Signatures of relic quantum nonequilibrium
  [Shortened Abstract:] This thesis explores the possibility that quantum
probabilities arose thermodynamically. A chief concern is the detection of
primordial `quantum nonequilibrium', since this is observably distinct from
textbook quantum physics. Chapters 2, 3, 4, and 5 are adaptations of references
[1,2,3], and [4] respectively.
  Chapter 2 proposes (information) entropy conservation as a minimal
requirement for a theory to feature classical-style thermodynamic relaxation.
The resulting structure is dubbed `the iRelax framework'. Both classical
mechanics and de Broglie-Bohm quantum theory are shown to be special cases.
Indications for a possible extension or unification of de Broglie-Bohm theory
are briefly highlighted.
  Chapter 3 examines ways in which quantum relaxation may be prevented. The
method of the drift-field is introduced. A systematic treatment of nodes is
given. A category of quantum states is found for which relaxation is
significantly impeded, and may not complete at all.
  Chapters 4 and 5 consider the possibility that primordial quantum
nonequilibrium may be conserved in the statistics of a species of relic
cosmological particle. Necessary factors for this to be the case are discussed
and illustrative scenarios are given both in terms of nonequilibrium particles
created by inflaton decay, as well as relic vacuum modes for species that
decoupled close to the Planck temperature. The search for so-called
`smoking-gun' spectral lines created by dark matter decay or annihilation is
argued to be a particularly promising setting for the detection of quantum
nonequilibrium. Unintuitive spectral effects relating to the contextuality of
quantum measurements are described. If such a suspected source of quantum
nonequilibrium were found, its subjection to a specifically quantum mechanical
test would confirm or deny the presence of the quantum nonequilibrium
conclusively.
