关于f（R）引力理论中密度扰动的演化
On the evolution of density perturbations in f(R) theories of gravity
摘要：
在引力的$f(R)$理论的背景下，我们研究了标量宇宙学扰动在度规形式下的演化。利用完全一般的方法，我们找到了纵向规范中物质密度扰动的精确四阶微分方程。在子哈勃模式的情况下，表达式减少到一个二阶方程，这是与文献中使用的标准（准静态）方程相比。我们表明，一般的$f(R)$功能的准静态近似是不合理的。然而，对于那些充分描述加速膨胀的当前阶段并满足局部重力检验的函数，它提供了对扰动演化的正确描述。
英文摘要：
In the context of $f(R)$ theories of gravity, we study the evolution of scalar cosmological perturbations in the metric formalism. Using a completely general procedure, we find the exact fourth-order differential equation for the matter density perturbations in the longitudinal gauge. In the case of sub-Hubble modes, the expression reduces to a second-order equation which is compared with the standard (quasi-static) equation used in the literature. We show that for general $f(R)$ functions the quasi-static approximation is not justified. However, for those functions adequately describing the present phase of accelerated expansion and satisfying local gravity tests, it provides a correct description for the evolution of perturbations.
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