\section*{Acknowledgments}
We thank Alberto Accomazzi and the SAO/NASA Astrophysics Data System team
at the Harvard-Smithsonian Center for Astrophysics for providing
access to the bibliographic data. The research content of this
article was discussed with Alyssa Goodman (Harvard-Smithsonian
Center for Astrophysics) and Matteo Cantiello (Kavli Institute for
Theoretical Physics, University of California, Santa Barbara). 
