"Since then" is a common phrase used to refer to something that happened in the past and continues to be relevant in the present. However, there are various synonyms that can be used to convey the same meaning. Some alternative phrases include "from that point on", "from that moment forward", "ever since", "since that time", "thereafter", "consequently", "as a result", and "henceforth". These phrases can be used interchangeably depending on the context in which they are used. They all emphasize a particular point in time when something significant occurred, and the subsequent effects that have followed.