We investigate a $(n+1)$-dimensional generalized Randall-Sundrum model with an anisotropic metric which has three different scale factors. One obtain a positive effective cosmological constant $\Omega_{eff}\sim10^{-124}$ (in Planck unit) which only need a solution $kr\simeq50-80$ without fine tuning, and both the visible and hidden brane tensions are positive which results in the two branes to be stable. Then, we find that the Hubble parameter is seem to be a constant in a large region near its minimum, thus causing the acceleration of the universe. Therefore, the fine tuning problem also can be solved in this model. Meanwhile, the scale of extra dimensions is smaller than the observed scale but greater than the Planck length. This demonstrates that the observed present acceleration of the universe is caused by the extra-dimensional evolution rather than dark energy.

中文翻译：

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广义 Randall-Sundrum 模型中由超维演化引起的宇宙加速

我们研究了具有各向异性度量的 $(n+1)$ 维广义 Randall-Sundrum 模型，该模型具有三个不同的比例因子。得到一个正的有效宇宙常数 $\Omega_{eff}\sim10^{-124}$（以普朗克单位），它只需要一个解 $kr\simeq50-80$ 无需微调，以及可见和隐藏的膜张力是正的，这导致两个膜是稳定的。然后，我们发现哈勃参数在其最小值附近的大区域内似乎是一个常数，从而导致宇宙加速。因此，在该模型中也可以解决微调问题。同时，额外维度的尺度小于观测尺度但大于普朗克长度。