  We study the spacetimes of the near-horizon regions in D3-brane, M2-brane and
M5-brane configurations, in cases where there is a pp-wave propagating along a
direction in the world-volume. While non-extremal configurations of this kind
locally have the same Carter-Novotny-Horsky-type metrics as those without the
wave, taking the BPS limit results instead in Kaigorodov-type metrics, which
are homogeneous, but preserve 1/4 of the supersymmetry, and have global and
local structures that are quite different from the corresponding anti-de Sitter
spacetimes associated with solutions where there is no pp-wave. We show that
the momentum density of the system is non-vanishing and held fixed under the
gravity decoupling limit. In view of the AdS/CFT correspondence, M-theory and
type IIB theory in the near-horizon region of these boosted BPS-configurations
specifies the corresponding CFT on the boundary in an infinitely-boosted frame
with constant momentum density. We model the microstates of such boosted
configurations (which account for the microscopic counting of near-extremal
black holes in D=7, D=9 and D=6) by those of a boosted dilute massless gas in a
d=4, d=3 and d=6 spacetime respectively. Thus we obtain a simple description
for the entropy of 2-charge black holes in D=7,9 and 6 dimensions. The paper
includes constructions of generalisations of the Kaigorodov and
Carter-Novotny-Horsky metrics in arbitrary spacetime dimensions, and an
investigation of their properties.
