminating different theoretical models of dark energy, and has the added advantage of not being dependent on the value of matter density. Thus we find that a cosmological parameter constructed from the first derivative of the data, for which the theoretical models of dark energy are sufficiently distant from each other, and which is independent of the matter density, performs the best in reconstructing dark energy from SNe data.

More on the low variance circles in CMB sky

Authors: V.G.Gurzadyan, R.Penrose
http://arxiv.org/abs/1012.1486v1
Two groups [3,4] have confirmed the results of our paper concerning the actual existence of low variance circles in the cosmic microwave background (CMB) sky. They also point out that the effect does not contradict the LCDM model - a matter which is not in dispute. We point out two discrepancies between their treatment and ours, however, one technical, the other having to do with the very understanding of what constitutes a Gaussian random signal. Both groups simulate maps using the CMB power spectrum for LCDM, while we simulate a pure Gaussian sky plus the WMAP's noise, which points out the contradiction with a common statement [3] that "CMB signal is random noise of Gaussian nature". For as it was shown in [5], the random component is a minor one in the CMB signal, namely, about 0.2. Accordingly, the circles we saw are a real structure of the CMB sky and they are not of a random Gaussian nature. Although the structures studied certainly cannot contradict the power spectrum, which is well fitted by LCDM model, we particularly emphasize that the low variance circles occur in concentric families, and this key fact cannot be explained as a purely random effect. It is, however a clear prediction of conformal cyclic cosmology.

Scale-dependent halo bias from scale-dependent growth

Authors: Kyle Parfrey, Lam Hui, Ravi K. Sheth
We derive a general expression for the large-scale halo bias, in theories with a scale-dependent linear growth, using the excursion set formalism. Such theories include modified gravity models, and models in which the dark energy clustering is non-negligible. A scale dependence is imprinted in both the formation and evolved biases by the scale-dependent growth. Mergers are accounted for in our derivation, which thus extends earlier work which focused on passive evolution. There is a simple analytic form for the bias for those theories in which the nonlinear collapse of perturbations is approximately the same as in general relativity. As an illustration, we apply our results to a simple Yukawa modification of gravity, and use SDSS measurements of the clustering of luminous red galaxies to constrain the theory's parameters.

arXiv: 7 December 2010

No evidence for anomalously low variance circles on the sky

Authors: Adam Moss, Douglas Scott, James P. Zibin

http://arxiv.org/abs/1012.1305v1

In a recent paper, Gurzadyan & Penrose claim to have found directions on the sky centred on which are circles of anomalously low variance in the cosmic microwave background (CMB). These features are presented as evidence for a particular picture of the very early Universe. We attempted to repeat the analysis of these authors, and we can indeed confirm that such variations do exist in the temperature variance for annuli around points in the data. However, we find that this variation is entirely expected in a sky which contains the usual CMB anisotropies. In other words, properly simulated Gaussian CMB data contain just the sorts of variations claimed. Gurzadyan & Penrose have not found evidence for pre-Big Bang phenomena, but have simply re-discovered that the CMB contains structure.

A statistical-mechanical explanation of dark matter halo properties

Authors: Dong-Biao Kang, Ping He
http://arxiv.org/abs/1012.1003v1
Cosmological $N$-body simulations have revealed many empirical relationships of dark matter halos, yet the physical origin of these halo properties still remains unclear. On the other hand, the attempts to establish the statistical mechanics for self-gravitating systems have encountered many formal difficulties, and little progress has been made for about fifty years. The aim of this work is to strengthen the validity of the statistical-mechanical approach we have proposed previously to explain the dark matter halo properties. By introducing an effective pressure instead of the radial pressure to construct the specific entropy, we use the entropy principle and proceed in a similar way as that of He & Kang, to obtain an entropy stationary equation. An equation of state for equilibrated dark halos is derived from this entropy stationary equation, by which the dark halo density profiles can be obtained. We also derive the anisotropy parameter and pseudo-phase-space density profile. All these predictions agree well with numerical simulations in the outer regions of dark halos. Our work provides further support to the idea that statistical mechanics for self-gravitating systems is viable.

Penrose’s Cyclic Cosmology

Penrose’s Cyclic Cosmology

arXiv: 6 December 2010

Searching for Chameleon-like Scalar Fields                                                                                                                                      Authors: S. A. Levshakov, P. Molaro, M. G. Kozlov, A. V. Lapinov, C. Henkel, D. Reimers, T. Sakai, I. I. Agafonova                            http://arxiv.org/abs/1012.0642v1

Using the 32-m Medicina, 45-m Nobeyama, and 100-m Effelsberg telescopes we found a statistically significant velocity offset Delta V = 27 +/- 3 m/s (1sigma) between the inversion transition in NH3(1,1) and low-J rotational transitions in N2H+(1-0) and HC3N(2-1) arising in cold and dense molecular cores in the Milky Way. Systematic shifts of the line centers caused by turbulent motions and velocity gradients, possible non-thermal hyperfine structure populations, pressure and optical depth effects are shown to be lower than or about 1 m/s and thus can be neglected in the total error budget. The reproducibility of Delta V at the same facility (Effelsberg telescope) on a year-to-year basis is found to be very good. Since the frequencies of the inversion and rotational transitions have different sensitivities to variations in mu = m_e/m_p, the revealed non-zero Delta V may imply that mu changes when measured at high (terrestrial) and low (interstellar) matter densities as predicted by chameleon-like scalar field models - candidates to the dark energy carrier. Thus we are testing whether scalar field models have chameleon-type interactions with ordinary matter. The measured velocity offset corresponds to the ratio Delta mu/mu = (mu_space - mu_lab)/mu_lab of (26 +/- 3)x10^{-9} (1sigma).

arXiv: 3 December 2010

Does environment affect the star formation histories of early-type galaxies?

Authors: Ignacio Ferreras (1), Anna Pasquali (2), Ben Rogers (3), ((1) MSSL/UCL, (2) ARI/Heidelberg, (3) King's College London)
http://arxiv.org/abs/1012.0310v1
Differences in the stellar populations of galaxies can be used to quantify the effect of environment on the star formation history. We target a sample of early-type galaxies from the Sloan Digital Sky Survey in two different environmental regimes: close pairs and a general sample where environment is measured by the mass of their host dark matter halo. We apply a blind source separation technique based on principal component analysis, from which we define two parameters that correlate, respectively, with the average stellar age (eta) and with the presence of recent star formation (zeta) from the spectral energy distribution of the galaxy. We find that environment leaves a second order imprint on the spectra, whereas local properties - such as internal velocity dispersion - obey a much stronger correlation with the stellar age distribution.

arXiv: 2 December 2010

The Helmholtz Hierarchy: Phase Space Statistics of Cold Dark Matter                                     Authors: Svetlin Tassev

We present a new formalism to study large-scale structure in the universe. The result is a hierarchy (which we call the "Helmholtz Hierarchy") of partial differential equations describing the phase space statistics of cold dark matter (CDM). The hierarchy features a physical ordering parameter which interpolates between the Zel'dovich approximation and fully-fledged gravitational interactions. The results incorporate the effects of stream crossing, and automatically generate a decay at high k for the CDM power spectrum as obtained in Renormalized Perturbation Theory. We show that the Helmholtz hierarchy is self-consistent and obeys causality to all orders.

Clarifying the effects of interacting dark energy on linear and nonlinear structure formation processes

http://arxiv.org/abs/1012.0002v1

Authors: Marco Baldi (Excellence Cluster Universe, Garching)
We present a detailed numerical study of the impact that cosmological models featuring a direct interaction between the Dark Energy component that drives the accelerated expansion of the Universe and Cold Dark Matter can have on the linear and nonlinear stages of structure formation. By means of a series of collisionless N-body simulations we study the influence that each of the different effects characterizing these cosmological models - which include among others a fifth force, a time variation of particle masses, and a velocity-dependent acceleration - separately have on the growth of density perturbations and on a series of observable quantities related to linear and nonlinear cosmic structures, as the matter power spectrum, the gravitational bias between baryons and Cold Dark Matter, the halo mass function and the halo density profiles. We perform our analysis applying and comparing different numerical approaches previously adopted in the literature, and we address the partial discrepancies recently claimed in a similar study by Li & Barrow (2010b) with respect to the first outcomes of Baldi et al. (2010), which are found to be related to the specific numerical approach adopted in the former work. Our results fully confirm the conclusions of Baldi et al. (2010) and show that when linear and nonlinear effects of the interaction between Dark Energy and Cold Dark Matter are properly disentangled, the velocity-dependent acceleration is the leading effect acting at nonlinear scales, and in particular is the most important mechanism in lowering the concentration of Cold Dark Matter halos.

arXiv: 1 December 2010

Astrophysical Probes of Fundamental Physics

Authors: C.J.A.P. Martins
http://arxiv.org/abs/1011.6591v1
The dramatic confrontation between new observations and theories of the early and recent universe makes cosmology one of the most rapidly advancing fields in the physical sciences. The universe is a unique laboratory in which to probe fundamental physics, the rationale being to start from fundamental physics inspired models and explore their consequences in sufficient quantitative detail to be able to identify key astrophysical and cosmological tests of the underlying theory (or developing new tests when appropriate). An unprecedented number of such tests will be possible in the coming years, by exploiting the ever improving observational data. In this spirit I will highlight some open issues in cosmology and particle physics and provide some motivation for this symposium.

Non-Gaussianity in Two-Field Inflation

Authors: Courtney M. Peterson, Max Tegmark
http://arxiv.org/abs/1011.6675v1
We derive semi-analytic formulae for the local bispectrum and trispectrum in general two-field inflation, and provide a simple geometric recipe for building observationally allowed models with observable non-Gaussianity. We show that the trispectrum can be expressed entirely in terms of spectral observables, which provides a new consistency relation unique to two-field inflation. We express the bispectrum in terms of model-independent physical quantities, with the exception of one model-dependent term that must be calculated and that represents the coupling between fields. We show that in order to generate observably large non-Gaussianity during inflation, the sourcing of curvature modes by isocurvature modes must be extremely sensitive to the initial conditions. We prove that this condition is satisfied only when neighboring trajectories through the two-dimensional field space dramatically diverge during inflation. Geometrically, this means that the inflaton must roll along a ridge in the potential V for many e-foldings, and that its trajectory must turn slightly (but not too sharply) in field space. Therefore, it follows that two-field scenarios with attractor solutions necessarily produce small non-Gaussianity. This explains why it has been so difficult to achieve large non-Gaussianity in two-field inflation, and why it has only been achieved in a narrow class of models like hybrid inflation and certain product potentials where the potential and/or the initial conditions are fine-tuned. Many of our conclusions generalize at least qualitatively to the case of general multi-field inflation.

arXiv: 30 November 2010

Probing the anisotropic local universe and beyond with SNe Ia data

Authors: Jacques Colin (Paris), Roya Mohayaee (Paris), Subir Sarkar (Oxford), Arman Shafieloo (Seoul)
http://arxiv.org/abs/1011.6292v1
The question of the transition to global isotropy from our anisotropic local Universe is studied using the Union 2 catalogue of Type Ia supernovae (SNe Ia). We construct a "residual" statistic sensitive to systematic shifts in their brightness in different directions and use this to search in different redshift bins for a preferred direction on the sky in which the SNe Ia are brighter or fainter relative to the 'standard' LCDM cosmology. At low redshift (z<0.05) we find that an isotropic model such as LCDM is barely consistent with the SNe Ia data at 2-3 sigma. A complementary maximum likelihood analysis of peculiar velocities confirms this finding -- there is a bulk flow of around 260 km/sec at z \sim 0.06, which disagrees with LCDM at 1-2 sigma. Since the Shapley concentration is believed to be largely responsible for this bulk flow, we make a detailed study of the infall region: the SNe Ia falling away from the Local Group towards Shapley are indeed significantly dimmer than those falling towards us and on to Shapley. Convergence to the CMB rest frame must occur well beyond Shapley (z>0.06) so the low redshift bulk flow can systematically bias any reconstruction of the expansion history of the Universe. At high redshifts z>0.15 the agreement between the SNe Ia data and the isotropic LCDM model does improve, however, the sparseness and low quality of the data means that LCDM cannot be singled out as the preferred cosmological model.

Matter perturbations in Galileon cosmology

Authors: Antonio De Felice, Ryotaro Kase, Shinji Tsujikawa
http://arxiv.org/abs/1011.6132v1
We study the evolution of matter density perturbations in Galileon cosmology where the late-time cosmic acceleration can be realized by a field kinetic energy. We obtain full perturbation equations at linear order in the presence of five covariant Lagrangians ${cal L}_i$ ($i=1,...,5$) satisfying a Galilean symmetry in the flat space-time. The equations for a matter perturbation as well as an effective gravitational potential are derived under a quasi-static approximation on sub-horizon scales. This approximation can reproduce full numerical solutions with high accuracy for the wavelengths relevant to large-scale structures. For the model parameters constrained by the background expansion history of the Universe the growth rate of matter perturbations is larger than that in the LCDM model, with the growth index $gamma$ today typically smaller than 0.4. We also find that, even on very large scales associated with the Integrated-Sachs-Wolfe (ISW) effect in Cosmic Microwave Background (CMB) temperature anisotropies, the effective gravitational potential exhibits a temporal growth during the transition from the matter era to the epoch of cosmic acceleration. These properties are useful to distinguish the Galileon model from the LCDM in future high-precision observations.

Theories of Dark Energy with Screening Mechanisms

Authors: Justin Khoury
http://arxiv.org/abs/1011.5909v1
Despite the overwhelming evidence for the existence of dark energy and dark matter, their underlying fundamental physics remains unknown. This review article explores the tantalizing possibility that the dark sector includes new light degrees of freedom that mediate long-range forces on cosmological scales. To ensure consistency with laboratory and solar system tests of gravity, some screening mechanism is necessary to "hide" these degrees of freedom locally. I will focus on two broad classes of screening theories, chameleons and symmetrons, which rely respectively on the scalar field acquiring a large mass or weak coupling in the presence of large ambient matter density.

Late-time cosmic acceleration: Dark gravity

Authors: Francisco S. N. Lobo
A central theme in cosmology is the perplexing fact that the Universe is undergoing an accelerating expansion. The latter, one of the most important and challenging current problems in cosmology, represents a new imbalance in the governing gravitational field equations. Several candidates, responsible for this expansion, have been proposed in the literature, in particular, dark energy models and modified gravity, amongst others. In this paper, we explore the possibility that the late-time cosmic acceleration is due to infra-red modifications of Einstein's theory of General Relativity, and review some of the modified theories of gravity that address this intriguing and exciting problem facing modern physics.

arXiv: 29 November 2010

NTT and NOT spectroscopy of SDSS-II supernovae

Authors: L. Ostman, J. Nordin, A. Goobar, R. Amanullah, M. Smith, J. Sollerman, V. Stanishev, M.D. Stritzinger, B.A. Bassett, T.M. Davis, E. Edmondson, J. A. Frieman, P.M. Garnavich, H. Lampeitl, G. Leloudas, J. Marriner, R.C. Nichol, K. Romer, M. Sako, D.P. Schneider, C. Zheng

http://arxiv.org/abs/1011.5869v1

Context. The SDSS-II Supernova Survey, conducted between 2005 and 2007, was designed to detect a large number of Type Ia supernovae (SNe Ia) around z~0.2, the redshift "gap" between low-z and high-z SN searches. The survey has provided multi-band photometric lightcurves for variable targets, and SN candidates were scheduled for spectroscopic observations, primarily to provide SN classification and accurate redshifts. We present SN spectra obtained in 2006 and 2007 using the NTT and the NOT. Aims. We provide an atlas of SN spectra in the range z =0.03-0.32 that complements the well-sampled lightcurves from SDSS-II in the forthcoming three-year SDSS SN cosmology analysis. The sample can, for example, be used for spectral studies of SNe Ia, which are critical for understanding potential systematic effects when SNe are used to determine cosmological distances. Methods. The spectra were reduced in a uniform manner, and special care was taken in estimating the uncertainties for the different processing steps. Host-galaxy light was subtracted when possible and the SN type fitted using the SuperNova IDentification code (SNID). We also present comparisons between spectral and photometric dating using SALT lightcurve fits to the photometry from SDSS-II, as well as the global distribution of our sample in terms of the lightcurve parameters: stretch and colour. Results. We report new spectroscopic data from 141 SNe Ia, mainly between -9 and +15 days from lightcurve maximum, including a few cases of multi-epoch observations. This homogeneous, host-galaxy subtracted, SN Ia spectroscopic sample is among the largest such data sets and unique in its redshift interval. The sample includes two potential SN 1991T-like SNe (SN 2006on and SN 2007ni) and one potential SN 2002cx-like SN (SN 2007ie). In addition, the new compilation includes spectra from 23 confirmed Type II and 8 Type Ib/c SNe.

The MOND Fundamental Plane

Authors: V.F. Cardone, G. Angus, A. Diaferio, C. Tortora, R. Molinaro
http://arxiv.org/abs/1011.5741v1
Modified Newtonian Dynamics (MOND) has been shown to be able to fit spiral galaxy rotation curves as well as giving a theoretical foundation for empirically determined scaling relations, such as the Tully - Fisher law, without the need for a dark matter halo. As a complementary analysis, one should investigate whether MOND can also reproduce the dynamics of early - type galaxies (ETGs) without dark matter. As a first step, we here show that MOND can indeed fit the observed central velocity dispersion $\sigma_0$ of a large sample of ETGs assuming a simple MOND interpolating functions and constant anisotropy. We also show that, under some assumptions on the luminosity dependence of the Sersic n parameter and the stellar M/L ratio, MOND predicts a fundamental plane for ETGs : a log - linear relation among the effective radius $R_{eff}$, $\sigma_0$ and the mean effective intensity $\langle I_e \rangle$. However, we predict a tilt between the observed and the MOND fundamental planes.

arXiv: 25 November 2010

 Imprints of a hemispherical power asymmetry in the seven-year WMAP data due to non-commutativity of space-time

Authors: N.E. Groeneboom, M. Axelsson, D. F. Mota, T. Koivisto
http://arxiv.org/abs/1011.5353v1

Non-commutative geometry at inflation can give arise to parity violating modulations of the primordial power spectrum. We develop the statistical tools needed for investigating whether these modulations are evident in the Cosmic Microwave Background (CMB). The free parameters of the models are two directional parameters (theta,phi), the signal amplitude A*, and a tilt parameter n* that modulates correlation power on different scales. The signature of the model corresponds to a kind of hemispherical power asymmetry. When analyzing the 7-year WMAP data we find a weak signature for a preferred direction in the Q-, V-, and W bands with direction (l,b) = (-225 deg,-25 deg) +- (20 deg, 20 deg), which is close to another previously discovered hemispherical power asymmetry. Although these results are intriguing, the significance of the detection in the W-, V- and Q-bands are nonzero at about 2 sigma, suggesting that the simplest parameterization of the leading correction represents only partially the effects of the space-time non-commutativity possibly responsible for the hemispherical asymmetry. Our constraints on the presence of a dipole are independent of its physical origin and prefer a blue-tilted spectral index n* ~ 0 with the amplitude A* ~ 0.18.

 

 

arXiv: 24 november 2010

Effect of a dark matter halo on the determination of black hole masses

Authors: Andreas Schulze, Karl Gebhardt
http://arxiv.org/abs/1011.5077v1

 Stellar dynamical modeling is a powerful method to determine the mass of black holes in quiescent galaxies. However, in previous work the presence of a dark matter halo has been ignored in the modeling. Gebhardt & Thomas (2009) showed that accounting for a dark matter halo increased the black-hole mass of the massive galaxy M87 by a factor of 2. We used a sample of 12 galaxies to investigate the effect of accounting for a dark matter halo in the dynamical modeling in more detail, and also updated the masses using improved modeling. The sample of galaxies possess HST and ground based observations of stellar kinematics. Their black-hole masses have been presented before, but without including a dark matter halo in the models. Without a dark halo, we find a mean increase in the estimated mass of 1.5 for the whole sample compared to previous results. We attribute this change to using a more complete orbit library. When we include a dark matter halo, along with the updated models, we find an additional increase in black-hole mass by a factor of 1.2 in the mean, much less than for M87. We attribute the smaller discrepancy in black-hole mass to using data that better resolves the black hole's sphere of influence. We redetermined the M-sigma and M-L relationships using our updated black-hole masses and find a slight increase in both normalization and intrinsic scatter.

 

arXiv: 23 November 2010

The Large-Scale 3-point correlation function of SDSS Luminous Red Galaxies

Authors: Felipe Marin (UChicago, Swinburne)

http://arxiv.org/abs/1011.4530v1

We present new measurements of the redshift-space three-point correlation function (3PCF) of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS). Using the largest dataset to date, the Data Release 7 (DR7) LRGs, and an improved binning scheme compared to previous measurements, we measure the LRG 3PCF on large scales up to ~90 Mpc/h, from the mildly non-linear to quasi-linear regimes. Comparing the LRG correlations to the dark matter two- and three-point correlation functions, obtained from N-body simulations we infer linear and non-linear bias parameters. As expected, LRGs are highly biased tracers of large scale structure, with a linear bias b1 ~ 2; the LRGs also have a large positive non-linear bias parameter, in agreement with predictions of galaxy population models. The use of the 3PCF to estimate biasing helps to also make estimates of the cosmological parameter {\sigma_8}, as well as to infer best-fit parameters of the Halo Occupation Distribution parameters for LRGs. We also use a large suite of public mock catalogs to characterize the error covariance matrix for the 3PCF and compare the variance among simulation results with jackknife error estimates.

 

 A new framework for analyzing the effects of small scale inhomogeneities in cosmology

Authors: Stephen R. Green, Robert M. Wald

http://arxiv.org/abs/1011.4920v1

 We develop a new, mathematically precise framework for treating the effects of nonlinear phenomena occurring on small scales in general relativity. Our approach is an adaptation of Burnett's formulation of the "shortwave approximation", which we generalize to analyze the effects of matter inhomogeneities as well as gravitational radiation. Our framework requires the metric to be close to a "background metric", but allows arbitrarily large stress-energy fluctuations on small scales. We prove that, within our framework, if the matter stress-energy tensor satisfies the weak energy condition (i.e., positivity of energy density in all frames), then the only effect that small scale inhomogeneities can have on the dynamics of the background metric is to provide an "effective stress-energy tensor" that is traceless and has positive energy density---corresponding to the presence of gravitational radiation. In particular, nonlinear effects produced by small scale inhomogeneities cannot mimic the effects of dark energy. We also develop "perturbation theory" off of the background metric. We derive an equation for the "long-wavelength part" of the leading order deviation of the metric from the background metric, which contains the usual terms occurring in linearized perturbation theory plus additional contributions from the small-scale inhomogeneities. Under various assumptions concerning the absence of gravitational radiation and the non-relativistic behavior of the matter, we argue that the "short wavelength" deviations of the metric from the background metric near a point $x$ should be accurately described by Newtonian gravity, taking into account only the matter lying within a "homogeneity lengthscale" of $x$. Finally, we argue that our framework should provide an accurate description of the actual universe.

 

 

arXiv: 22 November 2010

CMB Lensing - Power Without Bias

Authors: Blake D. Sherwin, Sudeep Das
http://arxiv.org/abs/1011.4510v1

We propose a novel bias-free method for reconstructing the power spectrum of the weak lensing deflection field from cosmic microwave background (CMB) observations. The proposed method is in contrast to the standard method of CMB lensing reconstruction where a reconstruction bias needs to be subtracted to estimate the lensing power spectrum. This bias depends very sensitively on the modeling of the signal and noise properties of the survey, and a misestimate can lead to significantly inaccurate results. Our method obviates this bias and hence the need to characterize the detailed noise properties of the CMB experiment. We illustrate our method with simulated lensed CMB maps with realistic noise distributions. This bias-free method can also be extended to create much more reliable estimators for other four-point functions in cosmology, such as those appearing in primordial non-Gaussianity estimators.

 

 Next-to-leading resummations in cosmological perturbation theory

Authors: Stefano Anselmi, Sabino Matarrese, Massimo Pietroni

http://arxiv.org/abs/1011.4477v1

 One of the nicest results in cosmological perturbation theory is the analytical resummaton of the leading corrections at large momentum, which was obtained by Crocce and Scoccimarro for the propagator. Using an exact evolution equation, we generalize this result, by showing that a class of next-to-leading corrections can also be resummed at all orders in perturbation theory. The new corrections modify the propagator by a few percent in the Baryonic Acoustic Oscillation range of scales, and therefore cannot be neglected in resummation schemes aiming at an accuracy compatible with future generation galaxy surveys. Similar tools can be employed to derive improved approximations for the Power Spectrum.

 

The Gauge-Invariant Bias of Dark Matter Haloes with Primordial non-Gaussianity

Authors: N. Bartolo (Univ. of Padova and INFN Padova), S. Matarrese (Univ. of Padova and INFN Padova), A. Riotto (CERN and INFN Padova)

http://arxiv.org/abs/1011.4374v1

The non-linear evolution of the halo population is followed by solving the continuity equation under the hypothesis that haloes move by the action of gravity. An exact and general formula for the Eulerian bias field of dark matter haloes in terms of the Lagrangian bias is expanded at second-order including the presence of primordial non-Gaussianity. Particular attention is paid in defining a gauge-invariant bias which is necessary when dealing with relativistic effects and measured quantities. We show that scale-dependent effects in the Eulerian bias arise both at first- and second-order independently from the presence of some primordial non-Gaussianity. Furthermore, the Eulerian bias inherits from the primordial non-Gaussianity not only a scale-dependence, but also a modulation with the angle of observation when sources with different biases are correlated.

 

 

arXiv: 19 November 2010

THINGS about MOND

Authors: G. Gentile, B. Famaey, W. J. G. de Blok

http://arxiv.org/abs/1011.4148v1

We present the analysis of 12 high-resolution galactic rotation curves from The HI Nearby Galaxy Survey (THINGS) in the context of modified Newtonian dynamics (MOND). These rotation curves were selected to be the most reliable for mass modelling, and they are the highest quality rotation curves currently available for a sample of galaxies spanning a wide range of luminosities. We fit the rotation curves with the "simple" and "standard" interpolating functions of MOND, and we find that the "simple" function yields better results. We also redetermine the value of a0, and find a median value very close to the one determined in previous studies, a0 = (1.22 +- 0.33) x 10^{-8} cm/s^2. Leaving the distance as a free parameter within the uncertainty of its best independently determined value leads to excellent quality fits for 75% of the sample. Among the three exceptions, two are also known to give relatively poor fits also in Newtonian dynamics plus dark matter. The remaining case (NGC 3198), presents some tension between the observations and the MOND fit, which might however be explained by the presence of non-circular motions, by a small distance, or by a value of a0 at the lower end of our best-fit interval, 0.9 x 10^{-8} cm/s^2. The best-fit stellar M/L ratios are generally in remarkable agreement with the predictions of stellar population synthesis models. We also show that the narrow range of gravitational accelerations found to be generated by dark matter in galaxies is consistent with the narrow range of additional gravity predicted by MOND.

 

Stellar Structure and Tests of Modified Gravity

Authors: Philip Chang, Lam Hui

http://arxiv.org/abs/1011.4107v1

Theories that attempt to explain cosmic acceleration by modifying gravity typically introduces a long-range scalar force that needs to be screened on small scales. One common screening mechanism is the chameleon, where the scalar force is screened in environments with a sufficiently deep gravitational potential, but acts unimpeded in regions with a shallow gravitational potential. This leads to a variation in the overall gravitational G with environment. We show such a variation can occur within a star itself, significantly affecting its evolution and structure, provided that the host galaxy is unscreened. The effect is most pronounced for red giants, which would be smaller by a factor of tens of percent and thus hotter by 100's of K, depending on the parameters of the underlying scalar-tensor theory. Careful measurements of these stars in suitable environments (nearby dwarf galaxies not associated with groups or clusters) would provide constraints on the chameleon mechanism that are four orders of magnitude better than current large scale structure limits, and two orders of magnitude better than present solar system tests.

 

The Consistency of the Dark-Matter-Interpreted Fermi-LAT Observations of the Galactic Center with a Millisecond Pulsar Population in the Central Stellar Cluster

Authors: Kevork N. Abazajian

http://arxiv.org/abs/1011.4275v2

 I show that the spectrum and morphology of the Fermi-LAT observation of the Galaxy center presented by the recent manuscript arXiv:1010.2752 are consistent with a millisecond pulsar population in the nuclear Central stellar cluster of the Milky Way. The Galaxy Center gamma-ray spectrum is consistent with the spectrum of four of eight globular clusters that have been detected in the gamma-ray. A dark matter annihilation interpretation cannot be ruled out, though no unique features exist that would require this conclusion.

 

 

arXiv: 16 November 2010

Fitting galactic rotation curves with conformal gravity and a global quadratic potential

Authors: Philip D. Mannheim, James G. O'Brien
http://arxiv.org/abs/1011.3495v1

We apply the conformal gravity theory to a sample of 110 spiral galaxies whose rotation curve data points extend well beyond the optical disk. With no free parameters other than galactic mass to light ratios, the theory is able to account for the systematics that is observed in this entire set of rotation curves without the need for any dark matter at all. In previous applications of the theory a central role was played by a universal linear potential term $V(r)=\gamma_0 c^2r/2$ that is generated through the effect of cosmology on individual galaxies, with the coefficient $\gamma_0=3.06\times 10^{-30}{\rm cm}^{-1}$ being of cosmological magnitude. Because the current sample is so big and encompasses some specific galaxies whose data points go out to quite substantial distances from galactic centers, we are able to identify an additional globally induced universal term in the data, a quadratic $V(r)=-\kappa c^2r^2/2$ term that is induced by inhomogeneities in the cosmic background. With $\kappa$ being found to be of magnitude $\kappa=9.54\times 10^{-54} {\rm cm}^{-2}$, through study of the motions of particles contained within galaxies we are thus able to both detect the presence of a global de Sitter-like component and provide a specific value for its strength. Our study suggests that invoking dark matter may be nothing more than an attempt to describe global physics effects such as these in purely local galactic terms.

 

Nonparametric Dark Energy Reconstruction from Supernova Data

Authors: Tracy Holsclaw, Ujjaini Alam, Bruno Sanso, Herbert Lee, Katrin Heitmann, Salman Habib, David Higdon

http://arxiv.org/abs/1011.3079v1

Understanding the origin of the accelerated expansion of the Universe poses one of the greatest challenges in physics today. Lacking a compelling fundamental theory to test, observational efforts are targeted at a better characterization of the underlying cause. If a new form of mass-energy, dark energy, is driving the acceleration, the redshift evolution of the equation of state parameter w(z) will hold essential clues as to its origin. To best exploit data from observations it is necessary to develop a robust and accurate reconstruction approach, with controlled errors, for w(z). We introduce a new, nonparametric method for solving the associated statistical inverse problem based on Gaussian Process modeling and Markov chain Monte Carlo sampling. Applying this method to recent supernova measurements, we reconstruct the continuous history of w out to redshift z=1.5.

 

Local Gravity versus Local Velocity: Solutions for $\beta$ and nonlinear bias

Authors: Marc Davis, Adi Nusser, Karen Masters, Christopher Springob, John P. Huchra, Gerard Lemson

http://arxiv.org/abs/1011.3114v1

(abridged) We perform a reconstruction of the cosmological large scale flows in the nearby Universe using two complementary observational sets. The first, the SFI++ sample of Tully-Fisher (TF) measurements of galaxies, provides a direct probe of the flows. The second, the whole sky distribution of galaxies in the 2MASS redshift survey (2MRS), yields a prediction of the flows given the cosmological density parameter, $\Omega$, and a biasing relation between mass and galaxies. We aim at an unbiased comparison between the peculiar velocity fields extracted from the two data sets and its implication on the cosmological parameters and the biasing relation. We expand the fields in a set of orthonormal basis functions, each representing a plausible realization of a cosmological velocity field. Our analysis completely avoids the strong error covariance in the smoothed TF velocities by the use of orthonormal basis functions and employs elaborate realistic mock data sets to extensively calibrate the errors in 2MRS predicted velocities. We relate the 2MRS galaxy distribution to the mass density field by a linear bias factor, $b$, and include a luminosity dependent, $\propto L^\alpha$, galaxy weighting. We assess the agreement between the fields as a function of $\alpha$ and $\beta=f(\Omega)/b$, where $f$ is the growth factor of linear perturbations. The agreement is excellent with a reasonable $\chi^2$ per degree of freedom. For $\alpha=0$ , we derive $0.28<\beta<0.37$ and $0.24<\beta<0.43$, respectively, at the 68.3% and 95.4% confidence levels (CLs). For $\beta=0.33$, we get $\alpha<0.25$ and $\alpha<0.5$, respectively, at the 68.3% and 95.4% CLs. We set a constraint on the fluctuation normalization, finding $\sigma_8 = 0.73 \pm 0.1$, in very good agreement with the latest WMAP results.

 

Distinguishability of scalar field models of dark energy with time variable equation of state parameter

Authors: Bohdan Novosyadlyj, Olga Sergijenko, Stepan Apunevych

http://arxiv.org/abs/1011.3474v1

The possibility of distinguishing of scalar field models of dark energy with different Lagrangians and time variable equation of state parameter by available observational data is analyzed. The multicomponent cosmological model with the scalar field with either Klein-Gordon or Dirac-Born-Infeld Lagrangians as dark energy and the monotonic decreasing and increasing equation of state parameters are considered. It is concluded that scalar field models of dark energy with decreasing and increasing EoS parameters should be distinguishable at the accuracy level of forthcoming observational data. The Lagrangians of scalar fields could be distinguished by expected observational data (Planck, SDSS etc.) in the case of decreasing EoS parameter, but are practically indistinguishable in the case of increasing one.

 

 

arXiv: November 2010

HIFLUGCS: Galaxy Cluster Scaling Relations between X-ray Luminosity, Gas Mass, Cluster Radius, and Velocity Dispersion

Authors: Y.-Y. Zhang, H. Andernach, C. A. Caretta, T. H. Reiprich, H. Boehringer, E. Puchwein, D. Sijacki, M. Girardi
http://arxiv.org/abs/1011.3018v1

We present relations between X-ray luminosity and velocity dispersion ($L-\sigma$), X-ray luminosity and gas mass ($L-M_{\rm gas}$), and between cluster radius and velocity dispersion ($r_{500}-\sigma$) for 62 galaxy clusters in the HIFLUGCS, an X-ray flux-limited sample minimizing bias toward any cluster morphology. Our analysis is based on in total ~1.3 Msec clean X-ray XMM-Newton data and on 13439 cluster member galaxies with redshifts. The presence of cool cores is one of the major contributors to the scatter of the $L-\sigma$ relation. When the cool-core corrected X-ray luminosity is used the intrinsic scatter is reduced to 0.27 dex. Even with the X-ray luminosity corrected for the cool core, the scatter caused by the presence of cool cores dominates for the low-mass systems. The scatter caused by the non-cool-core clusters does not strongly depend on the mass range, and becomes dominant in the high-mass regime. The observed $L-\sigma$ relation agrees with the self-similar prediction, matches that of a simulated sample with AGN feedback, disregarding six clusters with $<45$ cluster members with spectroscopic redshifts, and shows a common trend of increasing scatter toward the low-mass end, i.e., systems with $\sigma<500$ km/s. A comparison of observations with simulations indicates an AGN feedback driven impact in the low-mass regime. The best fits of the $L-M_{\rm gas}$ relations for the disturbed clusters and undisturbed clusters in the observational sample match very well the simulated samples with and without AGN feedback, respectively. This suggests one main cause of the scatter being AGN activities providing feedback in different phases, e.g. during a feedback cycle. The slope and scatter of the observed $r_{500}-\sigma$ relation is similar to that of the simulated sample with AGN feedback except for a small offset but still within the scatter.

 

Testing gravity using the growth of large scale structure in the Universe

Authors: Elise Jennings, Carlton M. Baugh, Silvia Pascoli

http://arxiv.org/abs/1011.2842v1

Future galaxy surveys hope to distinguish between the dark energy and modified gravity scenarios for the accelerating expansion of the Universe using the distortion of clustering in redshift space. The aim is to model the form and size of the distortion in order to infer the rate at which large scale structure grows. We test this hypothesis and assess the performance of current theoretical models for the redshift space distortion using very large volume N-body simulations of the gravitational instability process. We simulate competing cosmological models which have identical expansion histories - one is a quintessence dark energy model with a scalar field and the other is a modified gravity model with a time varying gravitational constant - and demonstrate that they do indeed produce different redshift space distortions. This is the first time this approach has been verified using a technique that can follow the growth of structure at the required level of accuracy. Our comparisons show that theoretical models for the redshift space distortion based on linear perturbation theory, which are currently in widespread use, give a surprisingly poor description of the simulation results. Furthermore, the application of such models can give rise to catastrophic systematic errors leading to an incorrect interpretation of the observations. We show that an improved model is able to extract the correct growth rate. Further enhancements to theoretical models of redshift space distortions, calibrated against simulations, are needed if we are to fully exploit the forthcoming high precision clustering measurements.

 

 

 

arXiv: 12 November 2010

Real-time Cosmology

Authors: Claudia Quercellini, Luca Amendola, Amedeo Balbi, Paolo Cabella, Miguel Quartin

http://arxiv.org/abs/1011.2646v1

In recent years the possibility of measuring the temporal change of radial and transverse position of sources in the sky in real time have become conceivable thanks to the thoroughly improved technique applied to new astrometric and spectroscopic experiments, leading to the research domain we call Real-time cosmology. We review for the first time great part of the work done in this field, analysing both the theoretical framework and some endeavor to foresee the observational strategies and their capability to constrain models. We firstly focus on real time measurements of the overall redshift drift and angular separation shift in distant source, able to trace background cosmic expansion and large scale anisotropy, respectively. We then examine the possibility of employing the same kind of observations to probe peculiar and proper acceleration in clustered systems and therefore the gravitational potential. The last two sections are devoted to the short time future change of the cosmic microwave background, as well as to the temporal shift of the temperature anisotropy power spectrum and maps. We conclude revisiting in this context the effort made to forecast the power of upcoming experiments like CODEX, GAIA and PLANCK in providing these new observational tools.

 

Level Crossing Analysis of Cosmic Microwave Background Radiation: A method for detecting cosmic strings

Authors: M. Sadegh Movahed, S. Khosravi

http://arxiv.org/abs/1011.2640v1

In this paper we study the footprint of cosmic string as the topological defects in the very early universe on the cosmic microwave background radiation. We develop the method of level crossing analysis in the context of the well-known Kaiser-Stebbins phenomenon for exploring the signature of cosmic strings. We simulate a Gaussian map by using the best fit parameter given by WMAP-7 and then superimpose cosmic strings effects on it as an incoherent and active fluctuations. In order to investigate the capability of our method to detect the cosmic strings for the various values of tension, $G\mu$, a simulated pure Gaussian map is compared with that of including cosmic strings. Based on the level crossing analysis, the superimposed cosmic string with $G\mu\gtrsim 4\times 10^{-9}$ in the simulated map without instrumental noise and the resolution $R=1'$ could be detected. In the presence of anticipated instrumental noise the lower bound increases just up to $G\mu\gtrsim 5.8\times 10^{-9}$.

 

The Peaks Formalism and the Formation of Cold Dark Matter Haloes

Authors: Aaron D. Ludlow, Cristiano Porciani

http://arxiv.org/abs/1011.2493v1

We use two cosmological simulations of structure formation to study the conditions under which dark matter haloes emerge from the linear density field. Our analysis focuses on matching sites of halo collapse to local density maxima, or "peaks", in the initial conditions of the simulations and provides a crucial test of the central ansatz of the peaks formalism. By identifying peaks on a variety of smoothed, linearly extrapolated density fields we demonstrate that as many as ~70% of well-resolved dark matter haloes form preferentially near peaks whose characteristic masses are similar to that of the halo, with more massive haloes showing a stronger tendency to reside near peaks initially. We identify a small but significant fraction of haloes that appear to evolve from peaks of substantially lower mass than that of the halo itself. We refer to these as "peakless haloes" for convenience. By contrasting directly the properties of these objects with the bulk of the proto-halo population we find two clear differences: 1) their initial shapes are significantly flatter and more elongated than the predominantly triaxial majority, and 2) they are, on average, more strongly compressed by tidal forces associated with their surrounding large scale structure. Using the two-point correlation function we show that peakless haloes tend to emerge from highly clustered regions of the initial density field implying that, at fixed mass, the accretion geometry and mass accretion histories of haloes in highly clustered environments differ significantly from those in the field. This may have important implications for understanding the origin of the halo assembly bias, of galaxy properties in dense environments and how environment affects the morphological transformation of galaxies near groups and rich galaxy clusters.

 

Linking haloes to galaxies: how many halo properties are needed?

Authors: Eyal Neistein, Simone M. Weinmann, Cheng Li, Michael Boylan-Kolchin

http://arxiv.org/abs/1011.2492v1

Recent studies emphasize that an empirical relation between the stellar mass of galaxies and the mass of their host dark matter subhaloes can predict the clustering of galaxies and its evolution with cosmic time. In this paper we study the assumptions made by this methodology using a semi-analytical model (SAM). To this end, we randomly swap between the locations of model galaxies within a narrow range of subhalo mass (M_infall). We find that shuffled samples of galaxies have different auto-correlation functions in comparison with the original model galaxies. This difference is significant even if central and satellite galaxies are allowed to follow a different relation between M_infall and stellar mass, and can reach a factor of 2 for massive galaxies at redshift zero. We analyze three features within SAMs that contribute to this effect: a) The relation between stellar mass and subhalo mass evolves with redshift for central galaxies, affecting satellite galaxies at the time of infall. b) The stellar mass of galaxies falling into groups and clusters at high redshift is different from the mass of central galaxies at the same time. c) The stellar mass growth for satellite galaxies after infall can be significant and depends on the infall redshift and the group mass. We show that the above is true for differing SAMs, and that the effect is sensitive to the treatment of dynamical friction and stripping of gas in satellite galaxies. We find that by using the FoF group mass at redshift zero in addition to M_infall, an empirical model is able to accurately reproduce the clustering properties of galaxies. On the other hand, using the infall redshift as a second parameter does not yield as good results because it is less correlated with stellar mass. Our analysis indicates that environmental processes are important for modeling the clustering and abundance of galaxies. (Abridged)

 

 

arXiv: 11 November 2010

Results from a Low-Energy Analysis of the CDMS II Germanium Data

Authors: CDMS Collaboration: Z. Ahmed, D. S. Akerib, S. Arrenberg, C. N. Bailey, D. Balakishiyeva, L. Baudis, D. A. Bauer, P. L. Brink, T. Bruch, R. Bunker, B. Cabrera, D. O. Caldwell, J. Cooley, P. Cushman, M. Daal, F. DeJongh, M. R. Dragowsky, L. Duong, S. Fallows, E. Figueroa-Feliciano, J. Filippini, M. Fritts, S. R. Golwala, J. Hall, R. Hennings-Yeomans, S. A. Hertel, D. Holmgren, L. Hsu, M. E. Huber, O. Kamaev, M. Kiveni, M. Kos, S. W. Leman, R. Mahapatra, V. Mandic, K. A. McCarthy, N. Mirabolfathi, D. Moore, H. Nelson, R. W. Ogburn, A. Phipps, M. Pyle, X. Qiu, E. Ramberg, W. Rau, A. Reisetter, T. Saab, B. Sadoulet, J. Sander, R. W. Schnee, D. N. Seitz, B. Serfass, K. M. Sundqvist, M. Tarka, P. Wikus, S. Yellin, J. Yoo, B. A. Young, J. Zhang

http://arxiv.org/abs/1011.2482v1

We report results from a reanalysis of data from the Cryogenic Dark Matter Search (CDMS II) experiment at the Soudan Underground Laboratory. Data taken between October 2006 and September 2008 using eight germanium detectors are reanalyzed with a lowered, 2 keV recoil-energy threshold, to give increased sensitivity to interactions from Weakly Interacting Massive Particles (WIMPs) with masses below ~10 GeV/c^2. This analysis provides stronger constraints than previous CDMS II results for WIMP masses below 9 GeV/c^2 and excludes parameter space associated with possible low-mass WIMP signals from the DAMA/LIBRA and CoGeNT experiments.

 

Redshift-Space Enhancement of Line-of-Sight Baryon Acoustic Oscillations in the SDSS Main-Galaxy Sample

Authors: H.J. Tian, Mark C. Neyrinck, Tamás Budavári, Alexander S. Szalay (JHU)

http://arxiv.org/abs/1011.2481v1

We show that redshift-space distortions of galaxy correlations have a strong effect on correlation functions with distinct, localized features, like the signature of the Baryon Acoustic Oscillations (BAO). Near the line of sight, the features become sharper as a result of redshift-space distortions. We demonstrate this effect by measuring the correlation function in Gaussian simulations and the Millennium Simulation. We also analyze the SDSS DR7 main-galaxy sample (MGS), splitting the sample into slices 2.5 degrees on the sky in various rotations. Measuring 2D correlation functions in each slice, we do see a sharp bump along the line of sight. Using Mexican-hat wavelets, we localize it to (110 +/- 10) Mpc/h. At a particular wavelet scale and location, employing the variance of the wavelet transform, we estimate its significance at about 4 sigma. We estimate that there is about a 0.2% chance of getting such a signal anywhere in the vicinity of the BAO scale from a power spectrum lacking a BAO feature. However, these estimates of the significances make some use of idealized Gaussian simulations, and thus are likely a bit optimistic.

 

Constraining Entropic Cosmology

Authors: Tomi S. Koivisto, David F. Mota, Miguel Zumalacarregui

http://arxiv.org/abs/1011.2226v1

It has been recently proposed that the interpretation of gravity as an emergent, entropic force might have nontrivial implications to cosmology. Here two approaches are investigated: in one, the Friedman equation receives entropic contributions from the usually neglected surface terms, and in another, the extra terms are derived from quantum corrections to the entropy formula. UV terms may drive inflation, avoiding a recently derived no-go theorem, though in some cases leading to a graceful exit problem. IR terms can generate dark energy, alleviating the cosmological constant problem. The quantum corrections are bounded by their implications to the BBN, and the surface terms are constrained in addition by their effect upon the behavior of matter. Likelihood analyses are performed to constrain the modifications by the SNeIa, BAO and CMB data. It is found that a monomial correction to the area-entropy formula results in late acceleration in very good agreement with observations, which then turn out to be compatible with positive curvature. The evolution of perturbations is deduced by assuming the Jebsen-Birkhoff theorem. Distinct signatures can then be identified in the large scale structure formation. Furthermore, it is shown that the visible universe satisfies the Bekenstein bound.

 

 

arXiv: 10 November 2010

Complementarity of Weak Lensing and Peculiar Velocity Measurements in Testing General Relativity

Authors: Yong-Seon Song (1,2), Gong-Bo Zhao (2), David Bacon (2), Kazuya Koyama (2), Robert C Nichol (2), Levon Pogosian (3) ((1) KIAS, (2) ICG Portsmouth,

http://arxiv.org/abs/1011.2106v1

We explore the complementarity of weak lensing and galaxy peculiar velocity measurements to better constrain modifications to General Relativity. We find no evidence for deviations from GR on cosmological scales from a combination of peculiar velocity measurements (for Luminous Red Galaxies in the Sloan Digital Sky Survey) with weak lensing measurements (from the CFHT Legacy Survey). We provide a Fisher error forecast for a Euclid-like space-based survey including both lensing and peculiar velocity measurements, and show that the expected constraints on modified gravity will be at least an order of magnitude better than with present data, i.e. we will obtain 5% errors on the modified gravity parametrization described here. We also present a model--independent method for constraining modified gravity parameters using tomographic peculiar velocity information, and apply this methodology to the present dataset.

 

CMB statistics in noncommutative inflation

Authors: Tomi S. Koivisto, David F. Mota

http://arxiv.org/abs/1011.2126v1

Noncommutative geometry can provide effective description of physics at very short distances taking into account generic effects of quantum gravity. Inflation amplifies tiny quantum fluctuations in the early universe to macroscopic scales and may thus imprint high energy physics signatures in the cosmological perturbations that could be detected in the CMB. It is shown here that this can give rise to parity-violating modulations of the primordial spectrum and odd non-Gaussian signatures. The breaking of rotational invariance of the CMB provides constraints on the scale of noncommutativity that are competitive with the existing noncosmological bounds, and could explain the curious hemispherical asymmetry that has been claimed to be observed in the sky. This introduces also non-Gaussianity with peculiar shape- and scale-dependence, which in principle allows an independent cross-check of the presence of noncommutativity at inflation.

 

The influence of inhomogeneities on the large-scale expansion of the universe

Authors: Hael Collins (The Niels Bohr International Academy and The Discovery Center)

http://arxiv.org/abs/1011.2046v1

The evolution of an inhomogeneous universe composed entirely of matter is followed from an early, nearly uniform state until the time when the inhomogeneities have begun to grow large. The particular distribution of matter studied in this article is chosen to have a periodic variation in only one of the directions, which is simple enough to allow the behavior of the metric to be solved analytically, well beyond a linear approximation based on the initial smallness of the fluctuation. This example provides an illustration of a universe where the inhomogeneities can affect its average expansion rate; and its simplicity allows a condition to be derived that tells when their presence should begin to become important. Since the averages of the non-uniform parts of the metric and the matter density grow faster than their uniform parts, the average expansion rate accelerates with the advent of the era governed by the inhomogeneities.

 

 

arXiv: 9 November 2010

Evolution of intermediate mass galaxies up to z~0.7 and studies of SNe Ia hosts

Authors: Myriam Rodrigues

http://arxiv.org/abs/1011.1884v1

PhD thesis

In the first part of this manuscript, I present the results on the properties of the interstellar medium and the stellar content of galaxies at z=0.6, from a representative sample of distant galaxies observed with the long slit spectrograph VLT/FORS2. This study has been realized in the framework of the ESO large program IMAGES "Intermediate MAss Galaxy Evolution Sequences", which aims to investigate the evolution of the main global properties of galaxies up to z~0.9. I discuss the implications of the observed chemical enrichment of the gas on the scenarios of galaxy formation. I also propose a new method to estimate reliable stellar masses in starburst galaxies using broadband photometry and their total star-formation rate.
In a second part, I present a new method to extract, with high accuracy, the sky in spectra acquired with a fiber-fed instrument. I have developed this code in the Framework of the phase A of an instrument proposed for the E-ELT: OPTIMOS-EVE. This is a multi-fiber spectrograph able to observe at optical and infrared wavelengths simultaneously.
In the third part, I show preliminary results from the CENTRA GEPI- survey at Calar Alto Observatory to study nearby galaxies, hosts of type Ia supernovae, using integral field spectroscopy. I present the first 2D maps of the gas and stellar populations of SNe Ia hosts. The results allow us to directly access the host properties in the immediate vicinity of the SNe Ia. This is a crucial step to investigate eventual correlations between galaxy properties and SNe Ia events and evolution, leading to systematic effects on the derivation of the cosmological parameters.

 

A study of relative velocity statistics in Lagrangian perturbation theory with PINOCCHIO

Authors: Lavinia Heisenberg (DPT/Geneve), Bjoern Malte Schaefer (ARI/Heidelberg), Matthias Bartelmann (ITA/Heidelberg)

http://arxiv.org/abs/1011.1559v1

Subject of this paper is a careful and detailed analysis of the PINOCCHIO algorithm for studying the relative velocity statistics of merging haloes in Lagrangian perturbation theory. Given a cosmological background model, a power spectrum of fluctuations as well as a Gaussian linear density contrast field $\delta_{\rm l}$ is generated on a cubic grid, which is then smoothed repeatedly with Gaussian filters. For each Lagrangian particle at position $\bmath{q}$ and each smoothing radius $R$, the collapse time, the velocities and ellipsoidal truncation are computed using Lagrangian Perturbation Theory. The collapsed medium is then fragmented into isolated objects by an algorithm designed to mimic the accretion and merger events of hierarchical collapse. Directly after the fragmentation process the mass function, merger histories of haloes and the statistics of the relative velocities at merging are evaluated. We reimplemented the algorithm in C++ and optimised the construction of halo merging histories. Comparing our results with the output of the Millennium simulation suggests that PINOCCHIO is well suited for studying relative velocities of merging haloes and is able to reproduce the pairwise velocity distribution.

 

Primordial non-Gaussianity in the Bispectrum of the Halo Density Field

Authors: Tobias Baldauf, Uros Seljak, Leonardo Senatore

http://arxiv.org/abs/1011.1513v1

The bispectrum vanishes for linear Gaussian fields and is thus a sensitive probe of non-linearities and non-Gaussianities in the cosmic density field. Hence, a detection of the bispectrum in the halo density field would enable tight constraints on non-Gaussian processes in the early Universe and allow inference of the dynamics driving inflation. We present a tree level derivation of the halo bispectrum arising from non-linear clustering, non-linear biasing and primordial non-Gaussianity. A diagrammatic description is developed to provide an intuitive understanding of the contributing terms and their dependence on scale, shape and the non-Gaussianity parameter fNL. We compute the terms based on a multivariate bias expansion and the peak-background split method and show that non-Gaussian modifications to the bias parameters lead to amplifications of the tree level bispectrum that were ignored in previous studies. Our results are in a good agreement with published simulation measurements of the halo bispectrum. Finally, we estimate the expected signal to noise on fNL and show that the constraint obtainable from the bispectrum analysis significantly exceeds the one obtainable from the power spectrum analysis.

 

The Wall of Fundamental Constants

Authors: Keith A. Olive, Marco Peloso, Jean-Philippe Uzan

http://arxiv.org/abs/1011.1504v1

We consider the signatures of a domain wall produced in the spontaneous symmetry breaking involving a dilaton-like scalar field coupled to electromagnetism. Domains on either side of the wall exhibit slight differences in their respective values of the fine-structure constant, alpha. If such a wall is present within our Hubble volume, absorption spectra at large redshifts may or may not provide a variation in alpha relative to the terrestrial value, depending on our relative position with respect to the wall. This wall could resolve the ``contradiction'' between claims of a variation of alpha based on Keck/Hires data and of the constancy of alpha based on VLT data. We derive the properties of the wall and the parameters of the underlying microscopic model required to reproduce the possible spatial variation of alpha. We discuss the constraints on the existence of the low-energy domain wall and describe its observational implications concerning the variation of the fundamental constants.

 

The nature of assembly bias - I. Clues from a LCDM cosmology

Authors: Ivan Lacerna, Nelson Padilla

http://arxiv.org/abs/1011.1498v1

We present a new proxy for the overdensity peak height for which the large-scale clustering of haloes of a given mass does not vary significantly with the assembly history. The peak height, usually taken to be well represented by the virial mass, can instead be approximated by the mass inside spheres of different radii, which in some cases can be larger than the virial radius and therefore include mass outside the individual host halo. The sphere radii are defined as r = $a$ delta_t + $b$ log_10(M_vir/M_nl), where delta_t is the age relative to the typical age of galaxies hosted by haloes with virial mass M_vir, M_nl is the non-linear mass, and $a$=0.2 and $b$=-0.02 are the free parameters adjusted to trace the assembly bias effect. Note that $r$ depends on both halo mass and age. In this new approach, some of the objects which were initially considered low-mass peaks belong to regions with higher overdensities. At large scales, i.e. in the two-halo regime, this model properly recovers the simple prescription where the bias responds to the height of the mass peak alone, in contrast to the usual definition (virial mass) that shows a strong dependence on additional halo properties such as formation time. The dependence on the age in the one-halo term is also remarkably reduced. The population of galaxies whose "peak height" changes with this new definition consists mainly of old stellar populations and are preferentially hosted by low-mass haloes located near more massive objects. The latter is in agreement with recent results which indicate that old, low-mass haloes would suffer truncation of mass accretion by nearby larger haloes or simply due to the high density of their surroundings, thus showing an assembly bias effect. The change in mass is small enough that the Sheth et al. (2001) mass function is still a good fit to the resulting distribution of new masses.

 

Ultraviolet Divergences in Cosmological Correlations

Authors: Steven Weinberg

http://arxiv.org/abs/1011.1630v1

A method is developed for dealing with ultraviolet divergences in calculations of cosmological correlations, which does not depend on dimensional regularization. An extended version of the WKB approximation is used to analyze the divergences in these calculations, and these divergences are controlled by the introduction of Pauli--Villars regulator fields. This approach is illustrated in the theory of a scalar field with arbitrary self-interactions in a fixed flat-space Robertson--Walker metric with arbitrary scale factor $a(t)$. Explicit formulas are given for the counterterms needed to cancel all dependence on the regulator properties, and an explicit prescription is given for calculating finite regulator-independent correlation functions. The possibility of infrared divergences in this theory is briefly considered.

 

 

arXiv: 8 November 2010

N-body Simulations for f(R) Gravity using a Self-adaptive Particle-Mesh Code

Authors: Gong-Bo Zhao (ICG, Portsmouth), Baojiu Li (Cambridge), Kazuya Koyama (ICG, Portsmouth)

http://arxiv.org/abs/1011.1257v1

We perform high resolution N-body simulations for f(R) gravity based on a self-adaptive particle- mesh code MLAPM. The Chameleon mechanism that recovers General Relativity on small scales is fully taken into account by self-consistently solving the non-linear equation for the scalar field. We independently confirm the previous simulation results, including the matter power spectrum, halo mass function and density profiles, obtained by Oyaizu et al. (Phys.Rev.D 78, 123524, 2008) and Schmidt et al. (Phys.Rev.D 79, 083518, 2009), and extend the resolution up to k~20 h/Mpc for the measurement of the matter power spectrum. Based on our simulation results, we discuss how the Chameleon mechanism affects the clustering of dark matter and halos on full non-linear scales.

arXiv: 5 November 2010

Cluster Abundance in f(R) Gravity Models

Authors: Simone Ferraro, Fabian Schmidt, Wayne Hu

http://arxiv.org/abs/1011.0992v1

As one of the most powerful probes of cosmological structure formation, the abundance of massive galaxy clusters is a sensitive probe of modifications to gravity on cosmological scales. In this paper, we present results from N-body simulations of a general class of f(R) models, which self-consistently solve the non-linear field equation for the enhanced forces. Within this class we vary the amplitude of the field, which controls the range of the enhanced gravitational forces, both at the present epoch and as a function of redshift. Most models in the literature can be mapped onto the parameter space of this class. Focusing on the abundance of massive dark matter halos, we compare the simulation results to a simple spherical collapse model. Current constraints lie in the large-field regime, where the chameleon mechanism is not important. In this regime, the spherical collapse model works equally well for a wide range of models and can serve as a model-independent tool for placing constraints on f(R) gravity from cluster abundance. Using these results, we show how constraints from the observed local abundance of X-ray clusters on a specific f(R) model can be mapped onto other members of this general class of models.

 

 

aarXiv: 4 November 2010

Cosmic acceleration and the challenge of modifying gravity

Authors: Mark Trodden

http://arxiv.org/abs/1011.0861v1

I briefly discuss the challenges presented by attempting to modify general relativity to obtain an explanation for the observed accelerated expansion of the universe. Foremost among these are the questions of theoretical consistency - the avoidance of ghosts in particular - and the constraints imposed by precision local tests of gravity within the solar system. For those models that clear these highly constraining hurdles, modern observational cosmology offers its own suite of tests, improving with upcoming datasets, that offer the possibility of ruling out modified gravity approaches or providing an intriguing hint of new infrared physics. In the second half of the talk, I discuss a recent approach to extracting cosmology from higher-dimensional induced gravity models.

 

 

arXiv: 3 November 2010

Towards a non-anthropic solution to the cosmological constant problem

Authors: Andrei Linde, Vitaly Vanchurin

http://arxiv.org/abs/1011.0119v1

Many probability measures in the multiverse depend exponentially on some observable parameters, giving rise to potential problems such as youngness bias, Q-catastrophe etc. In this paper we explore a possibility that the exponential runaway dependence should be viewed not as a problem, but as a feature that may help us to fix all parameters in the landscape, including the value of the cosmological constant, without using anthropic considerations.

 

Unified cosmic history in modified gravity: from F(R) theory to Lorentz non-invariant models

Authors: Shin'ichi Nojiri, Sergei D. Odintsov

http://arxiv.org/abs/1011.0544v1

Classical generalization of general relativity is considered as gravitational alternative for unified description of the early-time inflation with late-time cosmic acceleration. The structure and cosmological properties of number of modified theories, including traditional $F(R)$ and Ho\v{r}ava-Lifshitz $F(R)$ gravity, scalar-tensor theory, string-inspired and Gauss-Bonnet theory, non-local gravity, non-minimally coupled models, and power-counting renormalizable covariant gravity are discussed. Different representations and relations between such theories are investigated. It is shown that some versions of above theories may be consistent with local tests and may provide qualitatively reasonable unified description of inflation with dark energy epoch. The cosmological reconstruction of different modified gravities is made in great detail. It is demonstrated that eventually any given universe evolution may be reconstructed for the theories under consideration: the explicit reconstruction is applied to accelerating spatially-flat FRW universe. Special attention is paid to Lagrange multiplier constrained and conventional $F(R)$ gravities, for last theory the effective $\Lambda$CDM era and phantom-divide crossing acceleration are obtained. The occurrence of Big Rip and other finite-time future singularities in modified gravity is reviewed as well as its curing via the addition of higher-derivative gravitational invariants.