  With the recent discoveries of massive and highly luminous quasars at high
redshifts ($z\sim7$; e.g. Mortlock et al. 2011), the question of how black
holes (BHs) grow in the early Universe has been cast in a new light. In order
to grow BHs with $M_{\rm BH} > 10^9$ M$_{\odot}$ by less than a billion years
after the Big Bang, mass accretion onto the low-mass seed BHs needs to have
been very rapid (Volonteri & Rees, 2005). Indeed, for any stellar remnant seed,
the rate required would need to exceed the Eddington limit. This is the point
at which the outward force produced by radiation pressure is equal to the
gravitational attraction experienced by the in-falling matter. In principle,
this implies that there is a maximum luminosity an object of mass $M$ can emit;
assuming spherical accretion and that the opacity is dominated by Thompson
scattering, this Eddington luminosity is $L_{\rm{E}} = 1.38 \times 10^{38}
(M/M_{\odot})$ erg s$^{-1}$. In reality, it is known that this limit can be
violated, due to non-spherical geometry or various kinds of instabilities.
Nevertheless, the Eddington limit remains an important reference point, and
many of the details of how accretion proceeds above this limit remain unclear.
Understanding how this so-called super-Eddington accretion occurs is of clear
cosmological importance, since it potentially governs the growth of the first
supermassive black holes (SMBHs) and the impact this growth would have had on
their host galaxies (`feedback') and the epoch of reionization, as well as
improving our understanding of accretion physics more generally.
