Second-Order Gauge-Invariant Cosmological Perturbation Theory: Current Status
Figure 1
(a) The second term in (4) is the vector which points from the point to the point in the sense of the first-order correction. (b) If we look at the neighborhood of the point in detail, the vector may fail to point to in the sense of the second order. Therefore, it is necessary to add the second-order correction .