The accelerated expansion of the universe leads to a horizon beyond which we cannot see.   Structure arises from the quantum variance of early universe fields stretched across the horizon, according to the simplest theory fitting observations.  In concrete new models in string- (or `M-’) theory, the accelerating universe itself relies on the variance of extra dimensional fields, analogously to the quantum stability of atoms, stars and materials.   Theoretical calculations also suggest an enormous unobserved entropy associated with the de Sitter horizon.    Recent and ongoing work of multiple research groups has refined and sharpened the meaning of this entropy along with other thermodynamic quantities such as energy and temperature.  Timelike features – such as a fixed-metric boundary or an observer within the system – play a key role, as do novel deformations of (2d) field theories defining a family of quantum systems with a finite Hilbert space.  These structures combine naturally into a new proposal for holography of a bounded patch of (3d) de Sitter (at large radius and with matter included),  realizing the finite entropy as a microstate count in a non-gravitational boundary theory.

In addition to introducing these developments, the talk will briefly survey high-energy cosmology more broadly.   Please see the associated PCTS workshop for more!