  A crucial challenge in astrophysics over the coming decades will be to
understand the origins of supermassive black holes (SMBHs) that lie at the
centres of most, if not all, galaxies. The processes responsible for the
initial formation of these SMBHs and their early growth via accretion - when
they are seen as Active Galactic Nuclei (AGN) - remain unknown. To address this
challenge, we must identify low luminosity and obscured z>6 AGNs, which
represent the bulk of early SMBH growth. Sensitive X-ray observations are a
unique signpost of accretion activity, uncontaminated by star formation
processes, which prevent reliable AGN identification at other wavelengths (e.g.
optical, infrared). The Athena+ Wide Field Imager will enable X-ray surveys to
be carried out two orders of magnitude faster than with Chandra or XMM-Newton,
opening a new discovery space and identifying over 400 z>6 AGN, including
obscured sources. Athena+ will also play a fundamental role to enhance the
scientific return of future multiwavelength facilities that will probe the
physical conditions within the host galaxies of early SMBHs, which is vital for
understanding how SMBHs form, what fuels their subsequent growth, and to assess
their impact on the early Universe. Follow-up of samples of z>6 galaxies with
the Athena+ X-ray Integral Field Unit could also reveal the presence of highly
obscured AGNs, thanks to the detection of strong iron lines. Thus, Athena+ will
enable the first quantitative measurements of the extent and distribution of
SMBH accretion in the early Universe.
