Do Cosmic Rays Heat the Early Intergalactic Medium?
  Cosmic rays (CRs) govern the energetics of present-day galaxies and might
have also played a pivotal role during the Epoch of Reionization. In
particular, energy deposition by low-energy ($E \lesssim 10$ MeV) CRs
accelerated by the first supernovae, might have heated and ionized the neutral
intergalactic medium (IGM) well before ($z \approx 20$) it was reionized,
significantly adding to the similar effect by X-rays or dark matter
annihilations. Using a simple, but physically motivated reionization model, and
a thorough implementation of CR energy losses, we show that CRs contribute
negligibly to IGM ionization, but heat it substantially, raising its
temperature by $\Delta T=10-200$ K by $z=10$, depending on the CR injection
spectrum. Whether this IGM pre-heating is uniform or clustered around the first
galaxies depends on CR diffusion, in turn governed by the efficiency of
self-confinement due to plasma streaming instabilities that we discuss in
detail. This aspect is crucial to interpret future HI 21 cm observations which
can be used to gain unique information on the strength and structure of early
intergalactic magnetic fields, and the efficiency of CR acceleration by the
first supernovae.
