12,510 research outputs found
Extended Palatini action for general relativity and the natural emergence of the cosmological constant
In the Palatini action of general relativity the connection and the metric
are treated as independent dynamical variables. Instead of assuming a relation
between these quantities, the desired relation between them is derived through
the Euler-Lagrange equations of the Palatini action. In this manuscript we
construct an extended Palatini action, where we do not assume any a priori
relationship between the connection, the covariant metric tensor, and the
contravariant metric tensor. Instead we treat these three quantities as
independent dynamical variables. We show that this action reproduces the
standard Einstein field equations depending on a single metric tensor. We
further show that in this formulation the cosmological constant has an
additional theoretical significance. Normally the cosmological constant is
added to the Einstein field equations for the purpose of having general
relativity be consistent with cosmological observations. In the formulation
presented here, the nonvanishing cosmological constant also ensures the
self-consistency of the theory.Comment: in the revised version the original scalar matter action is replaced
with a general matter actio
Fabrication of titanium multi-wall Thermal Protection System (TPS) test panel arrays
Several arrays were designed and tested. Tests included vibrational and acoustical tests, radiant heating tests, and thermal conductivity tests. A feasible manufacturing technique was established for producing the protection system panels
Re-design and fabrication of titanium multi-wall Thermal Protection System (TPS) test panels
The Titanium Multi-wall Thermal Protection System (TIPS) panel was re-designed to incorporate Ti-6-2-4-2 outer sheets for the hot surface, ninety degree side closures for ease of construction and through panel fastness for ease of panel removal. Thermal and structural tests were performed to verify the design. Twenty-five panels were fabricated and delivered to NASA for evaluation at Langley Research Center and Johnson Space Center
On the Impact of Fair Best Response Dynamics
In this work we completely characterize how the frequency with which each
player participates in the game dynamics affects the possibility of reaching
efficient states, i.e., states with an approximation ratio within a constant
factor from the price of anarchy, within a polynomially bounded number of best
responses. We focus on the well known class of congestion games and we show
that, if each player is allowed to play at least once and at most times
any best responses, states with approximation ratio times the
price of anarchy are reached after best
responses, and that such a bound is essentially tight also after exponentially
many ones. One important consequence of our result is that the fairness among
players is a necessary and sufficient condition for guaranteeing a fast
convergence to efficient states. This answers the important question of the
maximum order of needed to fast obtain efficient states, left open by
[9,10] and [3], in which fast convergence for constant and very slow
convergence for have been shown, respectively. Finally, we show
that the structure of the game implicitly affects its performances. In
particular, we show that in the symmetric setting, in which all players share
the same set of strategies, the game always converges to an efficient state
after a polynomial number of best responses, regardless of the frequency each
player moves with
Thermal inactivation of Byssochlamys nivea in pineapple nectar combined with preliminary high pressure treatments
Byssochlamys nivea is a thermal resistant filamentous fungi and potential micotoxin producer. Recent studies have verified the presence of ascospores of such microorganism in samples of pineapple nectars. Although the majority of filamentous fungi have limited heat resistance and are easily destroyed by heat, Byssochlamys nivea ascospores have shown high thermal resistance. The aim of this work was to evaluate the application of linear and Weibull models on thermal inactivation (70, 80 and 90ºC) of Byssochlamys nivea ascospores in pineapple nectar after pretreatment with high pressure (550MPa or 650MPa during 15min). Following the treatments, survival curves were built up for each processing temperature and adjusted for both models. It was observed that survival curves at 90°C after high pressure pretreatment at 550 MPa/15 min did not fit well to linear and Weibull models. For all the other treatments, the Weibull model presented a better fit. At 90ºC without pressure treatment, the Weibull model also showed a better adjustment, having a larger R2 and a smaller RMSE. Regarding the process effectiveness, a 5-log reduction (t5), as recommended for pasteurization, was only achieved for Byssochlamys nivea ascospores presented in pineapple nectar at 90ºC/10.7 min with previous high pressure treatment of 650 MPa for 15 min. Considering the high intensity and energy demanding process with possibly product damage, other preventive and alternative treatments are being investigated
The Price of Anarchy for Selfish Ring Routing is Two
We analyze the network congestion game with atomic players, asymmetric
strategies, and the maximum latency among all players as social cost. This
important social cost function is much less understood than the average
latency. We show that the price of anarchy is at most two, when the network is
a ring and the link latencies are linear. Our bound is tight. This is the first
sharp bound for the maximum latency objective.Comment: Full version of WINE 2012 paper, 24 page
Air hydrodynamics of the ultrafast laser-triggered spark gap
We present space and time resolved measurements of the air hydrodynamics
induced by ultrafast laser pulse excitation of the air gap between two
electrodes at high potential difference. We explore both plasma-based and
plasma-free gap excitation. The former uses the plasma left in the wake of
femtosecond filamentation, while the latter exploits air heating by
multiple-pulse resonant excitation of quantum molecular wavepackets. We find
that the cumulative electrode-driven air density depression channel initiated
by the laser plays the dominant role in the gap evolution leading to breakdown
Oscillation regimes of a solid-state ring laser with active beat note stabilization : from a chaotic device to a ring laser gyroscope
We report experimental and theoretical study of a rotating diode-pumped
Nd-YAG ring laser with active beat note stabilization. Our experimental setup
is described in the usual Maxwell-Bloch formalism. We analytically derive a
stability condition and some frequency response characteristics for the
solid-state ring laser gyroscope, illustrating the important role of mode
coupling effects on the dynamics of such a device. Experimental data are
presented and compared with the theory on the basis of realistic laser
parameters, showing a very good agreement. Our results illustrate the duality
between the very rich non linear dynamics of the diode-pumped solid-state ring
laser (including chaotic behavior) and the possibility to obtain a very stable
beat note, resulting in a potentially new kind of rotation sensor
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