We present an unconventional approach for addressing the old cosmological constant (CC) problem in a class of F(R,G) models of modified gravity. For a CC of arbitrary size and sign the corresponding cosmological evolution follows an expansion history which strikingly resembles that of our real universe. The effects of the large CC are relaxed dynamically and there is no fine-tuning at any stage. In addition, the relaxation mechanism alleviates the coincidence problem. The upshot is that a large cosmological constant and the observed cosmic expansion history coexist peacefully in the Relaxed Universe. This model universe can be thought of as an interesting preliminary solution of the cosmological constant problem, in the sense that it provides a successful dynamical mechanism able to completely avoid the fine-tuning problem (the toughest aspect of the CC problem). However, since the Relaxed Universe is formulated within the context of modified gravity, it may still suffer of some of the known issues associated with these theories, and therefore it can be viewed only as a toy-model proposal towards a final solution of the CC problem.
Can We Live on the Brane in Schwarzschild-anti de Sitter Black Hole?
本文研究了以Schwarzschild-anti de Sitter黑洞（S-AdS BH）为精确解的d5高阶导数引力模型。在完备作用量中加入表面反项，当膜趋于无穷大时，表面反项由体AdS时空的有限性条件确定。因此，膜（可观测宇宙）是根据d5理论参数动态定义的（膜张力是固定的）。膜半径总是大于视界半径，4D宇宙本身可能是静态的或暴胀的。这个模型的自然一般化给出了AdS/CFT对应的次领先顺序的翘曲紧化（动态生成膜）。
