2,258 research outputs found
Asymptotically Safe Lorentzian Gravity
The gravitational asymptotic safety program strives for a consistent and
predictive quantum theory of gravity based on a non-trivial ultraviolet fixed
point of the renormalization group (RG) flow. We investigate this scenario by
employing a novel functional renormalization group equation which takes the
causal structure of space-time into account and connects the RG flows for
Euclidean and Lorentzian signature by a Wick-rotation. Within the
Einstein-Hilbert approximation, the -functions of both signatures
exhibit ultraviolet fixed points in agreement with asymptotic safety.
Surprisingly, the two fixed points have strikingly similar characteristics,
suggesting that Euclidean and Lorentzian quantum gravity belong to the same
universality class at high energies.Comment: 4 pages, 2 figure
The Standard Model of Particle Physics
Particle physics has evolved a coherent model that characterizes forces and
particles at the most elementary level. This Standard Model, built from many
theoretical and experimental studies, is in excellent accord with almost all
current data. However, there are many hints that it is but an approximation to
a yet more fundamental theory. We trace the development of the Standard Model
and indicate the reasons for believing that it is incomplete.Comment: 25 pages, 17 figures; accepted for publication in Rev. Mod. Physics
(APS centenary issue
Asymptotic safety in higher-derivative gravity
We study the non-perturbative renormalization group flow of higher-derivative
gravity employing functional renormalization group techniques. The
non-perturbative contributions to the -functions shift the known
perturbative ultraviolet fixed point into a non-trivial fixed point with three
UV-attractive and one UV-repulsive eigendirections, consistent with the
asymptotic safety conjecture of gravity. The implication of this transition on
the unitarity problem, typically haunting higher-derivative gravity theories,
is discussed.Comment: 8 pages; 1 figure; revised versio
Applying Length-Dependent Stochastic Context-Free Grammars to RNA Secondary Structure Prediction
Weinberg F, Nebel M. Applying Length-Dependent Stochastic Context-Free Grammars to RNA Secondary Structure Prediction. Algorithms. 2011;4(4):223--238
The Composite Spectrum of Strong Lyman-alpha Forest Absorbers
We present a new method for probing the physical conditions and metal
enrichment of the Intergalactic Medium: the composite spectrum of Ly-alpha
forest absorbers. We apply this technique to a sample of 9480 Ly-alpha
absorbers with redshift 2 < z < 3.5 identified in the spectra of 13,279
high-redshift quasars from the Sloan Digital Sky Survey (SDSS) Fifth Data
Release (DR5). Absorbers are selected as local minima in the spectra with 2.4 <
tau_Ly-alpha < 4.0; at SDSS resolution (~ 150km/s FWHM), these absorbers are
blends of systems that are individually weaker. In the stacked spectra we
detect seven Lyman-series lines and metal lines of O VI, N V, C IV, C III, Si
IV, C II, Al II, Si II, Fe II, Mg II, and O I. Many of these lines have peak
optical depths of < 0.02, but they are nonetheless detected at high statistical
significance. Modeling the Lyman-series measurements implies that our selected
systems have total H I column densities N_HI ~ 10^15.4cm-2. Assuming typical
physical conditions rho / = 10, T = 10^4 - 10^4.5 K, and [Fe/H]= -2
yields reasonable agreement with the line strengths of high-ionization species,
but it underpredicts the low-ionization species by two orders of magnitude or
more. This discrepancy suggests that the low ionization lines arise in dense,
cool, metal-rich clumps, present in some absorption systems.Comment: 7 pages, 4 figures, 1 table, accepted by ApJL, revisions mad
Subluminal OPERA Neutrinos
The OPERA collaboration has announced to have observed superluminal neutrinos
with a mean energy 17.5 GeV, but afterward the superluminal interpretation of
the OPERA results has been refuted theoretically by Cherenkov-like radiation
and pion decay. In a recent work, we have proposed a kinematical resolution to
this problem. A key idea in our resolution is that the OPERA neutrinos are not
superluminal but subluminal since they travel faster than the observed speed of
light in vacuum on the earth while they do slower than the true speed of light
in vacuum determining the causal structure of events. In this article, we dwell
upon our ideas and present some concrete models, which realize our ideas, based
on spin 0, 1 and 2 bosonic fields. We also discuss that the principle of
invariant speed of light in special relativity can be replaced with the
principle of a universal limiting speed.Comment: 17 page
Charge separation instability in an unmagnetized disk plasma around a Kerr black hole
In almost all of plasma theories for astrophysical objects, we have assumed
the charge quasi-neutrality of unmagnetized plasmas in global scales. This
assumption has been justified because if there is a charged plasma, it induces
electric field which attracts the opposite charge, and this opposite charge
reduces the charge separation. Here, we report a newly discovered instability
which causes a charge separation in a rotating plasma inside of an innermost
stable circular orbit (ISCO) around a black hole. The growth rate of the
instability is smaller than that of the disk instability even in the unstable
disk region and is forbidden in the stable disk region outside of the ISCO.
However, this growth rate becomes comparable to that of the disk instability
when the plasma density is much lower than a critical density inside of the
ISCO. In such case, the charge separation instability would become apparent and
cause the charged accretion into the black hole, thus charge the hole up.Comment: 15pages, 1 figur
Evolution of NASA's Near-Earth Tracking and Data Relay Satellite System (TDRSS)
NASA's Tracking and Data Relay Satellite System (TDRSS) is now in its 23rd year of operations and its spacecraft fleet includes three second-generation spacecraft launched since the year 2000; a figure illustrates the first generation TDRSS spacecraft. During this time frame the TDRSS has provided communications relay support to a broad range of missions, with emphasis on low-earth-orbiting (LEO) spacecraft that include unmanned science spacecraft (e.g., Hubble Space Telescope), and human spaceflight (Space Shuttle and Space Station). Furthermore, the TDRSS has consistently demonstrated its uniqueness and adaptability in several ways. First, its S- and K-band services, combined with its multi-band/steerable single-access (SA) antennas and ground-based configuration flexibility, have permitted the mission set to expand to unique users such as scientific balloons and launch vehicles. Second, the bent-pipe nature of the system has enabled the introduction of new/improved services via technology insertion and upgrades at each of the ground terminals; a specific example here is the Demand Access Service (DAS), which, for example, is currently providing science-alert support to NASA science missions Third, the bent-pipe nature of the system, combined with the flexible ground-terminal signal processing architecture has permitted the demonstration/vaIidation of new techniques/services/technologies via a real satellite channel; over the past 10+ years these have, for example, included demonstrations/evaluations of emerging modulation/coding techniques. Given NASA's emerging Exploration plans, with missions beginning later this decade and expanding for decades to come, NASA is currently planning the development of a seamless, NASA-wide architecture that must accommodate missions from near-earth to deep space. Near-earth elements include Ground-Network (GN) and Near-Earth Relay (NER) components and both must efficiently and seamlessly support missions that encompass: earth orbit, including dedicated science missions and lunar support/cargo vehicles; earth/moon transit; lunar in-situ operations; and other missions within approximately 2 million km of earth (e.g., at the sun/earth libration points). Given that the NER is an evolution of TDRSS, one element of this NASA-wide architecture development activity is a trade study of future NER architecture candidates. The present paper focuses on trade study aspects associated with the NER, highlights study elements, and provides representative interim results
Random generation of RNA secondary structures according to native distributions
Nebel M, Scheid A, Weinberg F. Random generation of RNA secondary structures according to native distributions. Algorithms for Molecular Biology. 2011;6(1): 24
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