Tuesday, June 28, 2011

arXiv: 29 June 2011

Galaxy Bias and non-Linear Structure Formation in General Relativity

arXiv:1106.5507v1
Length scales probed by large scale structure surveys are becoming closer to the horizon scale. Further, it has been recently understood that non-Gaussianity in the initial conditions could show up in a scale dependence of the bias of galaxies at the largest distances. It is therefore important to include General Relativistic effects. Here we provide a General Relativistic generalization of the bias, valid both for Gaussian and non-Gaussian initial conditions. The collapse of objects happens on very small scales, while long-wavelength modes are always in the quasi linear regime. Around every collapsing region, it is therefore possible to find a reference frame that is valid for all times and where the space time is almost flat: the Fermi frame. Here the Newtonian approximation is applicable and the equations of motion are the ones of the N-body codes. The effects of long-wavelength modes are encoded in the mapping from the cosmological frame to the local frame. For the linear bias, the effect of the long-wavelength modes on the dynamics is encoded in the local curvature of the Universe, which allows us to define a General Relativistic generalization of the bias in the standard Newtonian setting. We show that the bias due to this effect goes to zero as the squared ratio of the physical wavenumber with the Hubble scale for modes longer than the horizon, as modes longer than the horizon have no dynamical effects. However, the bias due to non-Gaussianities does not need to vanish for modes longer than the Hubble scale, and for non-Gaussianities of the local kind it goes to a constant. As a further application, we show that it is not necessary to perform large N-body simulations to extract information on long-wavelength modes: N-body simulations can be done on small scales and long-wavelength modes are encoded simply by adding curvature to the simulation and rescaling the coordinates.




arXiv: 28 June 2011

Dark matter halos around isolated ellipticals

arXiv:1106.5133v1 
We investigate the distribution of the luminous and the dark matter components in the isolated ellipticals NGC 7052 and NGC 7785, embedded in an emitting hot gas halo, by means of relevant X-ray and photometric data. In order to calculate the dark matter distribution in these rare objects, we performed an improved X-ray analysis of the XMM-Newton data of NGC 7785, and we used former results based on Chandra data of NGC 7052. For each object we also derived the stellar spheroid length scale from the surface photometry and the spheroid stellar mass from an analysis of the galaxy spectral energy distribution (SED). We find that a dark matter component is present in these objects. It is subdominant and mixed with the luminous matter inside the optical region half-light radius wide, while it dominates the gravitational potential at outer radii. On the whole, the dark halo structure is very similar to that found around spirals of comparable luminosity and it is well reproduced by a Burkert halo and a Sersic spheroid.



Sunday, June 26, 2011

arXiv: 27 June 2011

Growth rate of matter perturbations as a probe of large-scale magnetism

arXiv:1106.5043v1
The growth rate of matter perturbations is computed in a magnetized environment for the LambdaCDM and wCDM paradigms. It is argued that the baryons do not necessarily follow into the dark matter potential wells after they are released from the drag of the photons. The baryonic evolution equations inherit a forcing term whose explicit form depends on the plasma description and can be deduced, for instance, in the resistive magnetohydrodynamical approximation. After deriving an analytical expression for the growth rate applicable when dark energy does not cluster, the effects of relativistic corrections and of the inhomogeneities associated with the other species of the plasma are taken into account numerically. The spectral amplitudes and slopes of the stochastic magnetic background are selected to avoid appreciable distortions in the measured temperature and polarization anisotropies of the Cosmic Microwave Background. The growth of structures in the current paradigms of structure formation represents a complementary probe of large-scale magnetism in the same way as the shape of the growth factor and the associated indices can be used, in the conventional lore, to discriminate between competing scenarios of dark energy or even to distinguish different models of gravity.


Friday, June 24, 2011

arXiv:25 June 2011

Testing gravity with CAMB and CosmoMC

arXiv:1106.4543v1
We introduce a patch to the commonly used public codes CAMB and CosmoMC that allows the user to implement a general modification of the equations describing the growth of cosmological perturbations, while preserving the covariant conservation of the energy-momentum. This patch replaces the previously publicly released code MGCAMB, while also extending it in several ways. The new version removes the limitation of late-time-only modifications to the perturbed Einstein equations, and includes several parametrization introduced in the literature. To demonstrate the use of the patch, we obtain joint constraints on the neutrino mass and parameters of a scalar-tensor gravity model from CMB, SNe and ISW data as measured from the correlation of CMB with large scale structure.


Wednesday, June 22, 2011

arXiv: 23 June 2011

Cosmological apparent and trapping horizons

Valerio Faraoni (Bishop's University)
arXiv:1106.4427v1
The dynamics of particle, event, and apparent horizons in FLRW space are discussed. The apparent horizon is trapping when the Ricci curvature is positive. This simple criterion coincides with the condition for the Kodama-Hayward apparent horizon temperature to be positive, and also discriminates between timelike and spacelike character of the apparent horizon. We discuss also the entropy of apparent cosmological horizons in extended theories of gravity and we use the generalized 2nd law to discard an exact solution of Brans-Dicke gravity as unphysical.

Tuesday, June 21, 2011

arXiv: 22 June 2011

Hubble parameter data constraints on dark energy

arXiv:1106.4294v1 
We use Hubble parameter versus redshift data from Stern, et al(2010) and Gazta\~{n}aga, et al (2009) to place constraints on model parameters of constant and time-evolving dark energy cosmological models. These constraints are consistent with (through not as restrictive as) those derived from supernova Type Ia magnitude-redshift data. However, they are more restrictive than those derived from galaxy cluster angular diameter distance, and comparable with those from gamma-ray burst and lookback time data. A joint analysis of the Hubble parameter data with more restrictive baryon acoustic oscillation peak length scale and supernova Type Ia apparent magnitude data favors a spatially-flat cosmological model currently dominated by a time-independent cosmological constant but does not exclude time-varying dark energy.

Implicit Priors in Galaxy Cluster Mass and Scaling Relation Determinations

Adam Mantz (NASA/GSFC), Steven W. Allen (KIPAC, Stanford/SLAC)
arXiv:1106.4052v1
Deriving the total masses of galaxy clusters from observations of the intracluster medium (ICM) generally requires some prior information, in addition to the assumptions of hydrostatic equilibrium and spherical symmetry. Often, this information takes the form of particular parametrized functions used to describe the cluster gas density and temperature profiles. In this paper, we investigate the implicit priors on hydrostatic masses that result from this fully parametric approach, and the implications of such priors for scaling relations formed from those masses. We show that the application of such fully parametric models of the ICM naturally imposes a prior on the slopes of the derived scaling relations, favoring the self-similar model, and argue that this prior may be influential in practice. In contrast, this bias does not exist for techniques which adopt an explicit prior on the form of the mass profile but describe the ICM non-parametrically. Constraints on the slope of the cluster mass--temperature relation in the literature show a separation based the approach employed, with the results from fully parametric ICM modeling clustering nearer the self-similar value. Given that a primary goal of scaling relation analyses is to test the self-similar model, the application of methods subject to strong, implicit priors should be avoided. Alternative methods and best practices are discussed.

Predicted Constraints on Cosmic String Tension from Planck and Future CMB Polarization Measurements
Simon ForemanAdam MossDouglas Scott
arXiv:1106.4018v1
We perform a Fisher matrix calculation of the predicted uncertainties on estimates of the cosmic string tension Gmu from upcoming observational data (namely, cosmic microwave background power spectra from the Planck satellite and an idealized future polarization experiment). We employ simulations that are more general than others commonly used in the literature, leaving the mean velocity of strings, correlation length of the string network, and "wiggliness" (which parametrizes smaller-scale structure along the strings) as free parameters that can be observationally measured. In a new code, StringFast, we implement a method for efficient computation of the C_l spectra induced by a network of strings, which is fast enough to be used in Markov Chain Monte Carlo analyses of future data. Performing a calculation with the string parameters left free results in projected constraints on Gmu that are larger than those obtained by fixing their values a priori, typically by a factor of ~2-7. We also find that if Gmu is equal to the current observational maximum, Planck will be able to make a confident detection of strings. However, if Gmu is two orders of magnitude smaller, even a perfect, lensing-free measurement of polarization power spectra will not be able to detect a nonzero string tension at better than 2 sigma confidence.




Monday, June 20, 2011

arXiv: 21 June 2011

Disentangling non-Gaussianity, bias and GR effects in the galaxy distribution

Marco Bruni (ICG, Portsmouth), Robert Crittenden (ICG, Portsmouth), Kazuya Koyama (ICG, Portsmouth), Roy Maartens (Western Cape, ICG, Portsmouth),Cyril Pitrou (ICG, Portsmouth), David Wands (ICG, Portsmouth)
arXiv:1106.3999v1
Local non-Gaussianity, parametrized by $f_{\rm NL}$, introduces a scale-dependent bias that is strongest at large scales, precisely where General Relativistic (GR) effects also become significant. With future data, it should be possible to constrain $f_{\rm NL} = {\cal O}(1)$ with high redshift surveys. GR corrections to the power spectrum and ambiguities in the gauge used to define bias introduce effects similar to $f_{\rm NL}= {\cal O}(1)$, so it is essential to disentangle these effects. We show how to consistently include primordial non-Gaussianity in the observed angular power spectrum of the galaxy distribution and we discuss how to distinguish between the various effects, so as to extract an accurate non-Gaussianity signal.

Phenomenology of Gravitational Aether as a solution to the Old Cosmological Constant Problem

arXiv:1106.3955v1
One of the deepest and most long-standing mysteries in physics has been the huge discrepancy between the observed vacuum density and our expectations from theories of high energy physics, which has been dubbed the Old Cosmological Constant problem. One proposal to address this puzzle at the semi-classical level is to decouple quantum vacuum from space-time geometry via a modification of gravity that includes an incompressible fluid, known as Gravitational Aether. In this paper, we discuss classical predictions of this theory along with its compatibility with cosmological and experimental tests of gravity. We argue that deviations from General Relativity (GR) in this theory are sourced by pressure or vorticity. In particular, the theory predicts that the gravitational constant for radiation is 33% larger than that of non-relativistic matter, which is preferred by (most) cosmic microwave background (CMB), Lyman-Alpha forest, and Lithium-7 primordial abundance observations, while being consistent with other cosmological tests at ~2-sigma level. It is further shown that all Parametrized Post-Newtonian (PPN) parameters have the standard GR values aside from the anomalous coupling to pressure, which has not been directly measured. A more subtle prediction of this model (assuming irrotational aether) is that the (intrinsic) gravitomagnetic effect is 33% larger than GR prediction. This is consistent with current limits from LAGEOS and Gravity Probe B at ~2-sigma level.

Matter power spectra in dynamical-Dark Energy cosmologies

arXiv:1106.3987v1
(abridged) We used a suite of numerical cosmological simulations in order to investigate the effect of gas cooling and star formation on the large scale matter distribution. The simulations follow the formation of cosmic structures in five different Dark Energy models: the fiducial $\Lambda$CDM cosmology and four models where the Dark Energy density is allowed to have a non-trivial redshift evolution. For each cosmology we have a control run with dark matter only, in order to allow a direct assessment of the impact of baryonic processes. We found that the power spectra of gas and stars, as well as the total matter power spectrum, are in qualitative agreement with the results of previous works in the framework of the fiducial model, although several quantitative differences exist. We used the halo model in order to investigate the backreaction of gas and stars on the dark matter distribution, finding that it is very well reproduced by increasing the average dark matter halo concentration by 17%, irrespective of the mass. Moving to model universes dominated by dynamical Dark Energy, it turns out that they introduce a specific signature on the power spectra of the various matter components, that is qualitatively independent of the exact cosmology considered. This generic shape is well captured by the halo model, however the finer details of the dark matter power spectrum can be precisely captured only at the cost of a few slight modifications to the ingredients entering the model. The backreaction of baryons onto the dark matter distribution works pretty much in the same way as in the reference $\Lambda$CDM model. Nonetheless, the increment in average concentration is less pronounced than in the fiducial model (only $\sim 10%$), in agreement with a series of other clues pointing toward the fact that star formation is less efficient when Dark Energy displays a dynamical evolution.


Sunday, June 19, 2011

arXiv: 20 June 2011

The 6dF Galaxy Survey: Baryon Acoustic Oscillations and the Local Hubble Constant

arXiv:1106.3366v1 
We analyse the large-scale correlation function of the 6dF Galaxy Survey (6dFGS) and detect a Baryon Acoustic Oscillation (BAO) signal. The 6dFGS BAO detection allows us to constrain the distance-redshift relation at z_{\rm eff} = 0.106. We achieve a distance measure of D_V(z_{\rm eff}) = 456\pm27 Mpc and a measurement of the distance ratio, r_s(z_d)/D_V(z_{\rm eff}) = 0.336\pm0.015 (4.5% precision), where r_s(z_d) is the sound horizon at the drag epoch z_d. The low effective redshift of 6dFGS makes it a competitive and independent alternative to Cepheids and low-z supernovae in constraining the Hubble constant. We find a Hubble constant of H_0 = 67\pm3.2 km s^{-1} Mpc^{-1} (4.8% precision) that depends only on the WMAP-7 calibration of the sound horizon and on the galaxy clustering in 6dFGS. Compared to earlier BAO studies at higher redshift, our analysis is less dependent on other cosmological parameters. The sensitivity to H_0 can be used to break the degeneracy between the dark energy equation of state parameter w and H_0 in the CMB data. We determine that w = -0.97\pm0.13, using only WMAP-7 and BAO data from both 6dFGS and \citet{Percival:2009xn}. We also discuss predictions for the large scale correlation function of two future wide-angle surveys: the WALLABY blind H{\sc I} survey (with the Australian SKA Pathfinder, ASKAP), and the proposed TAIPAN all-southern-sky optical galaxy survey with the UK Schmidt Telescope (UKST). We find that both surveys are very likely to yield detections of the BAO peak, making WALLABY the first radio galaxy survey to do so. We also predict that TAIPAN has the potential to constrain the Hubble constant with 3% precision.

Cosmic Mach Number as A Sensitive Test of the Growth of Structure

Yin-Zhe Ma (Cambridge), Jeremiah P. Ostriker (Princeton, Cambridge), Gong-Bo Zhao (ICG, Portsmouth)
arXiv:1106.3327v1
In this Letter, we investigate the potential power of the Cosmic Mach Number (CMN), which is the ratio between the mean velocity and the velocity dispersion of galaxies as a function of cosmic scales, to constrain cosmologies. We first measure the CMN from 5 catalogues of galaxy peculiar velocity surveys at low redshift (0.002<z<0.03), and use them to contrast cosmological models. Overall, current data is consistent with the WMAP7 LCDM model. We find that the CMN is highly sensitive to the growth of structure on scales 0.01<k<0.1 h/Mpc in Fourier space. Therefore, modified gravity models, and models with massive neutrinos, in which the structure growth generally deviates from that in the LCDM model in a scale-dependent way, can be well differentiated from the LCDM model using future CMN data.


Saturday, June 18, 2011

arXiv: 17 June 2011

The Fundamental Plane of Early-Type Galaxies as a Confounding Correlation

arXiv:1106.3154v1
Early-type galaxies are characterized by many scaling relations. One of them, the so-called fundamental plane is a relatively tight correlation between three variables, and has resisted a clear physical understanding despite many years of intensive research. Here, we show that the correlation between the three variables of the fundamental plane can be the artifact of the effect of another parameter influencing all, so that the fundamental plane may be understood as a confounding correlation. Indeed, the complexity of the physics of galaxies and of their evolution suggests that the main confounding parameter must be related to the level of diversification reached by the galaxies. Consequently, many scaling relations for galaxies are probably evolutionary correlations.

Cosmology of the Galileon from Massive Gravity

Claudia de RhamLavinia Heisenberg (DPT and CAP Geneve)
arXiv:1106.3312v1
We covariantize the decoupling limit of massive gravity proposed in arXiv:1011.1232 and study the cosmology of this theory as a proxy, which embodies key features of the fully non-linear covariant theory. We first confirm that it exhibits a self-accelerating solution, similar to what has been found in arXiv:1010.1780, where the Hubble parameter corresponds to the graviton mass. For a certain range of parameters fluctuations relative to the self-accelerating background are stable and form an attractor solution. We also show that a degravitating solution can not be constructed in this covariantized proxy theory in a meaningful way. As for cosmic structure formation, we find that the helicity-0 mode of the graviton causes an enhancement relative to LCDM. For consistency we also compare proxy theories obtained starting from different frames in the decoupling limit and discuss the possibility of obtaining a non-representative proxy theory by choosing the wrong starting frame.


Planetary microlensing by lucky imaging method/Seminar Series 21

Speaker: Sedighe Sadjadian




Affiliation: Physics Department,Sharif University of Technology
Title:
Planetary microlensing by lucky imaging method
Date: 15 June 2011
Place: Seminar Series, Astronomy School, IPM



abstract:


Lucky imaging or lucky exposure technique improve angular resolution
of images by taking images faster that the atmospheric turbulence time
scale. In this method, by selecting the images with best strehl ratio,
recentering on the brightest image and combining them, a more resolved
image obtains. MindStep collaboration is supposed to use this method
in new observational season towards galactic bulge for detecting the
less massive exoplanets. Following this goal we investigate the best
strategy for using the lucky image camera in detecting exoplanets
according to the characteristic of Danish telescope. In this talk I
review all results from this study.


Gravitational Microlensing and Exoplanets/Seminar Series 20

Speaker: Martin Dominik


Affiliation:Physics and Astronomy department/ Saint Andrews, Scotland
Title:
 Gravitational Microlensing and Exoplanets
Date: 12-13-14-15
Place: A short Course, in Astronomy School, IPM.



abstract:

...

Understanding the saturation of magneto rotational instability in accretion disks/Seminar Series 19


Speaker: Dr.Mir Abbas Jalali


Affiliation: Mechanical Engineering Department,Sharif University of Technology
Title:
 Understanding the saturation of magneto rotational instability in accretion disks
Date: 8 June 2011
Place: Seminar Series, Astronomy School, IPM



abstract:

The governing magnetohydrodynamics equations of thin accretion disks
around black holes admit a set of unstable solutions, which are called
magneto rotational instabilities (MRI). The instabilities grow exponentially
for the inplane components of the velocity and magnetic field. Numerical
simulations, however, show that instabilities can be saturated due to
nonlinear effects though its exact physical mechanism is far from being
completely understood. After reviewing the literature, I introduce some
analytical and numerical procedures that may provide new insight into
the problem.

Spin 0 and spin 1/2 particles in a spherically symmetric static gravity and a Coulomb field /Seminar Series 18


Speaker: Dr. Liela Ramazan


Affiliation: Physics Department, Alzahra University
Title:
Spin 0 and spin 1/2 particles in a spherically symmetric static gravity and a Coulomb field
Date: 1 June 2011
Place: Seminar Series, Astronomy School, IPM



abstract:

 A relativistic particle in an attractive Coulomb field as well ‎as a static and spherically symmetric gravitational field is ‎studiedThe gravitational field is treated perturbatively ‎and the energy levels are obtained for both spin 0 (Klein-Gordon) ‎and spin 1/2 (Dirac) particlesThe results are shown to coincide ‎with each other as well as the result of the nonrelativistic ‎‎(Schrodinger) equation in the nonrelativistic limit.‎‎‎

Multi-dimensional astronomical data analysis using automatic pipeline/Seminar Series 17


Speaker: Dr. Mehdi Bazargan


Affiliation: Physics Department, Zanjan University
Title:
 Multi-dimensional astronomical data analysis using automatic pipeline
Date:25 May 2011
Place: Seminar Series, Astronomy School, IPM



abstract:

In the recent years, Artificial Neural Networks (ANN) have become
extremely popular as a tool for handling large data bases, particularly in
Astronomical application, with very large data available to the community.
The ANN can be applied for the classification of objects or extracting
parameters. I will be discussing the application of artificial neural
networks in the analysis of stellar spectra, mainly, application of these
techniques to the automatic classification of stellar spectra of stars.
Other applications of these tools such as estimating the nanoflare
parameters for the SUMER light curves in the corona of an active region,
Morphological classification of galaxies and the cosmological parameter
estimation, will be considered.

Structure Formation in MOND/Seminar Series 16


Speaker: Mirkamali
Affiliation:
 Physics Department, Sharif University of Technology
Title:Structure Formation in Mond
Date:22 May 2011
Place: Seminar Series, Cosmology seminars, SUT



abstract:
 
...

Effects of geometrical structure on electrical properties of free graphine/ Seminar series 15


Speaker: Dr. Naser Nafari
Affiliation:
Physics Department, Sharif University of Technology
Title:Effects of geometrical structure on electrical properties of free graphine
Date:22 May 2011
Place: Seminar Series, Colloquium, SUT






abstract:


...

Casimir Nanomechanics/Seminar Series 14


Speaker: Mir Faez Miri
Affiliation:
 Physics Department, Tehran University
Title: Casimir Nanomechanics
Date:15 May 2011
Place: Seminar Series, Colloquium, SUT



abstract:
 
...

Radio Astronomy With LOFAR /Seminar Series 13


Speaker: Halimee MirAghae
Affiliation:
 Physics Department, Sharif University of Technology
Title: Radio Astronomy with Lofar
Date:15 May 2011
Place: Seminar Series, Cosmology group, SUT



abstract:
 
...

arXiv: 16 June 2011

The core-cusp problem in cold dark matter halos and supernova feedback: Effects of Mass Loss

arXiv:1106.2864v1
The core-cusp problem remains as one of the unsolved discrepancies between observations and theories predicted by the standard paradigm of cold dark matter (CDM) cosmology. To solve this problem, we perform N-body simulations to study the nonlinear response of CDM halos to the variance of the gravitational potential induced by gas removal from galaxy centers. In this study, we focus on the timescale of the gas ejection, which is strongly correlated with stellar activities, and demonstrate that it is one of the key factors in determining the dynamical response of CDM halos. The results of simulations show that the power-low index of the mass-density profile of the dark matter halo correlated with the timescale of the mass loss, and it is flatter when the mass loss occurs over a short time than when it occurs over a long time. However, it is still larger than typical observational values; in other words, the central cusp remains for any mass loss model in the simulations. Moreover, for the slow mass-loss case, the final density profile of the dark matter halo recovers the universal density profiles predicted by the CDM cosmology. Therefore, mass loss driven by stellar feedback may not be an effective mechanism to flatten the central cusp.

Purely Kinetic Coupled Gravity

arXiv:1106.2815v1
Cosmic acceleration can be achieved not only with a sufficiently flat scalar field potential but through kinetic terms coupled to gravity. These derivative couplings impose a shift symmetry on the scalar field, aiding naturalness. We write the most general purely kinetic action not exceeding mass dimension six and obeying second order field equations. The result reduces to a simple form involving a coupling of the Einstein tensor with the kinetic term and can be interpreted as adding a new term to Galileon gravity in curved spacetime. We examine the cosmological implications of the effective dark energy and classify the dynamical attractor solutions, finding a quasistable loitering phase mimicking late time acceleration by a cosmological constant.

Oscillating Bispectra and Galaxy Clustering: A Novel Probe of Inflationary Physics with Large-Scale Structure

arXiv:1106.2806v1
Many models of inflation predict oscillatory features in the bispectrum of primordial fluctuations. Since it has been shown that primordial non-Gaussianity can lead to a scale-dependent halo bias, we investigate the effect of oscillations in the three-point function on the clustering of dark matter halos. Interestingly, we find that features in the inflaton potential such as oscillations or sharp steps get imprinted in the mass dependence of the non-Gaussian halo bias. In this paper, we focus on models displaying a sharp feature in the inflaton potential as well as Resonant non-Gaussianity. In both cases, we find a strong scale dependence for the non-Gaussian halo bias with a slope similar to that of the local model. In the resonant case, we find that the non-Gaussian bias oscillates with halo mass, a novel feature that is unique to this type of models. In the case of a sharp feature in the inflaton potential, we find that the clustering of halos is enhanced at the mass scale corresponding to the Fourier mode that exited the horizon when the inflaton was crossing the feature in the potential. Both of these are new effects that open the possibility of characterizing the inflationary potential with large-scale-structure surveys. We briefly discuss the prospects for detecting these non-Gaussian effects.

MOND cosmology from holographic principle

arXiv:1106.2966v1
We derive the MOND cosmology which is uniquely corresponding to the original MOND in galaxies via holographic approach of gravity. It inherits the key merit of MOND, that is, it reduces the byronic matter and mysterious non-byronic dark matter (dark matter for short) in the standard cosmology into byronic matter only. For the first time we derive the critical parameter in MOND, i.e., the transition acceleration $a_c$ on cosmological scale. We thus solve the long-standing coincidence problem $a_c\sim cH_{0}$. More interestingly, a term like age-graphic dark energy emerges naturally. In the frame of this MOND cosmology, we only need byronic matter to describe both dark matter and dark energy in standard cosmology.

The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs


arXiv:1106.2925v1
In this fourth part of the series presenting the Optical Gravitational Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact objects (MACHOs) we describe results of the OGLE-III monitoring of the Small Magellanic Cloud (SMC). Three sound candidates for microlensing events were found and yielded the optical depth tau_SMC-OIII=1.30+-1.01 10^{-7}, consistent with the expected contribution from Galactic disk and SMC self-lensing. We report that event OGLE-SMC-03 is the most likely a thick disk lens candidate, the first of such type found towards the SMC. In this paper we also combined all OGLE Large and Small Magellanic Cloud microlensing results in order to refine the conclusions on MACHOs. All but one of OGLE events are most likely caused by the lensing by known populations of stars, therefore we concluded that there is no need for introducing any special dark matter compact objects in order to explain the observed events rates. Potential black hole event indicates that similar lenses can contribute only about 2 per cent to the total mass of the halo, which is still in agreement with the expected number of such objects.

Casting Light on Dark Matter

arXiv:1106.2923v1 
The prospects for detecting a candidate supersymmetric dark matter particle at the LHC are reviewed, and compared with the prospects for direct and indirect searches for astrophysical dark matter. The discussion is based on a frequentist analysis of the preferred regions of the Minimal supersymmetric extension of the Standard Model with universal soft supersymmetry breaking (the CMSSM). LHC searches may have good chances to observe supersymmetry in the near future - and so may direct searches for astrophysical dark matter particles, whereas indirect searches may require greater sensitivity, at least within the CMSSM.






Tuesday, June 14, 2011

arXIv: 15 June 2011

Efficient computation of the non-linear matter power spectrum of LambdaCDM

:arXiv:1106.2607v1 
We address the issue of computing the non-linear matter power spectrum on mildly non-linear scales with efficient semi-analytic methods. We implemented M. Pietroni's Time Renormalization Group (TRG) method and its Dynamical 1-Loop (D1L) limit in a numerical module for the new Boltzmann code CLASS. Our publicly released module is valid for LambdaCDM models, and optimized in such a way to run in less than a minute for D1L, or in one hour (divided by number of nodes) for TRG. A careful comparison of the D1L, TRG and Standard 1-Loop approaches reveals that for a precise computation of mildly non-linear corrections at high redshift (z greater or equal to 2), the essential point is not to renormalize perturbations, but just to correctly implement initial conditions in a dynamical 1-loop approach. The very fast D1L algorithm then provides surprisingly good results, as can be checked by comparing with highly accurate simulations by M. Sato & T. Matsubara: for z greater or equal to 2, percent precision is achieved until at least k = 0.35h/Mpc, or until k = 0.18h/Mpc at z=1.

Monday, June 13, 2011

arXiv: 14 June 2011

Modified Gravity and Cosmology

arXiv:1106.2476v1
In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, Scalar-Tensor, Einstein-Aether, and Bimetric theories, as well as TeVeS, f(R), general higher-order theories, Horava-Lifschitz gravity, Galileons, Ghost Condensates, and models of extra dimensions including Kaluza-Klein, Randall-Sundrum, DGP, and higher co-dimension braneworlds. We also review attempts to construct a Parameterised Post-Friedmannian formalism, that can be used to constrain deviations from General Relativity in cosmology, and that is suitable for comparison with data on the largest scales. These subjects have been intensively studied over the past decade, largely motivated by rapid progress in the field of observational cosmology that now allows, for the first time, precision tests of fundamental physics on the scale of the observable Universe. The purpose of this review is to provide a reference tool for researchers and students in cosmology and gravitational physics, as well as a self-contained, comprehensive and up-to-date introduction to the subject as a whole.

Effective Field Theory for Inflation

arXiv:1106.2189v1
This is a version of the author's Ph.D. thesis. The methods of effective field theory are used to study generic theories of inflation with a single inflaton field and to perform a general analysis of the associated non-Gaussianities. We investigate the amplitudes and shapes of the various three and four-point correlators which are generated by different classes of single-field inflationary models. Besides the well-known results for the so called P(X,\phi) model and the ghost inflationary theories, which we recover, we point out that extrinsic curvature-generated interactions may give rise to large non-Gaussianities with distinctive features in the form of specific shape-functions (e.g. flat, orthogonal etc..) for the correlators.

The Sunyaev-Zeldovich Signal of the maxBCG SDSS Galaxy Clusters in WMAP
arXiv:1106.2185v1
The Planck Collaboration measured the Sunyaev-Zel'dovich (SZ) decrement of optically selected clusters from the Sloan Digital Sky Survey, finding that it falls significantly below expectations based on existing mass calibration of the maxBCG galaxy clusters. Resolving this tension requires either the data to go up, or the theoretical expectations to come down. Here, we use data from the Wilkinson Microwave Anisotropy Probe (WMAP) to perform an independent estimate of the SZ decrement of maxBCG clusters. The recovered signal is consistent with that obtained using Planck, though with larger error bars due to WMAP's larger beam size and smaller frequency range. Nevertheless, this detection serves as an independent confirmation of the magnitude of the effect, and demonstrates that the observed discrepancy must be theoretical in origin.

The Effect of Cosmological Background Dynamics on the Spherical Collapse in MOND


arXiv:1106.2274v1
The effect of background dynamics of the universe on formation of large scale structures in the framework of Modified Newtonian Dynamics (MOND) is investigated. A spherical collapse model is used for modeling the formation of the structures. This study is done in two extreme cases: ({\it i}) assuming a universe with a low-density baryonic matter without any cold dark matter and dark energy; ({\it ii}) a dark energy dominated universe with baryonic matter, without cold dark matter. We show that for the case ({\it ii}) the structures virialize at lower redshifts with larger radii compared to the low-density background universe. The dark energy slow downs the collapse of the structures. We show that our results are compatible with recent simulations of the structure formation in MOND.