{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"d8230e8062b505860de3b509e19c7319b585894a3adeda994519a4362d529634","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b","d65116928590606a95aa7e4541cc09c3031e8fac4600b9b93a8597ea51cdec2b"],"salt":"6ee7040d49b5dd05ace149ea836a8ee2572e32394a7fcd7ac061268aaa1b224e","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":"2","parents":[],"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 $B > \\sim 10^{-17} \\text{ to } 10^{-16}\\,\\mathrm{G}$ for coherence lengths $\\lambda_B > \\sim 1\\,\\mathrm{Mpc}$, while CMB and Faraday rotation give upper bounds of order $10^{-9}\\,\\mathrm{G}$. 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":"d65116928590606a95aa7e4541cc09c3031e8fac4600b9b93a8597ea51cdec2b","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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