分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We study the phase structure of quantum improved Schwarzschild-(A)dS black holes in asymptotically safe gravity. The quantum effect provides a repulsive force in the core region near singularity which stabilizes the thermodynamically unstable small black holes. It also creates a zero temperature state with finite size which could be a candidate for dark matter. Moreover, there is a new second order phase transition between small and large black holes for quantum improved Schwarzschild-Anti de Sitter black holes. We also discuss the black holes with different spatial topologies and find a notable duality.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We study the phase structure of quantum improved Schwarzschild-(A)dS black holes in asymptotically safe gravity. The quantum effect provides a repulsive force in the core region near singularity which stabilizes the thermodynamically unstable small black holes. It also creates a zero temperature state with finite size which could be a candidate for dark matter. Moreover, there is a new second order phase transition between small and large black holes for quantum improved Schwarzschild-Anti de Sitter black holes. We also discuss the black holes with different spatial topologies and find a notable duality.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-09-27
摘要： We study asymptotically AdS topological black hole solutions with k = 0 (plane symmetric) in the Einstein gravity with Gauss-Bonnet term, the dilaton and a “cosmo-logical constant” in various dimensions. We derive the field equations for suitable ansatz for general D dimensions. We determine the parameter regions including dilaton cou-plings where such solutions exist and construct black hole solutions of various masses numerically in D = 4, 5, 6 and 10 dimensional spacetime with (D − 2)-dimensional hypersurface of zero curvature.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-09-27
摘要： We study cosmological solutions in the low-energy effective heterotic string theory, which is the Einstein gravity with Gauss-Bonnet term and the dilaton. We show that the field equations are cast into an autonomous system for flat internal and external spaces, and derive all the fixed points in the system. We also examine the time evolution of the solutions and whether the solutions can give (transient) accelerated expansion of our four-dimensional space in the Einstein frame.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-09-27
摘要： We place observational constraints on a coupling between dark energy and dark matter by using 71 Type Ia supernovae (SNe Ia) from the first year of the five-year Supernova Legacy Survey (SNLS), the cosmic microwave background (CMB) shift parameter from the three-year Wilkinson Microwave Anisotropy Probe (WMAP), and the baryon acoustic oscillation (BAO) peak found in the Sloan Digital Sky Survey (SDSS). The interactions we study are (i) constant coupling #14; and (ii) varying coupling #14;(z) that depends on a redshift z, both of which have simple parametrizations of the Hubble parameter to confront with observational data. We find that the combination of the three databases marginalized over a present dark energy density gives stringent constraints on the coupling,−0.08 < #14; < 0.03 (95% CL) in the constant coupling model and −0.4 < #14;0 < 0.1 (95% CL) in the varying coupling model, where #14;0 is a present value. The uncoupled #3;CDM model (wX = −1 and #14; = 0) still remains a good fit to the data, but the negative coupling (#14; < 0) with the equation of state of dark energy wX < −1 is slightly favoured over the #3;CDM model.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-09-27
摘要： We discuss the realization of inflation and resulting cosmological perturbations in the low-energy effective string theory. In order to obtain nearly scale-invariant spectra of density perturbations and a suppressed tensor-to-scalar ratio, it is generally necessary that the dilaton field #30; is effectively decoupled from gravity together with the existence of a slowly varying dilaton potential. We also study the effect of second-order corrections to the tree-level action which are the sum of a Gauss-Bonnet term coupled to #30; and a kinetic term (∇#30;)4. We find that it is possible to realize observationally supported spectra of scalar and tensor perturbations provided that the correction is dominated by the (∇#30;)4 term even in the absence of the dilaton potential. When the Gauss-Bonnet term is dominant, tensor perturbations exhibit violent negative instabilities on small-scales about a de Sitter background in spite of the fact that scale-invariant scalar perturbations can be achieved.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-09-27
摘要： We develop a theoretical method of constructing the quintessence potential directly from the effective equation of state function w(z), which describes the properties of the dark energy. We apply our method to four parametrizations of equation of state parameter and discuss the general features of the resulting potentials. In particular, it is shown that the constructed quintessence potentials are all in the form of a runaway type.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-09-27
摘要： We study spherically symmetric, asymptotically flat black hole solutions in the low-energy e ective heterotic string theory, which is the Einstein gravity with Gauss-Bonnet term and the dilaton, in various dimensions. We derive the field equations for suitable ansatz for general D dimensions and construct black hole solutions of various masses numerically in D = 4, 5, 6 and 10 dimensional spacetime with (D − 2)-dimensional hypersurface with positive constant curvature. A detailed comparison with the non-dilatonic solutions is made. We also examine the thermodynamic properties of the solutions. It is found that the dilaton has significant e ects on the black hole solutions, and we discuss physical consequences.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-09-27
摘要： We investigate the cosmological evolution of the system of a Dirac-Born-Infeld field plus a perfect fluid. We analyze the existence and stability of scaling solutions for the AdS throat and the quadratic potential. We find that the scaling solutions exist when the equation of state of the perfect fluid is negative and in the ultra-relativistic limit.
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