  In this white paper, we discuss the concept of a next-generation X-ray
mission called BEST (Black hole Evolution and Space Time). The mission concept
uses a 3000 square centimeter effective area mirror (at 6 keV) to achieve
unprecedented sensitivities for hard X-ray imaging spectrometry (5-70 keV) and
for broadband X-ray polarimetry (2-70 keV). BEST can make substantial
contributions to our understanding of the inner workings of accreting black
holes, our knowledge about the fabric of extremely curved spacetime, and the
evolution of supermassive black holes. BEST will allow for time resolved
studies of accretion disks. With a more than seven times larger mirror area and
a seven times wider bandpass than GEMS, BEST will take X-ray polarimetry to a
new level: it will probe the time variability of the X-ray polarization from
stellar mass and supermassive black holes, and it will measure the polarization
properties in 30 independent energy bins. These capabilities will allow BEST to
conduct tests of accretion disk models and the underlying spacetimes. With
three times larger mirror area and ten times better angular resolution than
NuSTAR, BEST will be able to make deep field observations with a more than 15
times better sensitivity than NuSTAR. The mission will be able to trace the
evolution of obscured and unobscured black holes in the redshift range from
zero to six, covering the most important epoch of supermassive black hole
growth. The hard X-ray sensitivity of BEST will enable a deep census of
non-thermal particle populations. BEST will give us insights into AGN feedback
by measuring the particle luminosity injected by AGNs into the interstellar
medium (ISM) of their hosts, and will map the emission from particles
accelerated at large scale structure shocks. Finally, BEST has the potential to
constrain the equation of state of neutron stars (NS).
