arXiv: 17 May 2011

The Value of the Cosmological Constant

John D. Barrow, Douglas J. Shaw

arXiv:1105.3105v1 We make the cosmological constant, {\Lambda}, into a field and restrict the variations of the action with respect to it by causality. This creates an additional Einstein constraint equation. It restricts the solutions of the standard Einstein equations and is the requirement that the cosmological wave function possess a classical limit. When applied to the Friedmann metric it requires that the cosmological constant measured today, t_{U}, be {\Lambda} ~ t_{U}^(-2) ~ 10^(-122), as observed. This is the classical value of {\Lambda} that dominates the wave function of the universe. Our new field equation determines {\Lambda} in terms of other astronomically measurable quantities. Specifically, it predicts that the spatial curvature parameter of the universe is {\Omega}_{k0} \equiv -k/a_(0)^(2)H^2= -0.0055, which will be tested by Planck Satellite data. Our theory also creates a new picture of self-consistent quantum cosmological history.

arXiv: 16 May 2011

Open inflation in the landscape

Daisuke Yamauchi, Andrei Linde, Atsushi Naruko, Misao Sasaki, Takahiro Tanaka

arXiv:1105.2674v1

Open inflation scenario is attracting a renewed interest in the context of string landscape. Since there are a large number of metastable de Sitter vacua in string landscape, tunneling transitions to lower metastable vacua through the bubble nucleation occur quite naturally. Although the deviation of Omega_0 from unity is small by the observational bound, we argue that the effect of this small deviation on the large angle CMB anisotropies can be significant for tensor-type perturbation in open inflation scenario. We consider the situation in which there is a large hierarchy between the energy scale of the quantum tunneling and that of the slow-roll inflation in the nucleated bubble. If the potential just after tunneling is steep enough, a rapid-roll phase appears before the slow-roll inflation. In this case the power spectrum is basically determined by the Hubble rate during the slow-roll inflation. If such rapid-roll phase is absent, the power spectrum keeps the memory of the high energy density there in the large angular components. The amplitude of large angular components can be enhanced due to the effects of the wall fluctuation mode if the bubble wall tension is small. Therefore, one can construct some models in which the deviation of Omega_0 from unity is large enough to produce measurable effects. We also consider a more general class of models, where the false vacuum decay may occur due to Hawking-Moss tunneling, as well as the models involving more than one scalar field. We discuss scalar perturbations in these models and point out that a large set of such models is already ruled out by observational data, unless there was a very long stage of slow-roll inflation after the tunneling. These results show that observational data allow us to test various assumptions concerning the structure of the string theory potentials and the duration of the last stage of inflation.

Structure Formation with Scalar Field Dark Matter: The Fluid Approach

Abril Suárez, Tonatiuh Matos

:

arXiv:1101.4039v2 The properties of nearby galaxies that can be observed in great detail suggest that a better theory rather than cold dark matter (CDM) would describe in a better way a mechanism by which matter is more rapidly gathered into large-scale structure such as galaxies and groups of galaxies. In this work we develop and simulate a hydrodynamical approach for the early formation of structure in the Universe, this approach is based on the fact that dark matter is on the form of some kind of scalar field (SF) with a potential that goes as $\mu^2\Phi^2/2+\lambda\Phi^4/4$, we expect that the fluctuations coming from the SF will give us some information about the matter distribution we observe these days.

arXiv: 12 May 2011

Gravitational Wave Tests of General Relativity with the Parameterized Post-Einsteinian Framework

Neil Cornish, Laura Sampson, Nico Yunes, Frans Pretorius

arXiv:1105.2088v1 Gravitational wave astronomy has tremendous potential for studying extreme astrophysical phenomena and exploring fundamental physics. The waves produced by binary black hole mergers will provide a pristine environment in which to study strong field, dynamical gravity. Extracting detailed information about these systems requires accurate theoretical models of the gravitational wave signals. If gravity is not described by General Relativity, analyses that are based on waveforms derived from Einstein's field equations could result in parameter biases and a loss of detection efficiency. A new class of "parameterized post-Einsteinian" (ppE) waveforms has been proposed to cover this eventuality. Here we apply the ppE approach to simulated data from a network of advanced ground based interferometers (aLIGO/aVirgo) and from a future spaced based interferometer (LISA). Bayesian inference and model selection are used to investigate parameter biases, and to determine the level at which departures from general relativity can be detected. We find that in some cases the parameter biases from assuming the wrong theory can be severe. We also find that gravitational wave observations will beat the existing bounds on deviations from general relativity derived from the orbital decay of binary pulsars by a large margin across a wide swath of parameter space. A Fundamental Line for Elliptical Galaxies Preethi Nair, Sidney van den Bergh, Roberto G. Abraham arXiv:1105.2063v1 Recent studies have shown that massive galaxies in the distant universe are surprisingly compact, with typical sizes about a factor of three smaller than equally massive galaxies in the nearby universe. It has been suggested that these massive galaxies grow into systems resembling nearby galaxies through a series of minor mergers. In this model the size growth of galaxies is an inherently stochastic process, and the resulting size-luminosity relationship is expected to have considerable environmentally-dependent scatter. To test whether minor mergers can explain the size growth in massive galaxies, we have closely examined the scatter in the size-luminosity relation of nearby elliptical galaxies using a large new database (Nair & Abraham 2010) of accurate visual galaxy classifications. We demonstrate that this scatter is much smaller than has been previously assumed, and may even be so small as to challenge the plausibility of the merger driven hierarchical models for the formation of massive ellipticals.

On Rotation and Rotating Frames/ Seminar Series 12

Speaker: Dr. Mohammad Nouri

Affiliation: Physics Department, Tehran University
Title:On rotation and Rotation Frames
Date: 11 May 2011
Place: Seminar Series, IPM, Astronomy school.

abstract:
 

Unlike the Lorentz transformation, which replaces the Galilean transformation at high velocities 

there seems to be no such a consensus in the case of rotating frames and most of the authors use 

the usual Newtonian rotational transformation. In this talk I review and discuss some of the attempts 

made in generalizing the Newtonian transformation.

arXiv: 11 May 2011

Halo abundances and counts-in-cells: The excursion set approach with correlated steps

Aseem Paranjape, Tsz Yan Lam, Ravi K. Sheth

arXiv:1105.1990v1 The Excursion Set approach has been used to make predictions for a number of interesting quantities in studies of nonlinear hierarchical clustering. These include the halo mass function, halo merger rates, halo formation times and masses, halo clustering, analogous quantities for voids, and the distribution of dark matter counts in randomly placed cells. The approach assumes that all these quantities can be mapped to problems involving the first crossing distribution of a suitably chosen barrier by random walks. Most analytic expressions for these distributions ignore the fact that, although different $k$-modes in the initial Gaussian field are uncorrelated, this is not true in real space: the values of the density field at a given spatial position, when smoothed on different real-space scales, are correlated in a nontrivial way. As a result, the problem is to estimate first crossing distribution by random walks having correlated rather than uncorrelated steps. In 1990, Peacock & Heavens presented a simple approximation for the first crossing distribution of a single barrier of constant height by walks with correlated steps. We show that their approximation can be thought of as a correction to a specific analytic limit of the correlations. We then use this insight to extend their approach to treat moving barriers, as well as walks that are constrained to pass through a certain point before crossing the barrier. In all cases, comparison with numerical simulations shows reasonably good agreement. In particular, this old but still beautiful approximation is more accurate, and substantially easier to implement, than other more recently proposed treatments of the correlated steps problem. Is backreaction really small within concordance cosmology? Chris Clarkson, Obinna Umeh (Cape Town) arXiv:1105.1886v1 Smoothing over structures in general relativity leads to a renormalisation of the background, and potentially many other effects which are poorly understood. Observables such as the distance-redshift relation when averaged on the sky do not necessarily yield the same smooth model which arises when performing spatial averages. These issues are thought to be of technical interest only in the standard model of cosmology, giving only tiny corrections. However, when we try to calculate observable quantities such as the all-sky average of the distance-redshift relation, we find that perturbation theory delivers divergent answers in the UV and corrections to the background of order unity. There are further problems. Second-order perturbations are the same size as first-order, and fourth-order at least the same as second, and possibly much larger, owing to the divergences. Much hinges on a coincidental balance of 2 numbers: the primordial power, and the ratio between the comoving Hubble scales at matter-radiation equality and today. Consequently, it is far from obvious that backreaction is irrelevant even in the concordance model, however natural it intuitively seems Physical properties of 6dF dwarf galaxies Jean Michel Gomes, Polychronis Papaderos arXiv:1105.1882v1 Spectral synthesis is basically the decomposition of an observed spectrum in terms of the superposition of a base of simple stellar populations of various ages and metallicities, producing as output the star formation and chemical histories of a galaxy, its extinction and velocity dispersion. The STARLIGHT code provides one of the most powerful spectral synthesis tools presently available. We have applied this code to the entire Six-Degree-Field Survey (6dF) sample of nearby star-forming galaxies, selecting dwarf galaxy candidates with the goal of: (1) deriving the age and metallicity of their stellar populations and (2) creating a database with the physical properties of our sample galaxies together with the FITS files of pure emission line spectra (i.e. the observed spectra after subtraction of the best-fitting synthetic stellar spectrum). Our results yield a good qualitative and quantitative agreement with previous studies based on the Sloan Digital Sky Survey (SDSS). However, an advantage of 6dF spectra is that they are taken within a twice as large fiber aperture, much reducing aperture effects in studies of nearby dwarf galaxies. Primordial non-Gaussianity from the 21 cm Power Spectrum during the Epoch of Reionization Shahab Joudaki (UC Irvine), Olivier Dore (JPL, Caltech), Luis Ferramacho (CNRS, Toulouse), Manoj Kaplinghat (UC Irvine), Mario G. Santos (CENTRA-IST) arXiv:1105.1773v1 Primordial non-Gaussianity is a crucial test of inflationary cosmology. We consider the impact of non-Gaussianity on the ionization power spectrum from 21 cm emission during the epoch of reionization. We focus on the power spectrum on large scales at redshifts of 7 to 8 and explore the expected constraint on the local non-Gaussianity parameter f_NL for current and next-generation 21 cm experiments. We show that experiments such as SKA and MWA could measure f_NL values of order 10. This can be improved by an order of magnitude with a fast-Fourier transform telescope like Omniscope.

arXiv: 10 May 2011

An analytic approach to baryon acoustic oscillations

Francesco Montanari, Ruth Durrer

arXiv:1105.1514v2 The fitting formula for the location of the first acoustic peak in the matter power spectrum is revised. We discuss the physics that leads to baryon acoustic oscillations: the recombination history, the tight coupling approximation and the velocity overshoot effect. A new fitting formula is proposed, which is in accordance within 5% with numerical results for a suitable range of cosmological parameters, whereas previous results yield deviations of up to 20%. The crucial improvement turns out to be the accuracy of the recombination history. On the necessity of the revisions for the cosmological matter perturbations from the general relativity Jiro Matsumoto arXiv:1105.1419v1 The differential equations, which are used for matter perturbations, are usually derived from the Newton gravity and the Euler equation in the expanded universe. In this paper, by the explicit calculations of metric perturbation theory in $\Lambda$CDM model, we show that the equations do not include the corrections from Einstein's general relativity sufficiently. Even if we consider the matter perturbations in the galaxy distance scales, the corrections are not so small and we cannot neglect them.

QCD Phyase Diagram and Colour Super Conductivity

Speaker: FayazBaksh

Affiliation: Sharif University of Technology
Title:QCD Phase Diagram and Colour Super Conductivity
Date: 10 May 2011
Place: Seminar Series in Center of excellence/ Field, Particles and Cosmology

abstract:

...

A report on La Palma's Sky/ Seminar Series 10

Speaker: Shant Baghram

Affiliation: Sharif University of Technology
Title: A report on La Palma's sky
Date: 9 May 2011
Place: IPM, astronomy School, Journal Club

abstract:

 A report on 5 month training program on observational Cosmology and Support astronomer,

in Isaac Newton Group of Telescopes in La Palma Spain

arXiv: 9 May 2011

Mapping Growth and Gravity with Robust Redshift Space Distortions

Juliana Kwan, Geraint F. Lewis, Eric V. Linder

arXiv:1105.1194v1 Redshift space distortions caused by galaxy peculiar velocities provide a window onto the growth rate of large scale structure and a method for testing general relativity. We investigate through a comparison of N-body simulations to various extensions of perturbation theory beyond the linear regime, the robustness of cosmological parameter extraction, including the gravitational growth index, \gamma. We find that the Kaiser formula and some perturbation theory approaches bias the growth rate by 1-sigma or more relative to the fiducial at scales as large as k > 0.07 h/Mpc. This bias propagates to estimates of the gravitational growth index as well as \Omega_m and the equation of state parameter and presents a significant challenge to modelling redshift space distortions. We also determine an accurate fitting function for a combination of line of sight damping and higher order angular dependence that allows robust modelling of the redshift space power spectrum to substantially higher k.

General relativistic Mass and velocity profile within a cosmological structure/ Seminar Series 9

Speaker:Navid Razbin

Affiliation: Sharif University of Technology
Title:General relativistic Mass and velocity profile within a cosmological structure"
Date: 4 May 2011
Place: Cosmology Weekly seminars, SUT

abstract:

 Using a general relativistic toy model for structures within

a cosmological setting,
We are calculating the Geodesics within a structure such as a galaxy
or a cluster of galaxies, to
obtain the velocity profile of stars or galaxies. The profile defines
a Newtonian mass which is then compared to
different local gravitational masses, such as Misner-sharp or
Brown-York, to see how different they are.

Philosophy of Mathematics/ Seminar Series 8

Speaker: Siavash Shahshahani

Affiliation: Sharif University of Technology - Mathematics Department
Title: Philosophy of Mathematics
Date: 8 May 2011
Place: Physics Department Colloquium

abstract:

....

arXiv: 6 May 2011

Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone

Robin Wordsworth, François Forget, Franck Selsis, Ehouarn Millour, Benjamin Charnay, Jean-Baptiste Madeleine

arXiv:1105.1031v1 It has been suggested that the recently discovered exoplanet GJ581d might be able to support liquid water due to its relatively low mass and orbital distance. However, GJ581d receives 35% less stellar energy than Mars and is probably locked in tidal resonance, with extremely low insolation at the poles and possibly a permanent night side. Under such conditions, it is unknown whether any habitable climate on the planet would be able to withstand global glaciation and / or atmospheric collapse. Here we present three-dimensional climate simulations that demonstrate GJ581d will have a stable atmosphere and surface liquid water for a wide range of plausible cases, making it the first confirmed super-Earth (exoplanet of 2-10 Earth masses) in the habitable zone. We find that atmospheres with over 10 bar CO2 and varying amounts of background gas (e.g., N2) yield global mean temperatures above 0 degrees Celsius for both land and ocean-covered surfaces. Based on the emitted IR radiation calculated by the model, we propose observational tests that will allow these cases to be distinguished from other possible scenarios in the future. Accelerating Expansion of the Universe Writambhara Chakraborty arXiv:1105.1087v1 Ph.D Thesis (2010) This thesis concentrates on the accelerated expansion of the Universe recently explored by measurements of redshift and luminosity-distance relations of type Ia Supernovae. We have considered a model of the universe filled with modified Chaplygin gas and barotropic fluid. The role of dynamical cosmological constant has been explored with Modified Chaplygin Gas as the background fluid. Various phenomenological models for \Lambda have been studied in presence of the gravitational constant G to be constant or time dependent. A new form of the well known Chaplygin gas model has been presented by introducing inhomogeneity in the EOS. This model explains w=-1 crossing. An interaction of this model with the scalar field has also been investigated through a phenomenological coupling function. Tachyonic field has been depicted as dark energy model to represent the present acceleration of the Universe. A mixture of the tachyonic fluid has been considered with Generalized Chaplygin Gas to show the role of the later as a dark energy candidate in presence of tachyonic matter. A model of interaction has been studied with scalar field and the inhomogeneous ideal fluid. Two forms of the ideal fluid have been analysed. A power law expansion for the scale factor has been assumed to solve the equations for the energy densities. Brans-Dicke theory has been used to investigate the possibility of obtaining cosmic acceleration. For this purpose a constant and a variable \omega (Brans-Dicke parameter) have been considered. A self-interacting potential has been introduced to show its role in the evolution of the Universe. This model has been studied in presence of barotropic fluid and Generalized Chaplygin Gas. Relaxing a large cosmological constant in the astrophysical domain Florian Bauer, Joan Sola, Hrvoje Stefancic arXiv:1105.1030v1 We study the problem of relaxing a large cosmological constant in the astrophysical domain through a dynamical mechanism based on a modified action of gravity previously considered by us at the cosmological level. We solve the model in the Schwarzschild-de Sitter metric for large and small astrophysical scales, and address its physical interpretation by separately studying the Jordan's frame and Einstein's frame formulations of it. In particular, we determine the extremely weak strength of fifth forces in our model and show that they are virtually unobservable. Finally, we estimate the influence that the relaxation mechanism may have on pulling apart the values of the two gravitational potentials Psi and Phi of the metric, as this implies a departure of the model from General Relativity and could eventually provide an observational test of the new framework at large astrophysical scales, e.g. through gravitational lensing. A new Tolman test of a cosmic distance duality relation at 21 cm Satej Khedekar, Sayan Chakraborti arXiv:1105.1138v1 Under certain general conditions in an expanding universe, the luminosity distance (d_L) and angular diameter distance (d_A) are connected by the Etherington relation as d_L = d_A (1 + z)^2. The Tolman test suggests the use of objects of known surface brightness, to test this relation. In this letter, we propose the use of redshifted 21 cm signal from disk galaxies, where neutral hydrogen (HI) masses are seen to be almost linearly correlated with surface area, to conduct a new Tolman test. We construct simulated catalogs of galaxies, with the observed size-luminosity relation and realistic redshift evolution of HI mass functions, likely to be detected with the planned Square Kilometer Array (SKA). We demonstrate that these observations may soon provide the best implementation o f the Tolman test to detect any violation of the Etherington relation. Testing General Relativity using the Environmental Dependence of Dark Matter Halos Gong-Bo Zhao (ICG, Portsmouth), Baojiu Li (Cambridge), Kazuya Koyama (ICG, Portsmouth) arXiv:1105.0922v1 In this Letter, we investigate the environmental dependence of dark matter halos in theories that attempt to explain the accelerated expansion of the Universe by modifying general relativity (GR). Using high-resolution N-body simulations in f(R)gravity models which recover GR in dense environments by virtue of the chameleon mechanism, we find a strong environmentally-dependent difference between the lensing mass and dynamical mass estimates of dark matter halos. This environmental dependence of the halo properties can be used as a smoking gun to test GR observationally. Optimal strategies : theoretical approaches to the parametrization of the dark energy equation of state Seokcheon Lee arXiv:1105.0993v1 The absence of compelling theoretical model requires the parameterizing the dark energy to probe its properties. The parametrization of the equation of state of the dark energy is a common method. We explore the theoretical optimization of the parametrization based on the Fisher information matrix. As a suitable parametrization, it should be stable at high redshift and should produce the determinant of the Fisher matrix as large as possible. For the illustration, we propose one parametrization which can satisfy both criteria. By using the proper parametrization, we can improve the constraints on the dark energy even for the same data. We also show the weakness of the so-called principal component analysis method.

arXiv: 5 May 2011

Linear kinetic Sunyaev-Zel'dovich effect and void models for acceleration

James P. Zibin, Adam Moss

arXiv:1105.0909v1

There has been considerable recent interest in cosmological models in which the current apparent acceleration is due to a very large local underdensity, or void, instead of some form of dark energy. Here we examine a new proposal to constrain such models using the linear kinetic Sunyaev-Zel'dovich (kSZ) effect due to structure within the void. The simplified "Hubble bubble" models previously studied appeared to predict far more kSZ power than is actually observed, independently of the details of the initial conditions and evolution of perturbations in such models. We show that the constraining power of the kSZ effect is considerably weakened (though still impressive) under a fully relativistic treatment of the problem, and point out several theoretical ambiguities and observational shortcomings which further qualify the results. Nevertheless, we conclude that a very large class of void models is ruled out by the combination of kSZ and other methods.

Formation history, structure and dynamics of discs and spheroids in simulated Milky Way mass galaxies

Cecilia Scannapieco (1), Simon D.M White (2), Volker Springel (3), Patricia B. Tissera (4) ((1) Leibniz-Institute for Astrophysics Potsdam (AIP), (2) Max-Planck Institute for Astrophysics, (3) Heidelberg Institute for Theoretical Studies, (4) Institute for Astronomy and Space Physics)

arXiv:1105.0680v1  We study the stellar discs and spheroids in eight simulations of galaxy formation within Milky Way-mass haloes in a Lambda Cold Dark Matter cosmology. A first paper in this series concentrated on disc properties. Here, we extend this analysis to study how the formation history, structure and dynamics of discs and spheroids relate to the assembly history and structure of their haloes. We find that discs are generally young, with stars spanning a wide range in stellar age: the youngest stars define thin discs and have near-circular orbits, while the oldest stars form thicker discs which rotate ~2 times slower than the thin components, and have 2-3 times larger velocity dispersions. Unlike the discs, spheroids form early and on short time-scales, and are dominated by velocity dispersion. We find great variety in their structure. The inner regions are bar- or bulge-like, while the extended outer haloes are rich in complex non-equilibrium structures such as stellar streams, shells and clumps. Our discs have very high in-situ fractions, i.e. most of their stars formed in the disc itself. Nevertheless, there is a non-negligible contribution (~15 percent) from satellites that are accreted on nearly coplanar orbits. The inner regions of spheroids also have relatively high in-situ fractions, but 65-85 percent of their outer stellar population is accreted. We analyse the circular velocities, rotation velocities and velocity dispersions of our discs and spheroids, both for gas and stars, showing that the dynamical structure is complex as a result of the non-trivial interplay between cooling and SN heating. Dwarf galaxy populations in present-day galaxy clusters: I. Abundances and red fractions Simone M. Weinmann, Thorsten Lisker, Qi Guo, Hagen T. Meyer, Joachim Janz arXiv:1105.0674v1 We compare the galaxy population in the Virgo, Fornax, Coma and Perseus cluster to a state-of-the-art semi-analytic model, focusing on the regime of dwarf galaxies with luminosities from approximately 10^8 L_sun to 10^9 L_sun. We find that the number density profiles of dwarfs in observed clusters are reproduced reasonably well, and that the red fractions of model clusters provide a good match to Coma and Perseus. On the other hand, the red fraction among dwarf galaxies in Virgo is clearly lower than in model clusters. We argue that this is mainly caused by the treatment of environmental effects in the model. This explanation is supported by our finding that the colours of central ("field") dwarf galaxies are reproduced well, in contrast to previous claims. Finally, we report on several differences in the properties of galaxies in observed and simulated clusters that may indicate an underestimate of tidal disruption in the model: The dwarf-to-giant ratio is too high in the model; there is an unexplained flattening of the number density profiles of dwarfs in the centers of Virgo and Fornax; and the shape of the cluster luminosity function is different between model and observations (resembling a double Schechter-function in the observations, but not in the model). Some Adventures in the Search for a Modified Gravity Explanation for Cosmic Acceleration Mark Trodden arXiv:1105.0721v1  The discovery of cosmic acceleration has raised the intriguing possibility that we are witnessing the first breakdown of General Relativity on cosmological scales. In this article I will briefly review current attempts to construct a theoretically consistent and observationally viable modification of gravity that is capable of describing the accelerating universe. I will discuss f(R) models, and their obvious extensions, and the DGP model as an example of extra-dimensional implementations. I will then briefly describe the Galileon models and their very recent multifield and curved space extensions - a class of four-dimensional effective field theories encoding extra dimensional modifications to gravity. This article is dedicated to the career of my friend and former colleague, Joshua Goldberg, and is written to appear in his festschrift.

Gravitational Lensing in Cosmological Background/ Seminar Series 7

Speaker: Mojahed ParsiMood

Affiliation: Sharif University of Technology
Title: Gravitational Lensing in Cosmological Background
Date: 4 May 2011
Place: Astronomy seminar, Astronomy school IPM

abstract:

 In this talk, i want to investigate the problem of gravitational lensing in general relativity exactly. Considering a model for a lens in cosmological background, i solve the null geodesic equations in this model and compute deflection angle. Then i compare this results with the usual attitude to this problem and its relation to mass and dark matter. 
--
********************************
Shant Baghram
Ph.D. Student
Cosmology group
Department of Physics
Sharif University of Technology
Tehran, Iran
E-mail: baghram@physics.sharif.edu /

shant.baghramian@gmail.com
Homepage:http://physics.sharif.edu/~baghram/
*************************************

Steps toward phase space distribution function reconstruction of galaxy dark halos/ Seminar Series 6

Speaker: Laya Golchin
Affiliation: Sharif University of Technology, Physics department
Title: Steps toward phase space distribution function reconstruction of galaxy dark halos.
Date: 1 May  2011
Place: Cosmology weekly seminars, Physics department, Sharif University

Abstract:
In this talk I will mention some properties of galaxies
dark halo's density profiles and shapes that
we know from gravitational lensing and n-body simulations . then I
will briefly explain the FEM method
and its application to DF reconstruction of spherical stellar
systems and why we can't use
the same steps for non integrable systems. Finally I will introduce
the NAFF method ( Numerical Analysis of Fundamental Frequencies) since
it is one of steps toward the DF reconstruction of non integrable systems.

Gauge-Inflation from Non Abelian Guage Fields/ Seminar Series 5

Speaker: Azadeh Maleknejad
Affiliation: Alzahra University and IPM
Title: Gauge-Inflation from Non Abelian Guage Fields
Date: 27 April 2011
Place: Astronomy seminar, Astronomy school IPM

Abstract:
 

 In [arXiv:1102.1513] we introduced an inflationary scenario, Non-Abelian Gauge Field Inflation or gauge-flation for short, in which slow-roll inflation is driven by non-Abelian gauge field minimally coupled to gravity. By studying the phase diagrams of the theory, we show that getting enough number of e-folds during a slow-roll inflation is fairly robust to the choice of initial gauge field values. In addition, we present a detailed analysis of the cosmic perturbation theory in gauge-flation which has many special and interesting features compared the standard scalar-driven inflationary models. The specific gauge-flation model we study in this paper has two parameters, a cutoff scale Lambda and the gauge coupling g. Fitting our results with the current cosmological data fixes \Lambda\sim 10 H \sim 10^{15} GeV (H is the Hubble parameter) and g\sim 10^{-4}, which are in the natural range of parameters in generic particle physics beyond standard models. Our model also predicts a tensor-to-scalar ratio r>0.05, in the range detectable by the Planck satellite.

arXiv: 4 Mar 2011

Supersymmetric Galileons

Justin Khoury, Jean-Luc Lehners, Burt A. Ovrut

arXiv:1103.0003v1 

Galileon theories are of considerable interest since they allow for stable violations of the null energy condition. Since such violations could have occurred during a high-energy regime in the history of our universe, we are motivated to study supersymmetric extensions of these theories. This is carried out in this paper, where we construct generic classes of N=1 supersymmetric Galileon Lagrangians. They are shown to admit non-equivalent stress-energy tensors and, hence, vacua manifesting differing conditions for violating the null energy condition. The temporal and spatial fluctuations of all component fields of the supermultiplet are analyzed and shown to be stable on a large number of such backgrounds. In the process, we uncover a surprising connection between conformal Galileon and ghost condensate theories, allowing for a deeper understanding of both types of theories.

arXiv: 3 May 2011

The Atacama Cosmology Telescope: Evidence for Dark Energy from the CMB Alone

Blake D. Sherwin, Joanna Dunkley, Sudeep Das, John W. Appel, J. Richard Bond, C. Sofia Carvalho, Mark J. Devlin, Rolando Dunner, Thomas Essinger-Hileman, Joseph W. Fowler, Amir Hajian, Mark Halpern, Matthew Hasselfield, Adam D. Hincks, Renee Hlozek, John P. Hughes, Kent D. Irwin, Jeff Klein, Arthur Kosowsky, Tobias A. Marriage, Danica Marsden, Kavilan Moodley, Felipe Menanteau, Michael D. Niemack, Michael R. Nolta, Lyman A. Page, Lucas Parker, Erik D. Reese, Benjamin L. Schmitt, Neelima Sehgal, Jon Sievers, David N. Spergel, Suzanne T. Staggs,Daniel S. Swetz, Eric R. Switzer, Robert Thornton, Katerina Visnjic, Ed Wollack

arXiv:1105.0419v1 For the first time, measurements of the cosmic microwave background radiation (CMB) alone favor cosmologies with $w=-1$ dark energy over models without dark energy at a 3.2-sigma level. We demonstrate this by combining the CMB lensing deflection power spectrum from the Atacama Cosmology Telescope with temperature and polarization power spectra from the Wilkinson Microwave Anisotropy Probe. The lensing data break the geometric degeneracy of different cosmological models with similar CMB temperature power spectra. Our CMB-only measurement of the dark energy density $\Omega_\Lambda$ confirms other measurements from supernovae, galaxy clusters and baryon acoustic oscillations, and demonstrates the power of CMB lensing as a new cosmological tool. Rigging dark halos: why is hierarchical galaxy formation consistent with the inside-out build-up of thin discs? C. Pichon, D. Pogosyan, T. Kimm, A. Slyz, J. Devriendt, Y. Dubois arXiv:1105.0210v1 State-of-the-art hydrodynamical simulations show that gas inflow through the virial sphere of dark matter halos is focused (i.e. has a preferred inflow direction), consistent (i.e. its orientation is steady in time) and amplified (i.e. the amplitude of advected specific angular momentum increases with time). This is a consequence of the dynamics of the cosmic web within the neighbourhood of the halo, which produces steady, angular momentum rich, filamentary inflow of cold gas. On large scales, the dynamics within neighbouring patches drives matter out of the surrounding voids, into walls and filaments before it finally gets accreted onto virialised dark matter halos. As these walls/filaments constitute the boundaries of asymmetric voids, they naturally acquire a net transverse motion, which explains the angular momentum rich nature of the later infall which comes from further away (lever effect). We argue that this large-scale driven consistency explains why cold flows are so efficient at building up thin discs from the inside out. Seeking String Theory in the Cosmos Edmund J. Copeland, Levon Pogosian, Tanmay Vachaspati arXiv:1105.0207v1 We review the existence, formation and properties of cosmic strings in string theory, the wide variety of observational techniques that are being employed to detect them, and the constraints that current observations impose on string theory models. On new variational principles as alternatives to the Palatini method Tomi S. Koivisto arXiv:1103.2743v2 A variational principle was recently suggested by Goenner, where an independent metric generates the spacetime connection. It is pointed out here that the resulting theory is equivalent to the usual Palatini theory. However, a bimetric reformulation of the variational principle leads to theories which are physically distinct from both the metric and the metric-affine ones, even for the Einstein-Hilbert action. They are obtained at a decoupling limit of C-theories, which contain also other viable generalizations of the Palatini theories.

arXiv: 2 May 2011

Neutron Stars and the Cosmological Constant Problem

Farbod Kamiab, Niayesh Afshordi

arXiv:1104.5704v1

The gravitational aether theory is a modification of general relativity that decouples vacuum energy from gravity, and thus can potentially address the cosmological constant problem. The classical theory is distinguishable from general relativity only in the presence of relativistic pressure (or vorticity). Since the interior of neutron stars has high pressure and as their mass and radius can be measured observationally, they are the perfect laboratory for testing the validity of the aether theory. In this paper, we solve the equations of stellar structure for the gravitational aether theory and find the predicted mass-radius relation of non-rotating neutron stars using two different realistic proposals for the equation of state of nuclear matter. We find that the maximum neutron star mass predicted by the aether theory is 12% - 16% less than the maximum mass predicted by general relativity assuming these two equations of state. We also show that the effect of aether is similar to modifying the equation of state in general relativity. The effective pressure of the neutron star given by the aether theory at a fiducial density differs from the values given by the two nuclear equations of state to an extent that can be constrained using future gravitational wave observations of neutron stars in compact systems. This is a promising way to test the aether theory if further progress is made in constraining the equation of state of nuclear matter in densities above the nuclear saturation density.

Degree of randomness: numerical experiments for astrophysical signals

V.G.Gurzadyan, T.Ghahramanyan, S.Sargsyan

arXiv:1104.5688v1

Astrophysical and cosmological signals such as the cosmic microwave background radiation, as observed, typically contain contributions of different components, and their statistical properties can be used to distinguish one from another. A method developed originally by Kolmogorov is involved for the study of astrophysical signals of randomness of various degree. Numerical performed experiments based on the universality of Kolmogorov distribution and using a single scaling of the ratio of stochastic to regular components, reveal basic features in the behavior of generated signals including in terms of a critical value for that ratio, thus enable the application of this technique for various observational datasets.

CCC-predicted low-variance circles in CMB sky and LCDM

V. G. Gurzadyan, R. Penrose

arXiv:1104.5675v1

New analysis confirms our earlier claim [1], [7] of circles of notably low temperature variance, often in concentric sets, in the cosmic microwave background (CMB), discernable in WMAP data. Their reality can be interpreted as evidence of supermassive black-hole encounters in a previous aeon, as predicted by conformal cyclic cosmology (CCC) [2]. Counter arguments [4-6] pointed out that such circles arise, at similar frequency, also in simulated data using WMAP's CMB power spectrum, plus random input. We responded [7] that if such circles contribute to CMB, this influences the power spectrum, enhancing such circles in simulations. We confirm this here, but show that if the theoretical LCDM power spectrum is used instead, then the low-variance circles disappear. This is evidence that the LCDM model gives an incomplete explanation of the CMB, missing crucial information, which is provided by incorporating low-variance circles of CCC. The excellent agreement between theoretical LCDM and observed power spectrum, even for fairly large l-values, does not reveal this discrepancy, of relevance only at larger l-values where agreement is weak. We point out various non-random aspects of the circles, seen both in the true data and in simulations with WMAP power spectrum, but not with the theoretical LCDM spectrum. We also show the spatial distribution of concentric circle sets to be very non-random in the true WMAP data (perhaps owing to large-scale mass inhomogeneities distorting CCC's circle shapes), in complete contrast with simulations with WMAP power spectrum, where such circle sets are much sparser and closer to average temperature. These features are fully consistent with CCC (and with an earlier analysis [8] that the random Gaussian component in the CMB is only around 0.2 in the total CMB signal) but do not readily fit in with the random initial fluctuations of standard inflation.