The cosmological constant from the QCD Veneziano ghost
  We suggest that the solution to the cosmological vacuum energy puzzle is
linked to the infrared sector of the effective theory of gravity interacting
with standard model fields, with QCD fields specifically. We work in the
framework of low energy quantum gravity as an effective field theory. In
particular, we compute the vacuum energy in terms of QCD parameters and the
Hubble constant $H$ such that the vacuum energy is $\epsilon_{vac} \sim H \cdot
m_q\la\bar{q}q\ra /m_{\eta'} \sim (3.6\cdot 10^{-3} \text{eV})^4$, which is
amazingly close to the observed value today. The QCD ghost (responsible for the
solution of the $U(1)_A$ problem) plays a crucial r\^ole in the computation of
the vacuum energy, because the ghost's properties at very large but finite
distances slightly deviate (as $\sim H / \Lqcd $) from their infinite volume
Minkowski values. Another important prediction of this framework states that
the vacuum energy owes its existence to the asymmetry of the cosmos. Indeed,
this effect is a direct consequence of the embedding of our Universe on a
non-trivial manifold such as a torus with (slightly) different linear sizes.
Such a violation of cosmological isotropy is apparently indeed supported by
WMAP, and will be confirmed (or ruled out) by future PLANCK data.
