Comments on the vacuum energy decay

Understand

The {\em instability of vacuum energy} in de Sitter space as discussed recently by Polyakov is argued to be a generic feature when external gravitational fields are present.

• It is related to the existence of {\em forbidden} (by momentum conservation) decays derived in some detail by Bros, Epstein and Moschella.
• Some calculations are discussed in a conformally invariant setting, and a flat space model is presented.