Brane universe and multigravity: modification of gravity at large and small distances

We consider the modification of gravity at large distances in Brane Universes which was discussed recently [I.I. Kogan, S. Mouslopoulos, A. Papazoglou, G.G. Ross, J. Santiago, hep-ph/9912552; R. Gregory, V.A. Rubakov, S.M. Sibiryakov, hep-th/0002072; C. Csaki, J. Erlich, T.J. Hollowood, hep-th/0002161; G. Dvali, G. Gabadadze, M. Porrati, hep-th/0002190]. In these models the modification of gravity at large distances is connected to the existence of a negative tension brane(s) and an exponentially small tunneling factor. We discuss a general model which interpolates between the Bi-gravity + - + model [I.I. Kogan, S. Mouslopoulos, A. Papazoglou, G.G. Ross, J. Santiago, hep-ph/9912552] and the GRS model [R. Gregory, V.A. Rubakov, S.M. Sibiryakov, hep-th/0002072]. We also discuss the possible mechanism of stabilization of negative tension branes in an AdS background. Finally we show that extra degrees of freedom of massive gravitons do not lead to a disastrous contradiction with General Relativity if the stabilization condition integral dy root-G((5)) (T-mu(mu) - 2T(5)(5)) = 0 [P. Kanti, I.I. Kogan, K.A. Olive, M. Pospelov, hep-ph/9912266] is implemented (C) 2000 Elsevier Science B.V. All rights reserved.