{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"a67ec0fd45f3e88cf2af2fefe9707859a5bc153a17f0c74a28b28eae473bfb52","created":"2026-10-03T07:18:02Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011","758593ddd21934f5cafd29de60d162dd327db8579db4b33708c8bb9394fd068a"],"salt":"23ce128cce3931cd8cecdc049930fabc4a649a06823223fa27991aca33525d4d","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":[],"external_id":"cosmo.cosmic-magnetism.void-field-strength","kind":"well-posed","literature_status":"open","n":"1","parents":[{"note":"","parent_revision":"758593ddd21934f5cafd29de60d162dd327db8579db4b33708c8bb9394fd068a","relation":"special_case"}],"plain":"Very high-energy gamma rays from distant galaxies create electron-positron pairs that should produce lower-energy gamma rays, but those are missing, which suggests magnetic fields deflect the pairs. This gives a lower limit on the field, but nobody has measured its actual value.","posed_since":"2010","precise":"The absence of GeV cascade emission from TeV blazars gives $B > \\sim 1e-17 \\text{ to } 1e-16\\,\\mathrm{G}$ for coherence lengths $\\lambda_B > \\sim 1\\,\\mathrm{Mpc}$, while CMB and Faraday rotation give upper bounds of order $1e-9\\,\\mathrm{G}$. Measure $B$ and $\\lambda_B$ in voids, for example from cascade halos or time-delayed pair echoes. An answer is a measured value, not only a bound.","problem_ref":null,"references":"","settled_by":"Detection of extended pair halos or delayed echoes around TeV blazars and gamma-ray bursts with the Cherenkov Telescope Array Observatory and Fermi.","status_note":"A 2025 analysis revised the conservative Fermi plus Cherenkov-telescope lower bound, and 2025 work extended lower limits to $z \\sim 3$.","title":"What is the magnetic field strength in cosmic 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