arXiv: 25 Feb 2010

Accelerating expansion in the swisscheese model
Authors: Gyula Bene, Adelinda Csapo
arXiv:1002.4610v1
A version of the swisscheese model is investigated. Nonoverlapping spheres are cut from a flat Friedmann-Robertson-Walker (FRW) universe. The mass they contained before is compressed within each sphere to a smaller sphere with homogeneous density distribution. Hence the inner spheres form sections of some closed FRW model. Between the outer and inner spheres there is a vacuum, where, due to spherical symmetry, the Schwarzschield metric describes the gravitational field. Within the inner spheres the closed FRW metric is valid, while outside the cut spheres the flat FRW metric is relevant. The metric and its first derivatives are continuous across the bordering surfaces of the different regions. We discuss this model in detail, and show that such a model universe can have a realistic density distribution on large scales, and exhibits accelerating expansion for a limited period of time. Especially, we determine the Hubble diagram and discuss its properties.

Calculating the Hubble diagram by perturbation theory
Authors: Gyula Bene
arXiv:1002.4605v1
The effect of density fluctuations upon light propagation is calculated perturbatively in a matter dominated irrotational universe. The starting point is the perturbed metric (second order in the perturbation strength), while the output is the Hubble diagram. Density fluctuations cause this diagram to broaden to a strip. Moreover, the shift of the diagram mimics accelerated expansion.

Varying Constants: Constraints from Seasonal Variations
Authors: Douglas J. Shaw, John D. Barrow
arXiv:1002.4528v1
We analyse the constraints obtained from new atomic clock data on the possible time variation of the fine structure `constant' and the electron-proton mass ratio and show how they are strengthened when the seasonal variation of Sun's gravitational field at the Earth's surface is taken into account.

Constraints on Cosmological Dark Matter Annihilation from the Fermi-LAT Isotropic Diffuse Gamma-Ray Measurement
Authors:
The Fermi-LAT collaboration: A. A. Abdo, M. Ackermann, M. Ajello, L. Baldini, J. Ballet, G. Barbiellini, D. Bastieri, K. Bechtol, R. Bellazzini, B. Berenji, R. D. Blandford, E. D. Bloom, E. Bonamente, A. W. Borgland, A. Bouvier, J. Bregeon, A. Brez, M. Brigida, P. Bruel, T. H. Burnett, S. Buson, G. A. Caliandro, R. A. Cameron, P. A. Caraveo, S. Carrigan, J. M. Casandjian, C. Cecchi, O. C. Elik, A. Chekhtman, C. C. Cheung, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi, J. Conrad, S. Cutini, C. D. Dermer, A. de Angelis, F. de Palma, S. W. Digel, E. do Couto e Silva, P. S. Drell, R. Dubois, D. Dumora, Y. Edmonds, C. Farnier, C. Favuzzi, S. J. Fegan, W. B. Focke, P. Fortin, M. Frailis, Y. Fukazawa, P. Fusco, F. Gargano, D. Gasparrini, N. Gehrels, S. Germani, N. Giglietto, F. Giordano, T. Glanzman,
G. Godfrey, J. E. Grove, L. Guillemot, S. Guiriec, M. Gustafsson, D. Hadasch, A. K. Harding, D. Horan, R. E. Hughes, A. S. Johnson, W. N. Johnson, T. Kamae, H. Katagiri, J. Kataoka, N. Kawai, M. Kerr, J. Knodlseder, M. Kuss, J. Lande, L. Latronico, M. Llena Garde, F. Longo, F. Loparco, B. Lott, M. N. Lovellette, P. Lubrano, A. Makeev, M. N. Mazziotta, J. E. McEnery, C. Meurer, P. F. Michelson, W. Mitthumsiri, T. Mizuno, C. Monte, M. E. Monzani, A. Morselli, I. V. Moskalenko, S. Murgia, P. L. Nolan, J. P. Norris, E. Nuss, T. Ohsugi, N. Omodei, E. Orlando, J. F. Ormes, D. Paneque, J. H. Panetta, D. Parent, V. Pelassa, M. Pepe, M. Pesce-Rollins, F. Piron, S. Raino, R. Rando, A. Reimer, O. Reimer, T. Reposeur, A. Y. Rodriguez, M. Roth, H. F. W. Sadrozinski, A. Sander, P. M. Saz Parkinson, J. D. Scargle, A. Sellerholm, C. Sgro, E. J. Siskind, P. D. Smith, G. Spandre, P. Spinelli, J. L. Starck, M. S. Strickman, D. J. Suson, H. Takahashi, T. Tanaka, J. B. Thayer, J. G. Thayer, D. F. Torres, Y. Uchiyama, T. L. Usher, V. Vasileiou, N. Vilchez, V. Vitale, A. P. Waite, P. Wang, B. L. Winer, K. S. Wood, T. Ylinen, G. Zaharijas, M. Ziegleet al. (89 additional authors not shown)
arXiv:1002.4415v1
The first published Fermi large area telescope (Fermi-LAT) measurement of the isotropic diffuse gamma-ray emission is in good agreement with a single power law, and is not showing any signature of a dominant contribution from dark matter sources in the energy range from 20 to 100 GeV. We use the absolute size and spectral shape of this measured flux to derive cross section limits on three types of generic dark matter candidates: annihilating into quarks, charged leptons and monochromatic photons. Predicted gamma-ray fluxes from annihilating dark matter are strongly affected by the underlying distribution of dark matter, and by using different available results of matter structure formation we assess these uncertainties. We also quantify how the dark matter constraints depend on the assumed conventional backgrounds and on the Universe's transparency to high-energy gamma-rays. In reasonable background and dark matter structure scenarios (but not in all scenarios we consider) it is possible to exclude models proposed to explain the excess of electrons and positrons measured by the Fermi-LAT and PAMELA experiments. Derived limits also start to probe cross sections expected from thermally produced relics (e.g. in minimal supersymmetry models) annihilating predominantly into quarks. For the monochromatic gamma-ray signature, the current measurement constrains only dark matter scenarios with very strong signals.

Effects of standard and modified gravity on interplanetary ranges
Authors: Lorenzo Iorio
We numerically investigate the impact on the two-body range by several Newtonian and non-Newtonian dynamical effects for some Earth-planet pairs in view of the expected cm-level accuracy in future planned or proposed interplanetary ranging operations. The general relativistic gravitomagnetic Lense-Thirring effect should be modeled and solved-for in future, accurate ranging tests of Newtonian and post-Newtonian gravity because it falls within their measurability domain. It could a-priori "imprint" the determination of some of the target parameters of the tests considered. Moreover, the ring of the minor asteroids, Ceres, Pallas, Vesta and the Trans-Neptunian Objects (TNOs) act as sources of non-negligible systematic uncertainty on the larger gravitoelectric post-Newtonian signals from which it is intended to determine the parameters \gamma and \beta of the Parameterized Post Newtonian (PPN) formalism with very high precision (orders of magnitude better than the current 10^-4-10^-5 levels). Also other putative, non-conventional gravitational effects like a violation of the Strong Equivalence Principle (SEP), a secular variation of the Newtonian constant of gravitation G, and the Pioneer anomaly are considered. The presence of a hypothetical, distant planetary-sized body X could be detectable with future high-accuracy planetary ranging. Our analysis can, in principle, be extended also to future interplanetary ranging scenarios in which one or more spacecraft in heliocentric orbits are involved.

arXiv: 24 Feb 2010

A Couple of Issues Concerning Spinor Dark Energy
Authors: Hao Wei
arXiv:1002.4230v1
Recently, the so-called Elko spinor field has been proposed to be a candidate of dark energy. It is a non-standard spinor and has unusual properties. When the Elko spinor field is used in cosmology, its unusual properties could bring some interesting consequences. In the present work, we consider two different issues concerning the dark energy model described by the Elko spinor fields. Firstly, we discuss the cosmological coincidence problem in the spinor dark energy model by using the dynamical system method. Our results show that the coincidence problem should be taken to heart in the investigations of spinor dark energy models. Next, we turn to another issue. In this work, we propose a simple method to reconstruct spinor dark energy from the cosmological observations. We find that this reconstruction method works fairly well.

Current constraints on the cosmic growth history
Authors: Rachel Bean, Matipon Tangmatitham
arXiv:1002.4197v2
We present constraints on the cosmic growth history with recent cosmological data, allowing for deviations from Lambda CDM as might arise if cosmic acceleration is due to modifications to GR or inhomogeneous dark energy. We combine measures of the cosmic expansion history, from Type 1a supernovae, baryon acoustic oscillations and the CMB, with constraints on the growth of structure from recent galaxy, CMB and weak lensing surveys along with ISW-galaxy cross-correlations. Deviations from Lambda CDM are parameterized by phenomenological modifications to the Poisson equation and the relationship between the two Newtonian potentials. We find modifications that are present at the time the CMB is formed are tightly constrained through their impact on the well-measured CMB acoustic peaks. By contrast, constraints on late-time modifications to the growth history, as might arise if modifications are related to the onset of cosmic acceleration, are far weaker, but remain consistent with Lambda CDM at the 95% confidence level. For these late-time modifications we find that differences in the evolution on large and small scales could provide an interesting signature by which to search for modified growth histories with future wide angular coverage, large scale structure surveys.

A dark energy view of inflation
Authors: Stéphane Ilic, Martin Kunz, Andrew R. Liddle, Joshua A. Frieman
arXiv:1002.4196v1
Traditionally, inflationary models are analyzed in terms of parameters such as the scalar spectral index ns and the tensor to scalar ratio r, while dark energy models are studied in terms of the equation of state parameter w. Motivated by the fact that both deal with periods of accelerated expansion, we study the evolution of w during inflation, in order to derive observational constraints on its value during an earlier epoch likely dominated by a dynamic form of dark energy. We find that the cosmic microwave background and large-scale structure data is consistent with w_inflation=-1 and provides an upper limit of 1+w <~ 0.02. Nonetheless, an exact de Sitter expansion with a constant w=-1 is disfavored since this would result in ns=1.

arXiv: 23 Feb 2010

Pulsar Observations with the Fermi Large Area Telescope
Authors: Damien Parent
Ph. D. Thesis; Universite de Bordeaux 1; Centre d'Etudes Nucleaires de Bordeaux-Gradignan
arXiv:1002.4027v1
The Large Area Telescope (LAT) on Fermi, launched on 2008 June 11, is a space telescope to explore the high energy gamma-ray universe. The instrument covers the energy range from 20 MeV to 300 GeV with greatly improved sensitivity and ability to localize gamma-ray point sources. It detects gamma-rays through conversion to electron-positron pairs and measurement of their direction in a tracker and their energy in a calorimeter. This thesis presents the gamma-ray light curves and the phase-resolved spectral measurements of radio-loud gamma-ray pulsars detected by the LAT. The measurement of pulsar spectral parameters (i.e. integrated flux, spectral index, and energy cut-off) depends on the instrument response functions (IRFs). A method developed for the on-orbit validation of the effective area is presented using the Vela pulsar. The cut efficiencies between the real data and the simulated data are compared at each stage of the background rejection. The results are then propagated to the IRFs, allowing the systematic uncertainties of the spectral parameters to be estimated. The last part of this thesis presents the discoveries, using both the LAT observations and the radio and X ephemeredes, of new individual gamma-ray pulsars such as PSR J0205+6449, and the Vela-like pulsars J2229+6114 and J1048-5832. Timing and spectral analysis are investigated in order to constrain the gamma-ray emission model. In addition, we discuss the properties of a large population of gamma-ray pulsars detected by the LAT, including normal pulsars, and millisecond pulsars.

Common Radial Velocity vs. Rare Microlensing: Difficulties and Futures
Authors: Karan Molaverdikhani
arXiv:1002.4025v1
In this paper, effective factors for success of Microlensing and Radial Velocity methods were choose. A semi-Delphi process applied on the factors to evaluating them and finding the most important factors for present situation of ML and RV, with help from about 100 experts, in or related exoplanets detection. I found the public definition on "success of exoplanets detection methods" is not correct and we should change it, as some experts did it, in the form of fundamental questions in planetary science. Also, the views of "Special Experts" are different from other experts that help us to choose the right way in evaluating. The next step was choosing the best strategy for future and finally, from SWOT landscape and with a new objective of ML method (New Game Board Strategy) I suggested four critical future strategies for completing current strategic directions.

Why all these prejudices against a constant?
Authors: Eugenio Bianchi, Carlo Rovelli
arXiv:1002.3966v2
The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.

Cluster Weak Gravitational Lensing
Authors: Keiichi Umetsu (ASIAA)
Lectures given at the International School of Physics Enrico Fermi, Course CLXXII - "Astrophysics of Galaxy Clusters", Varenna, Italy,
arXiv:1002.3952v1
Weak gravitational lensing of background galaxies is a unique, direct probe of the distribution of matter in clusters of galaxies. We review several important aspects of cluster weak gravitational lensing together with recent advances in weak lensing techniques for measuring cluster lensing profiles and constraining cluster structure parameters.

Multi-scale cosmology and structure-emerging Dark Energy: a plausibility analysis
Authors: Alexander Wiegand, Thomas Buchert
arXiv:1002.3912v1
Cosmological backreaction suggests a link between structure formation and the expansion history of the Universe. In order to quantitatively examine this connection we dynamically investigate a volume partition of the Universe into over- and underdense regions. This allows to trace structure formation using the volume fraction of the overdense regions $\lambda_{\CM}$ as its characterizing parameter. Employing results from cosmological perturbation theory and extrapolating the leading mode into the nonlinear regime, we construct a three-parameter model for the effective cosmic expansion history, involving $\lambda_{\CM_{0}}$, the matter density $\Omega_{m}^{\CD_{0}}$ and the Hubble rate $H_{\CD_{0}}$ of today's Universe. Taking standard values for $\Omega_{m}^{\CD_{0}}$ and $H_{\CD_{0}}$ as well as a reasonable value for $\lambda_{\CM_{0}}$, that we derive from $N$--body simulations, we determine the corresponding amounts of backreaction and spatial curvature. We find that the obtained values that are sufficient to generate today's structure also lead to a $\Lambda$CDM-like behavior of the scale factor, parametrized by the same parameters $\Omega_{m}^{\CD_{0}}$ and $H_{\CD_{0}}$, but without a cosmological constant. However, the temporal behavior of $\lambda_{\CM}$ does not faithfully reproduce the structure formation history. Surprisingly, however, the model matches with structure formation with the assumption of a low matter content, $\Omega_{m}^{\CD_{0}}\approx 3%$, a result that hints to a different interpretation of part of the backreaction effect as kinematical Dark Matter.

Statefinder diagnosis for the Palatini $f(R)$ gravity theories
Authors: Song Li, Hao-Ran Yu, Tong-Jie Zhang
arXiv:1002.3867v1
The Palatini $f(R)$ gravity, is able to probably explain the late time cosmic acceleration without the need for dark energy, is studied. In this paper, we investigate a number of $f(R)$ gravity theories in Palatini formalism by means of statefinder diagnosis. We consider two types of $f(R)$ theories: (i) $f(R)=R+\alpha R^{m}-\beta R^{-n}$ and (ii) $f(R)=R+\alpha ln R+\beta$. We find that the evolutionary trajectories in the $s-r$ and $q-r$ planes for various types of the Palatini $f(R)$ theories reveal different evolutionary properties of the universe. Additionally, we use the observational $H(z)$ data to constrain models of $f(R)$ gravity.

Analytic formulae for CMB anisotropy in LTB cosmology
Authors: Keiki Saito, Akihiro Ishibashi, Hideo Kodama
arXiv:1002.3855v1
The local void model has lately attracted considerable attention since it can explain the present apparent accelerated expansion of the universe without introducing dark energy. However, in order to justify this model as an alternative cosmological model to the standard $\Lambda$CDM model (FLRW universe plus dark energy), one has to test the model by various observations, such as CMB temperature anisotropy, other than the distance-redshift relation of SNIa. For this purpose, we derive some analytic formulae that can be used to rigorously compare consequences of this model with observations of CMB anisotropy and to place constraints on the position of observers in the void model.

When scalar field is kinetically coupled to the Einstein tensor
Authors: Changjun Gao
arXiv:1002.4035v1
We explore the cosmic evolution of a scalar field when the kinetic term is coupled to the Einstein tensor. When the kinetic term is coupled to one Einstein tensor, we find that in the absence of other matter sources or in the presence of pressureless matter, the scalar would behave as the pressureless matter. This enables the scalar field to be the candidate of cold dark matter. By taking into account of a scalar potential in this case, we find the scalar field may play the role of both dark matter and dark energy. For sufficiently small exponential potential parameter $\zeta$, the equation of state of the scalar is $w\simeq -1$ in the total history of the Universe. We also find that the equation of state for the scalar can cross the phantom divide. But due to the kinetic energy is always positive, the scalar field is stable to classically perturbations. On the other hand, if the kinetic term is coupled to many more Einstein tensors, we find the equation of state is always approximately equals to -1 regardless whether the potential is flat or not. Thus the scalar may also be the candidate of inflaton field.

Enriched Phenomenology in Extended Palatini Theories
Authors: Gonzalo J. Olmo, Helios Sanchis-Alepuz, Swapnil Tripathi
arXiv:1002.3920v1
We show that extended theories of gravity with Lagrangian f(R,R_{\mu\nu}R^{\mu\nu}) in the Palatini formulation possess a phenomenology much richer than the simpler f(R) or f(R_{\mu\nu}R^{\mu\nu}) theories. In fact, we find that the scalars R and Q=R_{\mu\nu}R^{\mu\nu} can be written as algebraic functions of the energy density and pressure of the energy momentum tensor. In the simpler cases of f(R) or f(R_{\mu\nu}R^{\mu\nu}) theories, R and Q are just functions of the trace T of the energy-momentum tensor. As a result, in radiation dominated universes f(R) and f(R_{\mu\nu}R^{\mu\nu}) theories exhibit the same dynamics as general relativity with an effective cosmological constant. This is not the case of f(R,R_{\mu\nu}R^{\mu\nu}) models, in which R=R(\rho,P) and Q=Q(\rho,P) and, therefore, modified dynamics exists even for traceless sources.

Avoiding the Big Bang Singularity with Palatini f(R) Theories
Authors: Carlos Barragan, Gonzalo J. Olmo, Helios Sanchis-Alepuz
arXiv:1002.3919v1
We show that there exist modified theories of gravity in which the metric satisfies second-order equations and in which the Big Bang singularity is replaced by a cosmic bounce without violating any energy condition. In fact, the bounce is possible even for presureless dust. We give a characterization of such theories, which are formulated in the Palatini formalism, and discuss their dynamics in the region near the bounce. We consider spatially flat and non-flat homogeneous and isotropic universes.

Covariant gauge invariant theory of Scalar Perturbations in $f(R)$-gravity: a brief review
Authors: Sante Carloni
arXiv:1002.3868v1
review the state of the art of the investigation on the structure formation in $f(R)$-gravity based on the Covariant and Gauge Invariant approach to perturbations. A critical analysis of the results, in particular the presence of characteristic signature of these models, together with their meaning and their implication is given.

arXiv: 22 Feb 2010

Second-order matter perturbations in a LambdaCDM cosmology and non-Gaussianity
Authors: N. Bartolo, S. Matarrese, O. Pantano (Padova University), A. Riotto (INFN, Padova and CERN)
arXiv:1002.3759v1
We obtain exact expressions for the effect of primordial non-Gaussianity on the matter density perturbation up to second order in a LambdaCDM cosmology, fully accounting for the general relativistic corrections arising on scales comparable with the Hubble radius. We present our results both in the Poisson gauge and in the comoving and synchronous gauge, which are relevant for comparison to different cosmological observables.

On the possibility of braneworld quintessential inflation
Authors: Mafalda Dias, Andrew R Liddle
arXiv:1002.3703v1
We examine the possibility of achieving quintessential inflation, where the same field serves as both inflaton and quintessence, in the context of a five-dimensional braneworld. Braneworld cosmology provides an appropriate environment as it permits inflation with much steeper potentials than the conventional scenario, which is favourable to a late-time quintessence. We explore a wide space of models, together with contemporary observational data, to determine in which contexts such a picture is possible. We find that such a scenario, although attractive, is in fact impossible to achieve for the potentials studied due to the restrictiveness of current data.

Ideal Cosmic Shear Estimators Do Not Exist
Authors: Jun Zhang (UT Austin, UC Berkeley)
arXiv:1002.3615v1
A long standing problem in weak lensing is about how to construct a cosmic shear estimator from galaxy images. Ideally, for each shear component, one wants a single quantity from each galaxy image, whose ensemble average is equal to the true shear value. We prove that such ideal shear estimators do not exist in the presence of the point spread function. Alternatively, from each galaxy image, one can construct two quantities for each shear component, and use the ratio of the ensemble averages of the two quantities to recover the shear in an unbiased way. We show that the later is achievable using the shear measurement method of Zhang (2008). We also demonstrate that with the new but less ideal shear estimator, weak lensing statistics such as n-point correlations should be carried out in a slightly different way, but with little additional cost.

Measuring the "Reduced" Shear
Authors: Jun Zhang (UT Austin, UC Berkeley)
arXiv:1002.3614v1
We show that cosmic shear measurement can be made accurate to the second order in shear in the presence of a PSF and photon noise using an extension of the method of Zhang (2008). The sign of the second order correction is opposite to what is conventionally assumed. Neglecting the second order corrections can lead to a few percent uncertainties on cosmic shears, and becomes more important for cluster lensing mass reconstructions. Our shear measurement method is well defined mathematically. It does not require assumptions on the morphologies of galaxies and the point spread function. Contaminations to the shear signals from the background photon noise can be removed also in a well defined way. Using a large ensemble (10^7) of mock galaxies of unrestricted morphologies, we demonstrate that the shear recovery accuracy in this method reaches at least sub-percent levels even in the presence of large and correlated background noise. The recovery accuracy of shear-shear correlations are also tested under general conditions.

arXiv: 19 Feb 2010

The Cosmological Parameters 2010
Authors: Ofer Lahav, Andrew R Liddle
arXiv:1002.3488v1
This is a review article for The Review of Particle Physics 2010 (aka the Particle Data Book). It forms a compact review of knowledge of the cosmological parameters at the beginning of 2010. Topics included are Parametrizing the Universe; Extensions to the standard model; Probes; Bringing observations together; Outlook for the future.



Average luminosity distance in inhomogeneous universes
Authors: Valentin Kostov
Ph.D. thesis, University of Chicago 2010
arXiv:1002.3408v1
Using numerical ray tracing, the paper studies how the average distance modulus in an inhomogeneous universe differs from its homogeneous counterpart. The averaging is over all directions from a fixed observer not over all possible observers (cosmic), thus it is more directly applicable to our observations. Unlike previous studies, the averaging is exact, non-perturbative, and includes all possible non-linear effects. The inhomogeneous universes are represented by Sweese-cheese models containing random and simple cubic lattices of mass-compensated voids. The Earth observer is in the homogeneous cheese which has an Einstein - de Sitter metric. For the first time, the averaging is widened to include the supernovas inside the voids by assuming the probability for supernova emission from any comoving volume is proportional to the rest mass in it. Despite the well known argument for photon flux conservation, the average distance modulus correction at low redshifts is not zero due to the peculiar velocities. A formula for the maximum possible average correction as a function of redshift is derived and shown to be in excellent agreement with the numerical results. The actual average correction calculated in random and simple cubic void lattices is severely damped below the predicted maximal average. That is traced to cancelations between the corrections coming from the fronts and backs of different voids at the same redshift from the observer. The calculated correction at low redshifts allows one to readily predict the redshift at which the averaged fluctuation in the Hubble diagram is below a required precision and suggests a method to extract the background Hubble constant from low redshift data without the need to correct for peculiar velocities.



GRBs As Standard Candles: There Is No "Circularity Problem" (And There Never Was)
Authors: Carlo Graziani
arXiv:1002.3434v1
Beginning with the 2002 discovery of the "Amati Relation" of GRB spectra, there has been much interest in the possibility that this and other correlations of GRB phenomenology might be used to make GRBs into standard candles. One recurring apparent difficulty with this program has been that some of the primary observational quantities to be fit as "data" -- to wit, the isotropic-equivalent prompt energy $E_{iso}$ and the collimation-corrected "total" prompt energy energy $E_{\gamma}$ -- depend for their construction on the very cosmological models that they are supposed to help constrain. This is the so-called "Circularity Problem" of standard candle GRBs. This paper is intended to point out that the "Circularity Problem" is not in fact a problem at all, except to the extent that it amounts to a self-inflicted wound. It arises essentially because of an inapt choice of data variables -- "source-frame" variables such as $E_{iso}$, which are unnecessarily encumbered by cosmological considerations. If, instead, the empirical correlations of GRB phenomenology which are formulated in source-variables are {\it mapped to the primitive observational variables} (such as fluence) and compared to the observations in that space, then all taint of "circularity" disappears. I also describe procedures for encoding high-dimensional empirical correlations (such as between $E_{iso}$, $E_{pk}$, $t_{jet}$, and $T_{45}$) in a "Gaussian Tube" model that includes both the correlation and its intrinsic scatter, and how that source-variable model may easily be mapped to the space of primitive observables, to be convolved with the measurement errors and fashioned into a likelihood.

Quintessence and phantom cosmology with non-minimal derivative coupling
Authors: Emmanuel N. Saridakis, Sergey V. Sushkov arXiv:1002.3478v2 We investigate cosmological scenarios with a non-minimal derivative coupling between the scalar field and the curvature, examining both the quintessence and the phantom cases in zero and constant potentials. In general, we find that the universe transits from one de Sitter solution to another, determined by the coupling parameter. Furthermore, according to the parameter choices and without the need for matter, we can obtain a Big Bang, an expanding universe with no beginning, a cosmological turnaround, an eternally contracting universe, a Big Crunch, a Big Rip avoidance and a cosmological bounce. This variety of behaviors reveals the capabilities of the present scenario.

Exploring a matter-dominated model with bulk viscosity to drive the accelerated expansion of the Universe
Authors: Arturo Avelino, Ulises Nucamendi arXiv:1002.3605v1 We explore the viability of a bulk viscous matter-dominated Universe to explain the present accelerated expansion of the Universe. The model is composed by two fluids: a radiation component and a pressureless fluid with bulk viscosity of the form zeta = zeta_0 + zeta_1 H where zeta_0 and zeta_1 are constants and H is the Hubble parameter. The pressureless fluid characterizes both the baryon and dark matter components. We study all the possible scenarios for the Universe according to the values of zeta_0 and zeta_1 analyzing the behavior of the scale factor as well as the curvature scalar and the matter density. On the other hand, we test the model computing the best estimated values of zeta_0 and zeta_1 using the type Ia Supernovae (SNe Ia) and the shift parameter R of the Cosmic Microwave Radiation Anisotropies (CMB) probes. We find that the model fits well to both tests. We find also that from all the possible scenarios for the Universe, the preferred one by the best estimated values of (zeta_0, zeta_1) is that of an expanding Universe beginning with a Big-Bang, followed by an decelerated expansion at the early times, and with a smooth transition in recent times to an accelerated expansion epoch that is going to continue forever. The predicted age of the Universe is a little smaller than the mean value of the observational constraint coming from the oldest globular clusters but it is still inside of the confidence interval of this constraint (12.9 +- 2.9 Gyr). However, a possible drawback of the preferred model is the violation of the local second law of thermodynamics for early times (z around 1).

arXiv: 18 Feb 2010

Gravitational Lensing Effects in the LTB Model
Authors: Hajime Goto, Hideo Kodama
arXiv:1002.3161v1
In this talk, we discuss how to estimate the gravitational lensing effect of a local void on the CMB polarization by using the LTB model.

Testing gaussianity, homogeneity and isotropy with the cosmic microwave background
Authors: L. Raul Abramo, Thiago S. Pereira
arXiv:1002.3173v1
We review the basic hypotheses which motivate the statistical framework used to analyze the cosmic microwave background, and how that framework can be enlarged as we relax those hypotheses. In particular, we try to separate as much as possible the questions of gaussianity, homogeneity and isotropy from each other. We focus both on isotropic estimators of non-gaussianity as well as statistically anisotropic estimators of gaussianity, giving particular emphasis on their signatures and the enhanced "cosmic variances" that become increasingly important as our putative Universe becomes less symmetric. After reviewing the formalism behind some simple model-independent tests, we discuss how these tests can be applied to CMB data when searching for large scale "anomalies"

How BAO measurements can fail to detect quintessence
Authors: E. Jennings, C. M. Baugh, R. E. Angulo, S. Pascoli
arXiv:1002.3255v1
We model the nonlinear growth of cosmic structure in different dark energy models, using large volume N-body simulations. We consider a range of quintessence models which feature both rapidly and slowly varying dark energy equations of state, and compare the growth of structure to that in a universe with a cosmological constant. The adoption of a quintessence model changes the expansion history of the universe, the form of the linear theory power spectrum and can alter key observables, such as the horizon scale and the distance to last scattering. The difference in structure formation can be explained to first order by the difference in growth factor at a given epoch; this scaling also accounts for the nonlinear growth at the 15% level. We find that quintessence models which feature late $(z<2)$, w="-1$">

arXiv: 17 Feb 2010

Observational Constraints on the Modified Gravity Model (MOG) Proposed by Moffat: Using the Magellanic System
Authors: Hosein Haghi, Sohrab Rahvar
arXiv:1002.3136v1
A simple model for the dynamics of the Magellanic Stream (MS), in the framework of modified gravity models is investigated. We assume that the galaxy is made up of baryonic matter out of context of dark matter scenario. The model we used here is named Modified Gravity (MOG) proposed by Moffat (2005). In order to examine the compatibility of the overall properties of the MS under the MOG theory, the observational radial velocity profile of the MS is compared with the numerical results using the $\chi^2$ fit method. In order to obtain the best model parameters, a maximum likelihood analysis is performed. We also compare the results of this model with the Cold Dark Matter (CDM) halo model and the other alternative gravity model that proposed by Bekenstein (2004), so called TeVeS. We show that by selecting the appropriate values for the free parameters, the MOG theory seems to be plausible to explain the dynamics of the MS as well as the CDM and the TeVeS models.

Consistency of LCDM with Geometric and Dynamical Probes
Authors: L. Perivolaropoulos
arXiv:1002.3030v1
The LCDM cosmological model assumes the existence of a small cosmological constant in order to explain the observed accelerating cosmic expansion. Despite the dramatic improvement of the quality of cosmological data during the last decade it remains the simplest model that fits remarkably well (almost) all cosmological observations. In this talk I review the increasingly successful fits provided by LCDM on recent geometric probe data of the cosmic expansion. I also briefly discuss some emerging shortcomings of the model in attempting to fit specific classes of data (eg cosmic velocity dipole flows and cluster halo profiles). Finally, I summarize recent results on the theoretically predicted matter overdensity ($\delta_m=\frac{\delta \rho_m}{\rho_m}$) evolution (a dynamical probe of the cosmic expansion), emphasizing its scale and gauge dependence on large cosmological scales in the context of general relativity. A new scale dependent parametrization which describes accurately the growth rate of perturbations even on scales larger than 100h^{-1}Mpc is shown to be a straightforward generalization of the well known scale independent parametrization f(a)=\omms(a)^\gamma valid on smaller cosmological scales.

Slow-roll Extended Quintessence
Authors: Takeshi Chiba, Masaru Siino, Masahide Yamaguchi
arXiv:1002.2986v1
We derive the slow-roll conditions for a non-minimally coupled scalar field (extended quintessence) during the radiation/matter dominated era extending our previous results for thawing quintessence. We find that the ratio $\ddot\phi/3H\dot\phi$ becomes constant but negative, in sharp contrast to the ratio for the minimally coupled scalar field. We also find that the functional form of the equation of state of the scalar field asymptotically approaches that of the minimally coupled thawing quintessence.

arXiv: 16 Feb 2010

Decoherence of scalar cosmological perturbations
Authors: Mariano Franco, Esteban Calzetta
arXiv:1002.2916v1
In this paper we analyze the possibility of detecting nontrivial quantum phenomena in observations of the temperature anisotropy of the cosmic background radiation (CBR), for example, if the Universe could be found in a coherent superposition of two states corresponding to different CBR temperature self-correlations. Such observations are sensitive to scalar primordial fluctuations but insensitive to tensor fluctuations, which are therefore converted into an environment for the former. Even for a free inflaton field minimally coupled to gravity, scalar-tensor interactions induce enough decoherence among histories of the scalar fluctuations as to render them classical under any realistic probe of their amplitudes.

A Geometrical Approach to Strong Gravitational Lensing in f(R) Gravity
Authors: Anne Marie Nzioki, Peter K. S. Dunsby, Rituparno Goswami, Sante Carloni
arXiv:1002.2056v1
We present a framework for the study of lensing in spherically symmetric spacetimes within the context of f(R) gravity. Equations for the propagation of null geodesics, together with an expression for the bending angle are derived for any f(R) theory and then applied to an exact spherically symmetric solution of R^n gravity. We find that for this case more bending is expected for R^n gravity theories in comparison to GR and is dependent on the value of n and the value of distance of closest approach of the incident null geodesic.

A new framework for studying spherically symmetric static solutions in f(R) gravity
Authors: Anne Marie Nzioki, Sante Carloni, Rituparno Goswami, Peter K.S. Dunsby
arXiv:0908.3333v2
We develop a new covariant formalism to treat spherically symmetric spacetimes in metric} f(R) theories of gravity. Using this formalism we derive the general equations for a static and spherically symmetric metric in a general f(R)-gravity. These equations are used to determine the conditions for which the Schwarzschild metric is the only vacuum solution with vanishing Ricci scalar. We also show that our general framework provides a clear way of showing that the Schwarzschild solution is not a unique static spherically symmetric solution, providing some incite on how the current form of Birkhoff's theorem breaks down for these theories.

Interacting holographic tachyon model of dark energy
Authors: Alberto Rozas-Fernández, David Brizuela, Norman Cruz
arXiv:1002.2929v1
We propose a holographic tachyon model of dark energy with interaction between the components of the dark sector. The correspondence between the tachyon field and the holographic dark energy densities allows the reconstruction of the potential and the dynamics of the tachyon scalar field in a flat Friedmann-Robertson-Walker universe. We show that this model can describe the observed accelerated expansion of our universe with a parameter space given by the most recent observational results.

Testing the Newton second law in the regime of small accelerations
Authors: V. A. De Lorenci, M. Faundez-Abans, J. P. Pereira
arXiv:1002.2766v1
It has been pointed out that the Newtonian second law can be tested in the very small acceleration regime by using the combined movement of the Earth and Sun around the Galactic center of mass. It has been shown that there are only two brief intervals during the year in which the experiment can be completed, which correspond to only two specific spots on the Earth surface. An alternative experimental setup is presented to allow the measurement to be made on Earth at any location and at any time.

Evolution of Dark Energy Perturbations in Scalar-Tensor Cosmologies
Authors: J. C. Bueno Sanchez, L. Perivolaropoulos
arXiv:1002.2042v2
We solve analytically and numerically the generalized Einstein equations in scalar-tensor cosmologies to obtain the evolution of dark energy and matter linear perturbations. We compare our results with the corresponding results for minimally coupled quintessence perturbations. Our results for natural (O(1)) values of parameters in the Lagrangian which lead to a background expansion similar to LCDM are summarized as follows: 1. Scalar-Tensor dark energy density perturbations are amplified by a factor of about 10^6 compared to minimally coupled quintessence perturbations on scales less than about 100 h^{-1} Mpc. On these scales dark energy perturbations constitute a fraction of about 10% compared to matter density perturbations. 2. Scalar-Tensor dark energy density perturbations are anti-correlated with matter linear perturbations on sub-Hubble scales. Thus clusters of galaxies are predicted to overlap with voids of dark energy. 3. This anti-correlation of matter with negative pressure perturbations induces a mild amplification of matter perturbations by about 10% on sub-Hubble scales. 4. The evolution of scalar field perturbations on sub-Hubble scales, is scale independent and therefore it corresponds to a vanishing effective speed of sound (c_{s\Phi}=0). It also involves large oscillations at early times induced by the amplified effective mass of the field. This amplification is due to the non-minimal coupling of the field to the Ricci curvature scalar and (thus) to matter. No such oscillations are present in minimally coupled quintessence perturbations which are suppressed on sub-Hubble scales (c_{s\Phi}=1). Implications of our results include modified predictions for cluster halo profiles observed properties. These profiles are known to be in some tension with the predictions of LCDM.

arXiv: 15 Feb 2010

Cardy-Verlinde formula in FRW Universe with inhomogeneous generalized fluid and dynamical entropy bounds near the future singularity
Authors: Iver Brevik, Shin'ichi Nojiri, Sergei D. Odintsov, Diego Sáez-Gómez
arXiv:1002.1942v1
We derive a Cardy-Verlinde-like formula which relates the entropy of the closed FRW universe to its energy, and Casimir energy, for a multicomponent coupled fluid. The generalized fluid obeys an inhomogeneous equation of state. A viscous dark fluid is included, and we include also modified gravity using its fluid representation. It is demonstrated how such an expression reduces to the standard Cardy-Verlinde formula corresponding to the 2d CFT entropy only in some special cases. The dynamical entropy bound for a closed FRW universe with dark components is obtained. The universality of the dynamical entropy bound near a future singularity (of all four types), as well as near the Big Bang singularity, is investigated. It is demonstrated that except from some special cases of Type II and Type IV singularities the dynamical entropy bound is violated near the singularity even if quantum effects are taken into account. The dynamical entropy bound seems to be universal for the case of a regular universe, including the asymptotic de Sitter one.

arXiv: 12 Feb 2010

How to optimally parametrize deviations from General Relativity in the evolution of cosmological perturbations?
Authors: Levon Pogosian, Alessandra Silvestri, Kazuya Koyama, Gong-Bo Zhao
arXiv:1002.2382v1
The next generation of weak lensing surveys will trace the growth of large scale perturbations through a sequence of epochs, offering an opportunity to test General Relativity (GR) on cosmological scales. We review in detail the parametrization used in MGCAMB to describe the modified growth expected in alternative theories of gravity and generalized dark energy models. We highlight its advantages and examine several theoretical aspects. In particular, we show that the same set of equations can be consistently used on super-horizon and sub-horizon linear scales. We also emphasize the sensitivity of data to scale-dependent features in the growth pattern, and propose using Principal Component Analysis to converge on a practical set of parameters which is most likely to detect departures from GR. The connection with other parametrizations is also discussed.

How close can an Inhomogeneous Universe mimic the Concordance Model?
Authors: Peter Dunsby, Naureen Goheer, Bob Osano, Jean-Philippe Uzan
arXiv:1002.2397v1
Recently, spatially inhomogeneous cosmological models have been proposed as an alternative to the LCDM model, with the aim of reproducing the late time dynamics of the Universe without introducing a cosmological constant or dark energy. This paper investigates the possibility of distinguishing such models from the standard LCDM using background or large scale structure data. It also illustrates and emphasizes the necessity of testing the Copernican principle in order to confront the tests of general relativity with the large scale structure.

arXiv: 11 Feb 2010

A Computational Anthropic Principle: Where is the Hardest Problem in the Multiverse?
Authors: Navin Sivanandam
arXiv:1002.2178v1
The anthropic principle is an inevitable constraint on the space of possible theories. As such it is central to determining the limits of physics. In particular, we contend that what is ultimately possible in physics is determined by restrictions on the computational capacity of the universe, and that observers are more likely to be found where more complicated calculations are possible. Our discussion covers the inevitability of theoretical bias and how anthropics and computation can be an aid to imposing these biases on the theory landscape in a systematic way. Further, we argue for (as far as possible) top-down rather than bottom-up anthropic measures, contending that that the latter can often be misleading. We begin the construction of an explicit computational measure by examining the effect of the cosmological constant on computational bounds in a given universe, drawing from previous work on using entropy production as a proxy for observers by Bousso, Harnik, Kribs and Perez. In addition, we highlight a few of the additional computational considerations that may be used to extend such a measure.

A Geometrical Approach to Strong Gravitational Lensing in f(R) Gravity
Authors: Anne Marie Nzioki, Peter K. S. Dunsby, Rituparno Goswami, Sante Carloni
arXiv:1002.2056v1
We present a framework for the study of lensing in spherically symmetric spacetimes within the context of f(R) gravity. Equations for the propagation of null geodesics, together with an expression for the bending angle are derived for any f(R) theory and then applied to an exact spherically symmetric solution of R^n gravity. We find that for this case more bending is expected for R^n gravity theories in comparison to GR and is dependent on the value of n and the value of distance of closest approach of the incident null geodesic

Testing General Relativity with Current Cosmological Data
Authors: Scott F. Daniel, Eric V. Linder, Tristan L. Smith, Robert R. Caldwell, Asantha Cooray, Alexie Leauthaud, Lucas Lombriser
arXiv:1002.1962v1
Deviations from general relativity, such as could be responsible for the cosmic acceleration, would influence the growth of large scale structure and the deflection of light by that structure. We clarify the relations between several different model independent approaches to deviations from general relativity appearing in the literature, devising a translation table. We examine current constraints on such deviations, using weak gravitational lensing data of the CFHTLS and COSMOS surveys, cosmic microwave background radiation data of WMAP5, and supernova distance data of Union2. Markov Chain Monte Carlo likelihood analysis of the parameters over various redshift ranges yields consistency with general relativity at the 95% confidence level.

Drake Equation for the Multiverse: From the String Landscape to Complex Life
Authors: Marcelo Gleiser
arXiv:1002.1651v1
It is argued that selection criteria usually referred to as "anthropic conditions" for the existence of intelligent (typical) observers widely adopted in cosmology amount only to preconditions for primitive life. The existence of life does not imply in the existence of intelligent life. On the contrary, the transition from single-celled to complex, multi-cellular organisms is far from trivial, requiring stringent additional conditions on planetary platforms. An attempt is made to disentangle the necessary steps leading from a selection of universes out of a hypothetical multiverse to the existence of life and of complex life. It is suggested that what is currently called the "anthropic principle" should instead be named the "prebiotic principle."

arXiv: 10 Feb 2010

The dynamical equivalence of modified gravity revisited
Authors: Ippocratis D. Saltas, Mark Hindmarsh

arXiv:1002.1710v1

We revisit the dynamical equivalence between different representations of f(R) and f(G) gravity in vacuum, where G is the Gauss-Bonnet term, taking into account the relevant Gibbons-Hawking (GH) terms. As our basic tool we use the approach of the Helmholtz Lagrangian, as applied by Magnano et al, which is similar to the ordinary Hamiltonian formalism. In the R+f(G) case, starting from its scalar-tensor representation where the Gauss-Bonnet term is non-minimally coupled to a scalar field, application of a conformal transformation reveals that the former decouples from the latter only for dimensions of five or greater, however leading to a non-minimally coupled, fourth-order theory for the metric and the scalar field, in contrast to the case of f(R). We then re-express the same theory in a dynamically equivalent frame with four tensor fields and one scalar, with only two of the fields though being independent. The evolution of the two independent fields is described by a system of two coupled, second-order equations of motion. In the case of f(R) we revisit the equivalence of the full action to the Einstein-Hilbert one in an explicit way, using the conformal transformation technique as well as another, more fundamental one based on the use of the Helmholtz Lagrangian. We work at the action level, keeping track of all boundary contributions and derive the appropriate GH terms for each representation.

Evidence for a dark matter particle
Authors: Yukio Tomozawa

arXiv:1002.1938v1

A prediction and observational evidence for the mass of a dark matter particle are presented..

Primordial density perturbations with running spectral index: impact on non-linear cosmic structures
Authors: C. Fedeli, F. Finelli, L. Moscardini

arXiv:1002.1882v1

We explore the statistical properties of non-linear cosmic structures in a flat $\Lambda$CDM cosmology in which the index of the primordial power spectrum for scalar perturbations is allowed to depend on the scale. Within the inflationary paradigm, the running of the scalar spectral index can be related to the properties of the inflaton potential, and it is hence of critical importance to test it with all kinds of observations, which cover the linear and non-linear regime of gravitational instability. We focus on the amount of running $\alpha_{\mathrm{S},0}$ allowed by an updated combination of CMB anisotropy data and the 2dF Galaxy Redshift Survey. Our analysis constrains $\alpha_{\mathrm{S},0} = -0.051^{+0.047}_{-0.053}$ $(-0.034^{+0.039}_{-0.040})$ at 95% Confidence Level when (not) taking into account primordial gravitational waves in a ratio as predicted by canonical single field inflation, in agreement with other works. For the cosmological models best fitting the data both with and without running we studied the abundance of galaxy clusters and of rare objects, the halo bias, the concentration of dark matter halos, the Baryon Acoustic Oscillation, the power spectrum of cosmic shear, and the Integrated Sachs-Wolfe effect. We find that counting galaxy clusters in future X-ray and Sunyaev-Zel'dovich surveys could discriminate between the two models, more so if broad redshift information about the cluster samples will be available. Likewise, measurements of the power spectrum of cosmological weak lensing as performed by planned all-sky optical surveys such as EUCLID could detect a running of the primordial spectral index, provided the uncertainties

The Big Bang, Modern Cosmology and the Fate of the Universe: Impacts upon Culture
Authors: Lawrence M. Krauss (Arizona State University)

arXiv:1002.1952v2

Cosmological discoveries over the past century have completely changed our picture of our place in the universe. New observations have a realistic chance of probing nature on heretofore unimaginable scales, and as a result are changing the nature of fundamental science. Perhaps no other domain of science has an equal capacity to completely change our perspective of the world in which we live.

arXiv: 9 Feb 2010

Vector field models of modified gravity and the dark sector
Authors: J. Zuntz, T. G. Zlosnik, F. Bourliot, P. G. Ferreira, G. D. Starkman
arXiv:1002.0849v1
We present a comprehensive investigation of cosmological constraints on the class of vector field formulations of modified gravity called Generalized Einstein-Aether models. Using linear perturbation theory we generate cosmic microwave background and large-scale structure spectra for general parameters of the theory, and then constrain them in various ways. We investigate two parameter regimes: a dark-matter candidate where the vector field sources structure formation, and a dark-energy candidate where it causes late-time acceleration. We find that the dark matter candidate does not fit the data, and identify five physical problems that can restrict this and other theories of dark matter. The dark energy candidate does fit the data, and we constrain its fundamental parameters; most notably we find that the theory's kinetic index parameter $n_{\mathrm{ae}}$ can differ significantly from its $\Lambda$CDM value.



Comparing scalar-tensor gravity and f(R)-gravity in the Newtonian limit
Authors: S. Capozziello, A. Stabile, A. Troisi
arXiv:1002.1364v1

Recently, a strong debate has been pursued about the Newtonian limit (i.e. small velocity and weak field) of fourth order gravity models. According to some authors, the Newtonian limit of $f(R)$-gravity is equivalent to the one of Brans-Dicke gravity with $\omega_{BD} = 0$, so that the PPN parameters of these models turn out to be ill defined. In this paper, we carefully discuss this point considering that fourth order gravity models are dynamically equivalent to the O'Hanlon Lagrangian. This is a special case of scalar-tensor gravity characterized only by self-interaction potential and that, in the Newtonian limit, this implies a non-standard behavior that cannot be compared with the usual PPN limit of General Relativity. The result turns out to be completely different from the one of Brans-Dicke theory and in particular suggests that it is misleading to consider the PPN parameters of this theory with $\omega_{BD} = 0$ in order to characterize the homologous quantities of $f(R)$-gravity. Finally the solutions at Newtonian level, obtained in the Jordan frame for a $f(R)$-gravity, reinterpreted as a scalar-tensor theory, are linked to those in the Einstein frame.



Nonlocal Modification of Newtonian Gravity
Authors: Hans-Joachim Blome, Carmen Chicone, Friedrich W. Hehl, Bahram Mashhoon
arXiv:1002.1425v1
The Newtonian regime of a recent nonlocal extension of general relativity (GR) is investigated. Nonlocality is introduced via a scalar "constitutive" kernel in a special case of the translational gauge theory of gravitation, namely, the teleparallel equivalent of GR. In this theory, the nonlocal aspect of gravity simulates dark matter. A nonlocal and nonlinear generalization of Poisson's equation of Newtonian gravitation is presented. The implications of nonlocality for the gravitational physics in the solar system are briefly studied.



Interacting holographic dark energy with entropy corrections
Authors: Mubasher Jamil, M. Umar Farooq
arXiv:1002.1434v1
The holographic dark energy (HDE) is considered to be the most promising candidate of dark energy. Its definition is originally motivated from the entropy-area relation which depends on the theory of gravity under consideration. Recently a new definition of HDE is proposed with the help of quantum corrections to the entropy-area relation in the setup of loop quantum cosmology. Using this new definition, we investigate the model of interacting dark energy and derive its effective equation of state. Finally we establish a correspondence between generalized Chaplygin gas and entropy-corrected holographic dark energy.

Cross-correlation of the Lyman-\alpha Forest and HI 21 cm: A Probe of Cosmology
Authors: Tapomoy Guha Sarkar, Somnath Bharadwaj, Tirthankar Roy Choudhury, Kanan Datta arXiv:1002.1368v1 We present the cross-correlation of the redshifted 21-cm emission from neutral hydrogen (HI) in the post-reionization era with the Lyman-\alpha optical depth as a new probe of the large scale matter distribution in the redshift range z=2 to 3. Though the two signals originate from different astrophysical systems, they are both expected to trace the underlying dark matter distribution on large scales. The angular cross-correlation power spectrum estimator is found to be unaffected by the discrete quasar sampling, which only affects the noise in the estimate. Considering a quasar survey with high angular density, like the upcoming BOSS, we find that it will be possible to measure the cross-correlation at a higher level of precision than the 21-cm auto-correlation power spectrum using the same 21-cm observations. Further, the foregrounds and systematics in the 21-cm data and the Lyman-\alpha optical depth are expected to be uncorrelated, and hence these problems are expected to be considerably less severe in the cross-correlation as compared to the auto-correlations. The cross-correlation signal will be a new, independent probe of the astrophysics of the diffuse IGM, the growth of structure and the expansion history of the Universe.

Measuring the Speed of Dark: Detecting Dark Energy Perturbations
Authors: Roland de Putter, Dragan Huterer, Eric V. Linder arXiv:1002.1311v1 The nature of dark energy can be probed not only through its equation of state, but also through its microphysics, characterized by the sound speed of perturbations to the dark energy density and pressure. As the sound speed drops below the speed of light, dark energy inhomogeneities increase, affecting both CMB and matter power spectra. We show that current data can put no significant constraints on the value of the sound speed when dark energy is purely a recent phenomenon, but can begin to show more interesting results for early dark energy models. For example, the best fit model for current data has a slight preference for dynamics (w(a)\ne-1), degrees of freedom distinct from quintessence (c_s\ne1), and early presence of dark energy (Omega_ de(a<<1)\ne0).>

Constraints on variation in $\alpha$ and $m_e$ from WMAP 7-year data
Authors: Susana. J. Landau, Claudia G. Scóccola arXiv:1002.1603v1 We update the constraints on the time variation of the fine structure constant $\alpha$ and the electron mass $m_e$, using the latest CMB data, including the 7-yr release of WMAP. We made statistical analyses of the variation of each one of the constants and of their joint variation, together with the basic set of cosmological parameters. We used a modified version of CAMB and COSMOMC to account for these possible variations. We present bounds on the variation of the constants for different data sets, and show how results depend on them. When using the latest CMB data plus the power spectrum from Sloan Digital Sky Survey LRG, we find that $\alpha / \alpha_0=0.986 \pm 0.007$ at 1-$\sigma$ level, when the 6 basic cosmological parameters were fitted, and only variation in $\alpha$ was allowed. The constraints in the case of variation in both constants are $ \alpha / \alpha_0= 0.986 \pm 0.009$ and $m_e / m_{e0} = 0.999 \pm 0.035$. In the case of only variation in $m_e$, the bound is $m_e /m_{e0}=0.964 \pm 0.025$.

arXiv: 8 Feb 2010

Cosmological perturbations in a family of deformations of general relativity
Authors: Kirill Krasnov, Yuri Shtanov
arXiv:1002.1210v1
We study linear cosmological perturbations in a previously introduced family of deformations of general relativity characterized by the absence of new degrees of freedom. The homogeneous and isotropic background in this class of theories is unmodified and is described by the usual Friedmann equations. The theory of cosmological perturbations is modified and the relevant deformation parameter has the dimension of length. Gravitational perturbations of the scalar type can be described by a certain relativistic potential related to the matter perturbations just as in general relativity. A system of differential equations describing the evolution of this potential and of the stress-energy density perturbations is obtained. We find that the evolution of scalar perturbations proceeds with a modified effective time-dependent speed of sound, which, contrary to the case of general relativity, does not vanish even at the matter-dominated stage. In a broad range of values of the length parameter controlling the deformation, a specific transition from the regime of modified gravity to the regime of general relativity in the evolution of scalar perturbations takes place during the radiation domination. In this case, the resulting power spectrum of perturbations in radiation and dark matter is suppressed on the comoving spatial scales that enter the Hubble radius before this transition. We estimate the bounds on the deformation parameter for which this suppression does not lead to observable consequences. Evolution of scalar perturbations at the inflationary stage is modified but very slightly and the primordial spectrum generated during inflation is not noticeably different from the one obtained in general relativity.

Phantom boundary crossing and anomalous growth index of fluctuations in viable f(R) models of cosmic acceleration
Authors: Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
arXiv:1002.1141v1
Evolution of a background space-time metric and sub-horizon matter density perturbations in the Universe is numerically analyzed in viable $f(R)$ models of present dark energy and cosmic acceleration. It is found that viable models generically exhibit recent crossing of the phantom boundary $w_{\rm DE}=-1$. Furthermore, it is shown that, as a consequence of the anomalous growth of density perturbations during the end of the matter-dominated stage, their growth index evolves non-monotonically with time and may even become negative temporarily.

Large-scale inhomogeneities may improve the cosmic concordance of supernovae
Authors: Luca Amendola, Kimmo Kainulainen, Valerio Marra, Miguel Quartin
arXiv:1002.1232v1
We reanalyse the current data from the supernovae observations including the weak lensing effects caused by inhomogeneities. We compute the lensing probability distribution function for each background solution described by the parameters Omega_M, Omega_Lambda and w in the presence of inhomogeneities, which are designed to mimic the observed large-scale structures. We then perform a likelihood analysis in the space of FLRW-parameters and compare our results with the standard approach. We find that the inclusion of lensing can move the best-fit model significantly towards the cosmic concordance of the flat LCDM model, improving the agreement with the constraints coming from the cosmic microwave background and the baryon acoustic oscillations.

arXiv: 5 Feb 2010

Beyond Special and General Relativity. Neutrinos and Cosmology
Authors: Javier Indurain
arXiv:1002.1027v1
The widely accepted description of the present cosmic acceleration and the neutrino oscillations in terms of a cosmological constant and neutrino masses, respectively, seems conceptually not very satisfactory. From a phenomenological perspective, the possibility that those phenomena are instead the first windows to new physics at very low energies is explored.

arXiv: 4 Feb 2010

Variational principles in modified gravity: Field equations in f(R) gravity
Authors: Alejandro Guarnizo, Leonardo Castaneda, Juan M. Tejeiro
arXiv:1002.0617v1
We consider a detailed review of the variational principles and field equations in the metric formalism of f(R) gravity. First, we give a brief review of the Einstein-Hilbert action, including the Gibbons-York-Hawking boundary term given the lack of discussion in common text in the literature. Next we present in some detail the field equations in f(R) gravity, including the discussion about boundaries, and we compare with the Gibbons-York-Hawking term in General relativity. Here we discuss an additional boundary term similar to a Gibbons-York-Hawking term in General Relativity. The main goal of this paper is to get in a straightforward form the field equations in f(R) theory using elementary variational principles instead of a scalar-tensor approach.

arXiv: 3 Feb 2010

The string wave function across a Kasner singularity
Authors: Edmund J. Copeland, Gustavo Niz, Neil Turok
arXiv:1001.5291v1
A collision of orbifold planes in eleven dimensions has been proposed as an explanation of the hot big bang. When the two planes are close to each other, the winding membranes become the lightest modes of the theory, and can be effectively described in terms of fundamental strings in a ten dimensional background. Near the brane collision, the eleven-dimensional metric is an Euclidean space times a 1+1-dimensional Milne universe. However, one may expect small perturbations to lead into a more general Kasner background. In this paper we extend the previous classical analysis of winding membranes to Kasner backgrounds, and using the Hamiltonian equations, solve for the wave function of loops with circular symmetry. The evolution across the singularity is regular, and explained in terms of the excitement of higher oscillation modes. We also show there is finite particle production and unitarity is preserved.

Angular size test on the expansion of the Universe
Authors: Martin Lopez-Corredoira
arXiv:1002.0525v1
Assuming the standard cosmological model as correct, the average linear size of galaxies with the same luminosity is six times smaller at z=3.2 than at z=0, and their average angular size for a given luminosity is approximately proportional to 1/z. Neither the hypothesis that galaxies which formed earlier have much higher densities nor their luminosity evolution, mergers ratio, or massive outflows due to a quasar feedback mechanism are enough to justify such a strong size evolution. Also, at high redshift, the intrinsic ultraviolet surface brightness would be prohibitively high with this evolution, and the velocity dispersion much higher than observed. We explore here another possibility to overcome this problem by considering different cosmological scenarios that might make the observed angular sizes compatible with a weaker evolution. One of the models explored, a very simple phenomenological extrapolation of the linear Hubble law in a Euclidean static universe, fits the angular size vs. redshift dependence quite well, which is also approximately proportional to 1/z with this cosmological model. There are no free parameters derived ad hoc, although the error bars allow a slight size/luminosity evolution. The type Ia supernovae Hubble diagram can also be explained in terms of this model with no ad hoc fitted parameter. WARNING: I do not argue here that the true Universe is static. My intention is just to discuss which theoretical models provide a better fit to the data of observational cosmology.

Phantom behaviour and growth index anomalous evolution in viable f(R) gravity models
Authors: Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
arXiv:1002.0462v1
We present numerical calculation of the evolution of a background space-time metric and sub-horizon matter density perturbations in viable $f(R)$ gravity models of present dark energy and cosmic acceleration. We found that viable models generically exhibit recent crossing of the phantom boundary $w_{DE}=-1$. Moreover, as a consequence of the anomalous growth of density perturbations during the end of the matter-dominated stage, their growth index evolves non-monotonically with time and may even become negative.

Tachyonic Quintessence and a Preferred Direction in the Sky
Authors: Luca Grisa, Lorenzo Sorbo
arXiv:1002.0510v1
We show that quintessence, when it is described by a tachyonic field, can amplify a tiny primordial gradient generating a preferred direction in the sky. In its simplest realization, this mechanism only affects the Cosmic Microwave Background fluctuations at the quadrupole level. We briefly discuss how higher multipoles can also be affected, once the full structure of the quintessence potential is taken into account.

Exoplanetary Microlensing
Authors: B. Scott Gaudi (Ohio State University)
arXiv:1002.0332v2
Gravitational microlensing occurs when a foreground star happens to pass very close to our line of sight to a more distant background star. The foreground star acts as a lens, splitting the light from the source star into two images, which are typically unresolved. However, these images are also magnified, by an amount that depends on the angular lens-source separation. The relative lens-source motion results in a time-variable source magnification: a microlensing event. If the foreground star happens to host a planet with projected separation near the paths of these images, the planet will further perturb the images, resulting in a characteristic, short-lived signature of the planet. This chapter provides an introduction to the discovery and characterization of exoplanets with gravitational microlensing. The theoretical foundation of the method is reviewed, focusing on the phenomenology of planetary perturbations. The strengths and weaknesses of the microlensing technique are discussed, highlighting the fact that it is sensitive to low-mass planets with separations just beyond the snow line, orbiting stars located throughout the Galactic disk and foreground bulge. An overview of the practice of microlensing planet searches is given, with a discussion of some of the challenges with detecting and analyzing planetary perturbations. The chapter concludes with a review of the results that have been obtained to date, and a discussion of the near and long-term prospects for microlensing planet surveys. Ultimately, microlensing is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury, as well as to free floating planets, and will provide a crucial test of planet formation theories by determining the demographics of planets throughout the Galaxy.

arXiv: 2 Feb 2010

Projectable Horava-Lifshitz gravity in a nutshell
Authors: Silke Weinfurtner, Thomas P. Sotiriou, Matt Visser
arXiv:1002.0308v1
Approximately one year ago Horava proposed a power-counting renormalizable theory of gravity which abandons local Lorentz invariance. The proposal has been received with growing interest and resulted in various different versions of Horava-Lifshitz gravity theories, involving a colourful potpourri of new terminology. In this proceedings contribution we first motivate and briefly overview the various different approaches, clarifying their differences and similarities. We then focus on a model referred to as projectable Horava-Lifshitz gravity and summarize the key results regarding its viability.


Cosmological thinking: cultural heritage and challenge
Authors: Orfeu Bertolami
arXiv:1001.4196v1
The limitations of current technology do not allow one to foresee the expansion of the humankind beyond our planet for at least a few decades. Furthermore, the laws of physics, as for as they are known, preclude any form of traveling beyond the speed of light, as well as any viable and stable space-time shortcuts (wormholes, warp-drives, etc) that would facilitate cosmic traveling. Given the vastness of the Universe these are insurmountable obstacles for any {\it in situ} exploration of the cosmos beyond our most immediate cosmic neighbourhood. Nevertheless, the Universe is transparent and contains countless sources of visible light. Actually, in the last decades, technological developments have made possible to observe the cosmos throughout most of the electromagnetic spectrum as well as to perform dynamical studies that allow perceiving the presence of invisible components such as black holes, dark matter and dark energy. In this respect, humankind has then been given the opportunity to unravel the inner workings of the cosmos and through this process be part of the cosmic habitat. In this contribution various forms of cosmological thinking will be discussed, from some myths of creation till some of the latest scientific discoveries.



Radial asymptotics of Lemaitre-Tolman-Bondi dust models
Authors: Roberto A Sussman
arXiv:1002.0173v1
We examine the radial asymptotic behavior of spherically symmetric Lemaitre-Tolman-Bondi dust models by looking at their covariant scalars along radial rays, which are spacelike geodesics parametrized by proper length $\ell$, orthogonal to the 4-velocity and to the orbits of SO(3). By introducing quasi-local scalars defined as integral functions along the rays, we obtain a complete and covariant representation of the models, leading to an initial value parametrization in which all scalars can be given by scaling laws depending on two metric scale factors and two basic initial value functions. Considering regular "open" LTB models whose space slices allow for a diverging $\ell$, we provide the conditions on the radial coordinate so that its asymptotic limit corresponds to the limit as $\ell\to\infty$. The "asymptotic state" is then defined as this limit, together with asymptotic series expansion around it, evaluated for all metric functions, covariant scalars (local and quasi-local) and their fluctuations. By looking at different sets of initial conditions, we examine and classify the asymptotic states of parabolic, hyperbolic and open elliptic models admitting a symmetry center. We show that in the radial direction the models can be asymptotic to any one of the following spacetimes: FLRW dust cosmologies with zero or negative spatial curvature, sections of Minkowski flat space (including Milne's space), sections of the Schwarzschild--Kruskal manifold or self--similar dust solutions.



Voids as a Precision Probe of Dark Energy
Authors: Rahul Biswas, Esfandiar Alizadeh, Benjamin D. Wandelt
arXiv:1002.0014v1
A signature of the dark energy equation of state may be observed in the shape of voids. We estimate the constraints on cosmological parameters that would be determined from the ellipticity distribution of voids from future spectroscopic surveys already planned for the study of large scale structure. The constraints stem from the sensitivity of the distribution of ellipticity to the cosmological parameters through the variance of fluctuations of the density field smoothed at some length scale. This length scale can be chosen to be of the order of the comoving radii of voids at very early times when the fluctuations are Gaussian distributed. We use Fisher estimates to show that the constraints from void ellipticities are promising. Combining these constraints with other traditional methods results in the improvement of the Dark Energy Task Force Figure of Merit on the dark energy parameters by an order of hundred for future experiments. The estimates of these future constraints depend on a number of systematic issues which require further study using simulations. We outline these issues and study the impact of certain observational and theoretical systematics on the forecasted constraints on dark energy parameters.

arXiv: 1 Feb 2010

The matter Lagrangian and the energy-momentum tensor in modified gravity with non-minimal coupling between matter and geometry
Authors: T. Harko
arXiv:1001.5349v1
We show that in modified $f(R)$ type gravity models with non-minimal coupling between matter and geometry, both the matter Lagrangian, and the energy-momentum tensor, are completely and uniquely determined by the form of the coupling. This result is obtained by using the variational formulation for the derivation of the equations of motion in the modified gravity models with geometry-matter coupling, and the Newtonian limit for a fluid obeying a barotropic equation of state. The corresponding energy-momentum tensor of the matter in modified gravity models with non-minimal coupling is more general than the usual general-relativistic energy-momentum tensor for perfect fluids, and it contains a supplementary, equation of state dependent term, which could be related to the elastic stresses in the body, or to other forms of internal energy. Therefore, the extra-force induced by the coupling between matter and geometry never vanishes as a consequence of the thermodynamic properties of the system, or for a specific choice of the matter Lagrangian, and it is non-zero in the case of a fluid of dust particles.

The Birth Environment of the Solar System
Authors: Fred C. Adams
arXiv:1001.5444v2
This paper reviews our current understanding of the possible birth environments of our Solar System. Since most stars form within groups and clusters, the question becomes one of determining the nature of the birth aggregate of the Sun. This discussion starts by reviewing Solar System properties that provide constraints on our environmental history. We then outline the range of star-forming environments that are available in the Galaxy, and discuss how they affect star and planet formation. The nature of the solar birth cluster is constrained by many physical considerations, including radiation fields provided by the background environment, dynamical scattering interactions, and by the necessity of producing the short-lived radioactive nuclear species inferred from meteoritic measurements. Working scenarios for the solar birth aggregate can be constructed, as discussed herein, although significant uncertainties remain.