We discuss analytic continuation from d-dimensional Lorentzian de Sitter
(dSd​) to d-dimensional Lorentzian anti-de Sitter (AdSd​) spacetime. We
show that AdSd​, with opposite signature of the metric, can be obtained as
analytic continuation of a portion of dSd​. This implies that the dynamics of
(positive square-mass) scalar particles in AdSd​ can be obtained from the
dynamics of tachyons in dSd​. We discuss this correspondence both at the
level of the solution of the field equations and of the Green functions. The
AdS/CFT duality is obtained as analytic continuation of the dS/CFT duality.Comment: 17 pages, 1 figure, JHEP styl