What is the magnetic field strength in cosmic voids?
In plain words
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 statement
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.
What would settle it
Detection of extended pair halos or delayed echoes around TeV blazars and gamma-ray bursts with the Cherenkov Telescope Array Observatory and Fermi.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
A 2025 analysis revised the conservative Fermi plus Cherenkov-telescope lower bound, and 2025 work extended lower limits to $z \sim 3$.
Parent problems
- Special case of Were cosmic magnetic seed fields primordial or astrophysical? · open · from revision 1 The parent was revised since revision 1: it now states another question.
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