  In the current paradigm of cold dark matter cosmology, large-scale structures
are assembling through hierarchical clustering of matter. In this process, an
important role is played by megaparsec (Mpc)-scale cosmic shock waves, arising
in gravity-driven supersonic flows of intergalactic matter onto dark
matter-dominated collapsing structures such as pancakes, filaments, and
clusters of galaxies. Here, we report Very Large Array telescope observations
of giant (~2 Mpc by 1.6 Mpc), ring-shaped nonthermal radio-emitting structures,
found at the outskirts of the rich cluster of galaxies Abell 3376. These
structures may trace the elusive shock waves of cosmological large-scale matter
flows, which are energetic enough to power them. These radio sources may also
be the acceleration sites where magnetic shocks are possibly boosting
cosmic-ray particles with energies of up to 10^18 to 10^19 electron volts.
