The Radio Synchrotron Background -- A Cosmic Conundrum
  It has recently become apparent that the background level of diffuse radio
emission on the sky is significantly higher than the level that can result from
known extragalactic radio source classes or our Galaxy given our current
understanding of its large-scale structure.~ In contrast to the more well-known
and well-constrained cosmological and astrophysical backgrounds at microwave,
infrared, optical/UV, X-ray, and gamma-ray wavelengths, this ``radio
synchrotron background'' at radio wavelengths provides clear motivation for
considering the possibilities of new astrophysical sources and new
particle-based emission mechanisms.
