CPT-Symmetric Universe
  We propose that the state of the universe does {\it not} spontaneously
violate CPT. Instead, the universe after the big bang is the CPT image of the
universe before it, both classically and quantum mechanically. The pre- and
post-bang epochs comprise a universe/anti-universe pair, emerging from nothing
directly into a hot, radiation-dominated era. CPT symmetry selects a unique QFT
vacuum state on such a spacetime, providing a new interpretation of the
cosmological baryon asymmetry, as well as a remarkably economical explanation
for the cosmological dark matter. Requiring only the standard three-generation
model of particle physics (with right-handed neutrinos), a $\mathbb{Z}_2$
symmetry suffices to render one of the right-handed neutrinos stable. We
calculate its abundance from first principles: matching the observed dark
matter density requires its mass to be $4.8\times10^{8}~{\rm GeV}$. Several
other testable predictions follow: (i) the three light neutrinos are Majorana
and allow neutrinoless double $\beta$ decay; (ii) the lightest neutrino is
massless; and (iii) there are no primordial long-wavelength gravitational
waves. We mention connections to the strong CP problem and the arrow of time.
