Long-lived SEC violation via DM/DE couplings

TL;DR

The paper investigates how coupling dark matter (DM) to dark energy (DE) can induce an effective violation of the strong energy condition (SEC), enabling extended cosmic acceleration even when individual components would not sustain it. By analyzing DM/DE-coupled cosmologies with exponential potentials and masses, it derives a stable attractor with $\epsilon = \frac{d-1}{2}\frac{(1+w)}{1+\beta/\gamma}$, showing that large $\beta/\gamma$ can yield $\epsilon<1$ and prolonged acceleration without phantom behavior. It then confronts UV completions, arguing that the EFT cutoff and a black-hole bound constrain the duration of SEC-violating epochs, while exploring how towers of light states and negative potentials can be orchestrated to preserve parametric control and even realize controlled asymptotics in multi-field settings. The discussion highlights how these DM/DE-coupled mechanisms expand the landscape of UV-consistent routes to cosmic acceleration within string-inspired frameworks, though fully realized string embeddings remain an open challenge. Overall, the work proposes new avenues to realize long-lived SEC violation while respecting fundamental gravitational and quantum consistency conditions.

Abstract

We discuss a cosmological scenario where an effective violation of the strong energy condition (SEC) is realized through a coupling between SEC-fulfilling dark matter (DM) and dark energy (DE). Although the SEC-violating solutions might in principle last for an arbitrarily long time, we highlight several challenges that string realizations must face: most notably, these are the identification of suitable heavy states and their relationship with the theory cutoff. Furthermore, we discuss a black-hole argument that still allows for long-lived epochs of cosmic acceleration, but that prevents them from lasting forever. We also discuss negative potentials in the presence of a tower of light states, showing that the DM/DE coupling can push the theory towards regions of parametric control.

Figures (4)

• Figure 1: The potential $V = \Lambda \, \mathrm{e}^{- \kappa_d \gamma \varphi}$ (magenta) and the effective potential ${U = V + n_{ {\mathrm{DM}}, 0} \, (a_0/a)^{d-1} \, \mu \, \mathrm{e}^{\kappa_d \beta \varphi}}$ for a smaller (darker orange) and larger scale factor (lighter orange).
• Figure 2: With the orderings $\gamma_+ > \gamma_- > \alpha > 0$, and at a fixed scale factor, the effective potentials ${U_- = V_- + \rho_ {\mathrm{DM}}}$ (orange) and ${U_+ = V_+ + \rho_{ {\mathrm{DM}}}}$ (cyan), the pure field potential ${V_- + V_+}$ (green), and the full effective potential $U$ combining all terms, with a positive-energy minimum (magenta).
• Figure 3: A representation of the multi-field potential (teal) and DM/DE (blue) couplings, with the asymptotic field direction (purple); one may write $\epsilon = [(d-1)(1+w)/2]/(1+\beta_\infty/\gamma_\infty)$.
• Figure 4: The classification of the linearly-stable critical points in parameter space: the DM/DE-coupled solution (green), the scaling-like solution (orange), the kination/tracker solution (yellow), and a kinating solution (teal).