020 (2008), 0804.0232. 

[50] G. Caldera-Cabral, R. Maartens, and B. M. Schaefer, JCAP 0907 , 027 (2009), 0905.0492. 

[51] K. Koyama, R. Maartens, and Y.-S. Song, JCAP 0910 , 017 (2009), 0907.2126. 2 

[52] The definition of c P would coincide with the conventional 

rest-frame sound speed, ˆ c 2 s , if g = 3 H (1 + w )(ˆ c 2 s − c 2 a ) θ de where the adiabatic sound speed is c 2 a ≡ ˙ P de / ˙ ρ de . See W. Hu, Astrophys. J. 506 , 485 (1998), astro-ph/9801234 

