Something is happening with Alt-J. Even before this past weekend’s two sold-out shows at the Paradise kicked off its current tour, the British four-piece announced a September show at the Bank of America Pavilion. Perhaps that explains the tense giddiness of Saturday’s packed house: the spectacle of seeing a band that had outgrown the venue before it even played there.

First, though, that band’s going to have to come up with a concert that lasts longer than 50 minutes, including encores. It’s not clear whether Alt-J (whose name is actually the Greek letter delta; don’t ask) is up to the task. The band seemed to say everything it needed to say in the time allotted.