http://arxiv.org/abs/1708.00441
GAUGE INVARIANCE: The Sachs-Wolfe formula describing the Cosmic Microwave Background (CMB) temperature anisotropies is one of the most important relations in cosmology. Despite its importance, the gauge invariance of this formula has only been discussed at first order. Here we discuss the subtle issue of second-order gauge transformations on the CMB. By introducing two rules (needed to handle the subtle issues), we prove the gauge invariance of the second order Sachs-Wolfe formula and provide several compact expressions which can be useful for the study of gauge transformations on cosmology. THE RIVER-FRAME: we introduce a cosmological frame which we call the river-frame. In this frame, photons and any object can be thought as fishes swimming in the river and relations are easily expressed in either the metric or the covariant formalism then ensuring a transparent geometric meaning. Finally, our results show that the river-frame is useful to make perturbative and non-perturbative analysis. In particular, it was already used to obtain the fully non-linear generalization of the Sachs-Wolfe formula and is used here to describe second order perturbations.
O. Roldan
Mon, 7 Aug 17
39/54
Comments: 26 pages, 2 figures
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