18,745 research outputs found
Time Dependence of Particle Creation from Accelerating Mirrors
Particle production due to a quantized, massless, minimally coupled scalar
field in two-dimensional flat spacetime with an accelerating mirror is
investigated, with a focus on the time dependence of the process. We analyze
first the classes of trajectories previously investigated by Carlitz and Willey
and by Walker and Davies. We then analyze four new classes of trajectories, all
of which can be expressed analytically and for which several ancillary
properties can be derived analytically. The time dependence is investigated
through the use of wave packets for the modes of the quantized field that are
in the out vacuum state. It is shown for most of the trajectories studied that
good time resolution of the particle production process can be obtained.Comment: 21 pages, 5 figure
Black Hole - Moving Mirror II: Particle Creation
There is an exact correspondence between the simplest solution to Einstein's
equations describing the formation of a black hole and a particular moving
mirror trajectory. In both cases the Bogolubov coefficients in 1+1 dimensions
are identical and can be computed analytically. Particle creation is
investigated by using wave packets. The entire particle creation history is
computed, incorporating the early-time non-thermal emission due to the
formation of the black hole (or the early-time acceleration of the moving
mirror) and the evolution to a Planckian spectrum.Comment: Contribution to MG14 Proceedings, 5 pages, 4 figure
Mirror Reflections of a Black Hole
An exact correspondence between a black hole and an accelerating mirror is
demonstrated. It is shown that for a massless minimally coupled scalar field
the same Bogolubov coefficients connecting the "in" and "out" states occur for
a (1+1)D flat spacetime with a particular perfectly reflecting accelerating
boundary trajectory and a (1+1)D curved spacetime in which a null shell
collapses to form a black hole. Generalization of the latter to the (3+1)D case
is discussed. The spectral dynamics is computed in both (1+1)-dimensional
spacetimes along with the energy flux in the spacetime with a mirror. It is
shown that the approach to equilibrium is monotonic, asymmetric in terms of the
rate, and there is a specific time which characterizes the system when it is
the most out-of-equilibrium.Comment: 25 pages, 7 figure
Black Hole - Moving Mirror I: An Exact Correspondence
An exact correspondence is shown between a new moving mirror trajectory in
(1+1)D and a spacetime in (1+1)D in which a black hole forms from the collapse
of a null shell. It is shown that the Bogolubov coefficients between the "in"
and "out" states are identical and the exact Bogolubov coefficients are
displayed. Generalization to the (3+1)D black hole case is discussed.Comment: Contribution to MG14 Proceedings, 5 pages, 1 figur
Business cycle detrending of macroeconomic data via a latent business cycle index
We use Markov Chain Monte Carlo methods to augment a vector autoregressive system with a latent business cycle index that is negative during recessions and positive during expansions. We then sample counterfactual values of the macroeconomic variables in the case where the latent business cycle index is held constant at its median value. These counterfactual values represent posterior beliefs about how the economy would have evolved absent business cycle fluctuations. One advantage is that a VAR framework provides model-consistent counterfactual values in the same way that VARs provide model-consistent forecasts, so data series are not detrended in isolation from each other. We apply these methods to estimate the business cycle components of industrial production growth, consumer price inflation, the federal funds rate and the spread between long-term and short-term interest rates. These decompositions provide an explicitly counterfactual approach to deriving empirical business cycle facts that complements other approaches.Business cycles ; Time-series analysis
Symmetric factorization of the conformation tensor in viscoelastic fluid models
The positive definite symmetric polymer conformation tensor possesses a
unique symmetric square root that satisfies a closed evolution equation in the
Oldroyd-B and FENE-P models of viscoelastic fluid flow. When expressed in terms
of the velocity field and the symmetric square root of the conformation tensor,
these models' equations of motion formally constitute an evolution in a Hilbert
space with a total energy functional that defines a norm. Moreover, this
formulation is easily implemented in direct numerical simulations resulting in
significant practical advantages in terms of both accuracy and stability.Comment: 7 pages, 5 figure
Retroactive Anti-Jamming for MISO Broadcast Channels
Jamming attacks can significantly impact the performance of wireless
communication systems. In addition to reducing the capacity, such attacks may
lead to insurmountable overhead in terms of re-transmissions and increased
power consumption. In this paper, we consider the multiple-input single-output
(MISO) broadcast channel (BC) in the presence of a jamming attack in which a
subset of the receivers can be jammed at any given time. Further,
countermeasures for mitigating the effects of such jamming attacks are
presented. The effectiveness of these anti-jamming countermeasures is
quantified in terms of the degrees-of-freedom (DoF) of the MISO BC under
various assumptions regarding the availability of the channel state information
(CSIT) and the jammer state information at the transmitter (JSIT). The main
contribution of this paper is the characterization of the DoF region of the two
user MISO BC under various assumptions on the availability of CSIT and JSIT.
Partial extensions to the multi-user broadcast channels are also presented.Comment: submitted to IEEE Transactions on Information Theor
Research reports: 1991 NASA/ASEE Summer Faculty Fellowship Program
The basic objectives of the programs, which are in the 28th year of operation nationally, are: (1) to further the professional knowledge of qualified engineering and science faculty members; (2) to stimulate an exchange of ideas between participants and NASA; (3) to enrich and refresh the research and teaching activities of the participants' institutions; and (4) to contribute to the research objectives of the NASA Centers. The faculty fellows spent 10 weeks at MSFC engaged in a research project compatible with their interests and background and worked in collaboration with a NASA/MSFC colleague. This is a compilation of their research reports for summer 1991
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