Megaparsec-scale radio arcs glow at cluster edges with an efficiency the textbook mechanism cannot pay for. Radio relics, elongated synchrotron structures at cluster peripheries, trace merger shock fronts where electrons are accelerated and magnetic fields amplified; radio halos fill cluster volumes with diffuse emission attributed to turbulent re-acceleration. The accounting problem is sharpest for relics: X-ray and SZ measurements of the same shocks find Mach numbers of only 1.5 to 3, and diffusive shock acceleration at such weak shocks injects electrons from the thermal pool with efficiencies orders of magnitude too small to light the observed radio brightness; several relics imply acceleration efficiencies exceeding 100 percent under thermal-injection assumptions, a contradiction in terms. The standard repair is a pre-existing population: fossil relativistic electrons, deposited by past AGN activity, lingering at just the right places and energies for weak shocks to re-energize cheaply.
The repair works case by case and strains as a system: the fossil reservoirs must be ubiquitous (relics are common), aged but not too aged (re-acceleration needs seed energies), and distributed across megaparsec arcs at cluster outskirts where AGN bubbles rarely travel. Direct evidence remains anecdotal, a few relics traced to nearby AGN lobes, while most arcs stand without identified sources. Halos carry a parallel ledger: turbulent re-acceleration requires the same fossil seeds plus sustained turbulence, in tension with the measured ICM quiescence.
The standing is a recognized acceleration-efficiency crisis: LOFAR keeps finding relics and halos in ever less-disturbed systems, deepening the demand for ubiquitous fossil electrons, and the seed-population question has become the central open problem of cluster radio astronomy.