Every telescope is a time machine.
Look at the Moon and you see it a little more than a second ago.
The Sun appears roughly eight minutes in the past.
The nearest stars appear years in the past.
Distant galaxies can appear as they were billions of years ago.
Look far enough and astronomy becomes archaeology on the largest scale imaginable.
The universe is full of history.
Stars age.
Galaxies evolve.
Radiation carries ancient information.
Structures form.
Records accumulate.
The cosmic microwave background preserves evidence from a universe dramatically younger than the one we inhabit today.
Across enormous scales, those records appear to share a common historical orientation.
There is a direction from which evidence arrives and another toward which new consequences accumulate.
It is tempting to think that if scientists read enough of those records, they should eventually discover why the arrow points that way.
The Cosmological Arrow asks whether that inference actually follows.
Its answer is unexpectedly restrained.
A universe can be rich in history while remaining poor in evidence about the ultimate source of history's direction.