It’s interesting that they used “supersonic” since phenomenon of sound emerges when there is enough molecules to collide with each other to transmit sound waves

It’s interesting that they used “supersonic” since phenomenon of sound emerges when there is enough molecules to collide with each other to transmit sound waves, which is not the case in the outer space. But most probable speed of hydrogen molecules at 2.7K is surprisingly low - 150m/s. This speed can be used as “sound” speed approximation in the intergalactic space.