 Weekly meetings hosted by the "Gravitation Relativiste et Cosmologie" (GreCO) group at IAP Every Monday at 1.30pm To suggest a paper please send a message to bertone.AT.iap.fr Monday, 2 June 2008 @ 1.30pm, ** Salle du Conseil ** Weak Cosmic Censorship: As Strong as Ever(Submitted on 26 May 2008)
Abstract: Spacetime singularities that arise in gravitational collapse are always hidden inside of black holes. This is the essence of the weak cosmic censorship conjecture. The hypothesis, put forward by Penrose 40 years ago, is still one of the most important open questions in general relativity. In this Letter, we reanalyze extreme situations which have been considered as counterexamples to the weak cosmic censorship conjecture. In particular, we consider the absorption of scalar particles with large angular momentum by a black hole. Ignoring back reaction effects may lead one to conclude that the incident wave may overspin the black hole, thereby exposing its inner singularity to distant observers. However, we show that when back reaction effects are properly taken into account, the stability of the black-hole event horizon is irrefutable. We therefore conclude that cosmic censorship is actually respected in this type of gedanken experiments. The Dark Matter Annihilation Signal from Galactic Substructure: Predictions for GLAST(Submitted on 29 May 2008)
Abstract: We present quantitative predictions for the detectability of individual Galactic dark matter subhalos in gamma-rays from dark matter pair annihilations in their centers. Our method is based on a hybrid approach, employing the highest resolution numerical simulations available (including the recently completed one billion particle Via Lactea II simulation) as well as analytical models for the extrapolation beyond the simulations' resolution limit. We include a self-consistent treatment of subhalo boost factors, motivated by our numerical results, and a realistic treatment of the expected backgrounds that individual subhalos must outshine. We show that for reasonable values of the dark matter particle physics parameters (M_X ~ 50 - 500 GeV and <sigma*v> ~ 10^-26 - 10^-25 cm^3/s) GLAST may very well discover a few, even up to several dozen, such subhalos, at 5 sigma significance, and some at more than 20 sigma. We predict that the majority of luminous sources would be resolved with GLAST's expected angular resolution. For most observer locations the angular distribution of detectable subhalos is consistent with a uniform distribution across the sky. The brightest subhalos tend to be massive (median Vmax of 24 km/s) and therefore likely hosts of dwarf galaxies, but many subhalos with Vmax as low as 5 km/s are also visible. Typically detectable subhalos are 20 - 40 kpc from the observer, and only a small fraction are closer than 10 kpc. The total number of observable subhalos has not yet converged in our simulations, and we estimate that we may be missing up to 3/4 of all detectable subhalos. Testing Alternative Theories of Dark Matter with the CMB(Submitted on 28 May 2008)
Abstract: We propose a method to study and constrain modified gravity theories for dark matter using CMB temperature anisotropies and polarization. We assume that the theories considered here have already passed the matter power-spectrum test of large-scale structure. With this requirement met, we show that a modified gravity theory can be specified by parametrizing the time evolution of its dark-matter density contrast, which is completely controlled by the dark matter stress history. We calculate how the stress history with a given parametrization affects the CMB observables, and a qualitative discussion of the physical effects involved is supplemented with numerical examples. It is found that, in general, alternative gravity theories can be efficiently constrained by the CMB temperature and polarization spectra. There exist, however, special cases where modified gravity cannot be distinguished from the CDM model even by using both CMB and matter power spectrum observations, nor can they be efficiently restricted by other observables in perturbed cosmologies. Our results show how the stress properties of dark matter, which determine the evolutions of both density perturbations and the gravitational potential, can be effectively investigated using just the general conservation equations and without assuming any specific theoretical gravitational theory within a wide class. Vector Field Models of Inflation and Dark Energy(Submitted on 27 May 2008)
Abstract: Vector field models of inflation and dark energy are investigated. We consider nonminimal couplings of the vectors both in the matter and in the gravity sector. The cosmological equations, allowing anisotropic background expansion, are then studied as a dynamical system. We show the general conditions for the existence of scaling solutions for spatial fields. A vector with an inverse power-law potential, even if minimally coupled, scales with the matter component. For time-like fields in an isotropic background, we present a reconstruction method to deduce the form of the vector potential and couplings from a given expansion history. In particular, we reconstruct a vector-Gauss-Bonnet model which generates the concordance model acceleration at late times and supports an inflationary epoch at high curvatures. Thus, several classes of viable vector alternatives to the inflaton and quintessence scalar fields are found. These might help to make contact between the observed anomalies in the CMB and large scale structure and fundamental theories exhibiting vector fields. Predicting the cosmological constant with the scale-factor cutoff measure(Submitted on 15 May 2008)
Abstract: It is well known that anthropic selection from a landscape with a flat prior distribution of cosmological constant Lambda gives a reasonable fit to observation. However, a realistic model of the multiverse has a physical volume that diverges with time, and the predicted distribution of Lambda depends on how the spacetime volume is regulated. We study a simple model of the multiverse with probabilities regulated by a scale-factor cutoff, and calculate the resulting distribution, considering both positive and negative values of Lambda. The results are in good agreement with observation. In particular, the scale-factor cutoff strongly suppresses the probability for values of Lambda that are more than about ten times the observed value. We also discuss several qualitative features of the scale-factor cutoff, including aspects of the distributions of the curvature parameter Omega and the primordial density contrast Q. Dark matter annihilation effects on the first stars(Submitted on 26 May 2008)
Abstract: We study the effects of WIMP dark matter (DM) on the collapse and evolution of the first stars in the Universe. Using a stellar evolution code, we follow the pre-Main Sequence (MS) phase of a grid of metal-free stars with masses in the range 5-600 solar mass forming in the centre of a 1e6 solar mass halo at redhisft z=20. DM particles of the parent halo are accreted in the proto-stellar interior by adiabatic contraction and scattering/capture processes, reaching central densities of order 1e12 GeV/cm3 at radii of the order of the AU: energy release from annihilation reactions can effectively counteract the gravitational collapse. This induces a transient stalling phase (i.e. a "dark" star) lasting from 2.1e3 yr (M=600 solar mass) to 1.8e4 yr (M=9 solar mass). Later in the evolution, DM scattering/capture rate becomes high enough that energy deposition from annihilations significantly alters the pre-MS evolution of the star in a way that depends on DM (i) velocity dispersion, (ii) density, (iii) elastic scattering cross section with baryons. For our fiducial set of parameters (10 km/s, 1e11 GeV/cm3, 1e-38 cm2) we find that the evolution of stars of mass lower than 40 solar masses "freezes" on the HR diagram before reaching the ZAMS. Stars with bigger masses manage to ignite nuclear reactions; however, DM "burning" prolonges their lifetimes by a factor 2 (5) for a 600 (40) solar mass star. An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect(Submitted on 25 May 2008)
Abstract: We measure hot and cold spots on the microwave background associated with supercluster and supervoid structures identified in the Sloan Digital Sky Survey Luminous Red Galaxy catalog. The mean temperature deviation is 9.6 +/- 2.2 microK. We interpret this as a detection of the late-time Integrated Sachs-Wolfe (ISW) effect, in which cosmic acceleration from dark energy causes gravitational potentials to decay, heating or cooling photons passing through density crests or troughs. In a flat universe, the linear ISW effect is a direct signal of dark energy. The statistical significance of our detection is over 4 sigma, making it the clearest detection to date using a single galaxy dataset. Moreover, our method produces a compelling visual image of the effect. A Tree Theorem for Inflation(Submitted on 24 May 2008)
Abstract: It is shown that the generating function for tree graphs in the ``in-in'' formalism may be calculated by solving the classical equations of motion subject to certain constraints. This theorem is illustrated by application to the evolution of a single inflaton field in a Robertson--Walker background. Constraints on local primordial non-Gaussianity from large scale structure(Submitted on 23 May 2008)
Abstract: Recent work has shown that the local non-Gaussianity parameter f_nl induces a scale-dependent large scale structure bias, whose amplitude is growing with scale. Here we first rederive this result within the context of peak-background split formalism and show that it only depends on the assumption of universality of mass function, assuming halo bias only depends on mass. We then use extended Press-Schechter formalism to argue that this assumption may be violated and the scale dependent bias will depend on other properties, such as merging history of halos. In particular, in the limit of recent mergers we find the effect is suppressed. Next we use these predictions in conjunction with a compendium of large scale data to put a limit on the value of $\fnl$. When combining all data assuming that halo occupation depends only on halo mass, we get a limit of -29(-57)<f_nl<+69(+89) at 95% (99.7%) confidence. While we use a wide range of datasets, our combined result is dominated by the signal from the SDSS luminous red galaxy and photometric quasar samples. If the latter are modelled as recent mergers then the limits become -29(-85)<f_nl<+70(+90). These limits are comparable to the strongest current limits from the WMAP 5-year analysis, with no evidence of a positive signal in $\fnl$. The combination of our measurement with the WMAP $\fnl$ value gives -1(-23)<f_nl<+70(+86). While the method needs to be thoroughly tested against large scale structure simulations with realistic quasar and galaxy formation models, our results indicate that this is a competitive method relative to CMB and should be further pursued both observationally and theoretically. Searching for Dark Matter with X-ray Observations of Local Dwarf Galaxies(Submitted on 7 May 2008)
Abstract: A generic feature of weakly interacting massive particle (WIMP) dark matter models is the emission of photons over a broad energy band resulting from the stable yields of dark matter pair annihilation. Inverse Compton scattering off cosmic microwave background photons of energetic electrons and positrons produced in dark matter annihilation is expected to produce significant diffuse X-ray emission. Dwarf galaxies are ideal targets for this type of dark matter search technique, being nearby, dark matter dominated systems free of any astrophysical diffuse X-ray background. In this paper, we present the first systematic study of X-ray observations of local dwarf galaxies aimed at the search for WIMP dark matter. We outline the optimal energy and angular ranges for current telescopes, and analyze the systematic uncertainties connected to electron/positron diffusion. We do not observe any significant X-ray excess, and translate this null result into limits on the mass and pair annihilation cross section for particle dark matter. Our results indicate that X-ray observations of dwarf galaxies currently constrain dark matter models at the same level or even more strongly than gamma-ray observations of the same systems. The limits we find constrain portions of the supersymmetric parameter space, particularly if the effect of dark matter substructures is included. Finally, we comment on the role of future X-ray satellites (e.g. Constellation-X, XEUS) and on their complementarity with GLAST and other gamma-ray telescopes in the quest for particle dark matter. CMB and 21-cm Signals for Dark Matter with a Long-Lived Excited State(Submitted on 23 May 2008)
Abstract: Motivated by the eXciting Dark Matter (XDM) model of Finkbeiner & Weiner, hypothesized to explain the 511 keV signal in the center of the Milky Way, we consider the CMB and 21-cm signatures of models of dark matter with collisional long-lived excited states. We compute the relic excitation fraction from the early universe for a variety of assumptions about the collisional de-excitation cross-section and thermal decoupling. The relic excitation fraction can be as high as 1% for natural regions of parameter space, but could be orders of magnitude smaller. Since the lifetime of the excited state is naturally greater than 10^13s, we discuss the signatures of such relic excitation on cosmic microwave background (CMB) and high-z 21-cm observations. Such models have potentially richer astrophysical signals than the traditional WIMP annihilations and decays, and may have observable consequences for future generations of experiments. |