Generalized multi-Galileons, covariantized new terms, and the no-go theorem for non-singular cosmologies

Understand

It has been pointed out that non-singular cosmological solutions in second-order scalar-tensor theories generically suffer from gradient instabilities.

• We extend this no-go result to second-order gravitational theories with an arbitrary number of interacting scalar fields.
• Our proof follows directly from the action of generalized multi-Galileons, and thus is different from and complementary to that based on the effective field theory approach.
• Several new terms for generalized multi-Galileons on a flat background were proposed recently.