<<...I am currently working on a work of fiction that takes place in the dieing [sic] day of the universe...>>

The universe is not going to die, but all sentients within it will.

Long before the universe could possibly end the life-sustaining circumstances within it will become impossibly, satisically unlikely.

You needn't worry about how the universe will ultimately end, only how life within it will ultimately run out of ecological niches.