Interacting Dark-Sector Models

• Interacting dark sectors are defined as models where dark matter, dark energy, and hidden particles exchange energy via non-gravitational interactions to resolve cosmological anomalies.
• They employ theoretical frameworks such as string compactifications and effective field theories to predict deviations from ΛCDM in expansion history and structure formation.
• Observational and laboratory analyses, including CMB, BAO, and haloscope experiments, provide precise constraints, exemplified by measurements like ξ = -0.07 ± 0.03 and small-scale power suppression.

Interacting dark-sector scenarios encompass a class of theoretical extensions to the standard cosmological model in which distinct dark-sector components—typically dark matter (DM), dark energy (DE), and sometimes additional hidden-sector particles such as axions, axion-like particles (ALPs), or hidden photons—exchange energy, momentum, or quantum numbers via non-gravitational channels. These scenarios are motivated by empirical anomalies in the standard $Λ$CDM cosmology, such as the Hubble constant discrepancy, CMB lensing amplitude excess, curvature hints, sub-eV neutrino mass bounds, and evidence for non-minimal dark energy. Interacting dark sectors are also theoretically anticipated in models inspired by string compactifications, where hidden-sector fields (e.g., moduli, axions) and gauge bosons generically emerge. Theoretical, experimental, and observational efforts—such as the pan-European CA21106 “COSMIC WISPers” network—are directed at deploying and integrating laboratory, astrophysical, and cosmological probes to both constrain and potentially discover such interactions (Valentino, 4 Jan 2026, Brunelli, 20 Feb 2025, Aybas et al., 26 Mar 2025, Horns et al., 2014, Horns et al., 2013).

1. Theoretical Motivation and Model Classes

Interacting dark-sector models are motivated by the empirical tensions within $Λ$CDM and by extensions of the Standard Model in which additional weakly interacting slim particles (WISPs) naturally arise. In string-inspired constructions, the low-energy effective field theory generically contains axions, moduli, and hidden photons, with couplings dictated by the microscopic compactification geometry (Brunelli, 20 Feb 2025, Aybas et al., 26 Mar 2025).

Several model classes define interacting dark sectors:

• Dark Matter–Dark Energy Interaction (IDE): Non-gravitational energy transfer between DM and DE, modifying the standard continuity equations,

$$\dot\rho_c + 3H\rho_c = +Q, \qquad \dot\rho_x + 3H(1+w_x)\rho_x = -Q,$$

with coupling $Q=\xi H \rho_x$. For $\xi<0$ (energy transfer from DM to DE), the cosmological DM abundance $\Omega_c$ decreases, raising the local value of $H_0$ (Valentino, 4 Jan 2026).

• Dark Matter–Neutrino Scattering: Effective drag between DM and cosmic neutrinos, parameterized by

$$\sigma_{\nu c} = u_{\nu{\text{-DM}}}\,\sigma_T \left(\frac{T}{1~{\rm eV}}\right)^2,$$

with accompanying modifications to the Boltzmann hierarchy via drag terms (Valentino, 4 Jan 2026).

• Hidden-sector Gauge Mediators: Kinetic mixing between hidden photons ($\gamma'$) and the SM photon, or higher-dimensional couplings involving axions/ALPs, as in

$$\mathcal{L}_\text{mix} = -\frac{1}{4}F'_{\mu\nu} F'^{\mu\nu} - \frac{1}{2}\chi F_{\mu\nu} F'^{\mu\nu} + \frac{1}{2}m_{\gamma'}^2 A'_\mu A'^{\mu},$$

and

$$\mathcal{L}_{a\gamma\gamma} = -\frac{1}{4}g_{a\gamma\gamma} a F_{\mu\nu} \tilde{F}^{\mu\nu}$$

(Horns et al., 2014, Horns et al., 2013, Brunelli, 20 Feb 2025). This diversity enables model building tailored to specific cosmological and experimental signatures.

2. Phenomenological Implications and Observational Signatures

Interacting dark-sector models generically produce signatures distinguishable from those in $Λ$CDM:

• Modified Expansion History: IDE and related interactions modify the Friedmann equation through altered DM/DE evolution, impacting late-time distances ($D_V(z)$, $r_d$) and raising $H_0$ relative to CMB-inferred values (Valentino, 4 Jan 2026).
• CMB and Large-Scale Structure: Couplings such as nonzero $\xi$ change the growth of perturbations, suppress the CMB small-scale damping tail, and can alleviate $S_8$ tension. Dark radiation arising from hidden-sector decays appears as $\Delta N_\text{eff}>0$, constrained by current and future CMB/21cm surveys (Brunelli, 20 Feb 2025).
• Laboratory Signals: Hidden photon and axion-photon couplings are sought in haloscopes, helioscopes, and light-shining-through-walls experiments, with detection rates determined by the precise coupling parameters (e.g., mixing $\chi$, $g_{a\gamma}$, see below) (Horns et al., 2013, Horns et al., 2014).
• Astrophysical Constraints: Radio and gamma-ray astrophysics provide complementary limits on ultralight hidden sectors through, e.g., spectral irregularities or changes in stellar evolution.

3. Quantitative Constraints and Experimental Status

Post-Planck cosmology, supplemented by DESI BAO and local $H_0$ measurements, provides stringent parameter bounds. The salient results:

• Dark Matter–Dark Energy Coupling: Joint Planck+DESI BAO fits yield $\xi=-0.07\pm0.03$ (energy transfer DM$\to$DE), with $H_0=71\pm1~\mathrm{km\,s^{-1}\,Mpc^{-1}}$, fully alleviating the Hubble tension and preferring interacting dark sectors at $>95\%$ CL, though with only mild statistical preference over $Λ$CDM as quantified by $\Delta\ln\mathcal{Z}\simeq+1.0$ (Valentino, 4 Jan 2026).
• DM–Neutrino Scattering: Planck PR3 limits $u_{\nu\text{-DM}}<10^{-5}$ (95% CL), while Planck low-$\ell$+ACT+DESY3 combinations display a $\sim3\sigma$ preference for nonzero coupling, corresponding to potentially observable suppression in small-scale CMB power (Valentino, 4 Jan 2026).
• WISP Sector Constraints: Laboratory and astrophysics experiments probe axion/ALP-photon couplings down to $g_{a\gamma}\sim10^{-16}$–$10^{-14}~\text{GeV}^{-1}$ and hidden photon mixing $\chi\sim10^{-3}$–$10^{-6}$ over $m_a, m_{\gamma'} \sim10^{-19}$–$10^{-3}~\text{eV}$, with current and upcoming facilities (WISPDMX, broadband radiometers) providing cross-checks on astrophysical exclusions (Horns et al., 2013, Horns et al., 2014, Aybas et al., 26 Mar 2025).

4. Methodologies: Theory, Experiment, and Data Synthesis

Progress in probing interacting dark sectors proceeds via a combination of approaches:

• Theoretical Model Building: Construction of models with derived interaction terms (e.g., string compactification mass spectra and couplings) guides both parameter space selection and phenomenological interpretation (Brunelli, 20 Feb 2025, Aybas et al., 26 Mar 2025).
• Laboratory Searches: Haloscopes (e.g., WISPDMX with $\sim460$ L copper cavity and $Q \sim 4.6\times10^4$; target $g_{a\gamma}$ down to $10^{-15}$ GeV$^{-1}$), broadband radiometers (stellarator radiometry, dish antennae), and their associated signal power and exclusion metrics (Horns et al., 2013, Horns et al., 2014).
• Astrophysical Data Integration: Cross-correlation between laboratory exclusion limits and bounds from archival radio, CMB, and gamma-ray datasets, ensuring robust, model-independent constraints (Horns et al., 2013, Aybas et al., 26 Mar 2025).
• Cosmological Dataset Analysis: Joint MCMC and Bayesian approaches with metrics such as per-dataset $\chi^2$, parameter-shift indices, evidence ratios, and best practices for likelihood cross-validation, blind analyses, and independent code verification, as codified by CA21106 Working Group 2 (Valentino, 4 Jan 2026, Aybas et al., 26 Mar 2025).

5. Current Research Collaborations and Strategic Frameworks

The COST Action CA21106 “COSMIC WISPers” coordinates European research efforts across theory, cosmology, astrophysics, and laboratory experimentation in the search for WISPs and interacting dark sectors (Aybas et al., 26 Mar 2025). Key features include:

• Four Working Packages: WP1 (theory/model building), WP2 (cosmology/dark matter), WP3 (astrophysics), WP4 (direct searches).
• Cross-disciplinary milestones: coordinated workshops (e.g., quantum readout and axion string simulation training), design studies for new experimental proposals (IAXO-Phase II, MADMAX-2), and integration of lattice QCD, magnetar models, and exclusion data (Aybas et al., 26 Mar 2025).
• Metrics for impact: production of review articles, training events, community-building, unified data-analysis frameworks, and roadmap deliverables supporting both exclusion and discovery potential.

6. Implications for Cosmological Tensions and Future Prospects

Interacting dark-sector models currently provide the only empirically viable single-component extension capable of fully reconciling the $7\sigma$ Hubble tension within the constraints of existing CMB and BAO datasets; IDE scenarios, for instance, realize $H_0=71\pm1$ km s$^{-1}$ Mpc$^{-1}$ at $>95\%$ CL with negative DM$\to$DE coupling (Valentino, 4 Jan 2026). DM–neutrino interactions, by suppressing small-scale power, present a candidate explanation for the $S_8$ tension. The synergy of laboratory, radio, and cosmological probes provides comprehensive coverage of WISP and broader hidden-sector parameter space, from $m\sim10^{-19}$ eV up to the QCD axion band, with cross-checks between astrophysical and laboratory approaches as a pillar of current strategy (Horns et al., 2013, Horns et al., 2014, Aybas et al., 26 Mar 2025).

Future directions center around extending experimental reach (CMB-S4, LISA, 21 cm surveys, upgraded haloscopes/radiometers), systematizing multi-probe consistency tests, and refining model-space mapping from ultraviolet completions. The continuing integration of theory, data, and hardware initiatives is expected to sharply constrain or discover non-gravitational dark-sector interactions, with the CA21106 roadmap as the European template for this endeavor (Aybas et al., 26 Mar 2025, Valentino, 4 Jan 2026).