Phenomenology of an Open Effective Field Theory of Dark Energy

Abstract

All observational evidence for dark matter and dark energy is so far exclusively gravitational. Hence, the dark sector may be equivalently described by a theory of the spacetime metric whose dynamics is affected by interactions with an unknown environment. Adapting open-system techniques, we have recently constructed such a general theory of open gravitational dynamics. Here we study a minimal and concrete realization of this theory that describes the late-time acceleration of the Universe. Our model provides a good fit to recent baryon acoustic oscillation measurements by construction, while avoiding violations of the null energy condition. Moreover, it leads to a set of correlated and observationally testable predictions. Studying the modified cosmological perturbation theory and compared to the $Λ$CDM model we find: a dissipative suppression of the gravitational-wave luminosity distance relative to the electromagnetic one; a modification in the evolution of the Bardeen potentials with a clear signal in the gravitational slip; and an enhancement of structure formation at low redshift. We present semi-analytical estimates of the magnitude of these effects and show that they lie within the reach of current constraints while providing clear targets for upcoming cosmological surveys.

Figures (6)

• Figure 1: Evolution of effective equation of state $w_{\mathrm{eff}}$ in our model (ODF) compared that of CDM and dark energy in the CPL-parametrization. Parameters are set to the $w_0 w_a$CDM best-fit values from BAO + CMB data, with $1\sigma$-uncertainties shown DESI:2025zgx.
• Figure 2: Gravitational wave luminosity distance predicted by our model (ODF) for $(w_0, w_a)$ values within the BAO+CMB 95% confidence region, versus GWTC-4 constraints. Shaded regions show $68.3\%$ ($90\%$) credible intervals from GWTC-4 fits to the $\alpha_m$-parametrization, assuming a narrow $H_0$-prior LIGOScientific:2025jau.
• Figure 3: Bardeen potentials $\Phi$ and $\Psi$ predicted by our model (ODF), along with their average, and the corresponding evolution in $\Lambda$CDM and a matter-dominated universe. All fields are normalized to unity at the initial time.
• Figure 4: Gravitational slip $\eta(z)=\Phi(z)/\Psi(z)$ predicted by our model (ODF), versus BAO+CMB constraints DESI:2024hhd. The dashed line indicates $\Lambda$CDM, where $\eta=1$.
• Figure 5: Growth factor $D(a)$ in our model (ODF) compared to $\Lambda$CDM.