Munch: Monday, December 18, 2006

                               

Last Munch until January 8!

 

    usual time and place:  12:30, 6th floor conference room

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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       Munch Archive

Cosmological unification of string theories  hep-th/0612116  (last minute addition from Mark)

Authors: Simeon Hellerman, Ian Swanson
Comments: 51 pages, 5 figures
We present an exact solution of superstring theory that interpolates in time between an initial type 0 phase and a final phase whose physics is exactly that of the bosonic string. The initial theory is deformed by closed-string tachyon condensation along a lightlike direction. In the limit of large tachyon vev, the worldsheet conformal field theory precisely realizes the Berkovits-Vafa embedding of bosonic string theory into superstring theory. Our solution therefore connects the bosonic string dynamically with the superstring, settling a longstanding question about the relationship between the two theories.

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Sterile neutrino production via active-sterile oscillations: the quantum Zeno effect hep-ph/0612092 (suggested by Scott)

Authors: D. Boyanovsky, C. M. Ho
Comments: 18 pages, 4 figures
An important ingredient in the kinetic description of sterile neutrino production via active-sterile oscillations in a medium is the time averaged transition probability $<P_{a\to s}>$. We study the conditions for its quantum Zeno suppression at high temperature. In analogy with the neutral kaon system we point out that there are \emph{two} different relaxation rates corresponding to the propagating modes in the medium. We find these to be: $\Gamma_1(k) = \Gamma_{aa}(k) \cos^2\theta_m(k); \Gamma_2(k) = \Gamma_{aa}(k) \sin^2\theta_m(k)$ where $\Gamma_{aa}(k)\propto G^2_F k T^4$ is the active neutrino scattering rate and $\theta_m(k)$ is the mixing angle in the medium. This feature leads to a dramatic modification of the time averaged transition probability and to more restrictive conditions for quantum Zeno suppression: i) $\Gamma_{aa}(k) \gg \Delta E(k)$ with $\Delta E(k)$ the active-sterile oscillation frequency \emph{and} ii) an MSW resonance. For $m_s\sim \textrm{keV}$ sterile neutrinos with $\sin2\theta \lesssim 10^{-3}$ the conditions for quantum Zeno suppression are fulfilled \emph{only} near MSW resonances. Far away from these the effective sterile neutrino production rate is $\frac{\Gamma_{aa}(k)}{4} $ as a consequence of a wide separation of relaxation scales. Sterile neutrino production is favored at temperatures far away from MSW resonances.

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A Robust Estimator of the Small-Scale Galaxy Correlation Function  astro-ph/0612103

Authors: Nikhil Padmanabhan, Martin White, Daniel J. Eisenstein
Comments: 5 pages, 4 figures, submitted to MNRAS
Report-no: LBNL-62026
We present a new estimator, omega, of the small scale galaxy correlation function that is robust against the effects of redshift space distortions and large scale structures. The estimator is a weighted integral of the redshift space or angular correlation function and is a convolution of the real space correlation function with a localized filter. This allows a direct comparison with theory, without modeling redshift space distortions and the large scale correlation function. This has a number of advantages over the more traditional w_p estimator, including (i) an insensitivity to large scale structures and the details of the truncation of the line of sight integral, (ii) a compact kernel in xi(r), and (iii) being unbinned. We discuss how this estimator would be used in practice, applying it to a sample of mock galaxies selected from the Millennium simulation.

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Dark Energy with a Fine-toothed Comb  astro-ph/0612102

Authors: Eric V. Linder
Comments: 6 pages, 3 figures
The cosmological constant and many other possible origins for acceleration of the cosmic expansion possess variations in the dark energy properties slow on the Hubble time scale. Given that models with more rapid variation, or even phase transitions, are possible though, we examine the fineness in redshift with which cosmological probes can realistically be employed, and what constraints this could impose on dark energy behavior. In particular, we discuss various aspects of baryon acoustic oscillations, and their use to measure the Hubble parameter H(z). We find that cosmological probes have an innate resolution no finer than Delta z=0.2-0.3.

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Hints of Isocurvature Perturbations in the Cosmic Microwave Background  astro-ph/0611917 (suggested by Pasquale)

Authors: Reijo Keskitalo (HIP and Helsinki U.), Hannu Kurki-Suonio (Helsinki U.), Vesa Muhonen (HIP and Helsinki U.), Jussi Valiviita (ICG, Portsmouth)
Comments: 4 pages, 2 figures, revtex4 format
Report-no: HIP-2006-52/TH, ICG 06/43
The improved data on the cosmic microwave background (CMB) anisotropy allows a better determination of the adiabaticity of the primordial perturbation. Interestingly, we find that the CMB favors a significant contribution of a primordial isocurvature mode where the entropy perturbation is positively correlated with the primordial curvature perturbation and has a large spectral index (n_iso ~ 3). With 4 additional parameters we obtain a better fit to the CMB data by \Delta\chi^2 = 9.4 compared to an adiabatic model. At more than 95% C.L., the nonadiabatic contribution to the CMB temperature variance is nonzero; indeed positive. For the best-fit model it is 4%.

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Tentative detection of the gravitational magnification of type Ia supernovae  astro-ph/0612329

Authors: Jakob Jonsson, Tomas Dahlen, Ariel Goobar, Edvard Mortsell, Adam Riess
Comments: 7 pages, 2 figures
The flux from distant type Ia supernovae (SN) is likely to be amplified or de-amplified by gravitational lensing due to matter distributions along the line-of-sight. A gravitationally lensed SN would appear brighter or fainter than the average SN at a particular redshift. We estimate the magnification of 26 SNe in the GOODS fields and search for a correlation with the residual magnitudes of the SNe. The residual magnitude, i.e. the difference between observed and average magnitude predicted by the "concordance model" of the Universe, indicates the deviation in flux from the average SN. The linear correlation coefficient for this sample is r=0.29. For a similar, but uncorrelated sample, the probability of obtaining a correlation coefficient equal to or higher than this value is ~10%, i.e. a tentative detection of lensing at ~90% confidence level. Although the evidence for a correlation is weak, our result is in accordance with what could be expected given the small size of the sample.

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The Velocity Field of the Local Universe from Measurements of Type Ia Supernovae  astro-ph/0612137

Authors: Troels Haugboelle, Steen Hannestad, Bjarne Thomsen, Johan Fynbo, Jesper Sollerman, Saurabh Jha
Comments: 9 pages, 12 figures, submitted to ApJ. A version with high resolution figures can be found at this http URL
We present a measurement of the velocity flow of the local universe relative to the CMB rest frame, based on the Jha, Riess & Kirshner (2007) sample of 133 low redshift type Ia supernovae. At a depth of 4500 km/s we find a dipole amplitude of 279+-68 km/s in the direction (l,b) = (285+-18,-10+-15), consistent with earlier measurements and with the assumption that the local velocity field is dominated by the Great Attractor region. At a larger depth of 5900 km/s we find a shift in the dipole direction towards the Shapley concentration. We also present the first measurement of the quadrupole term in the local velocity flow at these depths. Finally, we have performed detailed studies based on N-body simulations of the expected precision with which the lowest multipoles in the velocity field can be measured out to redshifts of order 0.1. Our mock catalogues are in good agreement with current observations, and demonstrate that our results are robust with respect to assumptions about the influence of local environment on the type Ia supernova rate.

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High-resolution imaging of the cosmic mass distribution from gravitational lensing of pregalactic HI (suggested by Josh)

Authors: R. Benton Metcalf, S.D.M. White
Comments: 19 pages, 11 figures, submitted to MNRAS
Low-frequency radio observations of neutral hydrogen during and before the epoch of cosmic reionisation will provide ~ 1000 quasi-independent source planes, each of precisely known redshift, which can be used to reconstruct the projected mass distribution of foreground material. Structure in these source planes is linear and gaussian at high redshift (30 < z < 300) but is nonlinear and nongaussian during reionisation. At both epochs, significant power is expected down to sub-arcsecond scales. We demonstrate that this structure can, in principle, be used to make mass images with a formal signal-to-noise per pixel exceeding 10, even for pixels as small as an arc-second. With an ideal telescope, both resolution and signal-to-noise can exceed those of even the most optimistic idealised mass maps from galaxy lensing by more than an order of magnitude. Individual dark halos similar in mass to that of the Milky Way could be imaged with high signal-to-noise out to z ~ 10. Even with a much less ambitious telescope, a wide-area survey of 21 cm lensing would provide very sensitive constraints on cosmological parameters, in particular on dark energy. These are up to 20 times tighter than the constraints obtainable from comparably sized, very deep surveys of galaxy lensing, although the best constraints come from combining data of the two types. Any radio telescope capable of mapping the 21cm brightness temperature with good frequency resolution (~< 0.5 MHz) over a band of width ~> 10 MHz should be able to make mass maps of high quality.

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Cosmological Parameters from the SDSS DR5 Velocity Dispersion Function of Early-Type Galaxies through Radio-Selected Lens Statistics (suggested by Josh)

Authors: Kyu-Hyun Chae
Comments: ApJL, submitted
We improve strong lensing constraints on cosmological parameters in light of the new measurement of the velocity dispersion function of early-type galaxies based on the SDSS DR5 data and recent semi-analytical modeling of galaxy formation. Using both the number statistics of the CLASS statistical sample and the image separation distribution of the CLASS and the PANELS radio-selected lenses, we find the cosmological matter density $\Om = 0.25^{+0.13}_{-0.08}$ (68% CL) assuming evolutions of galaxies predicted by a semi-analytical model of galaxy formation and $\Om = 0.27^{+0.11}_{-0.09}$ assuming no evolution of galaxies for a flat cosmology with an Einstein cosmological constant. For a flat cosmology with a generalized dark energy, we find the non-evolving dark energy equation of state $w_x < -1.3$ ($w_x < -0.5$) at the 68% CL (95% CL).

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