Indications of a Late-Time Transition to a Strongly Interacting Dark Sector

TL;DR

This work examines whether a late-time transition to a strongly interacting dark sector can improve the description of cosmic expansion. It introduces a redshift-threshold coupling $Q_A = \alpha(z) H \rho_m$ with a sharp transition at $z_T$, such that $z<z_T$ activates energy transfer from dark energy to dark matter while $z>z_T$ reduces to $Λ$CDM; the data-driven constraint uses late-time probes ($z<2.5$) from PantheonPlus, Union3.0, DES-Dovekie, Cosmic Chronometers, and DESI DR2 BAO. Across three data combinations, the analyses locate $z_T$ around 0.37–0.62 with a large positive coupling $\alpha$ (strong late-time interaction), and the model provides a fit comparable to the CPL parametrization without requiring phantom crossing. Overall, the results support a late-onset, strong interaction in the dark sector that slows the present expansion relative to $Λ$CDM and offer a viable alternative to standard dark energy models, while remaining statistically equivalent to CPL within uncertainties.

Abstract

We explore the transition from the $Λ$CDM to an interacting dark sector by introducing a model with a redshift threshold that controls the onset of the energy transfer between the dark energy and the dark matter. Below the transition redshift, the interaction between dark matter and dark energy becomes active, while at earlier times the cosmological evolution coincides with that of $Λ$CDM. This approach allows us to determine the epoch in the comic history where the interacting effects have an impact in the description of the dark sector. We constrain the free parameters of the model using late-time cosmological observations, namely Cosmic Chronometers, DESI DR2 Baryonic Acoustic Oscillations, and Supernova data from the Pantheon Plus, Union3.0, and DES-Dovekie catalogues. The analysis provides an indication of a strong interacting term that describes energy transfer from dark energy to dark matter, which is activated at low redshifts. The PantheonPlus sample provides a threshold of $z_{T}<0.624$, the Union3.0 sample yields $z_{T}=0.400_{-0.23}^{+0.021}$, and the DES-Dovekie sample gives $z_{T}=0.371_{-0.26}^{+0.028}$. The model fits the data in a similar way to the CPL parametrization, without the dark energy to cross the phantom divide line.

Figures (5)

• Figure 1: Confidence space for the posterior parameters $\left\{ H_{0},\Omega_{m0},r_{drag},\alpha,z_{T}\right\}$ for the interacting model $Q_{A}$.
• Figure 2: Confidence space for the posterior parameters $\left\{ H_{0},\Omega_{m0},r_{drag},\alpha_{0}\right\} ~$for the interacting model $Q_{A}^{0}$.
• Figure 3: PP&OHD&BAO: Left Fig: Qualitative evolution of the $\Omega_{m}\left( z\right) ~$(solid lines) and $\Omega_{DE}\left( z\right)$ (dashed lines) for the $\Lambda$CDM, the $Q_{A}^{0}$ and $Q_{A}$ interacting models. Right Fig: Qualitative evolution of the deceleration parameter $q\left( z\right) =\frac{1}{2}\left( 1+3w_{tot}\left( z\right) \right)$. Plots are for the best-fit parameters. Blue lines are for the $\Lambda$CDM, orange and green lines are for the $Q_{A}^{0}$ and $Q_{A}$ interacting models respectively, red lines are for the CPL parametrization.
• Figure 4: U3&OHD&BAO: Left Fig: Qualitative evolution of the $\Omega_{m}\left( z\right) ~$(solid lines) and $\Omega_{DE}\left( z\right)$ (dashed lines) for the $\Lambda$CDM, the $Q_{A}^{0}$ and $Q_{A}$ interacting models. Right Fig: Qualitative evolution of the deceleration parameter $q\left( z\right) =\frac{1}{2}\left( 1+3w_{tot}\left( z\right) \right)$. Plots are for the best-fit parameters. Blue lines are for the $\Lambda$CDM, orange and green lines are for the $Q_{A}^{0}$ and $Q_{A}$ interacting models respectively, red lines are for the CPL parametrization.
• Figure 5: DD&OHD&BAO: Left Fig: Qualitative evolution of the $\Omega_{m}\left( z\right) ~$(solid lines) and $\Omega_{DE}\left( z\right)$ (dashed lines) for the $\Lambda$CDM, the $Q_{A}^{0}$ and $Q_{A}$ interacting models. Right Fig: Qualitative evolution of the deceleration parameter $q\left( z\right) =\frac{1}{2}\left( 1+3w_{tot}\left( z\right) \right)$. Plots are for the best-fit parameters. Blue lines are for the $\Lambda$CDM, orange and green lines are for the $Q_{A}^{0}$ and $Q_{A}$ interacting models respectively, red lines are for the CPL parametrization.