“…Nevertheless, such energy components have been known for some time [25,26,27] and their role as possible dark energy candidates was raised by Caldwell [28], who referred to DEC-violating sources as "phantom" components. The implications of these have since been investigated by several authors (for some examples see [16,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44]. Typically, DEC-violating sources with w < −1 are subject to violent instabilities, although these may conceivably be cured in models with higher-derivative kinetic terms [34,43,44].…”