Ultrasmall grains spend most of their time at
their vibrational ground state during the interval
of two photon absorption events (Section 3 and
Fig. 2;
Draine & Li 2001).
In the 1996 South African "Cold Dust" Symposium,
it was heatedly argued that these grains could obtain
a vibrational temperature less than the 2.7 K temperature
of the CMB so that they could be detected in absorption
against the CMB
(Duley & Poole 1998).
However, based on detailed modeling of the excitation
and de-excitation of these grains, we found that
even though these grains do have a large population in
the vibrational ground state, nevertheless the vibrational
levels are sufficiently excited that the grains would appear
in emission against the CMB with brightness temperature
9K (see
Draine & Li 2004b
for details).
Acknowledgments I thank G.J. Bendo, D.L. Block, F. Boulanger, B.T. Draine, R.C. Kennicutt, J.I. Lunine, D. Pfenniger, J.L. Puget, M.W. Regan, and C. Yuan for helpful discussions. I am grateful to D.L. Block and the organizing committee for inviting me to this stimulating symposium. I also thank my advisors B.T. Draine and the late J.M. Greenberg for guiding me to this fascinating field - cosmic dust.