Instabilities in tensorial nonlocal gravity
  We discuss the cosmological implications of nonlocal modifications of general relativity containing tensorial structures. Assuming the presence of standard radiation- and matter-dominated eras, we show that, except in very particular cases, the nonlocal terms contribute a rapidly growing energy density. These models therefore generically do not have a stable cosmological evolution. 
Henrik Nersisyan, Yashar Akrami, Luca Amendola, Tomi S. Koivisto,   Javier Rubio, Adam R. Solomon
gr-qc astro-ph.CO hep-th