  In this work, we present a few pieces of evidence in support of a possible
connection of the gravitational theory in dS space to the worldvolume theory of
unstable D-branes. We show that the action describing the geodesic motion of a
massive particle (or point-like object) in static dS space turns out to be the
same as that of the tachyon field theory for an unstable particle. The motion
along the radial direction from the origin, a locally Minkowski spacetime, to
the horizon, a locally Rindler space times a sphere, represents just the
tachyon condensation process, therefore providing a geometric picture of
tachyon condensation. We further study a scalar in global or flat dS and the
tachyon fluctuations in a homogeneous tachyon background, representing either
the full or half S-brane, on unstable D-branes and find that certain dynamics
of the dS universe corresponds to that of the homogeneous full or half S-brane.
The thermal temperature of tachyon radiation is found to agree with that felt
by any timelike observer in dS space. In string theory context, this
temperature is just the Hagedorn one, signaling a phase transition to closed
strings. An understanding of this transition in the bulk dS space is also
given.
