arXiv: 25 October 2010

Unifying Einstein and Palatini gravities

Authors: Luca Amendola, Kari Enqvist, Tomi Koivisto

http://arxiv.org/abs/1010.4776v1

We consider a novel class of $f(\R)$ gravity theories where the connection is related to the conformally scaled metric $\hat g_{\mu\nu}=C(\R)g_{\mu\nu}$ with a scaling that depends on the scalar curvature $\R$ only. We call them C-theories and show that the Einstein and Palatini gravities can be obtained as special limits. In addition, C-theories include completely new physically distinct gravity theories even when $f(\R)=\R$. With nonlinear $f(\R)$, C-theories interpolate and extrapolate the Einstein and Palatini cases and may avoid some of their conceptual and observational problems. We further show that C-theories have a scalar-tensor formulation, which in some special cases reduces to simple Brans-Dicke-type gravity. If matter fields couple to the connection, the conservation laws in C-theories are modified. The stability of perturbations about flat space is determined by a simple condition on the lagrangian.

 

Searching for Galactic Hidden Gas through interstellar scintillation: Results from a test with the NTT-SOFI detector

Authors: F. Habibi (LAL), Marc Moniez (LAL), R. Ansari (LAL), S. Rahvar

http://arxiv.org/abs/1010.4616v1

Aims: Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected at longer time scale when the light of remote stars crosses an interstellar molecular cloud, but it has never been observed at optical wavelength. In a favorable case, the light of a background star can be subject to stochastic fluctuations of order of a few percent at a characteristic time scale of a few minutes. Our ultimate aim is to discover or exclude such scintillation effects, in order to estimate the contribution of molecular hydrogen to the Galactic baryonic hidden mass. This feasibility study is a pathfinder towards an observational strategy to search for scintillation, probing sensitivity of future surveys and estimating the background level. Methods: Scintillation induced by molecular gas in visible dark nebulae as well as by hypothetical halo clumpuscules of cool molecular hydrogen ($\mathrm{H_2-He}$) has been searched for during two nights. We have taken long series of 10s infrared exposures with the ESO-NTT telescope toward stellar populations located behind visible nebulae and toward the SMC. We therefore searched for stars exhibiting stochastic flux variations similar to the ones expected from the scintillation effect. According to our simulations of the scintillation process, this search should allow one to detect (stochastic) transverse gradients of column density in cool Galactic molecular clouds of order of $\sim 3\times 10^{5}\,\mathrm{g/cm^2/10\,000\,km}$. Results: We found one light-curve which is compatible with a strong scintillation effect through a turbulent structure characterized by a diffusion radius $R_{diff}<100\, km$ in B68 nebula. Complementary observations are needed to clarify the status of such candidate, and no firm conclusion can be established from this single observation. We can also infer limits on the existence of turbulent dense cores (of number density $n>10^9\, cm^{-3}$) within the dark nebulae. As no candidate is found towards the Small Magellanic Cloud, we are also able to establish upper limits on the contribution of gas clumpuscules to the Galactic halo mass. Conclusions: The limits set by this test do not seriously constrain the known models, but we show that the short time-scale monitoring for a few $10^6 star\times hour$ in the visible band with a $>4$ meter telescope and a fast readout camera should allow one to quantify the contribution of turbulent molecular gas to the Galactic halo. The LSST (Large Synoptic Survey Telescope) is perfectly suitable for this search.

arXiv: 21 October 2010

The peculiar velocity field: constraining the tilt of the Universe

Authors: Yin-Zhe Ma, Christopher Gordon, Hume A. Feldman

 http://arxiv.org/abs/1010.4276v1

A large bulk flow, which is in tension with the Lambda Cold Dark Matter cosmological model, has been observed \cite{Watkins08,Feldman09}. In this letter, we provide a physical explanation for this very large bulk flow, based on the assumption that the cosmic microwave background (CMB) rest frame does not coincide with the matter rest frame, resulting in a "tilted Universe". We propose a model that takes into account the relative velocity of CMB frame with respect to to the matter rest frame (hereafter tilted velocity), and use Type Ia Supernovae (SN), ENEAR, SFI++, SMAC, and COMPOSITE galaxy catalogues to constrain this tilted velocity.
We find that: (1) the magnitude of the tilted velocity $u$ is around 400 km/s, and its direction is close to what is found by \cite{Watkins08}; for SN, SMAC and COMPOSITE catalogues, $u=0$ is excluded at the two to three sigma level; (2) the constraints on the magnitude of the tilted velocity can result in the constraints on the duration of inflation, due to the fact that inflation can neither be too long (no dipole effect) nor too short (very large dipole effect); (3) under certain assumptions, the constraints on the tilted velocity requires that inflation lasts at least 6 e-folds longer than that required to solve the horizon problem. This opens a new window for testing inflation and the models of the early Universe from observations of large scale structure.

 

 

A Testable Solution of the Cosmological Constant and Coincidence Problems

Authors: Douglas J. Shaw, John D. Barrow

http://arxiv.org/abs/1010.4262v1

We present a new solution to the cosmological constant (CC) and coincidence problems in which the observed value of the CC, Lambda, is linked to other observable properties of the universe. This is achieved by promoting the CC from a parameter which must to specified, to a field which can take many possible values. The observed value of Lambda = 1/(9.3 Gyrs)^2 (~ 10^(-120) in Planck units) is determined by a new constraint equation which follows from the application of a causally restricted variation principle. When applied to our visible universe, the model makes a testable prediction for the dimensionless spatial curvature of Omega_K0 = -0.0056 (s_b/0.5); where s_b ~ 1/2 is a QCD parameter. Requiring that a classical history exist, our model determines the probability of observing a given Lambda. The observed CC value, which we successfully predict, is typical within our model even before the effects of anthropic selection are included. When anthropic selection effects are accounted for, we find that the observed coincidence between t_Lambda = Lambda^(-1/2) and the age of the universe, t_{U}, is a typical occurrence in our model. In contrast to multiverse explanations of the CC problems, our solution is independent of the choice of a prior weighting of different Lambda-values and does not rely on anthropic selection effects. Our model includes no unnatural small parameters and does not require the introduction of new dynamical scalar fields or modifications to general relativity, and it can be tested by astronomical observations in the near future.

 

 

 

arXiv: 20 October 2010

Linear growth of matter density perturbations in $f(R,\GB)$ theories

Authors: Antonio De Felice, Teruaki Suyama

http://arxiv.org/abs/1010.3886v1

We derive the equation of matter density perturbations on sub-horizon scales around a flat Friedmann-Lema\^\i tre-Robertson-Walker background for the general Lagrangian density $f(R,\GB)$ that is a function of a Ricci scalar $R$ and a Gauss-Bonnet term $\GB$. We find that the effective gravitational constant generically scales as distance squared at small distances. The effect of this diminishing of the gravitational constant might be important in the gravitational dynamics of cosmic objects such as galaxies, which can be in principle tested by observations. We also provide the general expressions for the effective anisotropic stress, which is useful to constrain modified gravity models from observations of large-scale structure and weak lensing. We also find that there is a special class of theories which evade this unusual behaviour and that the condition to belong to this special class is exactly the same as the one for not having super-luminal modes with propagation speed proportional to their wavenumber.

 

Chameleon dark energy models with characteristic signatures

Authors: Radouane Gannouji, Bruno Moraes, David F. Mota, David Polarski, Shinji Tsujikawa, Hans A. Winther

http://arxiv.org/abs/1010.3769v1

In chameleon dark energy models, local gravity constraints tend to rule out parameters in which observable cosmological signatures can be found. We study viable chameleon potentials consistent with a number of recent observational and experimental bounds. A novel chameleon field potential, motivated by f(R) gravity, is constructed where observable cosmological signatures are present both at the background evolution and in the growth-rate of the perturbations. We study the evolution of matter density perturbations on low redshifts for this potential and show that the growth index today gamma_0 can have significant dispersion on scales relevant for large scale structures. The values of gamma_0 can be even smaller than 0.2 with large variations of gamma on very low redshifts for the model parameters constrained by local gravity tests. This gives a possibility to clearly distinguish these chameleon models from the Lambda-Cold-Dark-Matter model in future high-precision observations.

 

Gravitational Lensing

Authors: Matthias Bartelmann (Zentrum fuer Astronomie der Universitaet Heidelberg, Institut fuer Theoretische Astrophysik)

http://arxiv.org/abs/1010.3829v1

Gravitational lensing has developed into one of the most powerful tools for the analysis of the dark universe. This review summarises the theory of gravitational lensing, its main current applications and representative results achieved so far. It has two parts. In the first, starting from the equation of geodesic deviation, the equations of thin and extended gravitational lensing are derived. In the second, gravitational lensing by stars and planets, galaxies, galaxy clusters and large-scale structures is discussed and summarised.

 

On the Effects of Coupled Scalar Fields on Structure Formation

Authors: Baojiu Li, John D. Barrow

http://arxiv.org/abs/1010.3748v1

A coupling between a scalar field (representing the dark energy) and dark matter could produce rich phenomena in cosmology. It affects cosmic structure formation mainly through the fifth force, a velocity-dependent force that acts parallel to particle's direction of motion and proportional to its speed, an effective rescaling of the particle masses, and a modified background expansion rate. In many cases these effects entangle and it is difficult to see which is the dominant one. Here we perform N-body simulations to study their qualitative behaviour and relative importance in affecting the key structure formation observables, for a model with exponential scalar field coupling. We find that the fifth force, a prominent example of the scalar-coupling effects, is far less important than the rescaling of particle mass or the modified expansion rate. In particular, the rescaling of particle masses is shown to be the key factor leading to less concentration of particles in halos than in LCDM, a pattern which is also observed in previous independent coupled scalar field simulations

 

Self-Similar Solutions of Triaxial Dark Matter Halos

Authors: Yoram Lithwick, Neal Dalal

http://arxiv.org/abs/1010.3723v1

We investigate the collapse and internal structure of dark matter halos. We consider halo formation from initially scale-free perturbations, for which gravitational collapse is self-similar. Fillmore and Goldreich (1984) and Bertschinger (1985) solved the one dimensional (i.e. spherically symmetric) case. We generalize their results by formulating the three dimensional self-similar equations. We solve the equations numerically and analyze the similarity solutions in detail, focusing on the internal density profiles of the collapsed halos. By decomposing the total density into subprofiles of particles that collapse coevally, we identify two effects as the main determinants of the internal density structure of halos: adiabatic contraction and the shape of a subprofile shortly after collapse; the latter largely reflects the triaxiality of the subprofile. We develop a simple model that describes the results of our 3D simulations. In a companion paper, we apply this model to more realistic cosmological fluctuations, and thereby explain the origin of the nearly universal (NFW-like) density profiles found in N-body simulations

 

A generalized local ansatz and its effect on halo bias

Authors: Sarah Shandera, Neal Dalal, Dragan Huterer

http://arxiv.org/abs/1010.3722v1

Motivated by the properties of early universe scenarios that produce observationally large local non-Gaussianity, we perform N-body simulations with non-Gaussian initial conditions from a generalized local ansatz. The bispectra are schematically of the local shape, but with scale-dependent amplitude. We find that in such cases the size of the non-Gaussian correction to the bias of small and large mass objects depends on the amplitude of non-Gaussianity roughly on the scale of the object. In addition, some forms of the generalized bispectrum alter the scale dependence of the non-Gaussian term in the bias by a fractional power of k. These features may allow significant observational constraints on the particle physics origin of any observed local non-Gaussianity, distinguishing between scenarios where a single field or multiple fields contribute to the curvature fluctuations. While analytic predictions for the non-Gaussian bias agree qualitatively with the simulations, we find numerically a stronger observational signal than expected. This suggests that a more precise understanding of halo formation is needed to fully explain the consequences of primordial non-Gaussianity

 

Do we know the mass of a black hole? Mass of some cosmological black hole models

Authors: J. T. Firouzjaee, M. Parsi Mood, Reza Mansouri

http://arxiv.org/abs/1010.3971v1

Using a cosmological black hole model proposed recently, we have calculated the quasi-local mass of a collapsing structure within a cosmological setting due to different definitions put forward in the last decades to see how similar or different they are. It has been shown that the mass within the horizon follows the familiar Brown-York behavior. It increases, however, outside the horizon again after a short decrease, in contrast to the Schwarzschild case. Further away, near the void, outside the collapsed region, and where the density reaches the background minimum, all the mass definitions roughly coincide. They differ, however, substantially far from it. Generically, we are faced with three different Brown-York mass maxima: near the horizon, around the void between the overdensity region and the background, and another at cosmological distances corresponding to the cosmological horizon. While the latter two maxima are always present, the horizon mass maxima is absent before the onset of the central singularity.

 

 

 

arXiv: 19 October 2010

General relativistic effects on non-linear matter power spectrum

Authors: Donghui Jeong, Jinn-Ouk Gong, Hyerim Noh, Jai-chan Hwang

http://arxiv.org/abs/1010.3489v1

Non-linear nature of Einstein equation introduces genuine relativistic higher order corrections to the usual Newtonian fluid equations describing the evolution of cosmological matter perturbations. We study the effect of such novel non-linearities on the next-to-leading order matter power spectrum for the case of pressureless, irrotational fluid in a flat Friedmann background. We find that pure general relativistic corrections are negligibly small over all scales. Our result guarantees that one can safely use Newtonian cosmology even in non-linear regimes.

 

Large scale structure simulations of inhomogeneous LTB void models

Authors: David Alonso, Juan Garcia-Bellido, Troels Haugboelle, Julian Vicente

http://arxiv.org/abs/1010.3453v1

We perform numerical simulations of large scale structure evolution in an inhomogeneous Lemaitre-Tolman-Bondi (LTB) model of the Universe. We follow the gravitational collapse of a large underdense region (a void) in an otherwise flat matter-dominated Einstein-deSitter model. We observe how the (background) density contrast at the centre of the void grows to be of order one, and show that the density and velocity profiles follow the exact non-linear LTB solution to the full Einstein equations for all but the most extreme voids. This result seems to contradict previous claims that fully relativistic codes are needed to properly handle the non-linear evolution of large scale structures, and that local Newtonian dynamics with an explicit expansion term is not adequate. We also find that the (local) matter density contrast grows with the scale factor in a way analogous to that of an open universe with a value of the matter density Omage_M(r) corresponding to the appropriate location within the void.

 

Scale Invariance as a Solution to the Cosmological Constant Problem

Authors: Pankaj Jain, Subhadip Mitra, Sukanta Panda, Naveen K. Singh

http://arxiv.org/abs/1010.3483v1

We show that scale invariance may provide a solution to the fine tuning problem of the cosmological constant. We construct a generalization of the standard model of particle physics which displays exact quantum scale invariance. The action is invariant under global scale transformations in arbitrary dimensions. We introduce two additional scalar fields, beside the Higgs field. The scale symmetry is broken spontaneously in the matter sector of the theory. In the gravitational sector it is broken cosmologically. The scaling symmetry forbids the presence of cosmological constant in the action. Hence the contribution to the cosmological constant is identically zero from the matter sector within the full quantum theory. However the gravitational sector does lead to a non-zero cosmological constant after cosmological symmetry breaking. The value of the cosmological constant can be fitted to the observed value by an appropriate choice of the scalar self coupling parameter. No fine tuning is required at loop orders since the matter sector gives zero contribution to the cosmological constant.

 

 

 

 

arXiv: 18 October 2010

Topology of large scale structure as test of modified gravity

Authors: Xin Wang, Xuelei Chen, Changbom Park

http://arxiv.org/abs/1010.3035v1

The genus of the iso-density contours is a robust measure of the topology of large-scale structure, and relatively insensitive to galaxies biasing and redshift-space distortions. We show that the growth of density fluctuations is scale-dependent even in the linear regime in some modified gravity theories, which opens a possibility of testing the theories observationally. We propose to use the genus of the iso-density contours, an intrinsic measure of the topology of large-scale structure, as a statistic to be used in such tests. In Einstein's general theory of relativity density fluctuations are growing at the same rate on all scales in the linear regime and the topology of large-scale structure is conserved in time in comoving space because structures are growing homologously. In this theory we expect the genus-smoothing scale relation is time-independent. However, in modified gravity models where structures grow with different rates on different scales, the genus-smoothing scale relation should change in time and this can be used to test for the gravity models on large scales. We studied the case of the f(R) theory, DGP braneworld theory as well as the parameterized post-Friedmann (PPF) models. We also forecast how the modified gravity models can be constrained with optical/IR or 21cm surveys in the near future.

Charge, domain walls and dark energy

Authors: Jonathan A. Pearson

http://arxiv.org/abs/1010.3195v1

One idea to explain the mysterious dark energy which appears to pervade the Universe is that it is due to a network of domain walls which has frozen into some kind of static configuration, akin to a soap film. Such models predict an equation of state with w=P/rho=-2/3 and can be represented in cosmological perturbation theory by an elastic medium with rigidity and a relativistic sound speed. An important question is whether such a network can be created from random initial conditions. We consider various models which allow the formation of domain walls, and present results from an extensive set of numerical investigations. The idea is to give a mechanism which prevents the natural propensity of domain walls to collapse and lose energy, almost to the point where a domain wall network freezes in. We show that when domain walls couple to a field with a conserved charge, there is a parameter range for which charge condenses onto the domain walls, providing a resistive force to the otherwise natural collapse of the walls

arXiv: 15 October 2010

Cores and Cusps in Warm Dark Matter Halos

Authors: Francisco Villaescusa-Navarro, Neal Dalal

http://arxiv.org/abs/1010.3008v1

The apparent presence of large core radii in Low Surface Brightness galaxies has been claimed as evidence in favor of warm dark matter. Here we show that WDM halos do not have cores that are large fractions of the halo size: typically, r_core/r_200 < 0.001. This suggests an astrophysical origin for the large cores observed in these galaxies, as has been argued by other authors.

 

 

Modelling the shapes of the largest gravitationally bound objects

Authors: Graziano Rossi, Ravi K. Sheth, Giuseppe Tormen

http://arxiv.org/abs/1010.2839v1

We combine the physics of the ellipsoidal collapse model with the excursion set theory to study the shapes of dark matter halos. In particular, we develop an analytic approximation to the nonlinear evolution that is more accurate than the Zeldovich approximation; we introduce a planar representation of halo axis ratios, which allows a concise and intuitive description of the dynamics of collapsing regions and allows one to relate the final shape of a halo to its initial shape; we provide simple physical explanations for some empirical fitting formulae obtained from numerical studies. Comparison with simulations is challenging, as there is no agreement about how to define a non-spherical gravitationally bound object. Nevertheless, we find that our model matches the conditional minor-to-intermediate axis ratio distribution rather well, although it disagrees with the numerical results in reproducing the minor-to-major axis ratio distribution. In particular, the mass dependence of the minor-to-major axis distribution appears to be the opposite to what is found in many previous numerical studies, where low-mass halos are preferentially more spherical than high-mass halos. In our model, the high-mass halos are predicted to be more spherical, consistent with results based on a more recent and elaborate halo finding algorithm, and with observations of the mass dependence of the shapes of early-type galaxies. We suggest that some of the disagreement with some previous numerical studies may be alleviated if we consider only isolated halos.

arXive: 14 October 2010

Dark Matter Halos: The Dynamical Basis of Effective Empirical Models

Authors: A. Lapi (1,2), A. Cavaliere (1) (1-Univ. "Tor Vergata", Roma, Italy, 2-SISSA/ISAS, Trieste, Italy)

http://arxiv.org/abs/1010.2602v1

We investigate the dynamical basis of the classic empirical models (specifically, Sersic-Einasto and generalized NFW) that are widely used to describe the distributions of collisionless matter in galaxies. We submit that such a basis is provided by our \alpha-profiles, shown to constitute solutions of the Jeans dynamical equilibrium with physical boundary conditions. We show how to set the parameters of the empirical in terms of the dynamical models; we find the empirical models, and specifically Sersic-Einasto, to constitute a simple and close approximation to the dynamical models. Finally, we discuss how these provide an useful baseline for assessing the impact of the small-scale dynamics that may modulate the density slope in the central galaxy regions.

 

The Origin of Dark Matter Halo Profiles

Authors: Neal Dalal, Yoram Lithwick, Michael Kuhlen

http://arxiv.org/abs/1010.2539v1

A  longstanding puzzle of fundamental importance in modern cosmology has been the origin of the nearly universal density profiles of dark matter halos found in N-body simulations -- the so-called NFW profile. We show how this behavior may be understood, simply, by applying adiabatic contraction to peaks of Gaussian random fields. We argue that dynamical friction acts to reduce enormously the effect of random scatter in the properties of initial peaks, providing a key simplification. We compare our model predictions with results of the ultra-high resolution Via Lactea-II N-body simulation, and find superb agreement. We show how our model may be used to predict the distribution of halo properties like concentration. Our results suggest that many of the basic properties of halo structure may be understood using extremely simple physics.

 

Particle cosmology

Authors: A. Riotto (CERN)

http://arxiv.org/abs/1010.2642v1

In these lectures the present status of the so-called standard cosmological model, based on the hot Big Bang theory and the inflationary paradigm is reviewed. Special emphasis is given to the origin of the cosmological perturbations we see today under the form of the cosmic microwave background anisotropies and the large scale structure and to the dark matter and dark energy puzzles.

 

 

arXiv: 13 October 2010

Cluster number counts in quintessence models

Authors: Seokcheon Lee, Kin-Wang Ng

http://arxiv.org/abs/1010.2291v1

Even though the abundance and evolution of clusters have been used to study the cosmological parameters including the properties of dark energy owing to their pure dependence on the geometry of the Universe and the power spectrum, it is necessary to pay particular attention to the effects of dark energy on the analysis. We obtain the explicit dark energy dependent {\it rms} linear mass fluctuation $\sigma_8$ which is consistent with the CMB normalization with less than $2$ % errors for general constant dark energy equation of state, $\oQ$. Thus, we do not have any degeneracy between $\sigma_8$ and the matter energy density contrast $\Omo$. When we use the correct value of the critical density threshold $\delta_{c} = 1.58$ obtained recently \cite{09090826, 09100126} into the cluster number density $n$ calculation in the Press-Schechter (PS) formalism, $n$ increases as compared to the one obtained by using $\delta_{c} = 1.69$ by about $60$, $80$, and $110$ % at $z = 0$, $0.5$, and $1$, respectively. Thus, PS formalism predicts the cluster number consistent with both simulation and observed data at the high mass region. We also introduce the improved coefficients of Sheth-Tormen (ST) formalism, which is consistent with the recently suggested mass function \cite{10052239}. We found that changing $\oQ$ by $\Delta \oQ = -0.1$ from $\oQ = -1.0$ causes the changing of the comoving numbers of high mass clusters of $M = 10^{16} h^{-1} M_{\odot}$ by about $20$ and $40$ % at $z = 0$ and $1$, respectively.

 

Induced Gravity and the Attractor Dynamics of Dark Energy/Dark Matter

Authors: Jorge L. Cervantes-Cota, Roland de Putter, Eric V. Linder

http://arxiv.org/abs/1010.2237v1

Attractor solutions that give dynamical reasons for dark energy to act like the cosmological constant, or behavior close to it, are interesting possibilities to explain cosmic acceleration. Coupling the scalar field to matter or to gravity enlarges the dynamical behavior; we consider both couplings together, which can ameliorate some problems for each individually. Such theories have also been proposed in a Higgs-like fashion to induce gravity and unify dark energy and dark matter origins. We explore restrictions on such theories due to their dynamical behavior compared to observations of the cosmic expansion. Quartic potentials in particular have viable stability properties and asymptotically approach general relativity.

 

 

arXiv; 12 October 2010

Cosmic Acceleration and the Helicity-0 Graviton

Authors: Claudia de Rham, Gregory Gabadadze, Lavinia Heisenberg, David Pirtskhalava
http://arxiv.org/abs/1010.1780v1

We explore cosmology in the decoupling limit of a non-linear covariant extension of Fierz-Pauli massive gravity obtained recently in arXiv:1007.0443. In this limit the theory is a scalar-tensor model of a unique form defined by symmetries. We find that it admits a self-accelerated solution, with the Hubble parameter set by the graviton mass. The negative pressure causing the acceleration is due to a condensate of the helicity-0 component of the massive graviton, and the background evolution, in the approximation used, is indistinguishable from the \Lambda CDM model. Fluctuations about the self-accelerated background are stable for a certain range of parameters involved. Most surprisingly, the fluctuation of the helicity-0 field above its background decouples from an arbitrary source in the linearized theory.
We also show how massive gravity can remarkably screen an arbitrarily large cosmological constant in the decoupling limit, while evading issues with ghosts. The obtained static solution is stable against small perturbations, suggesting that the degravitation of the vacuum energy is possible in the full theory. Interestingly, however, this mechanism postpones the Vainshtein effect to shorter distance scales. Hence, fifth force measurements severely constrain the value of the cosmological constant that can be neutralized, making this scheme phenomenologically not viable for solving the old cosmological constant problem. We briefly speculate on a possible way out of this issue.

 

 

arXiv: 11 October 2010

Cosmography of f(R) - brane cosmology

Authors: Mariam Bouhmadi-Lopez, Salvatore Capozziello, Vincenzo F. Cardone

http://arxiv.org/abs/1010.1547v1

Cosmography is a useful tool to constrain cosmological models, in particular dark energy models. In the case of modified theories of gravity, where the equations of motion are generally quite complicated, cosmography can contribute to select realistic models without imposing arbitrary choices a priori. Indeed, its reliability is based on the assumptions that the universe is homogeneous and isotropic on large scale and luminosity distance can be "tracked" by the derivative series of the scale factor a(t). We apply this approach to induced gravity brane-world models where an f(R)-term is present in the brane effective action. The virtue of the model is to self-accelerate the normal and healthy DGP branch once the f(R)-term deviates from the Hilbert-Einstein action. We show that the model, coming from a fundamental theory, is consistent with the LCDM scenario at low redshift. We finally estimate the cosmographic parameters fitting the Union2 Type Ia Supernovae (SNeIa) dataset and the distance priors from Baryon Acoustic Oscillations (BAO) and then provide constraints on the present day values of f(R) and its second and third derivatives.

 

arXiv: 8 October 2010

Minimally Parametric Power Spectrum Reconstruction from the Lyman-alpha Forest

Authors: Simeon Bird (IoA/Cambridge), Hiranya V. Peiris (UCL), Matteo Viel (INAF-OATS/Trieste), Licia Verde (ICC/Barcelona)
http://arxiv.org/abs/1010.1519v1

Current results from the Lyman alpha forest assume that the primordial power spectrum of density perturbations follows a simple power law form. We present the first analysis of Lyman alpha data to study the effect of relaxing this strong assumption on primordial and astrophysical constraints. We perform a large suite of numerical simulations, using them to calibrate a minimally parametric framework for describing the power spectrum. Combined with cross-validation, a statistical technique which prevents over-fitting of the data, this framework allows us to reconstruct the power spectrum shape without strong prior assumptions. We find no evidence for deviation from scale-invariance; our analysis also shows that current Lyman alpha data do not have sufficient statistical power to robustly probe the shape of the power spectrum at these scales. In contrast, the ongoing Baryon Oscillation Sky Survey (BOSS) will be able to do so with high precision. Furthermore, this near-future data will be able to break degeneracies between the power spectrum shape and astrophysical parameters

 

Understanding the faint red galaxy population using large-scale clustering measurements from SDSS DR7

Authors: Ashley J. Ross, Rita Tojeiro, Will J. Percival

http://arxiv.org/abs/1010.1403v1

We use data from the SDSS to investigate the evolution of the large-scale galaxy bias as a function of luminosity for red galaxies. We carefully consider correlation functions of galaxies selected from both photometric and spectroscopic data, and cross-correlations between them, to obtain multiple measurements of the large-scale bias. We find, for our most robust analyses, a strong increase in bias with luminosity for the most luminous galaxies, an intermediate regime where bias does not evolve strongly over a range of two magnitudes in galaxy luminosity, and no evidence for an upturn in bias for fainter red galaxies. Previous work has found an increase in bias to low luminosities that has been widely interpreted as being caused by an increase in the satellite fraction. We can recover such an upturn in bias to faint luminosities if we push our measurements to small scales, and include galaxy clustering measurements along the line-of-sight, where we expect non-linear effects to be the strongest. The results that we expect to be most robust suggest that central galaxies dominate the observed low luminosity population of red galaxies rather than satellite galaxies in more massive haloes.

 

Needatool: A Needlet Analysis Tool for Cosmological Data Processing

Authors: Davide Pietrobon, Amedeo Balbi, Paolo Cabella, Krzysztof M. Gorski

http://arxiv.org/abs/1010.1371v1

We introduce NeedATool (Needlet Analysis Tool), a software for data analysis based on needlets, a wavelet rendition which is powerful for the analysis of fields defined on a sphere. Needlets have been applied successfully to the treatment of astrophysical and cosmological observations, and in particular to the analysis of cosmic microwave background (CMB) data. Usually, such analyses are performed in real space as well as in its dual domain, the harmonic one. Both spaces have advantages and disadvantages: for example, in pixel space it is easier to deal with partial sky coverage and experimental noise; in harmonic domain, beam treatment and comparison with theoretical predictions are more effective. During the last decade, however, wavelets have emerged as a useful tool for CMB data analysis, since they allow to combine most of the advantages of the two spaces, one of the main reasons being their sharp localisation. In this paper, we outline the analytical properties of needlets and discuss the main features of the numerical code, which should be a valuable addition to the CMB analyst's toolbox.

 

 

 

 

arXiv: 7 October 2010

f(T) gravity and local Lorentz invariance

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

http://arxiv.org/abs/1010.1041v1

We show that in theories of generalised teleparallel gravity, whose Lagrangians are algebraic functions of the usual teleparallel Lagrangian, the action and the field equations are not invariant under local Lorentz transformations. We also argue that these theories appear to have extra degrees of freedom with respect to general relativity. Both of these facts appear to have been overlooked but are crucial for assessing the viability of these theories as alternative explanations for the acceleration of the universe.

 

Statistical Classification Techniques for Photometric Supernova Typing

Authors: James Newling, Melvin Varughese, Bruce Bassett, Heather Campbell, Renée Hlozek, Martin Kunz, Hubert Lampeitl, Bryony Martin, Robert Nichol, David Parkinson, Mathew Smith

http://arxiv.org/abs/1010.1005v1

Future photometric supernova surveys will produce vastly more candidates than can be followed up spectroscopically, highlighting the need for effective classification methods based on lightcurves alone. Here we introduce boosting and kernel density estimation techniques which have minimal astrophysical input, and compare their performance on 20,000 simulated Dark Energy Survey lightcurves. We demonstrate that these methods are comparable to the best template fitting methods currently used, and in particular do not require the redshift of the host galaxy or candidate. However both methods require a training sample that is representative of the full population, so typical spectroscopic supernova subsamples will lead to poor performance. To enable the full potential of such blind methods, we recommend that representative training samples should be used and so specific attention should be given to their creation in the design phase of future photometric surveys.

 

The Accelerating Universe

Authors: Dragan Huterer (University of Michigan)

http://arxiv.org/abs/1010.1162v1

In this article we review the discovery of the accelerating universe using type Ia supernovae. We then outline ways in which dark energy - component that causes the acceleration - is phenomenologically described. We finally describe principal cosmological techniques to measure large-scale properties of dark energy. This chapter complements other articles in this book that describe theoretical understanding (or lack thereof) of the cause for the accelerating universe.

 

 

arXiv: 6 October 2010

Distance and time constraints on accelerating cosmological models

Authors: M.A. Dantas, J.S. Alcaniz, D. Mania, Bharat Ratra

http://arxiv.org/abs/1010.0995v1

The absence of guidance from fundamental physics about the mechanism behind cosmic acceleration has given rise to a number of alternative cosmological scenarios. These are based either on modifications of general relativistic gravitation theory on large scales or on the existence of new fields in Nature. In this paper we investigate the observational viability of some accelerating cosmological models in light of current measurements of lookback time as a function of redshift from passively evolving galaxies and recent estimates of the product of the cosmic microwave background acoustic scale and the baryonic acoustic oscillation peak scale. By using information-criteria model selection, we select the best-fit models and rank the alternative scenarios. We show that some of these models may provide a better fit to the data than does the current standard cosmological constant dominated ($\Lambda$CDM) model.

 

Rotation Speed of the First Stars

Authors: Athena Stacy, Volker Bromm, Abraham Loeb

http://arxiv.org/abs/1010.0997v1

We estimate the rotation speed of Population III (Pop III) stars within a minihalo at z ~ 20 using a smoothed particle hydrodynamics (SPH) simulation, beginning from cosmological initial conditions. We follow the evolution of the primordial gas up to densities of 10^12 cm^-3. Representing the growing hydrostatic cores with accreting sink particles, we measure the velocities and angular momenta of all particles that fall onto these protostellar regions. This allows us to record the angular momentum of the sinks and estimate the rotational velocity of the Pop III stars expected to form within them. The rotation rate has important implications for the evolution of the star, the fate encountered at the end of its life, and the potential for triggering a gamma-ray burst (GRB). We find that there is sufficient angular momentum to yield rapidly rotating stars (> 1000 km s^-1, or near break-up speeds). This indicates that Pop III stars likely experienced strong rotational mixing, impacting their structure and nucleosynthetic yields. A subset of them was also likely to result in hypernova explosions, and possibly GRBs.

 

Laboratory constraints on chameleon dark energy and power-law fields

Authors: Jason H. Steffen (1), Amol Upadhye (2), Al Baumbaugh (1), Aaron S. Chou (1), Peter O. Mazur (1), Ray Tomlin (1), Amanda Weltman (3), William Wester

http://arxiv.org/abs/1010.0988v1

We report results from the GammeV Chameleon Afterglow Search---a search for chameleon particles created via photon/chameleon oscillations within a magnetic field. This experiment is sensitive to a wide class of chameleon power-law models and dark energy models not previously explored. These results exclude five orders of magnitude in the coupling of chameleons to photons covering a range of four orders of magnitude in chameleon effective mass and, for individual chameleon models, exclude between 4 and 12 orders of magnitude in chameleon couplings to matter.

 

Constraining the expansion rate of the Universe using low-redshift ellipticals as cosmic chronometers

Authors: Michele Moresco, Raul Jimenez, Andrea Cimatti, Lucia Pozzetti

http://arxiv.org/abs/1010.0831v1

We present a new methodology to determine the expansion history of the Universe analyzing the spectral properties of early type galaxies (ETG). We found that for these galaxies the 4000\AA break is a spectral feature that correlates with the relative ages of ETGs. In this paper we describe the method, explore its robustness using theoretical synthetic stellar population models, and apply it using a SDSS sample of $\sim$14 000 ETGs. Our motivation to look for a new technique has been to minimise the dependence of the cosmic chronometer method on systematic errors. In particular, as a test of our method, we derive the value of the Hubble constant $H_0 = 72.3 \pm 2.8$ (68% confidence), which is not only fully compatible with the value derived from the Hubble key project, but also with a comparable error budget. Using the SDSS, we also derive, assuming w=constant, a value for the dark energy equation of state parameter $w = -0.8 \pm 0.2$. Given the fact that the SDSS ETG sample only reaches $z \sim 0.3$, this result shows the potential of the method. In future papers we will present results using the high-redshift universe, to yield a determination of H(z) up to $z \sim 1$.

 

Combining cluster observables and stacked weak lensing to probe dark energy: Self-calibration of systematic uncertainties

Authors: Masamune Oguri, Masahiro Takada

http://arxiv.org/abs/1010.0744v1

We develop a new method of combining cluster observables (number counts and cluster-cluster correlation functions) and stacked weak lensing signals of background galaxy shapes, both of which are available in a wide-field optical imaging survey. Assuming that the clusters have secure redshift estimates, we show that the joint experiment enables a self-calibration of important systematic errors including the source redshift uncertainty and the cluster mass-observable relation, by adopting a single population of background source galaxies for the lensing analysis. It allows us to use the relative strengths of stacked lensing signals at different cluster redshifts for calibrating the source redshift uncertainty, which in turn leads to accurate measurements of the mean cluster mass in each bin. In addition, our formulation of stacked lensing signals in Fourier space simplifies the Fisher matrix calculations, as well as the marginalization over the cluster off-centering effect, the most significant uncertainty in stacked lensing. We show that upcoming wide-field surveys yield stringent constraints on cosmological parameters including dark energy parameters, without any priors on nuisance parameters that model systematic uncertainties. Specifically, the stacked lensing information improves the dark energy FoM by a factor of 4, compared to that from the cluster observables alone. The primordial non-Gaussianity parameter can also be constrained with a level of f_NL~10. In this method, the mean source redshift is well calibrated to an accuracy of 0.1 in redshift, and the mean cluster mass in each bin to 5-10% accuracies, which demonstrates the success of the self-calibration of systematic uncertainties from the joint experiment. (Abridged)

 

Metric compatibility and torsionlessness in the Einstein-Palatini formulation of general relativity

Authors: Naresh Dadhich, Josep M. Pons

http://arxiv.org/abs/1010.0869v1

It has been shown recently that Einstein's general relativity equations are obtained within the Einstein-Palatini formalism with no restrictions whatsoever on the connection. We rederive and extend this result for the vielbein Einstein-Palatini formulation and in the process discover the underlying gauge symmetry and the corresponding gauge condition. So far as we know this had remained unnoticed in the previous considerations of this problem. We obtain in a very simple way Einstein's condition on the connection to yield the Levi Civita connection and we show that it is merely the gauge condition of the $U(1)^d$ gauge symmetry. We perform a canonical analysis and show by counting the degrees of freedom that the additional degrees introduced in the Palatini formalism are exactly acounted for by the $U(1)^d$ gauge symmetry. Finally we also include the fermions and note that this gauge symmetry still holds good implying metricity with torsion.

 

Inflationary Perturbations in Palatini Generalised Gravity

Authors: Nicola Tamanini, Carlo R. Contaldi

http://arxiv.org/abs/1010.0689v1

We examine the generation of primordial perturbations during an inflationary epoch in generalised theories of gravity when the equations of motion are derived using the Palatini variational principle. Both f(R) and Scalar-Tensor theories are considered and we compare our results with those obtained under the conventional metric formalism. Non-linear generalisations of the action lead to different theories under the two variational choices and we obtain distinct results for scalar and tensor spectral indices and their ratio. We find the following general result; inflation driven solely by f(R) modifications alone do not result in suitable curvature perturbations whilst Scalar-Tensor theories generate nearly scalar invariant curvature perturbations but no tensor modes.

 

 

 

 

arXiv: 5 October 2010

Observational constraints on Galileon cosmology

Authors: Savvas Nesseris, Antonio De Felice, Shinji Tsujikawa
http://arxiv.org/abs/1010.0407v1

We study the cosmology of a covariant Galileon field with five covariant Lagrangians and confront this theory with the most recent cosmological probes: the type Ia supernovae data (Constitution and Union2 sets), cosmic microwave background (WMAP7) and the baryon acoustic oscillations (SDSS7). In the Galileon cosmology with a late-time de Sitter attractor, there is a tracker that attracts solutions with different initial conditions to a common trajectory. Including the cosmic curvature K, we place observational constraints on two distinct cases: (i) the tracker, and (ii) the generic solutions to the equations of motion. We find that the tracker solution can be consistent with the individual observational data, but it is disfavored by the combined data analysis. The generic solutions fare quite well when a non-zero curvature parameter is taken into account, but the Akaike and Bayesian information criteria show that they are not particularly favored over the LCDM model.

 

The CMB Bispectrum, Trispectrum, non-Gaussianity, and the Cramer-Rao Bound

Authors: Marc Kamionkowski, Tristan L. Smith, Alan Heavens

http://arxiv.org/abs/1010.0251v1

Minimum-variance estimators for the parameter fnl that quantifies local-model non-Gaussianity can be constructed from the cosmic microwave background (CMB) bispectrum (three-point function) and also from the trispectrum (four-point function). Some have suggested that a comparison between the estimates for the values of fnl from the bispectrum and trispectrum allow a consistency test for the model. But others argue that the saturation of the Cramer-Rao bound by the bispectrum estimator implies that no further information on fnl can be obtained from the trispectrum. Here we elaborate the nature of the correlation between the bispectrum and trispectrum estimators for fnl. We show that the two estimators become statistically independent in the limit of large number of CMB pixels and thus that the trispectrum estimator does indeed provide additional information on fnl beyond that obtained from the bispectrum. We explain how this conclusion is consistent with the Cramer-Rao bound. Our discussion of the Cramer-Rao bound may be of interest to those doing Fisher-matrix parameter-estimation forecasts or data analysis in other areas of physics as well.

 

 

 

 

arXiv: 4 October 2010

Post-Decadal White Paper: A Dual-Satellite Dark-Energy/Microlensing NASA-ESA Mission

Authors: Andrew Gould

http://arxiv4.library.cornell.edu/abs/1010.0002v1

A confluence of scientific, financial, and political factors imply that launching two simpler, more narrowly defined dark-energy/microlensing satellites will lead to faster, cheaper, better (and more secure) science than the present EUCLID and WFIRST designs. The two satellites, one led by ESA and the other by NASA, would be explicitly designed to perform complementary functions of a single, dual-satellite dark-energy/microlensing ``mission''. One would be a purely optical wide-field camera, with large format and small pixels, optimized for weak-lensing, which because of its simple design, could be launched by ESA on relatively short timescales. The second would be a purely infrared satellite with marginally-sampled or under-sampled pixels, launched by NASA. Because of budget constraints, this would be launched several years later. The two would complement one another in 3 dark energy experiments (weak lensing, baryon oscillations, supernovae) and also in microlensing planet searches. Signed international agreements would guarantee the later NASA launch, and on this basis equal access of both US and European scientists to both data sets.

 

 

 Testing Modified Gravity with Gravitational Wave Astronomy

Authors: Carlos F. Sopuerta, Nicolas Yunes

http://arxiv.org/abs/1010.0062v1

The emergent area of gravitational wave astronomy promises to provide revolutionary discoveries in the areas of astrophysics, cosmology, and fundamental physics. One of the most exciting possibilities is to use gravitational-wave observations to test alternative theories of gravity. In this contribution we describe how to use observations of extreme-mass-ratio inspirals by the future Laser Interferometer Space Antenna to test a particular class of theories: Chern-Simons modified gravity.

 

 

 

arXiv: 1 October 2010

Cosmology of the selfaccelerating third order Galileon

Authors: David F. Mota, Marit Sandstad, Tom Zlosnik

http://arxiv.org/abs/1009.6151v1

In this paper we start from the original formulation of the galileon model with the original choice for couplings to gravity. Within this framework we find that there is still a subset of possible Lagrangians that give selfaccelerating solutions with stable spherically symmetric solutions. This is a certain constrained subset of the third order galileon which has not been explored before. We develop and explore the background cosmological evolution of this model drawing intuition from other even more restricted galileon models. The numerical results confirm the presence of selfacceleration, but also reveals a possible instability with respect to galileon perturbations.

 

 

arXiv: 30 September 2010

Rejuvenating Power Spectra II: the Gaussianized galaxy density field

Authors: Mark C. Neyrinck, István Szapudi, Alexander S. Szalay

http://arxiv.org/abs/1009.5680v1

We find that, even in the presence of discreteness noise, a Gaussianizing transform (producing a more-Gaussian one-point distribution) reduces nonlinearities in the power spectra of cosmological matter and galaxy density fields, in many cases drastically. Although Gaussianization does increase the effective shot noise, it also increases the power spectrum's fidelity to the linear power spectrum on scales where the shot noise is negligible. Gaussianizing also increases the Fisher information in the power spectrum in all cases and resolutions, although the gains are smaller in redshift space than in real space. We also find that the gain in cumulative Fisher information from Gaussianizing peaks at a particular grid resolution that depends on the sampling level.

 

 

 

 

arXiv: 29 September 2010

Varying constants, Gravitation and Cosmology

Authors: Jean-Philippe Uzan
http://arxiv.org/abs/1009.5514v1

Fundamental constants are a cornerstone of our physical laws. Any constant varying in space and/or time would reflect the existence of an almost massless field that couples to matter. This will induce a violation of the universality of free fall. It is thus of utmost importance for our understanding of gravity and of the domain of validity of general relativity to test for their constancy. We thus detail the relations between the constants, the tests of the local position invariance and of the universality of free fall. We then review the main experimental and observational constraints that have been obtained from atomic clocks, the Oklo phenomenon, Solar system observations, meteorites dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic microwave background and big bang nucleosynthesis. At each step we describe the basics of each system, its dependence with respect to the constants, the known systematic effects and the most recent constraints that have been obtained. We then describe the main theoretical frameworks in which the low-energy constants may actually be varying and we focus on the unification mechanisms and the relations between the variation of different constants. To finish, we discuss the more speculative possibility of understanding their numerical values and the apparent fine-tuning that they confront us with.

 

Baryonic acoustic oscillations simulations for the Large Synoptic Survey Telescope (LSST)

Authors: A.Gorecki (1), A.Abate (2), R.Ansari (2), A.Barrau (1), S.Baumont (1), M.Moniez (2), ((1) LPSC, Grenoble, (2) LAL, Orsay)

http://arxiv.org/abs/1009.5532v1

The baryonic acoustic oscillations are features in the spatial distribution of the galaxies which, if observed at different epochs, probe the nature of the dark energy. In order to be able to measure the parameters of the dark energy equation of state to high precision, a huge sample of galaxies has to be used. The Large Synoptic Survey Telescope will survey the optical sky with 6 filters from 300nm and 1100nm, such that a catalog of galaxies with photometric redshifts will be available for dark energy studies. In this article, we will give a rough estimate of the impact of the photometric redshift uncertainties on the computation of the dark energy parameter through the reconstruction of the BAO scale from a simulated photometric catalog.

 

The quantum origin of cosmic structure: theory and observations

Authors: Konstantinos Dimopoulos

http://arxiv.org/abs/1009.5466v1

The particle production process is reviewed, through which cosmic inflation can produce a scale invariant superhorizon spectrum of perturbations of suitable fields starting from their quantum fluctuations. Afterwards, in the context of the inflationary paradigm, a number of mechanisms (e.g. curvaton, inhomogeneous reheating etc.) through which such perturbations can source the curvature perturbation in the Universe and explain the formation of structures such as galaxies are briefly described. Finally, the possibility that cosmic vector fields also contribute to the curvature perturbation (e.g. through the vector curvaton mechanism) is considered and its distinct observational signatures are discussed, such as correlated statistical anisotropy in the spectrum and bispectrum of the curvature perturbation.

 

Nonparametric Reconstruction of the Dark Energy Equation of State

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

http://arxiv.org/abs/1009.5443v1

A basic aim of ongoing and upcoming cosmological surveys is to unravel the mystery of dark energy. In the absence of a compelling theory to test, a natural approach is to better characterize the properties of dark energy in search of clues that can lead to a more fundamental understanding. One way to view this characterization is the improved determination of the redshift-dependence of the dark energy equation of state parameter, w(z). To do this requires a robust and bias-free method for reconstructing w(z) from data that does not rely on restrictive expansion schemes or assumed functional forms for w(z). We present a new nonparametric reconstruction method that solves for w(z) as a statistical inverse problem, based on a Gaussian Process representation. This method reliably captures nontrivial behavior of w(z) and provides controlled error bounds. We demonstrate the power of the method on different sets of simulated supernova data; the approach can be easily extended to include diverse cosmological probes.

 

 

arXiv: 28 September 2010

Nonlinear clustering in models with primordial non-Gaussianity: the halo model approach

Authors: Robert E. Smith (UZurich, UBonn), Vincent Desjacques (UZurich), Laura Marian (UBonn)

http://arxiv.org/abs/1009.5085v1

We develop the halo model of large-scale structure as an accurate tool for probing primordial non-Gaussianity. In this study we focus on understanding the matter clustering at several redshifts. The primordial non-Gaussianity is modeled as a quadratic correction to the local Gaussian potential, and is characterized by the parameter f_NL. In our formulation of the halo model we pay special attention to the effect of halo exclusion, and show that this can potentially solve the long standing problem of excess power on large scales in this model. The model depends on the mass function, clustering and density profiles of halos. We test these ingredients using a large ensemble of high-resolution Gaussian and non-Gaussian numerical simulations. In particular, we provide a first exploration of how density profiles change in the presence of primordial non-Gaussianities. We find that for f_NL positive/negative high mass halos have an increased/decreased core density, so being more/less concentrated than in the Gaussian case. We also examine the halo bias and show that, if the halo model is correct, then there is a small asymmetry in the scale-dependence of the bias on very large scales, which arises because the Gaussian bias must be renormalized. We show that the matter power spectrum is modified by ~2.5% and ~3.5% on scales k~1.0 h/Mpc at z=0 and z=1, respectively. Our halo model calculation reproduces the absolute amplitude to within 10% and the ratio of non-Gaussian to Gaussian spectra to within 1%. We also measure the matter correlation functions and find similarly good agreement between the model and the data. We anticipate that this modeling will be useful for constraining f_NL from measurements of the shear correlation function in future weak lensing surveys such as Euclid.

 

 

 

arXiv: 27 September 2010

Large-scale BAO signatures of the smallest galaxies

Authors: Neal Dalal, Ue-Li Pen, Uros Seljak
http://arxiv.org/abs/1009.4704v1

Recent work has shown that at high redshift, the relative velocity between dark matter and baryonic gas is typically supersonic. This relative velocity suppresses the formation of the earliest baryonic structures like minihalos, and the suppression is modulated on large scales. This effect imprints a characteristic shape in the clustering power spectrum of the earliest structures, with significant power on 100 Mpc scales featuring highly pronounced baryon acoustic oscillations. The amplitude of these oscillations is orders of magnitude larger at z=20 than previously expected. This characteristic signature can allow us to distinguish the effects of minihalos on intergalactic gas at times preceding and during reionization. We illustrate this effect with the example of 21 cm emission and absorption from redshifts during and before reionization. This effect can potentially allow us to probe physics on kpc scales using observations on 100 Mpc scales.
We present sensitivity forecasts for FAST and Arecibo. Depending on parameters, this enhanced structure may be detectable by Arecibo at redshifts near z=15-20, and with appropriate instrumentation FAST could measure the BAO power spectrum with high precision. In principle, this effect could also pose a serious challenge for efforts to constrain dark energy using observations of the BAO feature at low redshift.

 

Investigating The Uncertainty On The BAO Scale Measured From Future Photometric And Spectroscopic Surveys

Authors: Alexandra Abate (1), Alexia Gorecki (2), Reza Ansari (1), Aurelien Barrau (2), Sylvain Baumont (2), Laurent Derome (2), Marc Moniez (1) ((1) LAL, Orsay, (2) LPSC, Grenoble)

http://arxiv.org/abs/1009.4769v1

The Large Synoptic Survey Telescope (LSST) is a wide (20,000 sq.deg.) and deep ugrizy imaging survey which will be sited at Cerro Pachon in Chile. A major scientific goal of LSST is to constrain dark energy parameters via the baryon acoustic oscillation (BAO) signal. Crucial to this technique is the measurement of well-understood photometric redshifts, derived from the survey ugrizy imaging. Here we present the results of the effect of simulated photometric redshift (PZ) errors on the reconstruction of the BAO signal. We generate many "Monte Carlo" simulations of galaxies from a model power spectrum using Fast Fourier Transform techniques. Mock galaxy properties are assigned using an algorithm that reproduces observed luminosity-color-redshift distributions from the GOODS survey. We also compare these results to those expected from a possible future spectroscopic survey such as BigBOSS.

 

 

arXiv: 24 September 2010

Energy conditions and entropy density of the universe

Authors: Ming-Jian Zhang (1), Cong Ma (1), Tong-Jie Zhang (1 and 2) ((1) Department of Astronomy, Beijing Normal University, (2) Center for High Energy Physics, Peking University)

http://arxiv.org/abs/1009.4513v1

In the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological model, the energy conditions provides model-independent bounds on the behavior of the distance modulus. However, this method can not provide us the detailed information about the violation between the energy conditions and the observation. In this paper, we present an extended analysis of the energy conditions based upon the entropy density of the universe. On the one hand, we find that these conditions imply that entropy density $s$ depends on Hubble parameter H(z). On the other hand, we compare the theoretical entropy density from the conservation law of energy-momentum tensor with that from the energy conditions using the observational Hubble parameter. We find that the two kinds of entropy density are in agreement, only when the present-day entropy density satisfies 0.0222 <= s_0 <= 0.7888. We also obtain that the strong energy condition (SEC) accords with the first law of thermodynamics in the redshift range z < 2.7, the null energy condition (NEC) at z<3.2, and the dominant energy condition (DEC) at z > 2.6. In addition, the energy conditions gives the deceleration parameter 0 <= q(z) <= 2, which is in a predicament of the accelerated expansion of the universe. In particular, the NEC suggests q(z) >= 5/3

 

 

arXiv: 23 September 2010

Fundamental physics in observational cosmology

Authors: Patrick Peter (IAP - France)
http://arxiv.org/abs/1009.4327v1

I discuss, through a few examples, how observational cosmology can provide insights on hypothetical fundamental physics phenomena or mechanisms, such as Grand Unified Theory, Superstring alternatives to the inflation paradigm, and inflation itself.

 

Observational constraints on the LLTB model

Authors: Valerio Marra, Mikko Paakkonen

http://arxiv.org/abs/1009.4193v1

We directly compare the concordance LCDM model to the inhomogeneous matter-only alternative represented by LTB void models. To achieve a "democratic" confrontation we explore LLTB models with non-vanishing cosmological constant and perform a global likelihood analysis in the parameter space of cosmological constant and void radius. In our analysis we carefully consider SNe, Hubble constant, CMB and BAO measurements, marginalizing over the age of the universe and the background curvature. We find that the LCDM model is not the only possibility compatible with the observations, and that a matter-only void model is a viable alternative to the concordance model only if the BAO constraints are relaxed. Moreover, we will show that the areas of the parameter space which give a good fit to the observations are always disconnected with the result that a small local void does not significantly affect the parameter extraction for LCDM models.

arXiv: 22 September 2010

Scale-Dependent Non-Gaussianity as a Generalization of the Local Model

Authors: Adam Becker, Dragan Huterer, Kenji Kadota

http://arxiv.org/abs/1009.4189v1

We generalize the local model of primordial non-Gaussianity by promoting the parameter fNL to a general scale-dependent function fNL(k). We calculate the resulting bispectrum and the effect on the bias of dark matter halos, and thus the extent to which fNL(k) can be measured from the large-scale structure observations. By calculating the principal components of fNL(k), we identify scales where this form of non-Gaussianity is best constrained and estimate the overlap with previously studied local and equilateral non-Gaussian models.

 

arXiv: 21 September 2010

Too big, too early? Multiple High-Redshift Galaxy Clusters: implications

Authors: Ben Hoyle, Raul Jimenez, Licia Verde

http://arxiv.org/abs/1009.3884v1

To date, 15 high-redshift (z>1.0) galaxy clusters with mass measurements have been observed, spectroscopically confirmed and are reported in the literature. These objects should be exceedingly rare in the standard LCDM model. We conservatively approximate the selection functions of these clusters' parent surveys, and quantify the tension between the abundances of massive clusters as predicted by the standard LCDM model and the observed ones. We alleviate the tension considering non-Gaussian primordial perturbations of the local type, characterized by the parameter fnl and derive constraints on fnl arising from the mere existence of these clusters. At the 95% confidence level, fnl>475 with cosmological parameters fixed to their most likely WMAP5 values, or fnl>370 if we marginalize over WMAP5 parameters priors. In combination with fnl constraints from Cosmic Microwave Background and halo bias, this determination implies a scale-dependence of fnl at 3 sigma. Given the assumptions made in the analysis, we expect any future improvements to the modeling of the non-Gaussian mass function, survey volumes, or selection functions to increase the significance of fnl>0 found here. In order to reconcile these massive, high-z clusters with an fnl=0, their masses would need to be systematically lowered by 1.5 sigma or the sigma_8 parameter should be ~4 sigma higher than CMB (and large-scale structure) constraints. The existence of these objects is a puzzle: it either represents a challenge to the LCDM paradigm or it is an indication that the mass estimates of clusters is dramatically more uncertain than we think.

 

Dark Before Light: Testing the Cosmic Expansion History through the Cosmic Microwave Background

Authors: Eric V. Linder, Tristan L. Smith

http://arxiv.org/abs/1009.3500v1

The cosmic expansion history proceeds in broad terms from a radiation dominated epoch to matter domination to an accelerated, dark energy dominated epoch. We investigate whether intermittent periods of acceleration are possible in the early universe -- between Big Bang nucleosynthesis (BBN) and recombination and beyond. We establish that the standard picture is remarkably robust: observations of anisotropies in the cosmic microwave background exclude any extra period of accelerated expansion between 1 \leq z \lesssim 10^5 (corresponding to 5\times10^{-4}\ {\rm eV} \leq T \lesssim 25\ {\rm eV}).

 

Keck/LRIS Spectroscopic Confirmation of Coma Cluster Dwarf Galaxy Membership Assignments

Authors: Kristin Chiboucas, R. Brent Tully, Ronald O. Marzke, Neil Trentham, Henry C. Ferguson, Derek Hammer, David Carter, Habib Khosroshahi

http://arxiv.org/abs/1009.3535v1

Keck/LRIS multi-object spectroscopy has been carried out on 140 of some of the lowest and highest surface brightness faint (19 < R < 22) dwarf galaxy candidates in the core region of the Coma Cluster. These spectra are used to measure redshifts and establish membership for these faint dwarf populations. The primary goal of the low surface brightness sample is to test our ability to use morphological and surface brightness criteria to distinguish between Coma Cluster members and background galaxies using high resolution HST/ACS images. Candidates were rated as expected members, uncertain, or expected background. From 93 spectra, 51 dwarf galaxy members and 20 background galaxies are identified. Our morphological membership estimation success rate is ~100% for objects expected to be members and better than ~90% for galaxies expected to be in the background. We confirm that low surface brightness is a very good indicator of cluster membership. High surface brightness galaxies are almost always background with confusion arising only from the cases of the rare compact elliptical galaxies. The more problematic cases occur at intermediate surface brightness. Many of these galaxies are given uncertain membership ratings, and these were found to be members about half of the time. Including color information will improve membership determination but will fail for some of the same objects that are already mis-identified when using only surface brightness and morphology criteria. Compact elliptical galaxies with B-V colors ~0.2 magnitudes redward of the red sequence in particular require spectroscopic follow-up. In a sample of 47 high surface brightness, UCD candidates, 19 objects have redshifts which place them in the Coma Cluster. Redshift measurements are presented and the use of indirect means for establishing cluster membership is discussed.

 

Highlights and Conclusions of the Chalonge 14th Paris Cosmology Colloquium 2010: `The Standard Model of the Universe: Theory and Observations'

Authors: H. J. de Vega, M.C. Falvella, N. G. Sanchez

http://arxiv.org/abs/1009.3494v1

The Chalonge 14th Paris Cosmology Colloquium was held on 22-24 July 2010 in Paris Observatory on the Standard Model of the Universe: News from WMAP7, BICEP, QUAD, SPT, AMI, ACT, Planck, QUIJOTE and Herschel; dark matter (DM) searches and galactic observations; related theory and simulations. %aiming synthesis, progress and clarification. P Biermann, D Boyanovsky, A Cooray, C Destri, H de Vega, G Gilmore, S Gottlober, E Komatsu, S McGaugh, A Lasenby, R Rebolo, P Salucci, N Sanchez and A Tikhonov present here their highlights of the Colloquium. Inflection points emerged: LambdaWDM (Warm DM) emerges impressively over LambdaCDM whose galactic scale problems are ever-increasing. Summary and conclusions by H. J. de Vega, M. C. Falvella and N. G. Sanchez stress among other points: (i) Primordial CMB gaussianity is confirmed. Inflation effective theory predicts a tensor to scalar ratio 0.05-0.04 at reach/border line of next CMB observations, early fast-roll inflation provides lowest multipoles depression. SZ amplitudes are smaller than expected: CMB and X-ray data agree but intracluster models need revision and relaxed/non-relaxed clusters distinction. (ii) cosmic ray positron excess is explained naturally by astrophysical processes, annihilating/decaying dark matter needs growing tailoring. (iii) Cored (non cusped) DM halos and warm (keV scale mass) DM are increasingly favored from theory and observations, naturally producing observed small scale structures, wimps turn strongly disfavoured. LambdaWDM 1 keV simulations well reproduce observations. Evidence that LambdaCDM does not work at small scales is staggering. P Biermann presents his live minutes of the Colloquium and concludes that a keV sterile neutrino is the most interesting DM candidate. Photos of the Colloquium are included.