# The QCD nature of Dark Energy - Astrophysics > Cosmology and Nongalactic Astrophysics

Abstract: The origin of the observed dark energy could be explained entirely within thestandard model, with no new fields required. We show how the low-energy sectorof the chiral QCD Lagrangian, once embedded in a non-trivial spacetime, givesrise to a cosmological vacuum energy density which can be can be presentedentirely in terms of QCD parameters and the Hubble constant $H$ as$ho \Lambda \simeq H \cdot m q\la\bar{q}q a -m {\eta-} \sim 4.3\cdot10^{-3} \text{eV}^4$. In this work we focus on the dynamics of the ghostfields that are essential ingredients of the aforementioned Lagrangian. Inparticular, we argue that the Veneziano ghost, being unphysical in the usualMinkowski QFT, becomes a physical degree of freedom if the universe isexpanding. As an immediate consequence, all relevant effects are naturally verysmall as they are proportional to the rate of expansion $H- \Lqcd \sim10^{-41}$. The co-existence of these two drastically different scales $\Lqcd\sim 100$ MeV and $H \sim 10^{-33}$ eV is a direct consequence of theauxiliary conditions on the physical Hilbert space that are necessary to keepthe theory unitary. The exact cancellation taking place in Minkowski space dueto this auxiliary condition is slightly violated when the system is upgraded toan expanding background. Nevertheless, this -tiny- effect is in fact thedriving force accelerating the universe today. We also derive the timedependent equation of state $wt$ for the dark energy component which tracksthe dynamics of the Veneziano ghost in a FLRW universe. Finally, we comment onhow the same physical phenomena can be recovered in Witten-s approach to theU1 problem when the ghost degree of freedom is not even present in thesystem.

Author: Federico R. Urban, Ariel R. Zhitnitsky

Source: https://arxiv.org/