8,924 research outputs found
Note on the super inflation in loop quantum cosmology
Phenomenological effect of the super-inflation in loop quantum cosmology
(LQC) is discussed. We investigate the case that the Universe is filled with
the interacting field between massive scalar field and radiation. Considering
the damping coefficient as a constant, the changes of the scale factor
during super-inflation with four different initial conditions are discussed,
and we find that the changes of the scale factor depends on the initial values
of energy density of the scalar field and radiation at the bounce point. But no
matter which initial condition is chosen, the radiation always dominated at the
late time. Moreover, we investigate whether the super-inflation can provide
enough e-folding number. For the super-inflation starts from the quantum bounce
point, the initial value of Hubble parameter , then it is possible
to solve the flatness problem and horizon problem. As an example, following the
method of \cite{Amoros-prd} to calculate particle horizon on the condition that
the radiation dominated at bounce point, and we find that the Universe has had
enough time to be homogeneous and isotopic.Comment: 9 pages, 4 figures. Physics Letters B, online publication complete:
13-NOV-201
Evaluating Robustness of Neural Networks with Mixed Integer Programming
Neural networks have demonstrated considerable success on a wide variety of
real-world problems. However, networks trained only to optimize for training
accuracy can often be fooled by adversarial examples - slightly perturbed
inputs that are misclassified with high confidence. Verification of networks
enables us to gauge their vulnerability to such adversarial examples. We
formulate verification of piecewise-linear neural networks as a mixed integer
program. On a representative task of finding minimum adversarial distortions,
our verifier is two to three orders of magnitude quicker than the
state-of-the-art. We achieve this computational speedup via tight formulations
for non-linearities, as well as a novel presolve algorithm that makes full use
of all information available. The computational speedup allows us to verify
properties on convolutional networks with an order of magnitude more ReLUs than
networks previously verified by any complete verifier. In particular, we
determine for the first time the exact adversarial accuracy of an MNIST
classifier to perturbations with bounded norm : for
this classifier, we find an adversarial example for 4.38% of samples, and a
certificate of robustness (to perturbations with bounded norm) for the
remainder. Across all robust training procedures and network architectures
considered, we are able to certify more samples than the state-of-the-art and
find more adversarial examples than a strong first-order attack.Comment: Accepted as a conference paper at ICLR 201
Eccentricity is not the only source of elliptic flow fluctuations
Sources of event-by-event elliptic flow fluctuations in relativistic
heavy-ion collisions are investigated in a multiphase transport model. Besides
the well-known initial eccentricity fluctuations, several other sources of
dynamical fluctuations are identified. One is fluctuations in initial parton
configurations at a given eccentricity. Second is quantum fluctuations in
parton interactions during system evolution. Third is fluctuations caused by
hadronization and final-state hadronic scatterings. The magnitudes of these
fluctuations are investigated relative to eccentricity fluctuations and average
flow magnitude. The fluctuations from the latter two sources are found to be
negative. The results may have important implications to the interpretation of
elliptic flow data.Comment: 5 pages, 3 figure
Online Algorithms Modeled After Mousehunt
In this paper we study a variety of novel online algorithm problems inspired
by the game Mousehunt. We consider a number of basic models that approximate
the game, and we provide solutions to these models using Markov Decision
Processes, deterministic online algorithms, and randomized online algorithms.
We analyze these solutions' performance by deriving results on their
competitive ratios.Comment: 14 pages, 2 figure
Non-Fermi liquid behavior in Bose-Fermi mixtures at two dimensions
In this paper we study the low temperature behaviors of a system of
Bose-Fermi mixtures at two dimensions. Within a self-consistent ladder diagram
approximation, we show that at nonzero temperatures the
fermions exhibit non-fermi liquid behavior. We propose that this is a general
feature of Bose-Fermi mixtures at two dimensions. An experimental signature of
this new state is proposed.Comment: 4 pages, 2 figures. supplementary materia
Factorization of event-plane correlations over transverse momentum in relativistic heavy ion collisions in a multi-phase transport model
Momentum-space azimuthal harmonic event planes (EP) are constructed from
final-state midrapidity particles binned in transverse momentum (pT ) in
sqrt(s_NN) = 200 GeV Au+Au collisions in a multi-phase transport (AMPT) model.
The EP correlations between pT bins, corrected by EP resolutions, are smaller
than unity. This indicates that the EP's decorrelate over pT in AMPT,
qualitatively consistent with data and hydrodynamic calculations. It is further
found that the EP correlations approximately factorize into single pT-bin EP
correlations to a common plane. This common plane appears to be the
momentum-space EP integrated over all pT, not the configuration space
participant plane (PP).Comment: 7 pages, 2 tables, 2 figure
Phenomenology analysis of duration inflation for Tachyon field in loop quantum cosmology
Assuming that the e-folding number is just determined by the change of the
scale factor, the tachyonic inflation theory in LQC has been discussed.
Considering the tachyon field with exponential potential and inverse quadratic
potential, we find that the evolutionary pictures of super inflation are
affected by the potentials and the initial conditions. However it cannot
provide enough e-folding number, no matter which condition is chosen. Therefore
a slow-rolling inflation is necessary. The e-folding number for slow-rolling
inflation depends on the values of the parameter of the exponential
potential and the initial conditions. To get enough e-folding number,
should be small. Based on the slow-rolling inflation happens immediately when
the super inflation ends, and the scale factor is continuously growing during
the whole inflation stage, we consider an e-folding number provided by the
whole inflationary stage, and we find that it is easier to get enough e-folding
number when the scale factor increases during all the inflation phase.Comment: 8 pages, 6 figure
Four paradoxes about the special theory of relativity
Various paradoxes about the relativity theory have been developed since the
birth of this theory. Each paradox somewhat shows people's query about the
relativity theory, and solving of each paradox demonstrates the correctness of
relativity theory once again. In this paper, four paradoxes about the special
theory of relativity are brought forward: displacement paradox, electromagnetic
transformation paradox, Doppler paradox and magnetic force paradox. We hope
some researchers can reasonably explain these paradoxes, and then knowledge of
the relativity theory will become more abundant.Comment: 12 pages, 7 figure
Direct numerical study of speed of sound in dispersed air-water two-phase flow
Speed of sound is a key parameter for the compressibility effects in
multiphase flow. We present a new approach to do direct numerical simulations
on the speed of sound in compressible two-phase flow, based on the stratified
multiphase flow model (Chang & Liou, JCP 2007). In this method, each face is
divided into gas-gas, gas-liquid, and liquid-liquid parts via reconstruction of
volume fraction, and the corresponding fluxes are calculated by Riemann
solvers. Viscosity and heat transfer models are included. The effects of
frequency (below the natural frequency of bubbles), volume fraction, viscosity
and heat transfer are investigated. With frequency 1 kHz, under viscous and
isothermal conditions, the simulation results satisfy the experimental ones
very well. The simulation results show that the speed of sound in air-water
bubbly two-phase flow is larger when the frequency is higher. At lower
frequency, for the phasic velocities, the homogeneous condition is better
satisfied. Considering the phasic temperatures, during the wave propagation an
isothermal bubble behavior is observed. Finally, the dispersion relation of
acoustics in two-phase flow is compared with analytical results below the
natural frequency. This work for the first time presents an approach to the
direct numerical simulations of speed of sound and other compressibility
effects in multiphase flow, which can be applied to study more complex
situations, especially when it is hard to do experimental study
Optical quasi-periodic oscillation and color behavior of blazar PKS 2155-304
PKS 2155-304 is a well studied BL Lac object in the southern sky. The
historical optical data during different period have been collected and
compiled. Light curves with a time span of 35 years have been constructed. The
R-band light curve has been analyzed by means of three methods: epoch folding
method, Jurkevich method and discrete correlation function (DCF) method. It is
derived that there is an evident periodic component of 317 days (i.e. 0.87 yr)
superposed on a long-term trend with large-amplitude variation in the light
curve. The variability of this source is accompanied by a slight color
variation, and the brightness and color index are correlated with each other.
On the long time-scale, PKS 2155-304 exhibits a tendency of
bluer-when-brighter, which means the spectrum becomes flatter when the source
brightens.Comment: 9 pages, 4 figures. Accepted for publication in RAA (Research in
Astronomy and Astrophysics
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