Primordial Perturbation of Dark Matter as a Novel Probe of Very Early
  Universe
  Dark matter(DM) is the only possible candidate which would be apart from the
thermal equilibrium before Big Bang nucleosynthesis(BBN) in accordance with
current DM searches. In this work, we report a generic scenario that primordial
perturbation of dark matter(PPDM) can be, effectively, generated and encoded
with primordial information of very early universe up to the reheating era. We
present an analytical solution of the whole evolution of PPDM. A novel and
strong constraint on the reheating process imposed by primordial gravitational
wave(PGW) is obtained for the first time. It indicates the ratio of PGW to
primordial curvature perturbation(PCP) is not only dependent on the slow-roll
spectral index but also, strongly, on the decay process of inflaton at
reheating. For very generic reheating process, our result provides a natural
explanation of the paucity of PGW in current observations.
