Theoretically time travel does less “violence” to our understanding of physics than does faster-than-light travel. However, that presupposes travel TO the future; not to the past.
Obviously there’s more to learn here but, if these things aren’t terrestrial and current events, then time travel seems vastly more likely to me than aliens.
There are many theoretical physics folks who argue travel to the past is possible—but most are believers in the “block universe” theory—a theory which makes no sense at all to me.
Just to continue:
The block universe theory allows you to travel in the past to pet Hitler’s cat but does not allow you to kill Hitler—crazy stuff—just makes no sense.