  We propose an algebra of operators along an observer's worldline as a
background-independent algebra in quantum gravity. In that context, it is
natural to think of the Hartle-Hawking no boundary state as a universal state
of maximum entropy, and to define entropy in terms of the relative entropy with
this state. In the case that the only spacetimes considered correspond to de
Sitter vacua with different values of the cosmological constant, this
definition leads to sensible results.
