arXiv: 14 June 2010

Effects of biasing on the matter power spectrum at large scales

Authors: Jose Beltrán Jiménez, Ruth Durrer

http://arxiv.org/abs/1006.2343v1

In this paper we study the effect of biasing on the power spectrum at large scales. We show that even though non-linear biasing does introduce a white noise contribution on large scales, the $P(k)\propto k^n$ behavior of the matter power spectrum on large scales, may still be visible and above the white noise for about one decade. We show, that the Kaiser biasing scheme which leads to linear bias of the correlation function on large scales generates a linear bias of the power spectrum on small scales (large wave number). We also discuss the effect of biasing on the baryon acoustic oscillations.

 

A large change in the predicted number of small halos due to a small amplitude oscillating inflaton potential

Authors: Luiz Felippe S. Rodrigues, Reuven Opher

http://arxiv.org/abs/1006.2159v1

A smooth inflaton potential is generally assumed when calculating the primordial power spectrum, implicitly assuming that a very small oscillation in the inflaton potential creates a negligible change in the predicted halo mass function. We show that this is not true. We find that a small oscillating perturbation in the inflaton potential in the slow-roll regime can alter significantly the predicted number of small halos. A class of models derived from supergravity theories gives rise to inflaton potentials with a large number of steps and many transplanckian effects may generate oscillations in the primordial power spectrum. The potentials we study are the simple quadratic (chaotic inflation) potential with superimposed small oscillations for small field values. Without leaving the slow-roll regime, we find that for a wide choice of parameters, the predicted number of halos change appreciably. For the oscillations beginning in the 10^7-10^8 solar masses range, for example, we find that only a 5% change in the amplitude of the chaotic potential causes a 50% suppression of the number of halos for masses between 10^7-10^8 solar masses and an increase in the number of halos for masses <10^6 solar masses by factors ~15-50. We suggest that this might be a solution to the problem of the lack of observed dwarf galaxies in the range 10^7-10^8 solar masses. This might also be a solution to the reionization problem where a very large number of Population III stars in low mass halos are required.

 

 

arXiv: 11 June 2010

Semielastic Dark Matter

Authors: David Krohn, Joshua T. Ruderman, Lian-Tao Wang

http://arxiv.org/abs/1006.1357v1
Many models have recently been proposed in which dark matter (DM) couples to Standard Model fields via a GeV-scale dark sector. We consider scenarios of this type where the DM mass, at the electroweak/TeV scale, is generated by the VEV of a singlet which also couples to the Higgs. Such a setup results in a distinct recoil spectrum with both elastic and inelastic components. We construct an explicit NMSSM-like realization of this setup, discuss constraints coming from the relic density, and include benchmark points which are consistent with current limits, yet visible at upcoming direct detection experiments.

 

Impact of Cluster Physics on the Sunyaev-Zel'dovich Power Spectrum

Authors: Laurie D. Shaw, Daisuke Nagai, Suman Bhattacharya, Erwin T. Lau

http://arxiv.org/abs/1006.1945v1

We use an analytic model to investigate the theoretical uncertainty on the thermal Sunyaev-Zel'dovich (SZ) power spectrum due to astrophysical uncertainties in the thermal structure of the intracluster medium (ICM). Our model accounts for star formation and energy feedback (from supernovae and Active Galactic Nuclei) as well as radially-dependent non-thermal pressure support due to random gas motions, the latter calibrated off the results of recent hydrodynamical simulations. We compare our model against X-ray observations of low redshift clusters, finding excellent agreement with observed pressure profiles. Varying the levels of feedback and non-thermal pressure support can significantly change both the amplitude and shape of the thermal SZ power spectrum. Increasing the feedback suppresses power at small angular scales, shifting the peak of the power spectrum to smaller angular scales. On the other hand, increasing the non-thermal pressure support has the opposite effect, significantly reducing power at large angular scales. In general, including non-thermal pressure at the level measured in simulations has a large effect on the power spectrum, reducing the amplitude by 50% at angular scales of a few arcminutes compared to a model without a non-thermal component. Our results demonstrate that measurements of the shape of the power spectrum can reveal useful information on important physical processes in groups and clusters, especially at high-redshift where there exists little observational data. Comparing with the recent SPT measurements of the small scale CMB power spectrum, we find our model reduces the tension between the value of sigma_8 measured from the SZ power spectrum and from cluster abundances.

 

Probability of the most massive cluster under non-Gaussian initial conditions

Authors: Laura Cayón, Christopher Gordon, Joseph Silk

http://arxiv.org/abs/1006.1950v1

Very massive high redshift clusters can be used to constrain and test the Lambda-CDM model. Taking into account the observational constraints of Jee et al. (2009) we have calculated the probability for the most massive cluster to be found in the range (5.2 - 7.6) x10^14M\odot, between redshifts 1.4<=z<=2.2 and under non-Gaussian initial conditions. Clusters constrain the non-Gaussianity on much smaller scales than current cosmic microwave background or halo bias data and so can be used to test for running of the non-Gaussianity parameter fNL. Combining with WMAP7 data, we find that on cluster scales there is a 92% probability for fNL > 0. If we assume that fNL > 0 we disfavor a scale invariant fNL at the 2 sigma level.

arXiv: 10 June 2010

Relic Abundance Predicts Universal Mass-Width Relations for Dark Matter Interactions

Authors: Mihailo Backovic, John P. Ralston

http://arxiv.org/abs/1006.1885v1

We find new and universal relations for the properties of dark matter particles consistent with standard relic abundances. Analysis is based on first characterizing the $s$-channel resonant annihilation process in great detail, keeping track of all velocity-dependence, the presence of multiple scales and treating each physical regime above, below, and close to thresholds separately. The resonant regime as well as extension to include non-resonant processes are then reduced to analytic formulas and inequalities that describe the full range of multi-dimensional numerical work. These results eliminate the need to recompute relic abundance model by model, and reduce calculations to verifying certain scale and parameter combinations are consistent. Remarkably simple formulas describe the relation between the total width of an $s$-channel intermediate particle, the masses and the couplings involved. Eliminating the width in terms of the mass produces new consistency relations between dark matter masses and the intermediate masses. The formulas are general enough to test directly whether new particles can be identified as dark matter. Resonance mass and total width are quantities directly observable at accelerators such as the LHC, and will be sufficient to establish whether new discoveries are consistent with the cosmological bounds on dark matter.

 

Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation?

Authors: P. Kroupa, B. Famaey, K. S. de Boer, J. Dabringhausen, M. S. Pawlowski, C. M. Boily, H. Jerjen, D. Forbes, G. Hensler, A. Del Popolo, M. Metz

http://arxiv.org/abs/1006.1647v1

Predictions of the Concordance Cosmological Model (CCM) of the structures in the environment of large spiral galaxies are compared with observed properties of Local Group galaxies. Five new possibly irreconcilable problems are uncovered. However, the Local Group properties provide hints that may lead to a solution of the above problems The DoS and bulge--satellite correlation suggest that dissipational events forming bulges are related to the processes forming phase-space correlated satellite populations. Such events are well known to occur since in galaxy encounters energy and angular momentum are expelled in the form of tidal tails, which can fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star clusters. If Local Group satellite galaxies are to be interpreted as TDGs then the sub-structure predictions of CCM are internally in conflict. All findings thus suggest that the CCM does not account for the Local Group observations and that therefore existing as well as new viable alternatives have to be further explored. These are discussed and natural solutions for the above problems emerge.

 

Gravitational waves from intermediate-mass black holes in young clusters

Authors: M. Mapelli, C. Huwyler, L. Mayer, Ph. Jetzer, A. Vecchio

http://arxiv.org/abs/1006.1664v1

Massive young clusters (YCs) are expected to host intermediate-mass black holes (IMBHs) born via runaway collapse. These IMBHs are likely in binaries and can undergo mergers with other compact objects, such as stellar mass black holes (BHs) and neutron stars (NSs). We derive the frequency of such mergers starting from information available in the Local Universe. Mergers of IMBH-NS and IMBH-BH binaries are sources of gravitational waves (GWs), which might allow us to reveal the presence of IMBHs. We thus examine their detectability by current and future GW observatories, both ground- and space-based. In particular, as representative of different classes of instruments we consider Initial and Advanced LIGO, the Einstein gravitational-wave Telescope (ET) and the Laser Interferometer Space Antenna (LISA). We find that IMBH mergers are unlikely to be detected with instruments operating at the current sensitivity (Initial LIGO). LISA detections are disfavored by the mass range of IMBH-NS and IMBH-BH binaries: less than one event per year is expected to be observed by such instrument. Advanced LIGO is expected to observe a few merger events involving IMBH binaries in a 1-year long observation. Advanced LIGO is particularly suited for mergers of relatively light IMBHs (~100 Msun) with stellar mass BHs. The number of mergers detectable with ET is much larger: tens (hundreds) of IMBH-NS (IMBH-BH) mergers might be observed per year, according to the runaway collapse scenario for the formation of IMBHs. We note that our results are affected by large uncertainties, produced by poor observational constraints on many of the physical processes involved in this study, such as the evolution of the YC density with redshift.[abridged]

 

Sandage-Loeb test for the new agegraphic and Ricci dark energy models

Authors: Jingfei Zhang, Li Zhang, Xin Zhang

http://arxiv.org/abs/1006.1738v1

The Sandage-Loeb (SL) test is a unique method to explore dark energy at the ``redshift desert'' ($2\lesssim z\lesssim 5$), an era not covered by any other dark energy probes, by directly measuring the temporal variation of the redshift of quasar (QSO) Lyman-$\alpha$ absorption lines. In this paper, we study the prospects for constraining the new agegraphic dark energy (NADE) model and the Ricci dark energy (RDE) model with the SL test. We show that, assuming only a ten-year survey, the SL test can constrain these two models with high significance.

 

 

 

 

arXiv: 9 June 2010

The CMB Bispectrum

Authors: J.R.Fergusson, M. Liguori, E.P.S. Shellard

http://arxiv.org/abs/1006.1642v1

We use a separable mode expansion estimator with WMAP data to estimate the bispectrum for all the primary families of non-Gaussian models. We review the late-time mode expansion estimator methodology which can be applied to any non-separable primordial and CMB bispectrum model, and we demonstrate how the method can be used to reconstruct the CMB bispectrum from an observational map. We extend the previous validation of the general estimator using local map simulations. We apply the estimator to the coadded WMAP 5-year data, reconstructing the WMAP bispectrum using $l<500$ multipoles and $n=31$ orthonormal 3D eigenmodes. We constrain all popular nearly scale-invariant models, ensuring that the theoretical bispectrum is well-described by a convergent mode expansion. Constraints from the local model $ \fnl=54.4\pm 29.4$ and the equilateral model $\fnl=143.5\pm 151.2$ ($\Fnl = 25.1\pm 26.4$) are consistent with previously published results. (Here, we use a nonlinearity parameter $\Fnl$ normalised to the local case, to allow more direct comparison between different models.) Notable new constraints from our method include those for the constant model $\Fnl = 35.1 \pm 27.4 $, the flattened model $\Fnl = 35.4\pm 29.2$, and warm inflation $\Fnl = 10.3\pm 27.2$. We investigate feature models surveying a wide parameter range in both the scale and phase, and we find no significant evidence of non-Gaussianity in the models surveyed. We propose a measure $\barFnl$ for the total integrated bispectrum and find that the measured value is consistent with the null hypothesis that CMB anisotropies obey Gaussian statistics. We argue that this general bispectrum survey with the WMAP data represents the best evidence for Gaussianity to date and we discuss future prospects, notably from the Planck satellite.

 

Fermi-LAT Sensitivity to Dark Matter Annihilation in Via Lactea II Substructure

Authors: Brandon Anderson, Michael Kuhlen, Robert Johnson, Piero Madau, Juerg Diemand

http://arxiv.org/abs/1006.1628v1

We present a study of the ability of the Fermi Gamma-ray Space Telescope to detect dark-matter annihilation signals from the Galactic subhalos predicted by the Via Lactea II N-body simulation. We implement an improved formalism for estimating the boost factor needed to account for the effect of dark-matter clumping on scales below the resolution of the simulation, and we incorporate a detailed Monte Carlo simulation of the response of the Fermi-LAT telescope, including a simulation of its all-sky observing mode integrated over a ten year mission. We find that for WIMP masses up to about 150 GeV in standard supersymmetric models with velocity-averaged cross section 3*10^-26 cm^3 s^-1, a few subhalos could be detectable with >5 standard deviations significance and would likely deviate significantly from the appearance of a point source.

 

 Notes on wormhole existence in scalar-tensor and F(R) gravity

Authors: K.A. Bronnikov, M.V. Skvortsova, A.A. Starobinsky

http://arxiv.org/abs/1005.3262v3

Some recent papers have claimed the existence of static, spherically symmetric wormhole solutions to gravitational field equations in the absence of ghost (or phantom) degrees of freedom. We show that in some such cases the solutions in question are actually not of wormhole nature while in cases where a wormhole is obtained, the effective gravitational constant G_eff is negative in some region of space, i.e., the graviton becomes a ghost. In particular, it is confirmed that there are no vacuum wormhole solutions of the Brans-Dicke theory with zero potential and the coupling constant \omega > -3/2, except for the case \omega = 0; in the latter case, G_eff < 0 in the region beyond the throat. The same is true for wormhole solutions of F(R) gravity: special wormhole solutions are only possible if F(R) contains an extremum at which G_eff changes its sign.

 

 

 

arXiv: 8 June 2010

The GMRT-EoR Experiment: A new upper limit on the neutral hydrogen power spectrum at z \approx 8.6

Authors: Gregory Paciga, Tzu-Ching Chang, Yashwant Gupta, Rajaram Nityanada, Julia Odegova, Ue-Li Pen, Jeffrey Peterson, Jayanta Roy, Kris Sigurdson

http://arxiv.org/abs/1006.1351v1

We present a new upper limit to the 21cm power spectrum during the Epoch of Reionization (EoR) which constrains reionization models with an unheated IGM. The GMRT-EoR experiment is an ongoing effort to make a statistical detection of the power spectrum of 21cm neutral hydrogen emission at redshift z~9. Data from this redshift constrain models of the (EoR), the end of the Dark Ages arising from the formation of the first bright UV sources, probably stars or mini-quasars. We present results from approximately 50 hours of observations at the Giant Metrewave Radio Telescope in India from December 2007. We describe radio frequency interference (RFI) localisation schemes which allow bright sources on the ground to be identified and physically removed. Singular-value decomposition is used to remove remaining broadband RFI by identifying ground sources with large eigenvalues. Foregrounds are modelled using a piecewise linear filter and the power spectrum is measured using cross-correlations of foreground subtracted images.

 

Systematic uncertainties in the determination of the local dark matter density

Authors: Miguel Pato, Oscar Agertz, Gianfranco Bertone, Ben Moore, Romain Teyssier

http://arxiv.org/abs/1006.1322v1

A precise determination of the local dark matter density and an accurate control over the corresponding uncertainties are of paramount importance for Dark Matter (DM) searches. Using very recent high-resolution numerical simulations of a Milky Way like object, we study the systematic uncertainties that affect the determination of the local dark matter density based on dynamical measurements in the Galaxy. In particular, extracting from the simulation with baryons the orientation of the Galactic stellar disk with respect to the DM distribution, we study the DM density for an observer located at $\sim$8 kpc from the Galactic center {\it on the stellar disk}, $\rho_0$. This quantity is found to be always larger than the average density in a spherical shell of same radius $\bar{\rho}_0$, which is the quantity inferred from dynamical measurements in the Galaxy, and to vary in the range $\rho_0/\bar{\rho}_0=1.01-1.41$. This implies that the actual dark matter density in the solar neighborhood is on average 21% larger than the value inferred from most dynamical measurements, and that the associated systematic errors are larger than the statistical errors recently discussed in the literature.

 

Spin down during quiescence of the fastest known accretion-powered pulsar

Authors: A. Papitto, A.Riggio, L.Burderi, T.Di Salvo, A.D'Aí, R.Iaria

http://arxiv.org/abs/1006.1303v1

We present a timing solution for the 598.89 Hz accreting millisecond pulsar, IGR J00291+5934, using Rossi X-Ray Timing Explorer data taken during the two outbursts exhibited by the source on 2008 August and September. We estimate the neutron star spin frequency and we refine the system orbital solution. The spin frequency, measured with a number of models, is compared with the value measured at the end of the last outburst exhibited by the source in 2004. We find that the source spins down during quiescence and, under conservative assumptions, we estimate the spin derivative value in the range nu_dot=-(1-7)E-15 Hz/s. We discuss possible scenarios that can account for the long-term neutron star spin down in terms of either magneto-dipole emission, gravitational waves emission, and propeller effect. The measured spin down translates in upper limits on the neutron star magnetic field, B<3E8 G, and on the neutron star mass quadrupole moment, $Q<1.8E36 g cm^2.

 

High-precision spectra for dynamical Dark Energy cosmologies from constant-w models

Authors: Luciano Casarini

http://arxiv.org/abs/1006.0981v1

Spanning the whole functional space of cosmologies with any admissible DE state equations w(a) seems a need, in view of forthcoming observations, namely those aiming to provide a tomography of cosmic shear. In this paper I show that this duty can be eased and that a suitable use of results for constant-w cosmologies can be sufficient. More in detail, I ``assign'' here six cosmologies, aiming to span the space of state equations w(a) = w_o + w_a(1-a), for w_o and w_a values consistent with WMAP5 and WMAP7 releases and run N-body simulations to work out their non-linear fluctuation spectra at various redshifts z. Such spectra are then compared with those of suitable auxiliary models, characterized by constant w. For each z a different auxiliary model is needed. Spectral discrepancies between the assigned and the auxiliary models, up to k ~ 2-3 h Mpc^{-1}, are shown to keep within 1%. Quite in general, discrepancies are smaller at greater z and exhibit a specific trend across the w_o and w_a plane. Besides of aiming at simplifying the evaluation of spectra for a wide range of models, this paper also outlines a specific danger for future studies of the DE state equation, as models fairly distant on the w_0 - w_a plane can be easily confused.

 

 

arXiv: 7 June 2010

Coupled dark matter-dark energy in light of near Universe observations

Authors: Laura Lopez Honorez, Beth A. Reid, Olga Mena, Licia Verde, Raul Jimenez
http://arxiv.org/abs/1006.0877v1

Cosmological analysis based on currently available observations are unable to rule out a sizeable coupling among the dark energy and dark matter fluids. We explore a variety of coupled dark matter-dark energy models, which satisfy cosmic microwave background constraints, in light of low redshift and near universe observations. We illustrate the phenomenology of different classes of dark coupling models, paying particular attention in distinguishing between effects that appear only on the expansion history and those that appear in the growth of structure. We find that while a broad class of dark coupling models are effectively models where general relativity (GR) is modified --and thus can be probed by a combination of tests for the expansion history and the growth of structure--, there is a class of dark coupling models where gravity is still GR, but the growth of perturbations is, in principle modified. While this effect is small in the specific models we have considered, one should bear in mind that an inconsistency between reconstructed expansion history and growth may not uniquely indicate deviations from GR. Our low redshift constraints arise from cosmic velocities, redshift space distortions and dark matter abundance in galaxy voids. We find that current data constrain the dimensionless coupling to be |xi|<0.2, but prospects from forthcoming data are for a significant improvement. Future, precise measurements of the Hubble constant, combined with high-precision constraints on the growth of structure, could provide the key to rule out dark coupling models which survive other tests. We shall exploit as well weak equivalence principle violation arguments, which have the potential to highly disfavour a broad family of coupled models.

 

The Accreting Millisecond X-ray Pulsar IGR J00291+5934: Evidence for a Long Timescale Spin Evolution

Authors: Alessandro Patruno (API, Univ. of Amsterdam)

http://arxiv.org/abs/1006.0815v1

Accreting Millisecond X-ray Pulsars like IGR J00291+5934 are important because it is possible to test theories of pulsar formation and evolution. They give also the possibility to constrain gravitational wave emission theories and the equation of state of ultra dense matter. Particularly crucial to our understanding is the measurement of the long term spin evolution of the accreting neutron star. An open question is whether these accreting pulsars are spinning up during an outburst and spinning down in quiescence as predicted by the recycling scenario. Until now it has been very difficult to measure torques, due to the presence of fluctuations in the pulse phases that compromise their measurements with standard coherent timing techniques. By applying a new method, I am now able to measure a spin up during an outburst and a spin down during quiescence. I ascribe the spin up (Fdot=5.1(3)x10^{-13}\Hz/s) to accretion torques and the spin down (Fdot=-3.0(8)x10^{-15} Hz/s) to magneto dipole torques, as those observed in radio pulsars. Both values nicely fit in the recycling scenario and I infer the existence of a magnetic field for the pulsar of B~2x10^{8} G. No evidence for an enhanced spin down due to gravitational wave emission is found. The accretion torques are smaller than previously reported and there is strong evidence for an ordered process that is present in all outbursts that might be connected with a motion of the hot spot on the neutron star surface.

 

Testing the Copernican and Cosmological Principles in the local universe with galaxy surveys

Authors: Francesco Sylos Labini, Yuri V. Baryshev

http://arxiv.org/abs/1006.0801v1

Cosmological density fields are assumed to be translational and rotational invariant, avoiding any special point or direction, thus satisfying the Copernican Principle. A spatially inhomogeneous matter distribution can be compatible with the Copernican Principle but not with the stronger version of it, the Cosmological Principle which requires the additional hypothesis of spatial homogeneity. We establish criteria for testing that a given density field, in a finite sample at low redshifts, is statistically and/or spatially homogeneous. The basic question to be considered is whether a distribution is, at different spatial scales, self-averaging. This can be achieved by studying the probability density function of conditional fluctuations. We find that galaxy structures in the SDSS samples, the largest currently available, are spatially inhomogeneous but statistically homogeneous and isotropic up to ~ 100 Mpc/h. Evidences for the breaking of self-averaging are found up to the largest scales probed by the SDSS data. The comparison between the results obtained in volumes of different size allows us to unambiguously conclude that the lack of elf-averaging is induced by finite-size effects due to long-range correlated fluctuations. We finally discuss the relevance of these results from the point of view of cosmological modeling.

 

Polarizing Bubble Collisions

Authors: Bartlomiej Czech, Matthew Kleban, Klaus Larjo, Thomas S. Levi, Kris Sigurdson

http://arxiv.org/abs/1006.0832v1

We predict the polarization of cosmic microwave background (CMB) photons that results from a cosmic bubble collision. The polarization is purely E-mode, symmetric around the axis pointing towards the collision bubble, and has several salient features in its radial dependence that can help distinguish it from a more conventional explanation for unusually cold or hot features in the CMB sky. The anomalous "cold spot" detected by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite is a candidate for a feature produced by such a collision, and the Planck satellite and other proposed surveys will measure the polarization on it in the near future. The detection of such a collision would provide compelling evidence for the string theory landscape.

 

 

arXiv: 4 June 2010

Baryon Acoustic Oscillations in 2D: Modeling Redshift-space Power Spectrum from Perturbation Theory

Authors: Atsushi Taruya, Takahiro Nishimichi, Shun Saito

http://arxiv.org/abs/1006.0699v1

We present an improved prescription for matter power spectrum in redshift space taking a proper account of both the non-linear gravitational clustering and redshift distortion, which are of particular importance for accurately modeling baryon acoustic oscillations (BAOs). Contrary to the models of redshift distortion phenomenologically introduced but frequently used in the literature, the new model includes the corrections arising from the non-linear coupling between the density and velocity fields associated with two competitive effects of redshift distortion, i.e., Kaiser and Finger-of-God effects. Based on the improved treatment of perturbation theory for gravitational clustering, we compare our model predictions with monopole and quadrupole power spectra of N-body simulations, and an excellent agreement is achieved over the scales of BAOs. Potential impacts on constraining dark energy and modified gravity from the redshift-space power spectrum are also investigated based on the Fisher-matrix formalism. We find that the existing phenomenological models of redshift distortion produce a systematic error on measurements of the angular diameter distance and Hubble parameter by 1~2%, and the growth rate parameter by ~5%, which would become non-negligible for future galaxy surveys. Correctly modeling redshift distortion is thus essential, and the new prescription of redshift-space power spectrum including the non-linear corrections can be used as an accurate theoretical template for anisotropic BAOs.

 

Effects of cosmological model assumptions on galaxy redshift survey measurements

Authors: Lado Samushia, Will J. Percival, Luigi Guzzo, Yun Wang, Andrea Cimatti, Carlton Baugh, James E. Geach, Cedric Lacey, Elisabetta Majerotto, Pia Mukherjee, Alvaro Orsi

http://arxiv.org/abs/1006.0609v1

The clustering of galaxies observed in future redshift surveys will provide a wealth of cosmological information. Matching the signal at different redshifts constrains the dark energy driving the acceleration of the expansion of the Universe. In tandem with these geometrical constraints, redshift-space distortions (RSD) depend on the build up of large-scale structure. As pointed out by many authors measurements of these effects are intrinsically coupled. We investigate this link, and argue that it strongly depends on the cosmological assumptions adopted when analysing data. Using representative assumptions for the parameters of the "Euclid" survey in order to provide a baseline future experiment, we show how the derived constraints change due to different model assumptions. We argue that even the assumption of a Friedman-Robertson-Walker (FRW) space-time is sufficient to reduce the importance of the coupling to a significant degree. Taking this idea further, we consider how the data would actually be analysed and argue that we should not expect to be able to simultaneously constrain multiple deviations from the standard $\Lambda$CDM model. We therefore consider different possible ways in which the Universe could deviate from the $\Lambda$CDM model, and show how the coupling between geometrical constraints and structure growth affects the measurement of such deviations.

 

The Sphericalization of Dark Matter Halos by Galaxy Disks

Authors: Stelios Kazantzidis (CCAPP/OSU), Mario G. Abadi (U.Cordoba), Julio F. Navarro (U.Victoria)

http://arxiv.org/abs/1006.0537v1

Cosmological simulations indicate that cold dark matter (CDM) halos should be triaxial. Verifying observationally this theoretical prediction is, however, less than straightforward because the assembly of galaxies is expected to modify the halo shapes and to render them more axisymmetric. We use a suite of N-body simulations to investigate quantitatively the effect of the growth of a central disk galaxy on the shape of triaxial dark matter halos. As expected, the halo responds to the presence of the disk by becoming more spherical. The net effect depends only weakly on the orientation of the disk relative to the halo principal axes or the timescale of disk assembly, but strongly on the overall gravitational importance of the disk. Our results show that exponential disks whose contribution peaks at less than ~50% of their circular velocity are unable to modify noticeably the shape of the gravitational potential of their surrounding halos. Many dwarf and low surface brightness galaxies are expected to be in this regime, and therefore their detailed kinematics could be used to probe halo triaxiality, one of the basic predictions of the CDM paradigm. We argue that the complex disk kinematics of the dwarf galaxy NGC 2976 might be the reflection of a triaxial halo. Such signatures of halo triaxiality should be common in galaxies where the luminous component is subdominant.

arXiv: 3 June 2010

Constraining Dark Matter Properties with Gamma-Rays from the Galactic Center with Fermi-LAT

Authors: Nicolas Bernal, Sergio Palomares-Ruiz (Lisbon, CFTP-IST)

http://arxiv.org/abs/1006.0477v1

We study the capabilities of the Fermi satellite to constrain particle dark matter properties, as annihilation cross section, mass and branching ratio into dominant annihilation channels. Besides the prompt gamma-ray flux, we also take into account the contribution from the electrons/positrons produced in dark matter annihilations to the gamma-ray signal via inverse Compton scattering off the interstellar photon background, which turns out to be crucial in the case of dark matter annihilations into mu+mu- and e+e- pairs. We study the signal dependence on different parameters like the region of observation, the density profile, the inclusion of systematic uncertainties in the gamma-ray background, the assumptions for the dark matter model and the uncertainties in the propagation model. If Fermi is able to distinguish a possible dark matter signal from the large gamma-ray background, we show that for dark matter masses below ~200 GeV, the Fermi experiment will likely be able to determine dark matter properties with good accuracy.

 

Investigation of the unique nulling properties of PSR B0818-41

Authors: Bhaswati Bhattacharyya (IUCAA, India), Yashwant Gupta (NCRA, India), Janusz Gil (University of Zielona Gora, Poland)

http://arxiv.org/abs/1006.0377v1

We report on the unique nulling properties of PSR B0818-41, using the GMRT at 325 and 610 MHz. We find following interesting behaviour just before and after the nulls: (i) The pulsar's intensity does not switch off abruptly at the null, but fades gradually, taking ~ 10 P1. Just after nulls intensity rises to a maximum over a short (less than one period) time scale. (ii) While the last active pulses before nulls are dimmer, the first few active pulses just after nulls outshine normal ones. This effect is very clear for inner region of pulsar profile, where mean intensity of last few active pulses just after nulls is ~ 2.8 times more than that for last active pulses just before nulls. (iii) There is a significant evolution of shape of the pulsar's profile, around nulls, especially at beginning of bursts: an enhanced bump of intensity in inner region, a change in ratio of strengths of the leading and trailing peaks towards a more symmetric profile, an increase in profile width of about 10%, and a shift of profile centre towards later longitudes. (iv) Just before nulls, the apparent drift rate becomes slower, transitioning to an almost phase stationary drift pattern. Further, when the pulsar comes out of null, the increased intensity is very often accompanied by what looks like a disturbed drift rate behaviour, which settles down to the regular drift pattern as the pulsar intensity returns to normal. Thus, we find some very specific and well correlated changes in the radio emission properties of PSR B0818-41 when the emission restarts after a null. These could imply that the phenomenon of nulling is associated with some kind of a "reset" of the pulsar radio emission engine. We also present plausible explanations for some of the observed behaviour, using the Partially Screened Gap model of the inner pulsar accelerator.

 

Dark energy from primordial inflationary quantum fluctuations

Authors: Christophe Ringeval, Teruaki Suyama, Tomo Takahashi, Masahide Yamaguchi, Shuichiro Yokoyama

http://arxiv.org/abs/1006.0368v1

We show that current cosmic acceleration can be explained by an almost massless scalar field experiencing quantum fluctuations during primordial inflation. Provided its mass does not exceed the Hubble parameter today, this field has been frozen during the cosmological ages to start dominating the universe only recently. By using supernovae data, completed with baryonic acoustic oscillations from galaxy surveys and cosmic microwave background anisotropies, we infer the energy scale of primordial inflation to be around a few TeV, which implies a negligible tensor-to-scalar ratio of the primordial fluctuations. Moreover, our model suggests that inflation lasted for an extremely long period thereby favouring a self-reproducing inflationary model. Dark energy could therefore be a natural consequence of cosmic inflation close to the electroweak energy scale.

 

X-ray Counterparts of Millisecond Pulsars in Globular Clusters

Authors: W. Becker, H.H. Huang, T. Prinz, (Max-Planck Institut für Extraterrestrische Physik)

http://arxiv.org/abs/1006.0335v1

We have systematically studied the X-ray emission properties of globular cluster millisecond pulsars in order to evaluate their spectral properties and luminosities in a uniform way. Cross-correlating the radio timing positions of the cluster pulsars with the high resolution Chandra images revealed 31 X-ray counterparts identified in nine different globular cluster systems, including those in 47 Tuc. Timing analysis has been performed for all sources corresponding to the temporal resolution available in the archival Chandra data. Making use of unpublished data on M28, M4 and NGC 6752 allowed us to obtain further constraints for the millisecond pulsar counterparts located in these clusters. Counting rate and energy flux upper limits were computed for those 36 pulsars for which no X-ray counterparts could be detected. Comparing the X-ray and radio pulse profiles of PSR J1821-2452 in M28 and the 47 Tuc pulsars PSR J0024-7204D,O,R indicated some correspondence between both wavebands. The X-ray efficiency of the globular cluster millisecond pulsars was found to be in good agreement with the efficiency Lx ~ 10^-3 Edot observed in Galactic field rotation-powered pulsars. Millisecond pulsars in the galactic plane and in globular clusters appear to show no distinct differences in their X-ray emission properties.

 

Observational Constraints to Ricci Dark Energy Model by Using: SN, BAO, OHD, fgas Data Sets

Authors: Lixin Xu, Yuting Wang

http://arxiv.org/abs/1006.0296v1

In this paper, we perform a global constraint on the Ricci dark energy model with both the flat case and the non-flat case, using the Markov Chain Monte Carlo (MCMC) method and the combined observational data from the cluster X-ray gas mass fraction, Supernovae of type Ia (397), baryon acoustic oscillations, current Cosmic Microwave Background, and the observational Hubble function. In the flat model, we obtain the best fit values of the parameters in $1\sigma, 2\sigma$ regions: $\Omega_{m0}=0.2927^{+0.0420 +0.0542}_{-0.0323 -0.0388}$, $\alpha=0.3823^{+0.0331 +0.0415}_{-0.0418 -0.0541}$, $Age/Gyr=13.48^{+0.13 +0.17}_{-0.16 -0.21}$, $H_0=69.09^{+2.56 +3.09}_{-2.37 -3.39}$. In the non-flat model, the best fit parameters are found in $1\sigma, 2\sigma$ regions:$\Omega_{m0}=0.3003^{+0.0367 +0.0429}_{-0.0371 -0.0423}$, $\alpha=0.3845^{+0.0386 +0.0521}_{-0.0474 -0.0523}$, $\Omega_k=0.0240^{+0.0109 +0.0133}_{-0.0130 -0.0153}$, $Age/Gyr=12.54^{+0.51 +0.65}_{-0.37 -0.49}$, $H_0=72.89^{+3.31 +3.88}_{-3.05 -3.72}$. Compared to the constraint results in the $\Lambda \textmd{CDM}$ model by using the same datasets, it is shown that the current combined datasets prefer the $\Lambda \textmd{CDM}$ model to the Ricci dark energy model.

 

Density perturbations in general modified gravitational theories

Authors: Antonio De Felice, Shinji Mukohyama, Shinji Tsujikawa

http://arxiv.org/abs/1006.0281v1

We derive the equations of linear cosmological perturbations for the general Lagrangian density $f (R,\phi, X)/2+L_c$, where $R$ is a Ricci scalar, $\phi$ is a scalar field, and $X=-(\nabla \phi)^2/2$ is a field kinetic energy. We take into account a nonlinear self-interaction term $L_c$ recently studied in the context of "Galileon" cosmology, which keeps the field equations at second order. Taking into account a scalar-field mass explicitly, the equations of matter density perturbations and gravitational potentials are obtained under a quasi-static approximation on sub-horizon scales. We also derive conditions for the avoidance of ghosts and Laplacian instabilities associated with propagation speeds. Our analysis includes most of modified gravity models of dark energy proposed in literature and thus it is convenient to test the viability of such models from both theoretical and observational points of view.

 

Primordial Non-Gaussianity in the Cosmic Microwave Background

Authors: Amit P.S. Yadav, Benjamin D. Wandelt

http://arxiv.org/abs/1006.0275v1

In the last few decades, advances in observational cosmology have given us a standard model of cosmology. We know the content of the universe to within a few percent. With more ambitious experiments on the way, we hope to move beyond the knowledge of what the universe is made of, to why the universe is the way it is. In this review paper we focus on primordial non-Gaussianity as a probe of the physics of the dynamics of the universe at the very earliest moments. We discuss 1) theoretical predictions from inflationary models and their observational consequences in the cosmic microwave background (CMB) anisotropies; 2) CMB--based estimators for constraining primordial non-Gaussianity with an emphasis on bispectrum templates; 3) current constraints on non-Gaussianity and what we can hope to achieve in the near future; and 4) non-primordial sources of non-Gaussianities in the CMB such as bispectrum due to second order effects, three way cross-correlation between primary-lensing-secondary CMB, and possible instrumental effects

 

Cosmographic constraints on a class of Palatini f(R) gravity

Authors: N. Pires, J. Santos, J. S. Alcaniz

http://arxiv.org/abs/1006.0264v1

Modified gravity, known as $f(R)$ gravity, has presently been applied to Cosmology as a realistic alternative to dark energy. For this kind of gravity the expansion of the Universe may accelerate while containing only baryonic and cold dark matter. The aim of the present investigation is to place cosmographic constraints on the class of theories of the form $f(R)=R - \alpha/R^n$ within the Palatini approach. Although extensively discussed in recent literature and confronted with several observational data sets, cosmological tests are indeed inconclusive about the true signal of $n$ in this class of theories. This is particularly important to define which kind of corrections (infra-red or high-energy) to general relativity this class of theory indeed represent. We shed some light on this question by examining the evolution of the deceleration parameter $q(z)$ for these theories. We find that for a large range of $\alpha$, models based on $f(R) = R - \alpha/R^{n}$ gravity in the Palatini approach can only have positive values for $n$, placing thus a broad restriction on this class of gravity.

 

Equivalence between Palatini and metric formalisms of f(R)-gravity by divergence free current

Authors: Salvatore Capozziello, Farhad Darabi, Daniele Vernieri

http://arxiv.org/abs/1006.0454v1

The equivalence between metric and Palatini formalisms in f(R)-gravity can be achieved in the general context of theories with divergence free current. This equivalence is a necessary result of a symmetry which is included in a particular conservation equation of the current. In fact the conservation equation, by an appropriate redefinition of the introduced auxiliary field, may be encoded in a massless scalar field equation.

 

The Post-Newtonian Limit of f(R)-gravity in the Harmonic Gauge

Authors: A. Stabile

http://arxiv.org/abs/1004.1973v2

A general analytic procedure is developed for the post-Newtonian limit of $f(R)$-gravity with metric approach in the Jordan frame by using the harmonic gauge condition. In a pure perturbative framework and by using the Green function method a general scheme of solutions up to $(v/c)^4$ order is shown. Considering the Taylor expansion of a generic function $f$ it is possible to parameterize the solutions by derivatives of $f$. At Newtonian order, $(v/c)^2$, all more important topics about the Gauss and Birkhoff theorem are discussed. The corrections to "standard" gravitational potential ($tt$-component of metric tensor) generated by an extended uniform mass ball-like source are calculated up to $(v/c)^4$ order. The corrections, Yukawa and oscillating-like, are found inside and outside the mass distribution. At last when the limit $f\rightarrow R$ is considered the $f(R)$-gravity converges in General Relativity at level of Lagrangian, field equations and their solutions.

 

 

arXiv: 2 June 2010

Gravitational Waves from Abelian Gauge Fields and Cosmic Strings at Preheating

Authors: Jean-Francois Dufaux, Daniel G. Figueroa, Juan Garcia-Bellido

http://arxiv.org/abs/1006.0217v1

Primordial gravitational waves provide a very important stochastic background that could be detected soon with interferometric gravitational wave antennas or indirectly via the induced patterns in the polarization anisotropies of the cosmic microwave background. The detection of these waves will open a new window into the early Universe, and therefore it is important to characterize in detail all possible sources of primordial gravitational waves. In this paper we develop theoretical and numerical methods to study the production of gravitational waves from out-of-equilibrium gauge fields at preheating. We then consider models of preheating after hybrid inflation, where the symmetry breaking field is charged under a local U(1) symmetry. We analyze in detail the dynamics of the system in both momentum and configuration space, and show that gauge fields leave specific imprints in the resulting gravitational wave spectra, mainly through the appearence of new peaks at characteristic frequencies that are related to the mass scales in the problem. We also show how these new features in the spectra correlate with string-like spatial configurations in both the Higgs and gauge fields that arise due to the appearance of topological winding numbers of the Higgs around Nielsen-Olesen strings. We study in detail the time evolution of the spectrum of gauge fields and gravitational waves as these strings evolve and decay before entering a turbulent regime where the gravitational wave energy density saturates.

 

The Hubble diagram for a system within dark energy: the location of the zero-gravity radius and the global Hubble rate

Authors: Pekka Teerikorpi, Arthur Davidovich Chernin

http://arxiv.org/abs/1006.0066v1

Here we continue to discuss the principle of the local measurement of dark energy using the normalized Hubble diagram describing the environment of a system of galaxies. We calculate the present locus of test particles injected a fixed time ago (\sim the age of the universe), in the standard \Lambda -cosmology and for different values of the system parameters (the model includes a central point mass M and a local dark energy density \rho_{loc}) and discuss the position of the zero-gravity distance R_v in the Hubble diagram. Our main conclusions are: 1) When the local DE density \rho_{loc} is equal to the global DE density \rho_v, the outflow reaches the global Hubble rate at the distance R_2 = (1+z_v)R_v, where z_v is the global zero-acceleration redshift (\approx 0.7 for the standard model). This is also the radius of the ideal Einstein-Straus vacuole. 2) For a wide range of the local-to-global dark energy ratio \rho_{loc}/\rho_v, the local flow reaches the known global rate (the Hubble constant) at a distance R_2 \ga 1.5 \times R_v. Hence, R_v will be between R_2/2 and R_2, giving upper and lower limits to \rho_{loc}/M. For the Local Group, this supports the view that the local density is near the global one.

 

The Relevance of the Cosmological Constant for Lensing

Authors: Mustapha Ishak, Wolfgang Rindler (The University of Texas at Dallas)

http://arxiv.org/abs/1006.0014v1

This review surveys some recent developments concerning the effect of the cosmological constant on the bending of light by a spherical mass in Kottler (Schwarzchild-de Sitter) spacetime. Some proposals of how such an effect may be put into a setting of gravitational lensing in cosmology are also discussed. The picture that emerges from this review is that it seems fair to assert that the contribution of $\Lambda$ to the bending of light has by now been well established, while putting the $\Lambda$ light-bending terms into a cosmological context is still subject to some interpretation and requires further work and clarification.

 

Inflationary solutions in asymptotically safe f(R) gravity

Authors: Alfio Bonanno, Adriano Contillo, Roberto Percacci

http://arxiv.org/abs/1006.0192v1

We discuss the existence of inflationary solutions in a class of renormalization group improved polynomial f(R) theories, which have been studied recently in the context of the asymptotic safety scenario for quantum gravity. These theories seem to possess a nontrivial ultraviolet fixed point, where the dimensionful couplings scale according to their canonical dimensionality. Assuming that the cutoff is proportional to the Hubble parameter, we obtain modified Friedmann equations which admit both power law and exponential solutions. We establish that for sufficiently high order polynomial the solutions are reliable, in the sense that considering still higher order polynomials is very unlikely to change the solution.

 

 

arXiv: 1 Jun 2010

Effect of low mass dark matter particles on the Sun

Authors: Marco Taoso, Fabio Iocco, Georges Meynet, Gianfranco Bertone, Patrick Eggenberger

http://arxiv.org/abs/1005.5711v1

We study the effect of dark matter (DM) particles in the Sun, focusing in particular on the possible reduction of the solar neutrinos flux due to the energy carried away by DM particles from the innermost regions of the Sun, and to the consequent reduction of the temperature of the solar core. We find that in the very low-mass range between 4 and 10 GeV, recently advocated to explain the findings of the DAMA and CoGent experiments, the effects on neutrino fluxes are detectable only for DM models with very small, or vanishing, self-annihilation cross section, such as the so-called asymmetric DM models, and we study the combination of DM masses and Spin Dependent cross sections which can be excluded with current solar neutrino data. Finally, we revisit the recent claim that DM models with large self-interacting cross sections can lead to a modification of the position of the convective zone, alleviating or solving the solar composition problem. We show that when the `geometric' upper limit on the capture rate is correctly taken into account, the effects of DM are reduced by orders of magnitude, and the position of the convective zone remains unchanged.

 

The spherical collapse model in time varying vacuum cosmologies

Authors: S. Basilakos, M. Plionis, J. Sola

http://arxiv.org/abs/1005.5592v1

We investigate the virialization of cosmic structures in the framework of flat FLRW cosmological models, in which the vacuum energy density evolves with time. In particular, our analysis focuses on the study of spherical matter perturbations, as the latter decouple from the background expansion and start to "turn around" and finally collapse. We generalize the spherical collapse model in the case when the vacuum energy is a running quantity of the Hubble rate, $\Lambda=\Lambda(H)$. A particularly well motivated model of this type is the so-called quantum field vacuum, in which $\Lambda(H)$ is a quadratic function, $\Lambda(H)=n_0+n_2\,H^2$, with $n_0\neq 0$. This model has been previously studied by our team using the latest high quality cosmological data to constraint its free parameters, as well as the predicted cluster formation rate. It turns out that the corresponding Hubble expansion history resembles that of the traditional $\Lambda$CDM cosmology. We use this $\Lambda(t)$CDM framework to illustrate the fact that the properties of the spherical collapse model (virial density, collapse factor, etc.) depend on the choice of the considered vacuum energy (homogeneous or clustered). In particular, if the distribution of the vacuum energy is clustered, then, under specific conditions, we can produce more concentrated structures with respect to the homogeneous vacuum energy case.

Binary stars as probes of dark substructures in dwarf galaxies

Authors: Jorge Penarrubia, Sergey E. Koposov, Matthew G. Walker, Gerry Gilmore, N. Wyn Evans, Craig D. Mackay

http://arxiv.org/abs/1005.5388v1

We use analytical and N-body methods to examine the survival of wide stellar binaries against repeated encounters with dark substructures orbiting in the dark matter haloes of dwarf spheroidal galaxies (dSphs). Our models adopt cosmologically-motivated conditions wherein dSphs are dark-matter dominated systems that form hierarchically and orbit about a host galaxy. Our analytical estimates show that wide binaries are disrupted at a rate that is proportional to the local density of dark substructures averaged over the life-time of the binary population. The fact that external tides can efficiently strip dark substructures from the outskirts of dSphs implies that the present number and distribution of binaries is strongly coupled with the mass evolution of individual galaxies. Yet we show that for the range of dynamical masses and Galactocentric distances spanned by Milky Way dSphs, a truncation in the separation function at a_max <~ 0.1 pc is expected in all these galaxies. An exception may be the Sagittarius dSph, which has lost most of is dark matter envelope to tides and is close to full disruption. Our simulations indicate that at separations larger than a_max the perturbed binary distribution scales as dN/da \propto a^{-2.1} independently of the mass and density of substructures. These results may be used to determine whether the binary separation function found in dwarf galaxies is compatible with the scale-free hierarchical picture that envisions the existence of dark substructures in all galactic haloes. We show that the ACS camera on board of the Hubble telescope may be able to test this prediction in dSphs at heliocentric distances <100 kpc, even if the binary fraction amounts only 10% of the stellar population.

 

$f(R)$ Gravity and Crossing the Phantom Divide Barrier

Authors: Yousef Bisabr

The $f(R)$ gravity models formulated in Einstein conformal frame are equivalent to Einstein gravity together with a minimally coupled scalar field. We shall explore phantom behavior of $f(R)$ models in this frame and compare the results with those of the usual notion of phantom scalar field.

 

Horizons and singularity in Clifton's spherical solution of f(R) vacuum

Authors: Valerio Faraoni (Bishop's University)

Due to the failure of Birkhoff's theorem, black holes in f(R) gravity theories in which an effective time-varying cosmological "constant" is present are, in general, dynamical. Clifton's exact spherical solution of R^(1+\delta) gravity, which is dynamical and describes a central object embedded in a spatially flat universe, is studied. It is shown that apparent black hole horizons disappear and a naked singularity emerges at late times.

 

 

arXiv: 31 May 2010

Astronomy's Greatest Hits: The 100 most Cited Papers in Each Year of the First Decade of the 21st Century (2000 - 2009)

Authors: Jay A. Frogel

http://arxiv.org/abs/1005.5377v1

This paper is based on the 100 most cited papers in astronomy for each year from 2000 to 2009 and from 1995 and 1990. The main findings are: The total number of authors of the top 100 articles per year has more than tripled. This is seen most strongly in papers with more than 6 authors. The yearly number of papers with 5 or fewer authors has declined over the same time period. The most highly cited papers tend to have the largest number of authors and visa versa. The distribution of normalized citation counts versus ranking is constant from year to year except for the top ranked half dozen or so papers. It is closely approximated by a power law. The papers that show the most divergence from the power law all have a high number of citations and are based on large surveys. The average page length of the top 100 papers is one and a half times that for astronomy papers in general. The same 5 journals (A&A, AJ, ApJ, ApJS, and MNRAS; Nature and Science are not included here) account for 80 to 85% of the total citations for each year of all the journals in the category "Astronomy and Astrophysics" by ISI's Journal Citation Reports. These same 5 journals account for 77% of the 1000 most cited papers. A significant number of articles originally ranked in the top 100 for a year, drop out after 2 to 3 years and are replaced by other articles. Most of the drop-outs deal with extra-galactic astronomy; their replacements deal with non-extra-galactic topics. Indicators of internet access to astronomical web sites such as data archives and journal repositories show increases of between factors of three and ten or more I propose that there are close complementarities between the communication capabilities that internet usage enables and the strong growth in numbers of authors of the most highly cited papers.

 

Can slow roll inflation induce relevant helical magnetic fields?

Authors: Ruth Durrer, Lukas Hollenstein, Rajeev Kumar Jain

http://arxiv.org/abs/1005.5322v1

We study the generation of helical magnetic fields during inflation induced by an axial coupling of the electromagnetic field to the inflaton. During slow roll inflation, we find that such a coupling always leads to a blue spectrum with $B^2 \propto k$. We also show that a short deviation from slow roll does not result in strong modifications to the shape of the spectrum. The magnetic energy density at the end of inflation is too small to back-react on the background dynamics of the inflaton. We calculate the evolution of the correlation length and the field amplitude during the inverse cascade and viscous damping of the helical magnetic field in the radiation era after inflation. The final magnetic fields turn out to be far too weak to provide the seeds for the observed fields in galaxies and clusters.

 

Thermodynamics in $f(R)$ gravity in the Palatini formalism

Authors: Kazuharu Bamba, Chao-Qiang Geng

http://arxiv.org/abs/1005.5234v1

We investigate thermodynamics of the apparent horizon in $f(R)$ gravity in the Palatini formalism with non-equilibrium and equilibrium descriptions. We demonstrate that it is more transparent to understand the horizon entropy in the equilibrium framework than that in the non-equilibrium one. Furthermore, we show that the second law of thermodynamics can be explicitly verified in both phantom and non-phantom phases for the same temperature of the universe outside and inside the apparent horizon.

arXiv: 28 May 2010

High Precision Astrometry with Adaptive Optics aided Imaging

Authors: Eva Meyer

http://arxiv.org/abs/1005.5091v1

More than 450 exoplanets are known and this number increases nearly every day. Only a few constraints on their orbital parameters and physical characteristics can be determined, as most exoplanets are detected indirectly. Measuring the astrometric signal of a planet by measuring the wobble of the host star yields the full set of orbital parameters. With this information the true masses of the planet candidates can be determined, making it possible to establish the candidates as real planets, brown dwarfs (BD) or low mass stars. In the context of this thesis, an M-dwarf with a BD candidate companion, discovered by radial velocity measurements, was observed within a monitoring program to detect the astrometric signal. Ground based adaptive optics aided imaging with ESO/NACO was used to establish its true nature (BD vs. star) and to investigate the prospects of this technique for exoplanet detection. The astrometric corrections necessary to perform high precision astrometry are described and their contribution to the overall precision is investigated. Due to large uncertainties in the pixel-scale and the orientation of the detector, no detection of the astrometric orbit signal was possible. The image quality of ground-based telescopes is limited by the turbulence in Earth's atmosphere. The induced distortions of the light can be measured and corrected with the adaptive optics technique. However, the correction is only useful within a small angle around the guide star. The novel correction technique of multi conjugated adaptive optics uses several guide stars to correct a larger field of view. The VLT/MAD instrument was built to demonstrate this technique. Observations with MAD are analyzed in terms of astrometric precision in this work. Two sets of data are compared, which were obtained in different correction modes: pure ground layer correction and full multi conjugated correction.

PhD Thesis

Cosmological Model-independent Gamma-ray Bursts Calibration and its Cosmological Constraint to Dark Energy

Authors: Lixin Xu

http://arxiv.org/abs/1005.5055v1

As so far, the higher redshift of Gamma-ray bursts (GRBs) can extend to $z\sim 8.1$ which makes it as complementary probe of dark energy to supernova Ia. However, the calibration of GRBs is still a big challenge when they are used to constrain cosmological models. Though, the absolute magnitude of GRBs is not known, the slopes of GRBs correlations can be used as a useful constraint to dark energy in a completely cosmological model in-denpendent way. In this paper, we follow Wang's model-independent distance measurement method and calculate their values by using $109$ GRBs events via the so-called Amati's relation. Then, these distances are used to constrain $\Lambda$CDM model.

 

Weighing the Galactic dark matter halo: a lower mass limit from the fastest halo star known

Authors: Norbert Przybilla, Alfred Tillich, Ulrich Heber, Ralf-Dieter Scholz

http://arxiv.org/abs/1005.5026v1

The mass of the Galactic dark matter halo is under vivid discussion. A recent study by Xue et al. (2008, ApJ, 684, 1143) revised the Galactic halo mass downward by a factor of ~2 relative to previous work, based on the line-of-sight velocity distribution of ~2400 blue horizontal-branch (BHB) halo stars. The observations were interpreted in a statistical approach using cosmological galaxy formation simulations, as only four of the 6D phase-space coordinates were determined. Here we concentrate on a close investigation of the stars with highest negative radial velocity from that sample. For one star, SDSSJ153935.67+023909.8 (J1539+0239 for short), we succeed in measuring a significant proper motion, i.e. full phase-space information is obtained. We confirm the star to be a Population II BHB star from an independent quantitative analysis of the SDSS spectrum - providing the first NLTE study of any halo BHB star - and reconstruct its 3D trajectory in the Galactic potential. J1539+0239 turns out as the fastest halo star known to date, with a Galactic rest-frame velocity of 694$^{+300}_{-221}$ km/s (full uncertainty range from Monte Carlo error propagation) at its current position. The extreme kinematics of the star allows a significant lower limit to be put on the halo mass in order to keep it bound, of M_halo$\ge1.7_{-1.1}^{+2.3}\times10^{12}$ Msun. We conclude that the Xue et al. results tend to underestimate the true halo mass as their most likely mass value is consistent with our analysis only at a level of 4%. However, our result confirms other studies that make use of the full phase-space information.

arXiv: 27 May 2010

Scattering of pulsar radio emission by the interstellar plasma

Authors: W. A. Coles, B. J. Rickett, J. J. Gao, G. Hobbs, J. P. W. Verbiest
http://arxiv.org/abs/1005.4914v1

We present simulations of scattering phenomena which are important in pulsar observations, but which are analytically intractable. The simulation code, which has also been used for solar wind and atmospheric scattering problems, is available from the authors. These simulations reveal an unexpectedly important role of dispersion in combination with refraction. We demonstrate the effect of analyzing observations which are shorter than the refractive scale. We examine time-of-arrival fluctuations in detail: showing their correlation with intensity and dispersion measure; providing a heuristic model from which one can estimate their contribution to pulsar timing observations; and showing that much of the effect can be corrected making use of measured intensity and dispersion. Finally, we analyze observations of the millisecond pulsar J0437$-$4715, made with the Parkes radio telescope, that show timing fluctuations which are correlated with intensity. We demonstrate that these timing fluctuations can be corrected, but we find that they are much larger than would be expected from scattering in a homogeneous turbulent plasma with isotropic density fluctuations. We do not have an explanation for these timing fluctuations.

 

Cosmic strings and their induced non-Gaussianities in the cosmic microwave background

Authors: Christophe Ringeval

http://arxiv.org/abs/1005.4842v1

Motivated by the fact that cosmological perturbations of inflationary quantum origin were born Gaussian, the search for non-Gaussianities in the cosmic microwave background (CMB) anisotropies is considered as the privileged probe of non-linear physics in the early universe. Cosmic strings are active sources of gravitational perturbations and incessantly produce non-Gaussian distortions in the CMB. Even if, on the currently observed angular scales, they can only contribute a small fraction of the CMB angular power spectrum, cosmic strings could actually be the main source of its non-Gaussianities. In this article, after having reviewed the basic cosmological properties of a string network, we present the signatures Nambu-Goto cosmic strings would induce in various observables ranging from the one-point function of the temperature anisotropies to the bispectrum and trispectrum. It is shown that string imprints are significantly different than those expected from the primordial type of non-Gaussianity and could therefore be easily distinguished.

 

The X-ray spectrum of the newly discovered accreting millisecond pulsar IGR J17511-3057

Authors: A.Papitto, A.Riggio, T.Di Salvo, L.Burderi, A.D'Aì, R.Iaria, E.Bozzo, M.T.Menna

http://arxiv.org/abs/1005.4827v1

We report on an XMM-Newton observation of the accreting millisecond pulsar, IGR J17511-3057. Pulsations at 244.8339512(1) Hz are observed with an RMS pulsed fraction of 14.4(3)%. A precise solution for the P_orb=12487.51(2)s binary system is derived. The measured mass function indicates a main sequence companion with a mass between 0.15 and 0.44 Msun. The XMM-Newton spectrum of the source can be modelled by at least three components, multicoloured disc emission, thermal emission from the NS surface and thermal Comptonization emission. Spectral fit of the XMM-Newton data and of the RXTE data, taken in a simultaneous temporal window, constrain the Comptonization parameters: the electron temperature, kT_e=51(+6,-4) keV, is rather high, while the optical depth (tau=1.34(+0.03,-0.06)) is moderate. The energy dependence of the pulsed fraction supports the interpretation of the cooler thermal component as coming from the accretion disc, and indicates that the Comptonizing plasma surrounds the hot spots on the NS surface, which provide the seed photons. Signatures of reflection, such as a broadened iron K-alpha emission line and a Compton hump at 30 keV ca., are also detected. We derive from the smearing of the reflection component an inner disc radius of ~> 40 km for a 1.4 Msun neutron star, and an inclination between 38{\deg} and 68{\deg}. XMM-Newton also observed two type-I X-ray bursts, probably ignited in a nearly pure helium environment. No photospheric radius expansion is observed, thus leading to an upper limit on the distance to the source of 10 kpc. A lower limit of 6.5 kpc can be also set if it is assumed that emission during the decaying part of the burst involves the whole neutron star surface. Pulsations observed during the burst decay are compatible with being phase locked, and have a similar amplitude, than pre-burst pulsations.

 

High accuracy power spectra including baryonic physics in dynamical Dark Energy models

Authors: Luciano Casarini, Andrea V. Maccio', Silvio A. Bonometto, Greg S. Stinson

http://arxiv.org/abs/1005.4683v1

The next generation mass probes will obtain information on non--linear power spectra P(k,z) and their evolution, allowing us to investigate the nature of Dark Energy. To exploit such data we need high precision simulations, extending at least up to scales of k\simeq 10 h^-1 Mpc, where the effects of baryons can no longer be neglected.
In this paper, we present a series of large scale hydrodynamical simulations for LCDM and dynamical Dark Energy (dDE) models, in which the equation of state parameter is z-dependent. The simulations include gas cooling, star formation and Supernovae feedback. They closely approximate the observed star formation rate and the observationally derived star/Dark Matter mass ratio in collapsed systems. Baryon dynamics cause spectral shifts exceeding 1% at k > 2-3 hMpc^-1 compared to pure n-body simulations in the LCDM simulations. This agrees with previous studies, although we find a smaller effect (~50%) on the power spectrum amplitude at higher k's. dDE exhibits similar behavior, even though the dDE simulations produce ~20% less stars than the analogous LCDM cosmologies. Finally, we show that the technique introduced in Casarini et al. to obtain spectra for any $w(z)$ cosmology from constant-w models at any redshift still holds when gas physics is taken into account. While this relieves the need to explore the entire functional space of dark energy state equations, we illustrate a severe risk that future data analysis could lead to misinterpretation of the DE state equation.

arXiv: 26 May 2010

New limits on the population of normal and millisecond pulsars in the Large and Small Magellanic Clouds

Authors: J.P. Ridley, D.R. Lorimer

http://arxiv.org/abs/1005.4653v1

We model the potentially observable populations of normal and millisecond radio pulsars in the Large and Small Magellanic Clouds (LMC and SMC) where the known population currently stands at 19 normal radio pulsars. Taking into account the detection thresholds of previous surveys, and assuming optimal period and luminosity distributions based on studies of Galactic pulsars, we estimate there are (1.79 +/- 0.20) x 10^4 and (1.09 +/- 0.16) x 10^4 normal pulsars in the LMC and SMC respectively. When we attempt to correct for beaming effects, and the fraction of high-velocity pulsars which escape the clouds, we estimate birth rates in both the LMC and SMC to be comparable and in the range 0.5--1 pulsar per century. Although higher than estimates for the rate of core-collapse supernovae in the clouds, these pulsar birth rates are consistent with historical supernova observations in the past 300 yr. A substantial population of active radio pulsars (of order a few hundred thousand) have escaped the LMC and SMC and populate the local intergalactic medium. For the millisecond pulsar (MSP) population, the lack of any detections from current surveys leads to respective upper limits (at the 95% confidence level) of 15,000 for the LMC and 23,000 for the SMC. Several MSPs could be detected by a currently ongoing survey of the SMC with improved time and frequency resolution using the Parkes multibeam system. Giant-pulse emitting neutron stars could also be seen by this survey

 

 

Lopsidedness of cluster galaxies in modified gravity

Authors: Xufen Wu, HongSheng Zhao, Benoit Famaey

http://arxiv.org/abs/1005.4629v1

We point out an interesting theoretical prediction for elliptical galaxies residing inside galaxy clusters in the framework of modified Newtonian dynamics (MOND), that could be used to test this paradigm. Apart from the central brightest cluster galaxy, other galaxies close enough to the centre experience a strong gravitational influence from the other galaxies of the cluster. This influence manifests itself only as tides in standard Newtonian gravity, meaning that the systematic acceleration of the centre of mass of the galaxy has no consequence. However, in the context of MOND, a consequence of the breaking of the strong equivalence principle is that the systematic acceleration changes the own self-gravity of the galaxy. We show here that, in this framework, initially axisymmetric elliptical galaxies become lopsided along the external field's direction, and that the centroid of the galaxy, defined by the outer density contours, is shifted by a few hundreds parsecs with respect to the densest point.

 

Search for Gravitational Waves from Compact Binary Coalescence in LIGO and Virgo Data from S5 and VSR1

Authors: the LIGO Scientific Collaboration, the Virgo Collaboration

http://arxiv.org/abs/1005.4655v1

We report the results of the first search for gravitational waves from compact binary coalescence using data from the LIGO and Virgo detectors. Five months of data were collected during the concurrent S5 (LIGO) and VSR1 (Virgo) science runs. The search focused on signals from binary mergers with a total mass between 2 and 35 Msun. No gravitational waves are identified. The cumulative 90%-confidence upper limits on the rate of compact binary coalescence are calculated for non-spinning binary neutron stars, black hole-neutron star systems, and binary black holes to be 8.7x10^-3, 2.2x10^-3 and 4.4x10^-4 yr^-1 L_10^-1 respectively, where L_10 is 10^10 times the blue solar luminosity. These upper limits are compared with astrophysical expectations