118528998
Note on self-gravitating radiation in AdS spacetime
Recently Vaganov and Hammersley investigated independently the equilibrium self-gravitating radiation in higher (d >= 4)-dimensional, spherically symmetric anti-de Sitter space. It was found that in 4 = 11, the temperature, mass, and entropy of the self-gravitating configuration are monotonic functions of the central energy density, asymptoting to their maxima as the central density goes to infinity. In this paper we investigate the equilibrium self-gravitating radiation in higher-dimensional, plane-symmetric anti-de Sitter space. We find that there exist essential differences from the spherically syrnmetric case: In each dimension (d >= 4), there are maximal mass (density), maximal entropy (density), and maximal temperature configurations; they do not appear at the same central energy density; the oscillation behavior appearing in the spherically symmetric case does not happen in this case; and the mass (density), as a function of the central energy density, increases first and reaches its maximum at a certain central energy density and then decreases monotonically in 4 = 8, besides the maximum, the mass (density) of the equilibrium configuration has a minimum: the mass (density) first increases and reaches its maximum, then decreases to its minimum, and then increases to its asymptotic value monotonically. The reason causing the difference is discussed.
15707844
Holography, UV/IR Relation, Causal Entropy Bound, and Dark Energy
The constraint on the total energy in a given spatial region is given from holography by the mass of a black hole that just fits in that region, which leads to an UV/IR relation: the maximal energy density in that region is propotional to M(p)(2)/L(2), wher M(p) is the Planck mass and L is the spatial scale of that region under consideration. Assuming that maximal black hole in the universe is formed through gravitational collapse of perturbations in the universe, then the "Jeans" scale of the perturbations gives a causal connection scale R(CC). For gravitational perturbations, R(CC)(-2) = Max ((H) over dot + 2H(2), -(H) over dot) for a flat universe. We study the cosmological dynamics of the corresponding vacuum energy density by choosing the causal connection scale as the IR cutoff in the UV/IR relation, in the cases of the vacuum energy density as an independently conserved energy component and an effective dynamical cosmological constant, respectively. It turns out that only the case with the choice R(CC)(-2) = (H) over dot + 2H(2), could be consistent with the current cosmological observations when the vacuum density appears as an independently conserved energy component. In this case, the model is called holographic Ricci scalar dark energy model in the literature.
118029650
Future CMB integrated-Sachs-Wolfe-lensing bispectrum constraints on modified gravity in the parametrized post-Friedmann formalism
We forecast the constraints on both Hu-Sawicki model and Bertschinger-Zukin model of modified gravity within the Parameterized Post-Friedmann (PPF) formalism for the Planck satellite experiment by performing the joint analysis of ISW-Lensing bispectrum and CMB power spectrum. We find that, even considering the temperature-temperature mode of CMB power spectrum only, Planck data are expected to reduce the error bars on the modified gravity parameter $B_0$ (related to the present value of Compton wavelength of the extra scalar degree of freedom) at least one order magnitude compared with WMAP. The spectrum-bispectrum joint analysis can further improve the results by a factor ranging from 1.14 to 5.32 depending on the specific modified gravity model. One of our main results is that the cross-correlation between ISW-Lensing bispectrum and power spectrum can be safely neglected when performing the joint analysis. For simplicity, we only investigate the likelihood of one parameter ({$B_0$}) and fix all other cosmological parameters to their best-fit values in WMAP7yr results.
118634823
Gauss–Bonnet term on vacuum decay
We study the effect of the Gauss-Bonnet term on vacuum decay process in the Colennan-De Luccia formalism. The Gauss-Bonnet term has ail exponential Coupling With the real scalar held, which appears in the low energy effective action of string theories. We Calculate numerically the instanton Solution, which describes the process of vacuum decay, and obtain the critical size Of bubble. We find that the Gauss-Bonnet term has a nontrivial effect on the false Vacuum decay, depending oil the Gauss-Bonnet coefficient. (C) 2008 Elsevier B.V. All rights reserved.
118657913
Constraining models of f(R) gravity with Planck and WiggleZ power spectrum data
In order to explain cosmic acceleration without invoking "dark" physics, we consider f(R) modified gravity models, which replace the standard Einstein-Hilbert action in General Relativity with a higher derivative theory. We use data from the WiggleZ Dark Energy survey to probe the formation of structure on large scales which can place tight constraints on these models. We combine the large-scale structure data with measurements of the cosmic microwave background from the Planck surveyor. After parameterizing the modification of the action using the Compton wavelength parameter B0, we constrain this parameter using ISiTGR, assuming an initial non-informative log prior probability distribution of this cross-over scale. We find that the addition of the WiggleZ power spectrum provides the tightest constraints to date on B0 by an order of magnitude, giving log10(B0) < -4.07 at 95% confidence limit. Finally, we test whether the effect of adding the lensing amplitude ALens and the sum of the neutrino mass ∑mν is able to reconcile current tensions present in these parameters, but find f(R) gravity an inadequate explanation.
118426631
Acoustic signatures in the Cosmic Microwave Background bispectrum from primordial magnetic fields
Using the full radiation transfer function, we numerically calculate the CMB angular bispectrum seeded by the compensated magnetic scalar density mode. We find that, for the string inspired primordial magnetic fields characterized by index nB = −2.9 and mean-field amplitude Bλ = 9nG, the angular bispectrum is dominated by two primordial magnetic shapes. The first magnetic shape looks similar to the one from local-type primordial curvature perturbations, so both the amplitude and profile of the Komatsu-Spergel estimator (reduced bispectrum) seeded by this shape are almost the same as those of the primary CMB anisotropies. However, for different parameter sets (l1,l2), this ``local-type'' reduced bispectrum oscillates around different asymptotic values in the high-l3 regime because of the effect of the Lorentz force, which is exerted by the primordial magnetic fields on the charged baryons. This feature is different from the standard case where all modes approach to zero asymptotically in the high-l limit. On the other hand, the second magnetic shape appears only in the primordial magnetic field model. The amplitude of the Komatsu-Spergel estimator sourced by the second shape diverges in the low-l regime because of the negative slope of shape. In the high-l regime, this amplitude is approximately equal to that of the first estimator, but with a reversal phase.
117131297
Noise Kernel and Stress Energy Bi-Tensor of Quantum Fields in Conformally-Optical Metrics: Schwarzschild Black Holes
In Paper II [N. G. Phillips and B. L. Hu, previous abstract] we presented the details for the regularization of the noise kernel of a quantum scalar field in optical spacetimes by the modified point separation scheme, and a Gaussian approximation for the Green function. We worked out the regularized noise kernel for two examples: hot flat space and optical Schwarzschild metric. In this paper we consider noise kernels for a scalar field in the Schwarzschild black hole. Much of the work in the point separation approach is to determine how the divergent piece conformally transforms. For the Schwarzschild metric we find that the fluctuations of the stress tensor of the Hawking flux in the far field region checks with the analytic results given by Campos and Hu earlier [A. Campos and B. L. Hu, Phys. Rev. D {\bf 58} (1998) 125021; Int. J. Theor. Phys. {\bf 38} (1999) 1253]. We also verify Page's result [D. N. Page, Phys. Rev. {\bf D25}, 1499 (1982)] for the stress tensor, which, though used often, still lacks a rigorous proof, as in his original work the direct use of the conformal transformation was circumvented. However, as in the optical case, we show that the Gaussian approximation applied to the Green function produces significant error in the noise kernel on the Schwarzschild horizon. As before we identify the failure as occurring at the fourth covariant derivative order.
118510249
Effects of Symmetry Energy in the Reaction 40Ca+124Sn at 140 MeV/nucleon
The density-dependent symmetry energy is a hot topic in nuclear physics. Many laboratories over the world are planning to perform related experiments to probe the symmetry energy. Based on the semiclassical Boltzmann-Uehling-Uhlenbeck (BUU) transport model, we study the effects of nuclear symmetry energy in the central reaction 40Ca+124Sn at 140MeV/nucleon in the laboratory system. It is found that the rapidity distribution of free nucleon's neutron-to-proton ratio is sensitive to the symmetry energy, especially at large rapidities. The free neutron-to-proton ratios at small or large rapidities may reflect high or low density behavior of nuclear symmetry energy. To probe the density dependence of nuclear symmetry energy, it is better to give the kinetic distribution and the rapidity distribution of emitted nucleons at the same time.
121974058
Finite temperature quantum fields in expanding universes
Abstract The thermodynamics of an ideal relativistic quantum gas in expansion is studied. It is found that only for conformally invariant fields in conformally static spacetime can thermal equilibrium be strictly maintained. A finite temperature theory can be defined under the condition of quasi equilibrium when the background expansion is nearly adiabatic. The high temperature expansion of the energy density for massive nonconformal fields in Robertson-Walker universes and for conformal fields in Bianchi Type-I universes are calculated. The importance of these results on phase transition and quantum processes in the early universe is discussed.
118622161
Stochastic Gravity: Theory and Applications
Whereas semiclassical gravity is based on the semiclassical Einstein equation with sources given by the expectation value of the stress-energy tensor of quantum fields, stochastic semi-classical gravity is based on the Einstein-Langevin equation, which has, in addition, sources due to the noise kernel. The noise kernel is the vacuum expectation value of the (operator-valued) stress-energy bitensor, which describes the fluctuations of quantum-matter fields in curved spacetimes. A new improved criterion for the validity of semiclassical gravity may also be formulated from the viewpoint of this theory. In the first part of this review we describe the fundamentals of this new theory via two approaches: the axiomatic and the functional. The axiomatic approach is useful to see the structure of the theory from the framework of semiclassical gravity, showing the link from the mean value of the stress-energy tensor to the correlation functions. The functional approach uses the Feynman-Vernon influence functional and the Schwinger-Keldysh closed-time-path effective action methods. In the second part, we describe three applications of stochastic gravity. First, we consider metric perturbations in a Minkowski spacetime, compute the two-point correlation functions of these perturbations and prove that Minkowski spacetime is a stable solution of semiclassical gravity. Second, we discuss structure formation from the stochastic-gravity viewpoint, which can go beyond the standard treatment by incorporating the full quantum effect of the inflaton fluctuations. Third, using the Einstein-Langevin equation, we discuss the backreaction of Hawking radiation and the behavior of metric fluctuations for both the quasi-equilibrium condition of a black-hole in a box and the fully nonequilibrium condition of an evaporating black hole spacetime. Finally, we briefly discuss the theoretical structure of stochastic gravity in relation to quantum gravity and point out directions for further developments and applications.
119340655
Accelerated detector-quantum field correlations: From vacuum fluctuations to radiation flux
In this paper we analyze the interaction of a uniformly accelerated detector with a quantum field in (3+1)D spacetime, aiming at the issue of how kinematics can render vacuum fluctuations the appearance of thermal radiance in the detector (Unruh effect) and how they engender flux of radiation for observers afar. Two basic questions are addressed in this study: (a) How are vacuum fluctuations related to the emitted radiation? (b) Is there emitted radiation with energy flux in the Unruh effect? We adopt a method which places the detector and the field on an equal footing and derive the two-point correlation functions of the detector and of the field separately with full account of their interplay. From the exact solutions, we are able to study the complete process from the initial transient to the final steady state, keeping track of all activities they engage in and the physical effects manifested. We derive a quantum radiation formula for a Minkowski observer. We find that there does exist a positive radiated power of quantum nature emitted by the detector, with a hint of certain features of the Unruh effect. We further verify that the total energy of the dressed detector and a partmore » of the radiated energy from the detector is conserved. However, this part of the radiation ceases in steady state. So the hint of the Unruh effect in radiated power is actually not directly from the energy flux that the detector experiences in Unruh effect. Since all the relevant quantum and statistical information about the detector (atom) and the field can be obtained from the results presented here, they are expected to be useful, when appropriately generalized, for addressing issues of quantum information processing in atomic and optical systems, such as quantum decoherence, entanglement, and teleportation.« less