  CMB observations provide a precise measurement of the primordial power
spectrum on large scales, corresponding to wavenumbers $10^{-3}$ Mpc$^{-1}$ < k
< 0.1 Mpc$^{-1}$, [1-8]. Luminous red galaxies and galaxy clusters probe the
matter power spectrum on overlapping scales (0.02 Mpc$^{-1}$ < k < 0.7
Mpc$^{-1}$ [9-18]), while the Lyman-alpha forest reaches slightly smaller
scales (0.3 Mpc$^{-1} < k < 3$ Mpc$^{-1}$; [19]). These observations indicate
that the primordial power spectrum is nearly scale-invariant with amplitude
close to $2 \times 10^{-9}$, [5, 20-25]. They also strongly support Inflation
and motivate us to obtain constraints reaching to smaller scales on the
primordial curvature power spectrum and by implication on Inflation. One could
obtain limits to much higher values of $k < 10^5$ Mpc$^{-1}$ and with less
sensitivity even higher to $k < 10^{19}- 10^{23}$ Mpc$^{-1}$ using limits from
CMB spectral distortions(SD)and on ultracompact minihalo objects(UCMHs)and
Primordial Black Holes(PBHs). In this paper, we revisit and collect all the
known constraints on both PBHs and UCMHs. We show that unless one uses SD, PBHs
give us very relaxed bounds on the primordial curvature perturbations. UCMHs
are very informative over a reasonable $k$ range($3 < k < 10^6$ Mpc$^{-1}$)and
lead to significant upper-bounds on the curvature spectrum. We review the
conditions under which the tighter constraints on the UCMHs could imply
extremely strong bounds on the fraction of Dark Matter that could be PBHs.
Failure to satisfy these conditions would lead to over production of the UCMHs,
which is inconsistent with the observations. Therefore, we can almost rule out
PBH within their overlap scales with the UCMHs. We consider the UCMH bounds
from experiments such as $\gamma$-rays, Neutrinos, Reionization, pulsar-timing
and SD. We show that they lead to comparable results independent of the form of
DM.
