Ludovic Van Waerbeke, Ariel Zhitnitsky

We present a physically motivated model of dark energy (DE) rooted in the topological structure of the Quantum ChromoDynamic (QCD) vacuum. In this framework, DE emerges as the Universe expands, from the difference in vacuum energy between the expanding Universe and Minkowski spacetime, driven by QCD vacuum topological sectors. This leads to a modification of the dark energy term in the Friedmann equation, which then scales with the Hubble parameter as $\rho_{\rm DE}(t) \propto H(t)$ when dark energy dominates the expansion of the Universe. The QCD scale, $\Lambda_{\rm QCD} \sim 100~{\rm MeV}$, naturally sets the energy density of DE and provides a compelling explanation for why its impact on cosmic expansion becomes significant only in the recent cosmological epoch. Importantly, the entire framework is grounded in Standard Model (SM) physics, involving no new fields or coupling constants. Key predictions of the model include: (a) A present-day DE equation of state $w_{\rm DE} > -1$, asymptotically approaching the de Sitter limit $w_{\rm DE} = -1$ in the future, with the corresponding asymptotic Hubble constant $\overline{H}$ set by $\Lambda_{\rm QCD}$. (b) At redshifts $z \ge 0$, $w_{\rm DE}$ can lie above or below $-1$ and may cross this boundary multiple times, behavior qualitatively consistent with recent DESI observations. (c) The solution to the Friedmann equation in this framework can deviate from the canonical $\Lambda$CDM form at $z \ge 0$. (d) If such deviation occurs, it can be tested using cosmological observations, CMB anisotropies, BAO, SNIa, and large-scale structure, which we propose to explore in future work. (e) Finally, we point out a potential connection between our framework and the observed $H_0$ tension.