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arXiv 1003.2850hep-th
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酉引力理论中黑洞熵的界限

Bounds on Black Hole Entropy in Unitary Theories of Gravity

Ram Brustein, A. J. M. Medved

摘要:

我们考虑酉和弱耦合引力理论,扩展爱因斯坦引力,并减少到它在大距离渐近。我们的讨论仅限于这样的理论,类似于爱因斯坦引力,包含黑洞作为半经典状态在一定范围内的尺度。我们表明,在给定的尺度下,这些黑洞的熵必须大于理论中的基本光物种的数量。我们的界限是根据观察得出的,即黑洞的熵必须大于它的质量和视界半径的乘积(单位是普朗克常数除以光速),而且对于任何半经典黑洞,这个乘积必须大于轻物质的数量。对于服从我们假设的理论,这个界限解决了“物种问题”:黑洞熵的几何、物种独立性与普通热力学熵与物种数的比例之间的紧张关系。然后,我们表明,当黑洞在爱因斯坦的理论相比,在一个固定的质量值的扩展理论,爱因斯坦黑洞的熵将永远是最小的。类似的考虑也适用于反德西特空间中黑膜的熵密度。

英文摘要:

We consider unitary and weakly coupled theories of gravity that extend Einstein gravity and reduce to it asymptotically at large distances. Our discussion is restricted to such theories that, similarly to Einstein gravity, contain black holes as semiclassical states in a range of scales. We show that, at a given scale, the entropy of these black holes has to be larger than the number of elementary light species in the theory. Our bound follows from the observation that the black hole entropy has to be larger than the product of its mass and horizon radius (in units of Planck's constant divided by the speed of light) and the fact that, for any semiclassical black hole, this product has to be larger than the number of light species. For theories that obey our assumptions, the bound resolves the "species problem": the tension between the geometric, species-independent nature of black hole entropy and the proportionality of ordinary thermodynamic entropy to the number of species. We then show that, when black holes in Einstein's theory are compared to those in the extended theories at a fixed value of mass, the entropy of the Einstein black holes will always be minimal. Similar considerations are also applied to the entropy density of black branes in anti-de Sitter space.

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