Interacting photon–baryon fluid, warm dark matter, and the first acoustic peak Júlio C. Fabris, Alan M. Velasquez-Toribio, Winfried Zimdahl and Ilya L. Shapiro Eur. Phys. J. C, 74 7 (2014) 2968 Published online: 19 July 2014 DOI: 10.1140/epjc/s10052-014-2968-2