  In this Letter, we investigate the potential power of the Cosmic Mach Number
(CMN), which is the ratio between the mean velocity and the velocity dispersion
of galaxies as a function of cosmic scales, to constrain cosmologies. We first
measure the CMN from 5 catalogues of galaxy peculiar velocity surveys at low
redshift (0.002<z<0.03), and use them to contrast cosmological models. Overall,
current data is consistent with the WMAP7 LCDM model. We find that the CMN is
highly sensitive to the growth of structure on scales 0.01<k<0.1 h/Mpc in
Fourier space. Therefore, modified gravity models, and models with massive
neutrinos, in which the structure growth generally deviates from that in the
LCDM model in a scale-dependent way, can be well differentiated from the LCDM
model using future CMN data.
