Vacuum Transitions in Two-Dimensions and their Holographic Interpretation

We calculate amplitudes for 2D vacuum transitions by means of the Euclidean methods of Coleman-De Luccia (CDL) and Brown-Teitelboim (BT), as well as the Hamiltonian formalism of Fischler, Morgan and Polchinski (FMP). The resulting similarities and differences in between the three approaches are compared with their respective 4D realisations. For CDL, the total bounce can be expressed as the product of relative entropies, whereas, for the case of BT and FMP, the transition rate can be written as the difference of two generalised entropies, ultimately enabling to circumvent the need to resort to detailed balance. By means of holographic arguments, we show that the Euclidean methods, as well as the Lorentzian cases without non-extremal black holes, provide examples of an AdS$_2$/CFT$_1 \subset$ AdS$_3$/CFT$_2$ correspondence. Such embedding is not possible in the presence of islands for which the setup corresponds to AdS$_2$/CFT$_1 \not\subset$ AdS$_3$/CFT$_2$. We find that whenever an island is present, up-tunnelling is possible.