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) and quantify their complementarity to dedicated cosmological probes, such as the Dark Energy Spectroscopic Instrument (DESI) and Euclid mission. In scalar–tensor frameworks with universal conformal coupling A(φ), screening (chameleon/symmetron, Vainshtein) suppresses fifth forces in deep Solar potentials, consistent with the gravitational-wave speed bound |c_T /c− 1| 10−15. We treat Solar–System tests as hypothesis-driven probes: we assemble quantitative guardrails (MICROSCOPE η, Cassini γ, LLR G/G and SEP, ephemeris limits on ρDM and Yukawa α(λ), and clock-network searches for ultralight DM), place them alongside current cosmology posteriors, and provide an explicit map from cosmology-level linear response (e.g., µlin,0) to local residuals using the screening relations (thin–shell and Vainshtein). Guided by systematics-gated criteria, we outline a focused near-term program—solar-conjunction Shapiro-delay and Doppler/range tests, sustained mm-class lunar laser ranging (LLR), global optical-clock links, refined ephemerides, and spaceborne atom interferometry (AIS)—with realistic sensitivities |γ− 1| ∼ few × 10^−6, η∼ 10^−16–10^−17 (AIS/LLR), |˙G/G| few × 10−14 yr−1, and factor-2 improvements in AU-scale Yukawa/DM-density bounds, plus 3–10× gains in ultralight-DM couplings from clock networks. We conclude that primary DE discovery remains with multi-probe cosmology (geometry and growth), while a hypothesis-driven Solar–System program supplies high-leverage falsification and selective discovery windows for DM with ultralight mediators or long-range forces.