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 $A(\ensuremath{\phi})$ and chameleon/Vainshtein screening, we map cosmological responses ${\ensuremath{\mu}(z,k),\mathrm{\ensuremath{\Sigma}}(z,k)}$ inferred by DESI and Euclid to thin-shell or Vainshtein residuals in deep Solar potentials ${\mathrm{\ensuremath{\Phi}}}_{N}$. 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 $|\ensuremath{\gamma}\ensuremath{-}1|\ensuremath{\sim}\mathrm{few}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, an AU-scale Yukawa tail, or a narrowband 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 ${V(\ensuremath{\phi}),A(\ensuremath{\phi})}$. In a guardrail branch, Solar-System tests enforce universal constraints (EEP; PPN parameters $\ensuremath{\gamma}$, $\ensuremath{\beta}$; and $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{G}/G$) and close unscreened or weakly screened corners indicated by cosmology. Under realistic near-term conditions we forecast, per conjunction, $|\ensuremath{\gamma}\ensuremath{-}1|\ensuremath{\lesssim}(2\ensuremath{-}5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ (Ka-/X-band or optical Shapiro), ${\ensuremath{\eta}}_{\mathrm{EEP}}\ensuremath{\sim}(1\ensuremath{-}10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ (drag-free AIS), $|\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{G}/G|\ensuremath{\sim}(3\ensuremath{-}5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\text{ }\text{ }\mathrm{y}{\mathrm{r}}^{\ensuremath{-}1}$ [sub-mm-class lunar laser ranging (LLR)], a uniform $\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}$ tightening of AU-scale Yukawa/DM-density bounds, and $(3\ensuremath{-}10)\ifmmode\times\else\texttimes\fi{}$ improved ULDM-coupling reach from clock networks. For a conformal benchmark, ${\ensuremath{\mu}}_{\mathrm{lin},0}=0.10$ implies $\ensuremath{\chi}\ensuremath{\simeq}\sqrt{{\ensuremath{\mu}}_{\mathrm{lin},0}/2}$ and a Sun thin shell $\mathrm{\ensuremath{\Delta}}R/R\ensuremath{\lesssim}(1/3\ensuremath{\chi})\sqrt{|\ensuremath{\gamma}\ensuremath{-}1|/2}=2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at $|\ensuremath{\gamma}\ensuremath{-}1|=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$; Vainshtein screening at 1 AU yields $|\ensuremath{\gamma}\ensuremath{-}1|\ensuremath{\lesssim}{10}^{\ensuremath{-}11}$, naturally below near-term reach. We recommend a cost-effective $\mathrm{guardrail}+\mathrm{discovery}$ portfolio with explicit triggers for escalation to dedicated missions.