arXiv: 5 January 2010

Future Oscillations around Phantom Divide in f(R) Gravity

Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama

http://arxiv.org/abs/1101.0744v1

It is known that scalar-tensor theory of gravity admits regular crossing of the phantom divide line $w_{\DE}=-1$ for dark energy, and existing viable models of present dark energy for its particular case -- $f(R)$ gravity -- possess one such crossing in the recent past, after the end of the matter dominated stage. It was recently noted that during the future evolution of these models the dark energy equation of state $w_{\DE}$ may oscillate with an infinite number of phantom divide crossings. In this paper we present an analytical condition for the existence of this effect and investigate it numerically. With the increase of the present mass of the scalaron (the scalar particle appearing in $f(R)$ gravity) beyond the border of the existence of such oscillations, their amplitude is shown to decrease very fast, so the effect quickly becomes very small even in the infinite future.

f(R) Gravity and its Cosmological Implications

Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama

http://arxiv.org/abs/1101.0716v1

We have investigated the evolution of a homogeneous isotropic background of the Universe and inhomogeneous subhorizon matter density perturbations in viable $f(R)$ models of present dark energy and cosmic acceleration analytically and numerically. It is found that viable $f(R)$ models generically exhibit recent crossing of the phantom boundary $w_{\rm DE}=-1$. Furthermore, it is shown that the growth index of perturbations depends both on time and wavenumber. This anomalous growth may explain properties of the observational matter power spectrum from the SDSS data and can also partially counteract the spectrum suppression by massive neutrinos making larger values of the total sum of neutrino rest masses possible.

arXiv: 4 January 2011

The Sunyaev-Zel'dovich Array: Constraining a new pressure profile for fitting SZE observations of galaxy clusters

Tony Mroczkowski

http://arxiv.org/abs/1101.0372v1

Thesis, Defended in Sept 2008, Submitted to Columbia University in March 2009

The Sunyaev-Zel'dovich Array (SZA), an eight element interferometer designed to probe the Sunyaev-Zel'dovich effect (SZE) from galaxy clusters, which I helped construct and operate, is described here (Part I). I then use SZA observations to investigate the utility of a new, self-similar pressure profile for fitting SZE observations of galaxy clusters (Part II). 
The SZA 30-GHz receiver system probes angular scales ~1-5'. A model that can accurately describe a cluster's pressure profile over a correspondingly broad range of radii is therefore required. In the analysis presented here, I fit a 2-parameter, radial pressure profile, derived from simulations and detailed X- ray analysis of relaxed clusters, to SZA observations of three clusters with exceptionally high quality X-ray data. From the joint analysis of the SZE and X-ray data, I derive physical properties of the cluster, such as gas and total mass, gas fraction and the integrated Compton y -parameter. 
The parameters derived from the joint fit to SZE+X-ray data agree well with a detailed, independent, X-ray-only analysis of these same clusters. When combined with X-ray imaging data, this new pressure profile yields an independent estimate of the electron temperature profile that is in good agreement with spectroscopic X-ray determinations. In addition to yielding relationships between cluster observables and physical cluster properties, this model could prove to be a useful tool in helping to constrain the temperatures of high redshift clusters, for which X-ray spectroscopic data are difficult to obtain.

Modified Gravity: the CMB, Weak Lensing and General Parameterisations

Shaun A. Thomas, Stephen A. Appleby, Jochen Weller

http://arxiv.org/abs/1101.0295v1

We examine general physical parameterisations for viable gravitational models in the $f(R)$ framework. This is related to the mass of an additional scalar field, called the scalaron, that is introduced by the theories. Using a simple parameterisation for the scalaron mass $M(a)$ we show there is an exact correspondence between the model and popular parameterisations of the modified Poisson equation $\mu(a,k)$ and the ratio of the Newtonian potentials $\eta(a,k)$. However, by comparing the aforementioned model against other viable scalaron theories we highlight that the common form of $\mu(a,k)$ and $\eta(a,k)$ in the literature does not accurately represent $f(R)$ behaviour. We subsequently construct an improved description for the scalaron mass (and therefore $\mu(a,k)$ and $\eta(a,k)$) which captures their essential features and has benefits derived from a more physical origin. We study the scalaron's observational signatures and show the modification to the background Friedmann equation and CMB power spectrum to be small. We also investigate its effects in the linear and non linear matter power spectrum--where the signatures are evident--thus giving particular importance to weak lensing as a probe of these models. Using this new form, we demonstrate how the next generation Euclid survey will constrain these theories and its complementarity to current solar system tests. In the most optimistic case Euclid, together with a Planck prior, can constrain a fiducial scalaron mass $M_{0} = 9.4 \times 10^{-30}{\rm eV}$ at the $\sim 20 %$ level. However, the decay rate of the scalaron mass, with fiducial value $\nu = 1.5$, can be constrained to $\sim 3%$ uncertainty.

Modified gravity models of dark energy

Shinji Tsujikawa

http://arxiv.org/abs/1101.0191v1

We review recent progress of modified gravity models of dark energy--based on f(R) gravity, scalar-tensor theories, braneworld gravity, Galileon gravity, and other theories. In f(R) gravity and Brans-Dicke theory it is possible to design viable models consistent with local gravity constraints under a chameleon mechanism, while satisfying conditions for the cosmological viability. The Dvali-Gabadazde-Porrati braneworld model can be compatible with local gravity constraints through a nonlinear field self-interaction arising from a brane-bending mode, but the self-accelerating solution contains a ghost mode in addition to the tension with observational data about the cosmic expansion history. The extension of the field self-interaction to more general forms satisfying a Galilean symmetry in the flat space-time allows a possibility to avoid the appearance of ghosts and Laplacian instabilities, while the late-time cosmic acceleration can be realized by the field kinetic energy. We study the evolution of cosmological perturbations in those models to place constraints on model parameters from the observations of large-scale structure, cosmic microwave background, and weak lensing. We also briefly review other modified gravitational models of dark energy-- such as those based on Gauss-Bonnet gravity and Lorentz-violating theories.

Hydrostatic equilibrium and stellar structure in f(R)-gravity

S. Capozziello, M. De Laurentis, S.D. Odintsov, A. Stabile

http://arxiv.org/abs/1101.0219v1

We investigate the hydrostatic equilibrium of stellar structure by taking into account the modi- fied La\'e-Emden equation coming out from f(R)-gravity. Such an equation is obtained in metric approach by considering the Newtonian limit of f(R)-gravity, which gives rise to a modified Poisson equation, and then introducing a relation between pressure and density with polytropic index n. The modified equation results an integro-differential equation, which, in the limit f(R) \rightarrow R, becomes the standard La\'e-Emden equation. We find the radial profiles of gravitational potential by solving for some values of n. The comparison of solutions with those coming from General Relativity shows that they are compatible and physically relevant.

arXiv: 30 December 2010

COSMOS weak-lensing constraints on modified gravity

Ismael Tereno, Elisabetta Semboloni, Tim Schrabback

http://arxiv.org/abs/1012.5854v1

The observed acceleration of the universe, explained through dark energy, could alternatively be explained through a modification of gravity that would also induce modifications in the evolution of cosmological perturbations. We use new weak lensing data from the COSMOS survey to test for deviations from General Relativity. The departure from GR is parametrized in a model-independent way that consistently parametrizes the two-point cosmic shear amplitude and growth. Using CMB priors, we perform a likelihood analysis. We find constraints on the amplitude of the signal that do not indicate a deviation from General Relativity.

arXiv: 27 December 2010

CMB Polarization in Einstein-Aether Theory

Masahiro Nakashima, Tsutomu Kobayashi

http://arxiv.org/abs/1012.5348v1

We study the impact of modifying the vector sector of gravity on the CMB polarization. We employ the Einstein-aether theory as a concrete example. The Einstein-aether theory admits dynamical vector perturbations generated during inflation, leaving imprints on the CMB polarization. We derive the perturbation equations of the aether vector field in covariant formalism and compute the CMB B-mode polarization using the modified CAMB code. It is found that the amplitude of the B-mode signal from the aether field can surpass the one from the inflationary gravitational waves. The shape of the spectrum is clearly understood in an analytic way using the tight coupling approximation.

arXiv: 24 December 2010

Caching and Interpolated Likelihoods: Accelerating Cosmological Monte Carlo Markov Chains

Adam Bouland, Richard Easther, Katherine Rosenfeld

http://arxiv.org/abs/1012.5299v1

We describe a novel approach to accelerating Monte Carlo Markov Chains. Our focus is cosmological parameter estimation, but the algorithm is applicable to any problem for which the likelihood surface is a smooth function of the free parameters and computationally expensive to evaluate. We generate a high-order interpolating polynomial for the log-likelihood using the first points gathered by the Markov chains as a training set. This polynomial then accurately computes the majority of the likelihoods needed in the latter parts of the chains. We implement a simple version of this algorithm as a patch (InterpMC) to CosmoMC and show that it accelerates parameter estimatation by a factor of between two and four for well-converged chains. The current code is primarily intended as a "proof of concept", and we argue that there is considerable room for further performance gains. Unlike other approaches to accelerating parameter fits, we make no use of precomputed training sets or special choices of variables, and InterpMC is almost entirely transparent to the user.

Nonlinear matter spectra in growing neutrino quintessence

N. Brouzakis, V. Pettorino, N. Tetradis, C. Wetterich

http://arxiv.org/abs/1012.5255v1

We investigate the nonlinear power spectra of density perturbations and acoustic oscillations in growing neutrino quintessence. In this scenario, the neutrino mass has a strong dependence on the quintessence field. The induced coupling stops the evolution of the field when the neutrinos become nonrelativistic, and triggers the transition to the accelerating phase of the cosmological expansion. At redshifts around five, the neutrino fluctuations are still linear and acoustic oscillations are present in the neutrino power spectrum, induced by the acoustic oscillations in the baryonic and dark-matter sectors. The neutrino perturbations become nonlinear at redshifts around three. The mode coupling generated by the nonlinearities erases the oscillations in the neutrino spectrum at some redshift above two. There is a potential danger that at later times the influence of the gravitational potentials induced by the neutrino inhomogeneities could erase the oscillations from the baryonic and dark-matter spectra, making the scenario incompatible with observations. For the scenario to be viable, the neutrino-induced gravitational potentials in the range of baryonic acoustic oscillations should not grow to average values much larger than 10^{-4}. The magnitude of the expected potentials is still not known reliably, as the process of structure formation is poorly understood in growing neutrino quintessence.

Physics of the Cosmic Microwave Background and the Planck Mission

Hannu Kurki-Suonio

http://arxiv.org/abs/1012.5204v1

This lecture is a sketch of the physics of the cosmic microwave background. The observed anisotropy can be divided into four main contributions: variations in the temperature and gravitational potential of the primordial plasma, Doppler effect from its motion, and a net red/blueshift the photons accumulate from traveling through evolving gravitational potentials on their way from the primordial plasma to here. These variations are due to primordial perturbations, probably caused by quantum fluctuations in the very early universe. The ongoing Planck satellite mission to observe the cosmic microwave background is also described.

Towards a UV Completion for Chameleon Scalar Theories

Kurt Hinterbichler, Justin Khoury, Horatiu Nastase

http://arxiv.org/abs/1012.4462v1

Chameleons are scalar fields that couple directly to ordinary matter with gravitational strength, but which nevertheless evade the stringent constraints on tests of gravity because of properties they acquire in the presence of high ambient matter density. Chameleon theories were originally constructed in a bottom-up, phenomenological fashion, with potentials and matter couplings designed to hide the scalar from experiments. In this paper, we attempt to embed the chameleon scenario within string compactifications, thus UV completing the scenario. We look for stabilized potentials that can realize a screening mechanism, and we find that the volume modulus rather generically works as a chameleon, and in fact the supersymmetric potential used by Kachru, Kallosh, Linde and Trivedi (KKLT) is an example of this type. We consider all constraints from tests of gravity, allowing us to put experimental constraints on the KKLT parameters.

Theory of planet formation

Christoph Mordasini, Hubert Klahr, Yann Alibert, Willy Benz, Kai-Martin Dittkrist

http://arxiv.org/abs/1012.5281v1

We review the current theoretical understanding how growth from micro-meter sized dust to massive giant planets occurs in disks around young stars. After introducing a number of observational constraints from the solar system, from observed protoplanetary disks, and from the extrasolar planets, we simplify the problem by dividing it into a number of discrete stages which are assumed to occur in a sequential way. In the first stage - the growth from dust to kilometer sized planetesimals - the aerodynamics of the bodies are of central importance. We discuss both a purely coagulative growth mode, as well as a gravoturbulent mode involving a gravitational instability of the dust. In the next stage, planetesimals grow to protoplanets of roughly 1000 km in size. Gravity is now the dominant force. The mass accretion can be strongly non-linear, leading to the detachment of a few big bodies from the remaining planetesimals. In the outer planetary system (outside a few AU), some of these bodies can become so massive that they eventually accrete a large gaseous envelope. This is the stage of giant planet formation, as understood within the core accretion-gas capture paradigm. We also discuss the direct gravitational collapse model where giant planets are thought to form directly via a gravitational fragmentation of the gas disk. In the inner system, protoplanets collide in the last stage - probably after the dispersal of the gaseous disk - in giant impacts until the separations between the remaining terrestrial planets become large enough to allow long term stability. We finish the review with some selected questions.

A much lower density for the transiting extrasolar planet WASP-7

John Southworth, M. Dominik, U. G. Jorgensen, S. Rahvar, C. Snodgrass, K. Alsubai, V. Bozza, P. Browne, M. Burgdorf, S. Calchi Novati, P. Dodds, S. Dreizler, F. Finet, T. Gerner, S. Hardis, K. Harpsoe,C. Hellier, T. C. Hinse, M. Hundertmark, N. Kains, E. Kerins, C. Liebig, L. Mancini, M. Mathiasen, M. T. Penny, S. Proft, D. Ricci, K. Sahu, G. Scarpetta, S. Schafer, F. Schonebeck, J. Surdej

http://arxiv.org/abs/1012.5181v1

We present the first high-precision photometry of the transiting extrasolar planetary system WASP-7, obtained using telescope defocussing techniques and reaching a scatter of 0.68 mmag per point. We find that the transit depth is greater and that the host star is more evolved than previously thought. The planet has a significantly larger radius (1.330 +/- 0.093 Rjup versus 0.915 +0.046 -0.040 Rjup) and much lower density (0.41 +/- 0.10 rhojup versus 1.26 +0.25 -0.21 rhojup) and surface gravity (13.4 +/- 2.6 m/s2 versus 26.4 +4.4 -4.0 m/s2) than previous measurements showed. Based on the revised properties it is no longer an outlier in planetary mass--radius and period--gravity diagrams. We also obtain a more precise transit ephemeris for the WASP-7 system.

arXiv: 23 December 2010

The ISW-tSZ cross correlation: ISW extraction out of pure CMB data

N. Taburet (1), C. Hernandez-Monteagudo (2), N. Aghanim (1), M. Douspis (1), R. A. Sunyaev (2 and 3) ((1) Institut d'Astrophysique Spatiale - Orsay - France, (2) Max-Planck Institut fur Astrophysik - Garching - Germany, (3) Space Research Institute, Russian Academy of Sciences - Moscow - Russia)

http://arxiv.org/abs/1012.5036v1

If Dark Energy introduces an acceleration in the universal expansion then large scale gravitational potential wells should be shrinking, causing a blueshift in the CMB photons that cross such structures (Integrated Sachs-Wolfe effect, [ISW]). Galaxy clusters are known to probe those potential wells. In these objects, CMB photons also experience inverse Compton scattering off the hot electrons of the intra-cluster medium, and this results in a distortion with a characteristic spectral signature of the CMB spectrum (the so-called thermal Sunyaev-Zel'dovich effect, [tSZ]). Since both the ISW and the tSZ effects take place in the same potential wells, they must be spatially correlated. We present how this cross ISW-tSZ signal can be detected in a CMB-data contained way by using the frequency dependence of the tSZ effect in multi frequency CMB experiments like {\it Planck}, {\em without} requiring the use of external large scale structure tracers data. We find that by masking low redshift clusters, the shot noise level decreases significantly, boosting the signal to noise ratio of the ISW--tSZ cross correlation. We also find that galactic and extragalactic dust residuals must be kept at or below the level of ~0.04 muK^2 at l=10, a limit that is a factor of a few below {\it Planck}'s expectations for foreground subtraction. If this is achieved, CMB observations of the ISW-tSZ cross correlation should also provide an independent probe for the existence of Dark Energy and the amplitude of density perturbations.

arXiv: 22 December 2010

Intrinsic brightness of SDSS objects is similar at all redshifts in de Sitter space

John B. Miller, Thomas E. Miller

http://arxiv.org/abs/1012.4516v1

The redshift-luminosity distributions for well-defined galaxies and quasars in the Sloan Digital Sky Survey (SDSS) are compared for the two redshift-distance relations of a Hubble redshift and a de Sitter redshift. Assuming a Hubble redshift, SDSS data can be interpreted as luminosity evolution following the Big Bang. In contrast, given a de Sitter redshift, the intrinsic brightness of objects at all redshifts is roughly the same. In a de Sitter universe, 95 per cent of SDSS galaxies and quasars fall into a magnitude range of only 2.8, and 99.7 per cent are within 5.4 mag. The comparable Hubble luminosity ranges are much larger: 95 per cent within 6.9, and 99.7 per cent within 11.5 mag. De Sitter space is now widely discussed, but the de Sitter redshift is hardly mentioned.

Lessons from Classical Gravity about the Quantum Structure of Spacetime

T. Padmanabhan

http://arxiv.org/abs/1012.4476v1

I present the theoretical evidence which suggests that gravity is an emergent phenomenon like gas dynamics or elasticity with the gravitational field equations having the same status as, say, the equations of fluid dynamics/elasticity. This paradigm views a wide class of gravitational theories - including Einstein's theory - as describing the thermodynamic limit of the statistical mechanics of "atoms of spacetime". The evidence for this paradigm is hidden in several classical features of the gravitational theories and depends on just one quantum mechanical input, viz. the existence of Davies-Unruh temperature of horizons. I discuss several conceptual ingredients of this approach.

arXiv: 21 December 2010

Cosmological Limits on Hidden Sector Dark Matter

Subinoy Das, Kris Sigurdson (UBC)

http://arxiv.org/abs/1012.4458v1

We explore the model-independent constraints from cosmology on a dark-matter particle with no prominent standard model interactions that interacts and thermalizes with other particles in a hidden sector. Without specifying detailed hidden-sector particle physics, we characterize the relevant physics by the annihilation cross section, mass, and temperature ratio of the hidden to visible sectors. While encompassing the standard cold WIMP scenario, we do not require the freeze-out process to be nonrelativistic. Rather, freeze-out may also occur when dark matter particles are semirelativistic or relativistic. We solve the Boltzmann equation to find the conditions that hidden-sector dark matter accounts for the observed dark-matter density, satisfies the Tremaine-Gunn bound on dark-matter phase space density, and has a free-streaming length consistent with cosmological constraints on the matter power spectrum. We show that for masses <1.4 keV no region of parameter space satisfies all these constraints. This is a gravitationally-mediated lower bound on the dark-matter mass for any model in which the primary component of dark matter once had efficient interactions -- even if it has never been in equilibrium with the standard model.

Generalizations of teleparallel gravity and local Lorentz symmetry

Thomas P. Sotiriou, Baojiu Li, John D. Barrow

http://arxiv.org/abs/1012.4039v1

We analyze the relation between teleparallelism and local Lorentz invariance. We show that generic modifications of the teleparallel equivalent to general relativity will not respect local Lorentz symmetry. We clarify the reasons for this and explain why the situation is different in general relativity. We give a prescription for constructing teleparallel equivalents for known theories. We also explicitly consider a recently proposed class of generalized teleparallel theories, called f(T) theories of gravity, and show why restoring local Lorentz symmetry in such theories cannot lead to sensible dynamics, even if one gives up teleparallelism.

arXiv: 20 December 2010

Cosmological Consequences of Exponential Gravity in Palatini Formalism

M. Campista, B. Santos, J. Santos, J.S. Alcaniz

http://arxiv.org/abs/1012.3943v1

We investigate cosmological consequences of a class of exponential $f(R)$ gravity in the Palatini formalism. By using the current largest type Ia Supernova sample along with determinations of the cosmic expansion at intermediary and high-$z$ we impose tight constraints on the model parameters. Differently from other $f(R)$ models, we find solutions of transient acceleration, in which the large-scale modification of gravity will drive the Universe to a new decelerated era in the future. We also show that a viable cosmological history with the usual matter-dominated era followed by an accelerating phase is predicted for some intervals of model parameters.

Galaxy And Mass Assembly (GAMA): Galaxies at the faint end of the Halpha luminosity function

S. Brough (AAO), A.M. Hopkins, R.G. Sharp, M. Gunawardhana, D. Wijesinghe, A.S.G. Robotham, S.P. Driver, I.K. Baldry, S.P. Bamford, J. Liske, J. Loveday, P. Norberg, J.A. Peacock, J.H. Bland-Hawthorn,M.J.I. Brown, E. Cameron, S.M. Croom, C.S. Frenk, C. Foster, D.T. Hill, D.H. Jones, L.S. Kelvin, K. Kuijken, R.C. Nichol, H.R. Parkinson, K. Pimbblet, C.C. Popescu, M. Prescott, W.J. Sutherland, E. Taylor,D. Thomas, R.J. Tuffs, E. van Kampen

http://arxiv.org/abs/1012.3771v1

We present an analysis of the properties of the lowest Halpha-luminosity galaxies (L_Halpha<4x10^32 W; SFR<0.02 Msun/yr) in the Galaxy And Mass Assembly (GAMA) survey. These galaxies make up the the rise above a Schechter function in the number density of systems seen at the faint end of the Halpha luminosity function. Above our flux limit we find that these galaxies are principally composed of intrinsically low stellar mass systems (median stellar mass =2.5x10^8 Msun) with only 5/90 having stellar masses M>10^10 Msun. The low SFR systems are found to exist predominantly in the lowest density environments (median density ~0.02 galaxy Mpc^-2 with none in environments more dense than ~1.5 galaxy Mpc^-2). Their current specific star formation rates (SSFR; -8.5 < log(SSFR[yr^-1])<-12.) are consistent with their having had a variety of star formation histories. The low density environments of these galaxies demonstrates that such low-mass, star-forming systems can only remain as low-mass and forming stars if they reside sufficiently far from other galaxies to avoid being accreted, dispersed through tidal effects or having their gas reservoirs rendered ineffective through external processes.

arXiv: 17 December 2010

Testing the Gaussianity and Statistical Isotropy of the Universe

Authors: Dragan Huterer, Eiichiro Komatsu, Sarah Shandera
http://arxiv.org/abs/1012.3744v1
The last few years have seen a surge in excitement about measurements of statistics of the primordial fluctuations beyond the power spectrum. New ideas for precision tests of Gaussianity and statistical isotropy in the data are developing simultaneously with proposals for a wide range of new theoretical possibilities. From both the observations and theory, it has become clear that there is a huge discovery potential from upcoming measurements. In this Special Issue of Advances in Astronomy we have collected articles that summarize the theoretical predictions for departures from Gaussianity or statistical isotropy from a variety of potential sources, together with the observational approaches to test these properties using the CMB or large-scale structure. We hope this collection provides an accessible entry point to these topics as they currently stand, indicating what direction future developments may take and demonstrating why these questions are so compelling. The Special Issue is available at this http URL, and individual articles are also available on the arXiv.

Recovering cores and cusps in dark matter haloes using mock velocity field observations

Authors: Rachel Kuzio de Naray, Tobias Kaufmann
http://arxiv.org/abs/1012.3471v1
We present mock DensePak Integral Field Unit (IFU) velocity fields, rotation curves, and halo fits for disc galaxies formed in spherical and triaxial cuspy dark matter haloes, and spherical cored dark matter haloes. The simulated galaxies are "observed" under a variety of realistic conditions to determine how well the underlying dark matter halo can be recovered and to test the hypothesis that cuspy haloes can be mistaken for cored haloes. We find that the appearance of the velocity field is distinctly different depending on the underlying halo type. We also find that we can successfully recover the parameters of the underlying dark matter halo. Cuspy haloes appear cuspy in the data and cored haloes appear cored. Our results suggest that the observed cores in dark matter-dominated galaxies are genuine discrepancies from the predictions of LCDM and cannot be ascribed to systematic errors or non-circular motions.

Toward a Universal Formulation of the Halo Mass Function

Authors: P.S. Corasaniti, I. Achitouv

http://arxiv.org/abs/1012.3468v1
We compute the dark matter halo mass function in the context of the Excursion Set formalism for a diffusive barrier model with linearly drifting average, which captures the main features of the ellipsoidal collapse. We use a path-integral method to evaluate the corrections due to the sharp filtering of the linear density fluctuation field in real space. This allows us to consistently confront the model predictions with N-body simulation data. We find a remarkable agreement with the numerical results of Tinker et al. (2008) with deviations no greater than 5% over the range of masses probed by the simulations. This indicates that the Excursion Set in combination with an accurate modelling of the halo collapse threshold can provide a robust estimation of the mass function.

arXiv: 16 December 2010

Modelling the evolution of galaxies as a function of environment

Authors: Gabriella De Lucia
http://arxiv.org/abs/1012.3326v1
In this review, I provide an overview of theoretical aspects related to the evolution of galaxies as a function of environment. I discuss the main physical processes at play, their characteristic time-scales and environmental dependency, and comment on their treatment in the framework of hierarchical galaxy formation models. I briefly summarize recent results and the main open issues.

Lyman α radiative transfer in the high-redshift, dusty Universe

Authors: Peter Laursen
Ph.D. thesis, 204 coherently written pages
http://arxiv.org/abs/1012.3175v1
The significance of the Ly{\alpha} emission line as a probe of the high-redshift Universe has long been established. Originating mainly in the vicinity of young, massive stars and in association with accretion of large bulks of matter, it is ideal for detecting young galaxies, the fundamental building blocks of our Universe. Since many different processes shape the spectrum and the spatial distribution of the Ly{\alpha} photons in various ways, a multitude of physical properties of galaxies can be unveiled. However, this also makes the interpretation of Ly{\alpha} observations notoriously difficult. Because Ly{\alpha} is a resonant line, it scatters on neutral hydrogen, having its path length from the source to our telescopes vastly increased, and taking it through regions of unknown physical conditions. In this work, a numerical code capable of calculating realistically the radiative transfer of Ly{\alpha} is presented. The code is capable of performing the radiative transfer in an arbitrary and adaptively refined distribution of Ly{\alpha} source emission, temperature and velocity field of the interstellar and intergalactic medium, as well as density of neutral and ionized hydrogen, and, particularly important, dust. Accordingly, it is applied to galaxies simulated at high resolution, yielding a number of novel and interesting results, most notably the escape fractions of Ly{\alpha} photons, the effect of dust on the line profile, and the impact of the transfer through the intergalactic medium. Furthermore, the remarkable detection of Ly{\alpha} emission from a so-called "damped Ly{\alpha} absorber" --- a special type of objects thought to be the progenitor of present-day's galaxies --- is presented, and the potential of the code for interpreting observations is demonstrated.

Anisotropic stress and stability in modified gravity models

Authors: Ippocratis D. Saltas, Martin Kunz
The existence of anisotropic stress of a purely geometrical origin seems to be a characteristic of higher order gravity models, and has been suggested as a probe to test these models observationally, for example in weak lensing experiments. In this paper, we seek to find a class of higher order gravity models of f(R,G) type that would give us a zero anisotropic stress and study the consequences for the viability of the actual model. For the special case of a de Sitter background, we identify a subclass of models with the desired property. We also find a direct link between anisotropic stress and the stability of the model as well as the presence of extra degrees of freedom, which seems to be a general feature of higher order gravity models. Particularly, trying to make the effective anisotropic stress small, one approaches a singularity which renders the model non-viable.

arXiv: 15 December 2010

The effect of peculiar velocities on supernova cosmology

Authors: Tamara M Davis, Lam Hui, Joshua A Frieman, Troels Haugbølle, Richard Kessler, Benjamin Sinclair, Jesper Sollerman, Bruce Bassett, John Marriner, Edvard Mörtsell, Robert C Nichol, Michael W Richmond, Masao Sako, Donald P Schneider

http://arxiv.org/abs/1012.2912v1
We present an analysis of peculiar velocities and their effect on supernova cosmology. In particular, we study (a) the corrections due to our own motion, (b) the effects of correlations in peculiar velocities induced by large-scale structure, and (c) uncertainties arising from a possible local under- or over-density. For all of these effects we present a case study of their impact on the cosmology derived by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SN Survey). Correcting supernova redshifts for the CMB dipole slightly over-corrects nearby supernovae that share some of our local motion. We show that while neglecting the CMB dipole would cause a shift in the derived equation of state of Delta w ~ 0.04 (at fixed matter density) the additional local-motion correction is currently negligible (Delta w<0.01). We use a covariance-matrix approach to statistically account for correlated peculiar velocities. This down-weights nearby supernovae and effectively acts as a graduated version of the usual sharp low-redshift cut. Neglecting coherent velocities in the current sample causes a systematic shift of ~2% in the preferred value of w and will therefore have to be considered carefully when future surveys aim for percent-level accuracy. Finally, we perform n-body simulations to estimate the likely magnitude of any local density fluctuation (monopole) and estimate the impact as a function of the low-redshift cutoff. We see that for this aspect the low-z cutoff of z=0.02 is well-justified theoretically, but that living in a putative local density fluctuation leaves an indelible imprint on the magnitude-redshift relation.

arXiv: 14 December 2010

Cosmic acceleration a new review

Authors: O.A. Lemets, D.A. Yerokhi

http://arxiv.org/abs/1012.2756v1
Recent observations of near supernova show that the acceleration expansion of Universe decreases. This phenomenon is called the transient acceleration. In the second part of work we consider the 3-component Universe composed of a scalar field, interacting with the dark matter on the agegraphic dark energy background. We show that the transient acceleration appears in frame of such a model. The obtained results agree with the latest cosmological observations, namely, the 557 SNIa sample (Union2) was released by the Supernova Cosmology Project (SCP) Collaboration.

Estimating f_NL and g_NL from Massive High-Redshift Galaxy Clusters

Authors: Kari Enqvist, Shaun Hotchkiss, Olli Taanila
http://arxiv.org/abs/1012.2732v1
There are observations of 15 high-redshift massive galaxy clusters, which have an extremely small probability with a purely Gaussian initial curvature perturbation. Here we revisit the estimation of the contribution of non-Gaussianities to the cluster mass function and point out serious problems that have resulted in the application of the mass function out of the range of its validity. We remedy the situation and show that the values of f_NL previously claimed to completely reconcile (i.e. at ~100% confidence) the existence of the clusters with LambdaCDM are unphysically small. However, for WMAP cosmology and at 95% confidence, we arrive at the limit f_NL>411, which is similar to previous estimates. We also explore the possibility of a large g_NL as the reason for the observed excess of the massive galaxy clusters. This scenario, g_NL>2*10^6, appears to be in more agreement with CMB and LSS limits for the non-Gaussianity parameters and could also provide an explanation for the overabundance of large voids in the early universe.

Constraints on scalar-tensor theories of gravity from observations

Authors: Seokcheon Lee
In spite of their original discrepancy, both dark energy and modified theory of gravity can be parameterized by the effective equation of state (EOS) $\omega$ for the expansion history of the Universe. A useful model independent approach to the EOS of them can be given by so-called Chevallier-Polarski-Linder (CPL) parametrization where two parameters of it ($\omega_{0}$ and $\omega_{a}$) can be constrained by the geometrical observations which suffer from degeneracies between models. The linear growth of large scale structure is usually used to remove these degeneracies. This growth can be described by the growth index parameter $\gamma$ and it can be parameterized by $\gamma_{0} + \gamma_{a} (1 - a)$ in general. We use the scalar-tensor theories of gravity (STG) and show that the discernment between models is possible only when $\gamma_a$ is not negligible. We show that the linear density perturbation of the matter component as a function of redshift severely constrains the viable subclasses of STG in terms of $\omega$ and $\gamma$. From this method, we can rule out or prove the viable STG in future observations. When we use $Z(\phi) =1$, $F$ shows the convex shape of evolution in a viable STG model. The viable STG models with $Z(\phi) = 1$ are not distinguishable from dark energy models when we strongly limit the solar system constraint.

The Dawn of the Red: Star formation histories of group galaxies over the past 5 billion years

Authors: Sean L. McGee (1,2), Michael L. Balogh (1), David J. Wilman (3), Richard G. BowerJohn S. Mulchaey (4), Laura C. Parker (5), Augustus Oemler Jr. (4) ((1) Waterloo, (2) Durham, (3) MPE, (4) OCIW, (5) McMaster) (2),
http://arxiv.org/abs/1012.2388v1
We examine the star formation properties of group and field galaxies in two surveys, the Sloan Digital Sky Survey (SDSS; at z ~ 0.08) and the Group Environment and Evolution Collaboration (GEEC; at z ~ 0.4). Using UV imaging from the GALEX space telescope, along with optical and, for GEEC, near infrared photometry, we compare the observed spectral energy distributions to large suites of stellar population synthesis models. This allows us to accurately determine star formation rates and stellar masses. We find that star forming galaxies of all environments undergo a systematic lowering of their star formation rate between z=0.4 and z=0.08 regardless of mass. Nonetheless, the fraction of passive galaxies is higher in groups than the field at both redshifts. Moreover, the difference between the group and field grows with time and is mass-dependent, in the sense the the difference is larger at low masses. However, the star formation properties of star forming galaxies, as measured by their average specific star formation rates, are consistent within the errors in the group and field environment at fixed redshift. The evolution of passive fraction in groups between z=0.4 and z=0 is consistent with a simple accretion model, in which galaxies are environmentally affected 3 Gyrs after falling into a ~ 10E13 Msun group. This long timescale appears to be inconsistent with the need to transform galaxies quickly enough to ensure that star forming galaxies appear similar in both the group and field, as observed.

arXiv: 13 December 2010

The clustering of galaxies and galaxy clusters: constraints on primordial non-Gaussianity from future wide-field surveys

Authors: Cosimo Fedeli, Carmelita Carbone, Lauro Moscardini, Andrea Cimatti
http://arxiv.org/abs/1012.2305v1
We investigate the constraints on primordial non-Gaussianity with varied bispectrum shapes that can be derived from the power spectrum of galaxies and clusters of galaxies detected in future wide field optical/near-infrared surveys. Having in mind the proposed ESA space mission \emph{Euclid} as a specific example, we combine the spatial distribution of spectroscopically selected galaxies with that of weak lensing selected clusters. We use the physically motivated halo model in order to represent the correlation function of arbitrary tracers of the Large Scale Structure in the Universe. As naively expected, we find that galaxies are much more effective in jointly constrain the level of primordial non-Gaussianity $f_\mathrm{NL}$ and the amplitude of the matter power spectrum $\sigma_8$ than clusters of galaxies, due to the much lower abundance of the latter that is not adequately compensated by the larger effect on the power spectrum. Nevertheless, combination of the galaxy power spectrum with the cluster-galaxy cross spectrum can decrease the error on the determination of $f_\mathrm{NL}$ by up to a factor of $\sim 2$. This decrement is particularly evident for the less studied non-Gaussian bispectrum shapes, the so-called enfolded and the orthogonal ones. Setting constraints on these models can shed new light on various aspects of the physics of the early Universe, and it is hence of extreme importance. By combining the power spectra of clusters and galaxies with the cluster-galaxy cross spectrum we find constraints on primordial non-Gaussianity of the order $\Delta f_\mathrm{NL} \sim $ a few, competitive and possibly superior to future CMB experiments.

Molecular hydrogen in the cosmic recombination epoch

Authors: Esfandiar Alizadeh, Christopher M. Hirata
http://arxiv.org/abs/1012.2378v1
The advent of precise measurements of the cosmic microwave background (CMB) anisotropies has motivated correspondingly precise calculations of the cosmic recombination history. Cosmic recombination proceeds far out of equilibrium because of a "bottleneck" at the $n=2$ level of hydrogen: atoms can only reach the ground state via slow processes: two-photon decay or Lyman-$\alpha$ resonance escape. However, even a small primordial abundance of molecules could have a large effect on the interline opacity in the recombination epoch and lead to an additional route for hydrogen recombination. Therefore, this paper computes the abundance of the H$_2$ molecule during the cosmic recombination epoch. Hydrogen molecules in the ground electronic levels X$^1\Sigma^+_g$ can either form from the excited H$_2$ electronic levels B$^1\Sigma^+_u$ and C$^1\Pi_u$ or through the charged particles H$_2^+$, HeH$^+$ and H$^-$. We follow the transitions among all of these species, resolving the rotational and vibrational sub-levels. Since the energies of the X$^1\Sigma^+_g$--B$^1\Sigma^+_u$ (Lyman band) and X$^1\Sigma^+_g$-C$^1\Pi_u$ (Werner band) transitions are near the Lyman-$\alpha$ energy, the distortion of the CMB spectrum caused by escaped H Lyman-line photons accelerates both the formation and the destruction of H$_2$ due to this channel relative to the thermal rates. This causes the populations of H$_2$ molecules in X$^1\Sigma^+_g$ energy levels to deviate from their thermal equilibrium abundances. We find that the resulting H$_2$ abundance is $10^{-17}$ at $z=1200$ and $10^{-13}$ at $z=800$, which is too small to have any significant influence on the recombination history.

The growth factor of matter perturbations in an f(R) gravity

Authors: Xiangyun Fu, Puxun Wu, Hongwei Yu
http://arxiv.org/abs/1012.2249v1
The growth of matter perturbations in the $f(R)$ model proposed by Starobinsky is studied in this paper. Three different parametric forms of the growth index are considered respectively and constraints on the model are obtained at both the $1\sigma$ and $2\sigma$ confidence levels, by using the current observational data for the growth factor. It is found, for all the three parametric forms of the growth index examined, that the Starobinsky model is consistent with the observations only at the $2\sigma$ confidence level.

Exoplanet atmospheres: a brand-new and rapidly expanding research field

Authors: Mercedes Lopez-Morales
http://arxiv.org/abs/1012.2157v1
The field of exoplanets is quickly expanding from just the detection of new planets and the measurement of their most basic parameters, such as mass, radius and orbital configuration, to the first measurements of their atmospheric characteristics, such as temperature, chemical composition, albedo, dynamics and structure. Here I will overview some the main findings on exoplanet atmospheres thus far, first from space and just in the past two year also from the ground.

Filter Measurements for ACAM / Seminar Series (1)

Speaker: Annemieke Janssen, PhD Student
Affiliation:Rijks Universiteit Groningen & ING student
Title: Filter Measurements for ACAM
Date: 10 December 2010
Place: ING+NOT Astro-seminar- Santa Cruz-La Palma- Spain

Abstract:

During the last three months the Transmitted Wavefront Distortions (TWD)
of almost all 50-mm filters have been measured. These distortions are
important to know since they affect the image quality of ACAM
observations. For other instruments the TWD is less critical since filters
are usually placed in the focal plane, where they only affect the image
quality locally. ACAM is a bit of an exception having the filters placed
near the pupil plane, where wavefront distortions over the whole filter
area affect the image quality.

We started analysing the results, partly with Zemax and partly with
pinhole experiments. In the last case, calibration lamps and a pinhole in
ACAM create a very tiny spot on the CCD. The effect of a filter placed in
the beam can now be compared with the measured TWD. The first results will
be discussed in an half-hour presentation.

arXiv: 10 December 2010

First Observational Tests of Eternal Inflation

Authors: Stephen M. Feeney (UCL), Matthew C. Johnson (Perimeter Institute), Daniel J. Mortlock (Imperial College London), Hiranya V. Peiris (UCL)
http://arxiv.org/abs/1012.1995v1
The eternal inflation scenario predicts that our observable universe resides inside a single bubble embedded in a vast multiverse, the majority of which is still undergoing super-accelerated expansion. Many of the theories giving rise to eternal inflation predict that we have causal access to collisions with other bubble universes, opening up the possibility that observational cosmology can probe the dynamics of eternal inflation. We present the first observational search for the effects of bubble collisions, using cosmic microwave background data from the WMAP satellite. Using a modular algorithm that is designed to avoid a posteriori selection effects, we find four features on the CMB sky that are consistent with being bubble collisions. If this evidence is corroborated by upcoming data from the Planck satellite, we will be able to gain insight into the possible existence of the multiverse.

Structure formation in f(R) gravity: a distinguishing probe between the dark energy and modified gravity

Structure formation in f(R) gravity: a distinguishing probe between the dark energy and modified gravity

Author :Shant Baghram and Sohrab Rahvar

Affiliations :Department of Physics, Sharif University of Technology, P.O.Box 11365–9161, Tehran, Iran E-mail:baghram@physics.sharif.edu rahvar@sharif.edu

Journal :Journal of Cosmology and Astroparticle Physics Create an alert

Issue :Volume 2010, December 2010

Citation

shant Baghram and Sohrab Rahvar JCAP12(2010)008

doi: 10.1088/1475-7516/2010/12/008

arXiv: 9 December 2010

Constraints on a $f(R)$ gravity dark energy model with early scaling evolution

Authors: Chan-Gyung Park, Jai-chan Hwang, Hyerim Noh

http://arxiv.org/abs/1012.1662v1

The modified gravity with $f(R)=R^{1+\epsilon}$ ($\epsilon>0$) allows a scaling solution where the density of gravity sector follows the density of the dominant fluid. We present initial conditions of background and perturbation variables during the scaling evolution regime in the modified gravity. As a possible dark energy model we consider a gravity with a form $f(R)=R^{1+\epsilon}+qR^{-n}$ ($-1<n \le 0$) where the second term drives the late-time acceleration. We show that our $f(R)$ gravity parameters are very sensitive to the baryon perturbation growth and baryon density power spectrum, and present observational constraints on the model parameters. Our analysis suggests that only the parameter space extremely close to the $\Lambda\textrm{CDM}$ model is allowed.


Are There Echoes From The Pre-Big Bang Universe? A Search for Low Variance Circles in the CMB Sky

Authors: Amir Hajian

http://arxiv.org/abs/1012.1656v1
The existence of concentric low variance circles in the CMB sky, generated by black-hole encounters in an aeon preceding our big bang, is a prediction of the Conformal Cyclic Cosmology. Detection of three families of such circles in WMAP data was recently reported by Gurzadyan & Penrose (2010). We reassess the statistical significance of those circles by comparing with Monte Carlo simulations of the CMB sky with realistic modeling of the anisotropic noise in WMAP data. We find that the circles are not anomalous and that all three groups are consistent at 3sigma level with a Gaussian CMB sky as predicted by inflationary cosmology model.