By calculating the power spectrum from the pt correlations data we determined the sound of the little bang at the last moment of the collision. But to know how this sound developed, we also determined how the acoustic horizon expanded with time. Just like the horizon defines how far we can see, the acoustic horizon defines what sounds we can hear. We needed to know the time evolution of the speed of sound in the medium to determine the acoustic horizon. The speed of sound versus energy density has been calculated from lattice QCD [1] and the energy density versus time has been calculated in a hydrodynamic calculation [2]. We put these two together to get find the speed of sound versus time and the acoustic horizon.

[1] A. Bazavov et al., Phys. Rev. D 80 (2009), 014504
[2] Kolb, P., Heavy Ion Phys. 21, 243-248, (2004)

Power Spectrum
Speed of Sound
Acoustic Horizon
The Sound