Munch: Monday, March 17th, 2008

arXiv:0802.4452 [ps, pdf, other]

Title: Analytic calculations of the spectra of ultra-high energy cosmic ray nuclei. I. The case of CMB radiation

Authors: R. Aloisio, V. Berezinsky, S. Grigorieva

We present a new method for analytic calculation of diffuse spectra of ultra-high energy nuclei. Nuclei propagating in the intergalactic medium are photo-disintegrated and decrease their Lorentz factor due to the interaction with CMB and infrared (IR) radiations. Our method allows the calculation of the diffuse spectra of primary nuclei as well as of secondary nuclei and protons. Our computation scheme is based on four main elements: {\it (i)} using the Lorentz factor instead of energy, {\it (ii)} calculation of the evolution trajectories backward in time which describe how atomic number $A$ and Lorentz factor $\Gamma$ change with redshift $z$, {\it (iii)} using the kinetic equations for calculations of spectra, {\it (iv)} conservation of the number of particles along each nucleus evolution trajectory. In the present paper we consider nuclei interaction only with CMB, this case is particularly suitable to explain our computation scheme. In a forthcoming paper (II) IR radiation will be also included. The main physical results obtained in this paper are as follows. The Helium nuclei strongly dominate the flux of secondary nuclei at energies $1\times 10^{18} - 3\times 10^{19}$ eV. All other secondary nuclei have a strong peak in the $E^3J(E)$ spectrum at $\Gamma \sim 4\times 10^9$, which coincides with a characteristic scale: the critical Lorentz factor. The spectrum of secondary protons is universal, i.e. it does not depend on the primary nuclei species $A_0$. In the case of Iron primaries $(A_0=56)$, and IR radiation included, the secondary proton spectrum intersects the Iron spectrum at $E=2\times 10^{19}$ eV and starts to dominate at higher energies.

arXiv:0803.1738 [ps, pdf, other]

Title: Planck and reionization history: a model selection view

Authors: Pia Mukherjee, Andrew R. Liddle

We use Bayesian model selection tools to forecast the Planck satellite's ability to distinguish between different models for the reionization history of the Universe, using the large angular scale signal in the cosmic microwave background polarization spectrum. We find that Planck is not expected to be able to distinguish between an instantaneous reionization model and a two-parameter smooth reionization model, and if so then it will not be able to distinguish between different two-parameter models either. However Bayesian model averaging will be needed to obtain unbiased estimates of the optical depth to reionization. We also generalize our results to a hypothetical future cosmic variance limited microwave anisotropy survey. There the outlook is much more optimistic.

Science 7 March 2008: Vol. 319. no. 5868, pp. 1367

Title: Fiber-Optical Analog of the Event Horizon

Authors: Thomas G. Philbin et al.

The physics at the event horizon resembles the behavior of waves in moving media. Horizons are formed where the local speed of the medium exceeds the wave velocity. We used ultrashort pulses in microstructured optical fibers to demonstrate the formation of an artificial event horizon in optics. We observed a classical optical effect: the blue-shifting of light at a white-hole horizon. We also showed by theoretical calculations that such a system is capable of probing the quantum effects of horizons, in particular Hawking radiation.

arXiv:0803.0324 [ps, pdf, other]

Title: Discovery of the Dust-Enshrouded Progenitor of the Type IIn SN 2008S with Spitzer

Authors: J.L. Prieto, M.D. Kistler, T.A. Thompson, H. Yuksel, C.S. Kochanek, K.Z. Stanek, J.F. Beacom, P. Martini, A. Pasquali, J. Bechtold

We report the discovery of the progenitor of the recent type IIn supernova 2008S in the nearby galaxy NGC 6946. Surprisingly, the progenitor was not found in deep, pre-supernova optical images of its host galaxy taken with the Large Binocular Telescope, but only through examination of archival Spitzer mid-IR data. A source coincident with the supernova position is clearly detected in the 4.5, 5.8, and 8.0 micron IRAC bands, showing no evident variability in a three-year light curve leading up to the explosion, yet undetected at 3.6 and 24 micron. The distinct presence of ~440 K dust, along with stringent LBT limits on the optical fluxes, suggests that the progenitor of SN 2008S died in a shroud of its own dust. The inferred luminosity of 3.5x10^4 Lsun implies a modest progenitor mass of ~10 Msun. We conclude that type IIn supernovae need not always result from the death of very massive eta Carinae-like objects.

arXiv:0803.0859 [ps, pdf, other]

Title: Speedy sound and cosmic structure

Authors: Joao Magueijo

If the speed of sound were vastly larger in the early Universe a near scale-invariant spectrum of density fluctuations could have been produced even if the Universe did not submit to conventional solutions to the horizon problem. We examine how the mechanism works, presenting full mathematical solutions and their heuristics. We then discuss several concrete models based on scalar fields and hydrodynamical matter which realize this mechanism, but stress that the proposed mechanism is more fundamental and general.

arXiv:0803.0663 [ps, pdf, other]

Title: The Cosmological Constant Problem and Inflation in the String Landscape

Authors: Qing-Guo Huang, S.-H. Henry Tye

An earlier paper points out that a quantum treatment of the string landscape is necessary. It suggests that the wavefunction of the universe is mobile in the landscape until the universe reaches a meta-stable site with its cosmological constant $\Lambda_0$ smaller than the critical value $\Lambda_c$, where $\Lambda_c$ is estimated to be exponentally small compared to the Planck scale. Since this site has an exponentially long lifetime, it may well be today's universe. We investigate specific scenarios based on this quantum diffusion property of the cosmic landscape and find a plausible scenario for the early universe. In the last fast tunneling to the $\Lambda_0$ ($<\Lambda_c$) site in this scenario, all energies are stored in the nucleation bubble walls, which are released to radiation only after bubble collisions and thermalization. So the $\Lambda_0$ site is chosen even if $\Lambda_0$ plus radiation is larger than $\Lambda_c$, as long as the radiation does not destabilize the $\Lambda_0$ vacuum. A consequence is that inflation must happen before this last fast tunneling, so the inflationary scenario that emerges naturally is extended brane inflation, where the brane motion includes a combination of rolling, fast tunnelings, slow-roll, hopping and percolation in the landscape. We point out that, in the brane world, radiation during nucleosynthesis are mostly on the standard model branes (brane radiation, as opposed to radiation in the bulk). This distinction may lead to interesting dynamics.

arXiv:0803.0749 [ps, pdf, other]

Title: Dangerous implications of a minimum length in quantum gravity

Authors: Cosimo Bambi, Katherine Freese

The existence of a minimum length and a generalization of the Heisenberg uncertainty principle seem to be two fundamental ingredients required in any consistent theory of quantum gravity. In this letter we show that they naturally predict dangerous processes which somehow must be suppressed. For example, long-lived virtual super--Planck mass black holes may lead to rapid proton decay. Possible solutions of this puzzle are briefly discussed.