The QCD nature of Dark Energy
  The origin of the observed dark energy could be explained entirely within the
standard model, with no new fields required. We show how the low-energy sector
of the chiral QCD Lagrangian, once embedded in a non-trivial spacetime, gives
rise to a cosmological vacuum energy density which can be can be presented
entirely in terms of QCD parameters and the Hubble constant $H$ as
$\rho_\Lambda \simeq H \cdot m_q\la\bar{q}q\ra /m_{\eta'} \sim (4.3\cdot
10^{-3} \text{eV})^4$. In this work we focus on the dynamics of the ghost
fields that are essential ingredients of the aforementioned Lagrangian. In
particular, we argue that the Veneziano ghost, being unphysical in the usual
Minkowski QFT, becomes a physical degree of freedom if the universe is
expanding. As an immediate consequence, all relevant effects are naturally very
small as they are proportional to the rate of expansion $H/ \Lqcd \sim
10^{-41}$. The co-existence of these two drastically different scales ($\Lqcd
\sim 100 $ MeV and $H \sim 10^{-33}$ eV) is a direct consequence of the
auxiliary conditions on the physical Hilbert space that are necessary to keep
the theory unitary. The exact cancellation taking place in Minkowski space due
to this auxiliary condition is slightly violated when the system is upgraded to
an expanding background. Nevertheless, this "tiny" effect is in fact the
driving force accelerating the universe today. We also derive the time
dependent equation of state $w(t)$ for the dark energy component which tracks
the dynamics of the Veneziano ghost in a FLRW universe. Finally, we comment on
how the same physical phenomena can be recovered in Witten's approach to the
U(1) problem when the ghost degree of freedom is not even present in the
system.
