Munch: Monday, June 25, 2007

                               


 

 WHERE: 6TH FLOOR CONFERENCE ROOM
WHEN  : 12:30, MONDAY

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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 arXiv:0706.1775 [ps, pdf, other] : Title: Galaxy-CMB Cross-Correlation as a Probe of Alternative Models of Gravity
Authors: Fabian Schmidt, Michele Liguori, Scott Dodelson
Comments: 10 pages, 6 figures, submitted to Phys. Rev. D
Bekenstein's alternative to general relativity, TeVeS, reduces to Modified Newtonian Dynamics (MOND) in the galactic limit. On cosmological scales, the (potential well <-> overdensity) relationship is quite different than in standard general relativity. Here we investigate the possibility of cross-correlating galaxies with the cosmic microwave background (CMB) to probe this relationship. At redshifts of order 2, the sign of the CMB-galaxy correlation differs in TeVeS from that in general relativity. We show that this effect is detectable and hence can serve as a powerful discriminator of these two models of gravity.

arXiv:0706.1982 [ps, pdf, other]
Title: Limits on primordial power spectrum resolution: An inflationary flow analysis
Authors: Brian A. Powell, William H. Kinney
Comments: 10 pages, 4 figures. The flow code used in this paper is available at this http URL
We investigate the ability of current and future CMB data to reliably constrain the form of the primordial power spectrum generated during inflation. We attempt to identify more exotic power spectra that yield equally good fits to the data as simple power-law spectra. In order to test a wide variety of spectral shapes, we combine the flow formalism, which is a method of stochastic model generation, with a numerical integration of the mode equations of quantum fluctuations. This allows us to handle inflation models that yield spectra that are not well described by the standard spectral parameterization. Using the latest WMAP data-set, we find a high degree of variation in possible spectral shapes. In particular, we find strongly running spectra arising from fast-rolling inflaton fields providing equally good fits to the data as power-law spectra arising from slowly-rolling fields. Current data poorly constrains the spectrum on scales (k < 0.01 h{\rm Mpc}^{-1}), where the error due to cosmic variance is large. Among the statistically degenerate models, we identify spectra with strong running on these larger scales, but with reduced running at smaller scales. These models predict values for the tensor-to-scalar ratio, (r), that lie outside the 2-(\sigma) confidence interval obtained from SDSS+WMAP data for spectra that are parametrized as power-laws or spectra with constant running. By considering more generalized power spectra, we therefore open up regions of parameter space excluded for simpler models.

arXiv:0706.3203 [ps, pdf, other]
Title: Detecting Axion-Like Particles With Gamma Ray Telescopes
Authors: Dan Hooper, Pasquale D. Serpico
Comments: 4 pages, 3 eps figures
We propose that axion-like particles (ALPs) with a two-photon vertex, consistent with all astrophysical and laboratory bounds, may lead to a detectable signature in the spectra of high-energy gamma ray sources. This occurs as a result of gamma rays being converted into ALPs in the magnetic fields of efficient astrophysical accelerators according to the ``Hillas criterion", such as jets of active galactic nuclei or hot spots of radio galaxies. The discovery of such an effect is possible by GLAST in the 1-100 GeV range and by ground based gamma ray telescopes in the TeV range.

arXiv:0706.2864 [ps, pdf, other]
Title: Evidence for nearby universe structures in the ultra-high energy sky
Authors: A. Cuoco, G. Miele, P. D. Serpico
Comments: 8 pages, 7 figures
We compare the clustering properties of the combined dataset of ultra-high energy cosmic rays events, reported by the AGASA, HiRes, Yakutsk and Sugar collaborations, with a catalogue of galaxies of the local universe (redshift z<~0.06). We find that the data reproduce particularly well the clustering properties of the nearby universe within z <~0.02. There is no statistically significant cross-correlation between data and structures, although intriguingly the nominal cross-correlation chance probability for displacements within~50 deg. drops from ~50% to ~10% using the catalogue with a smaller horizon. Also, we discuss the impact on the robustness of the results of deflections in some galactic magnetic field models used in the literature. These results suggest a relevant role of magnetic fields (possibly extragalactic ones, too) and/or possibly some heavy nuclei fraction in the UHECRs. The importance of a confirmation of these hints by Auger data is emphasized.

arXiv:0706.2395 [ps, pdf, other]
Title: Combining Weak Lensing Tomography with Halo Clustering to Probe Dark Energy
Authors: Charles Shapiro (Chicago, KICP), Scott Dodelson (Chicago, FNAL)
Comments: 4 pages, 2 color figures
Two methods of constraining the properties of dark energy are weak lensing tomography (WLT) and cluster counting (CC). Uncertainties in mass calibration of clusters can be reduced by using the properties of halo clustering (HC). However, within a single survey, WLT and HC probe the same density fluctuations. We explore the question of whether this information can be used twice -- once in WLT and then again in HC to calibrate cluster masses -- or whether the combined constraints are weaker than the sum of the individual constraints. For a survey like the Dark Energy Survey (DES), we find that the WLT signals from source galaxies at high redshifts are indeed highly correlated with HC at lower redshifts. Surprisingly, this correlation does not degrade cosmological constraints for a DES-like survey, and in fact, constraints are marginally improved since the correlations themselves act as additional observables. This considerably simplifies the analysis for a DES-like survey: when treated as independent experiments, combined constraints from WLT and SCC are accurate if not slightly conservative. Our findings mirror those of Takada and Bridle, who investigated correlations between WLT and CC.

arXiv:gr-qc/0703060 [ps, pdf, other] : Title: Stability of spherically symmetric solutions in modified theories of gravity
Authors: Michael D. Seifert
Comments: ReVTex; 19 pages, 3 figures. v2: references added, submitted to PRD
In recent years, a number of alternative theories of gravity have been proposed as possible resolutions of certain cosmological problems or as toy models for possible but heretofore unobserved effects. However, the implications of such theories for the stability structures such as stars have not been fully investigated. We use our "generalized variational principle", described in a previous work, to analyze the stability of static spherically symmetric solutions to spherically symmetric perturbations in three such alternative theories: Carroll et al.'s f(R) gravity, Jacobson & Mattingly's "Einstein-aether theory", and Bekenstein's TeVeS. We find that in the presence of matter, f(R) gravity is highly unstable; that the stability conditions for spherically symmetric curved vacuum Einstein-aether backgrounds are the same as those for linearized stability about flat spacetime, with one exceptional case; and that the ``kinetic terms'' of vacuum TeVeS are indefinite in a curved background, leading to an instability.

 arXiv:0706.1071 [ps, pdf, other]  (suggested by Scott)
Title: Anisotropic Magnification Distortion of the 3D Galaxy Correlation: I. Real Space
Authors: Lam Hui, Enrique Gaztanaga, Marilena LoVerde
Comments: 15 pages, revised to include a discussion of the pioneering paper by Matsubara (2000)
It is well known gravitational lensing, mainly via magnification bias, modifies the observed galaxy/quasar clustering. Such discussions have largely focused on the 2D angular correlation. Here and in a companion paper (Paper II) we explore how magnification bias distorts the 3D correlation function and power spectrum, as first considered by Matsubara. The interesting point is: the distortion is anisotropic. Magnification bias preferentially enhances the observed correlation in the line-of-sight (LOS) orientation, especially on large scales. For example at LOS separation of ~100 Mpc/h, where the intrinsic galaxy-galaxy correlation is rather weak, the observed correlation can be enhanced by lensing by a factor of a few, even at a modest redshift of z ~ 0.35. The opportunity: this lensing anisotropy is distinctive, making it possible to separately measure the galaxy-galaxy, galaxy-magnification and magnification-magnification correlations, without measuring galaxy shapes. The anisotropy is distinguishable from the well known distortion due to peculiar motions, as will be discussed in Paper II. The challenge: the magnification distortion of the galaxy correlation must be accounted for in interpreting data as precision improves. For instance, the ~100 Mpc/h baryon acoustic oscillation scale in the correlation function is shifted by up to ~3% in the LOS orientation, and up to ~0.6% in the monopole, depending on the galaxy bias, redshift and number count slope. The corresponding shifts in the inferred Hubble parameter and angular diameter distance, if ignored, could significantly bias measurements of the dark energy equation of state. Lastly, magnification distortion offers a plausible explanation for the well known excess correlations seen in pencil beam surveys.

arXiv:0705.1356 [pdf]  (suggested by Jeter)
Title: Missing Mass in Collisional Debris from Galaxies
Authors: F. Bournaud, P.-A. Duc, E. Brinks, M. Boquien, P. Amram, U. Lisenfeld, B. S. Koribalski, F. Walter, V. Charmandaris
Comments: Published in Science. Complete manuscript with appendix available at this http URL . Published online in Science Express May 10 issue at this http URL
Journal-ref: Science, 316, 1166 (2007)
Recycled dwarf galaxies can form in the collisional debris of massive galaxies. Theoretical models predict that, contrary to classical galaxies, they should be free of non-baryonic Dark Matter. Analyzing the observed gas kinematics of such recycled galaxies with the help of a numerical model, we demonstrate that they do contain a massive dark component amounting to about twice the visible matter. Staying within the standard cosmological framework, this result most likely indicates the presence of large amounts of unseen, presumably cold, molecular gas. This additional mass should be present in the disks of their progenitor spiral galaxies, accounting for a significant part of the so-called missing baryons.

arXiv:0706.3019 [ps, pdf, other]
Title: Neutrino Backgrounds to Dark Matter Searches
Authors: Jocelyn Monroe, Peter Fisher
Comments: 5 pages, 6 figures
Neutrino coherent scattering cross sections can be as large as 10^{-39} cm^2, while current dark matter experiments have sensitivities to WIMP coherent scattering cross sections five orders of magnitude smaller; future experiments plan to have sensitivities to cross sections as small as 10^{-48} cm^2. With large target masses and few keV recoil energy detection thresholds, neutral current coherent scattering of solar neutrinos becomes an irreducible background in dark matter searches. In the current zero-background analysis paradigm, neutrino coherent scattering will limit the achievable sensitivity to dark matter scattering cross sections, at the level of 10^{-46} cm^2.
he initial flux per correlation length is decreased, there is a point at which infinite strings disappear, as in the Hagedorn transition.

arXiv:0706.2151 [ps, pdf, other] : Title: Nonlocal Cosmology
Authors: S. Deser (Caltech and Brandeis), R. P. Woodard (U. of Florida)
Comments: 7 pages, no figures, uses LaTeX 2epsilon
We explore nonlocally modified models of gravity, inspired by quantum loop corrections, as a mechanism for explaining current cosmic acceleration. These theories enjoy two major advantages: they allow a delayed response to cosmic events, here the transition from radiation to matter dominance, and they avoid the usual fine tuning; instead, emulating Dirac's dictum, the required large numbers come from the large time scales involved.