119229309
Non-Gaussian features from the inverse volume corrections in loop quantum cosmology
In this paper we study the non-Gaussian features of the primordial fluctuations in loop quantum cosmology with the inverse volume corrections. The detailed analysis is performed in the single field slow-roll inflationary models. However, our results reflect the universal characteristics of bispectrum in loop quantum cosmology. The main corrections to the scalar bispectrum come from two aspects: one is the modifications to the standard Bunch-Davies vacuum, and the other is the corrections to the background dependent variables, such as slow-roll parameters. Our calculations show that the loop quantum corrections make f(NL) of the inflationary models increase 0.1%. Moreover, we find that two new shapes of non-Gaussian signal arise, which we name F-1 and F-2. The former gives a unique loop quantum feature, which is less correlated with the local, equilateral, and single types, while the latter is highly correlated with the local one.
119107827
Holographic thermalization and gravitational collapse in a spacetime dominated by quintessence dark energy
In this paper, the thermalization has been studied holographically. Explicitly in the gravity side, we consider the gravitational collapse of a thin shell of dust in a spacetime dominated by quintessence dark energy. With the thermalization probes such as the normalized geodesic length and minimal area surface, we study the effect of the state parameter for the quintessence dark energy on the thermalization. Our results show that the smaller the state parameter of quintessence is, the harder the plasma to thermalize. We also investigate the thermalization velocity and thermalization acceleration. We hope our results here can shed light on the nature of the quintessence dark energy.
54926043
Thermodynamics in Loop Quantum Cosmology
Loop quantum cosmology (LQC) is very powerful to deal with the
behavior of early universe. Moreover, the
effective loop quantum cosmology gives a successful description of
the universe in the semiclassical region. We consider the apparent
horizon of the Friedmann-Robertson-Walker universe as a
thermodynamical system and investigate the thermodynamics of LQC
in the semiclassical region. The effective density and effective
pressure in the modified Friedmann equation from LQC not only
determine the evolution of the universe in LQC scenario but
also are actually found to be the thermodynamic
quantities. This result comes from the energy definition in
cosmology (the Misner-Sharp gravitational energy) and is
consistent with thermodynamic laws. We prove that within the
framework of loop quantum cosmology, the elementary equation of
equilibrium thermodynamics is still valid.
123143121
Gravitational wave detection: stochastic resonance method with matched filtering
Along with the development of the interferometric gravitational wave detector, we enter into an epoch of the gravitational wave astronomy, which will open a brand new window for astrophysics to observe our universe. However, the gravitational wave detection is a typical weak signal detection, and this weak signal is buried in a strong instrument noise. To our knowledge, almost all of the data analysis methods in gravitational wave detection at present are based on a matched filtering. So it is desirable to take advantage of stochastic resonance methods. However, the all of the stochastic resonance methods are general based on a Fourier transformation and fall short of the matched filtering as a usable technique. In this paper we relate the stochastic resonance to the matched filtering. Our results show that the stochastic resonance can indeed be combined with the matched filtering for both the periodic and the non-periodic input signal. This encouraging result will be the first step to apply the stochastic resonance to the matched filtering in gravitational wave detection. Moreover, based on the matched filtering, we firstly propose a novel measurement method for the stochastic resonance which is valid for both the periodic and the non-periodic driven signal.
119237835
Dynamical Horizon Entropy Bound Conjecture in Loop Quantum Cosmology
The covariant entropy bound conjecture is an important hint for the quantum gravity, with several versions available in the literature. For cosmology, Ashtekar and Wilson-Ewing ever show the consistence between the loop gravity theory and one version of this conjecture. Recently, He and Zhang [J. High Energy Phys. 10(2007)077] proposed a version for the dynamical horizon of the universe,which validates the entropy bound conjecture for the cosmology filled with perfect fluid in the classical scenario when the universe is far away from the big bang singularity. However,their conjecture breaks down near big bang region. We examine this conjecture in the context of the loop quantum cosmology. With the example of photon gas,this conjecture is protected by the quantum geometry effects as expected.
118624166
Averaged null energy condition in loop quantum cosmology
Wormholes and time machines are objects of great interest in general relativity. However, to support them it needs exotic matters which are impossible at the classical level. Semiclassical gravity introduces the quantum effects into the stress-energy tensor and constructs many self-consistent wormholes. But they are not traversable due to the averaged null energy condition. Loop quantum gravity (LQG) significantly modifies the Einstein equation in the deep quantum region. If we write the modified Einstein equation in the form of the standard one but with an effective stress-energy tensor, it is convenient to analyze the geometry in LQG through the energy condition. Loop quantum cosmology (LQC), an application of LQG, has an effective stress-energy tensor which violates some kinds of local energy conditions. So it is natural that the inflation emerges in LQC. In this paper, we investigate the averaged null energy condition in LQC in the framework of the effective Hamiltonian, and we find that the effective stress-energy tensor in LQC violates the averaged null energy condition in the massless scalar field coupled model.
16864354
Evidence for single top quark production at D0
We present evidence for the production of single top quarks at the Fermilab Tevatron p collider. Using a 0.9 fb-1 dataset collected with D0 detector, the analysis is performed in the electron+jets and muon+jets decay modes, taking special care in modeling the large backgrounds, applying a new powerful b-quark tagging algorithm and using three multivariate techniques to extract the small signal. The combined measured production cross section is σ(p tb + X, tqb + X) = 4.7 ± 1.3 pb. The probability to measure a cross section at this value or higher in the absence of signal is 0.014%, corresponding to a 3.6 standard deviation significance. We use the cross section measurement to directly determine the CKM matrix element that describes the Wtb coupling and find 0.68 < |Vtb|≤ 1 at 95% C.L. within the standard model.
36263241
New optimization methods for converging perturbative series with a field cutoff
We take advantage of the fact that in lambda phi ^4 problems a large field cutoff phi_max makes perturbative series converge toward values exponentially close to the exact values, to make optimal choices of phi_max. For perturbative series terminated at even order, it is in principle possible to adjust phi_max in order to obtain the exact result. For perturbative series terminated at odd order, the error can only be minimized. It is however possible to introduce a mass shift in order to obtain the exact result. We discuss weak and strong coupling methods to determine the unknown parameters. The numerical calculations in this article have been performed with a simple integral with one variable. We give arguments indicating that the qualitative features observed should extend to quantum mechanics and quantum field theory. We found that optimization at even order is more efficient that at odd order. We compare our methods with the linear delta-expansion (LDE) (combined with the principle of minimal sensitivity) which provides an upper envelope of for the accuracy curves of various Pade and Pade-Borel approximants. Our optimization method performs better than the LDE at strong and intermediate coupling, but not at weak coupling where it appears less robust and subject to further improvements. We also show that it is possible to fix the arbitrary parameter appearing in the LDE using the strong coupling expansion, in order to get accuracies comparable to ours.
118646964
Large Field Cutoffs in Lattice Gauge Theory
In pure gauge SU(3) near beta = 6, weak and strong coupling expansions break down and the MC method seems to be the only practical alternative. We discuss the possibility of using a modified version of perturbation theory which relies on a large field cutoff and has been successfully applied to the double-well potential (Y. M., PRL 88 141601). Generically, in the case of scalar field theory, the weak coupling expansion is unable to reproduce the exponential suppression of the large field configurations. This problem can be solved by introducing a large field cutoff. The value of this cutoff can be chosen to reduce the discrepancy with the original problem. This optimization can be approximately performed using the strong coupling expansion and bridges the gap between the two expansions. We report recent attempts to extend this procedure for SU(3) gauge theory on the lattice. We compare gauge invariant and gauge dependent (in the Landau gauge) criteria to sort the configurations into ``large-field'' and ``small-field'' configurations. %We discuss the effects of discarding the large field configurations. We discuss the convergence of lattice perturbation theory and the way it can be modified in order to obtain results similar to the scalar case.
17303015
Must time machines be unstable against vacuum fluctuations
In the appearance of an absorption material, the quantum vacuum fluctuations of all kinds of fields may be smoothed out and the spacetime with time machines may be stable against vacuum fluctuations. The chronology protection conjecture may break down and the anti-chronology protection conjecture may hold: there is no law of physics preventing the appearance of closed timelike curves.
100860790
A curve-based material recognition method in MeV dual-energy X-ray imaging system
High-energy dual-energy X-ray digital radiography imaging is mainly used in the material recognition of cargo inspection. We introduce the development history and principle of the technology and describe the data process flow of our system. The system corrects original data to get a dual-energy transparence image. Material categories of all points in the image are identified by the classification curve, which is related to the X-ray energy spectrum. For the calibration of classification curve, our strategy involves a basic curve calibration and a real-time correction devoted to enhancing the classification accuracy. Image segmentation and denoising methods are applied to smooth the image. The image contains more information after colorization. Some results show that our methods achieve the desired effect.