Solar System Experiments in the Search for Dark Energy and Dark Matter

We reassess the realistic discovery reach of Solar-System experiments for dark energy (DE) and dark matter (DM), making explicit the bridge from cosmology-level linear responses to local, screened residuals. In scalar-tensor frameworks with a universal conformal coupling $A(\phi)$ and chameleon/Vainshtein screening, we map cosmological responses $\{\mu(z,k),\Sigma(z,k)\}$ inferred by DESI and \emph{Euclid} to thin-shell or Vainshtein residuals in deep Solar potentials $\Phi_N$. We emphasize a two-branch strategy. In a detection-first branch, a verified local anomaly -- an Einstein equivalence principle (EEP) violation, a Shapiro-delay signal with $|\gamma-1|\sim\mathrm{few}\times 10^{-6}$, an AU-scale Yukawa tail, or a ultralight DM (ULDM) line in clocks/atom interferometers in space (AIS) -- triggers a joint refit of cosmology and Solar-System data under a common microphysical parameterization $\{V(\phi),A(\phi)\}$. In a guardrail branch, Solar-System tests enforce constraints (EEP; PPN parameters $\gamma,\beta$; and $\dot G/G$) and close unscreened or weakly screened corners indicated by cosmology. We forecast, per conjunction, $|\gamma-1|\lesssim (2-5)\times 10^{-6}$ (Ka-/X-band or optical Shapiro), $\eta_{EEP}\sim (1\mbox{--}10)\times 10^{-17}$ (drag-free AIS), $|\dot G/G|\sim(3-5)\times10^{-15}\,\mathrm{yr^{-1}}$ (sub-mm-class LLR), a uniform ~2x tightening of AU-scale Yukawa/DM-density bounds, and $(3-10)\times$ improved ULDM-coupling reach from clocks. For a conformal benchmark, $\mu_{ lin,0}=0.10$ implies $\chi\simeq \sqrt{\mu_{lin,0}/2}$ and a Sun thin shell $\Delta R/R\lesssim (1/3\chi)\sqrt{|\gamma-1|/2}=2.4\times 10^{-3}$ at $|\gamma-1|=5\times 10^{-6}$; Vainshtein screening at 1 AU yields $|\gamma-1|\lesssim 10^{-11}$, naturally below near-term reach. We recommend a cost-effective guardrail+discovery portfolio with explicit triggers for escalation to dedicated missions.