Super-acceleration as Signature of Dark Sector Interaction

TL;DR

The paper shows that a quintessence field coupled to dark matter can reproduce an effective phantom-like equation of state $w_{ m eff}<-1$ without introducing ghosts. By coupling DM to a scalar via $f(rac{ phi}{M_{ m Pl}})$, the DM density redshifts as $ ho_{ m DM}\sim rac{f( phi/M_{ m Pl})}{a^{3}}$, leading to $w_{ m eff}=rac{w_ phi}{1-x}$ with $xigl(0igr) imesto 0$ today and $x>0$ in the past; for increasing $f$, this can yield $w_{ m eff}<-1$ while preserving a healthy total fluid. An explicit example with $V( phi)=M^{4}(M_{ m Pl}/ phi)^{oldsymbol{\alpha}}$ and $f( phi)=e^{oldsymbol{eta} phi/M_{ m Pl}}$ shows $rac{ phi_{0}}{M_{ m Pl}} oughly rac{oldsymbol{oldsymbol{ alpha}}}{oldsymbol{oldsymbol{eta}}}rac{oldsymbol{ Omega}_{ m DE}^{(0)}}{oldsymbol{ Omega}_{ m DM}^{(0)}}$ and $w_{ m eff} o rac{w_ phi}{1-x}$ with $w_ phi oughly -1$, yielding $w_{ m eff}<-1$ for $z aisebox{-0.15ex}{$ ilde{>}$}0.1$. The model remains compatible with current data and predicts testable signatures in the growth of structure, CMB/distance measures, and cluster dynamics, offering a falsifiable alternative to phantom dark energy. Future surveys could distinguish this scenario from $oldsymbol{ m Lambda CDM}$ and phantom models via percent-level differences in $d_L$, $d_A$, and the matter power spectrum, as well as potential biases between baryons and dark matter.

Abstract

We show that an interaction between dark matter and dark energy generically results in an effective dark energy equation of state of w<-1. This arises because the interaction alters the redshift-dependence of the matter density. An observer who fits the data treating the dark matter as non-interacting will infer an effective dark energy fluid with w<-1. We argue that the model is consistent with all current observations, the tightest constraint coming from estimates of the matter density at different redshifts. Comparing the luminosity and angular-diameter distance relations with LambdaCDM and phantom models, we find that the three models are degenerate within current uncertainties but likely distinguishable by the next generation of dark energy experiments.

Figures (4)

• Figure 1: Redshift evolution of $w_{\rm eff}$ (solid line) and $w_{\phi}$ (dash line). As advocated, $w_{\rm eff}<-1$ in the recent past due to the interaction with the dark matter.
• Figure 2: Upper panel shows the luminosity distance ($d_L$) as function of redshift for our model (solid) a phantom model with $w=-1.2$ (dash-dot) and $\Lambda$CDM (dash). We have fixed $\Omega_{\rm DM}^{(0)}=0.3$. Lower panel shows the percentage difference between our model and phantom (dash-dot), and between our model and $\Lambda$CDM (dash), respectively.
• Figure 3: Same as in Figure \ref{['dL1']}, except $\Omega_{\rm DM}^{(0)}=0.4$ for the interacting scalar field dark matter model in this case. This gives equal $d_{\rm A}(z_{\rm rec})$ for all three models.
• Figure 4: Upper panel shows the matter power spectrum ($\Delta^2(k)$) over the relevant range of scales for our model (solid) and $\Lambda$CDM (dash) with $\Omega_{\rm DM}^{(0)}=0.4$ and $0.3$, respectively. Lower panel shows the percentage difference between the two curves, which is well within current experimental accuracy.