Is the DBI scalar field as fragile as other $k$-essence fields?

Understand

Caustic singularity formations in shift-symmetric $k$-essence and Horndeski theories on a fixed Minkowski spacetime were recently argued.

• In $n$ dimensions, this singularity is the $(n-2)$-dimensional plane in spacetime at which second derivatives of a field diverge and the field loses single-valued description for its evolution.
• This does not necessarily imply a pathological behavior of the system but rather invalidates the effective description.
• The effective theory would thus have to be replaced by another to describe the evolution thereafter.