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Title: What is the magnetic field strength in cosmic voids?
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.
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.
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.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status note: A 2025 analysis revised the conservative Fermi plus Cherenkov-telescope lower bound, and 2025 work extended lower limits to $z \sim 3$.
Posed since: 2010