  Motivated by the fact that QCD critical point (CP) belongs to the same
universality class as the liquid-gas transition, the dynamical density
fluctuations around the CP is analyzed using relativistic fluid dynamics for a
viscous fluid. It is shown that relativistic effects on the dynamical structure
factor of the density fluctuation appears only in the width of the sound and
thermal modes through a modification of the transport/kinetic coefficients. It
is found that the mechanical density fluctuation which is enhanced by the
relativistic effects are attenuated, whereas the entropy fluctuation in turn
becomes more prominent around the QCD CP. This attenuation of the density
fluctuation around the QCD CP may imply that if the suppression or even total
disappearance of Mach cone is observed as the incident energy of the
relativistic heavy-ion collisionsis lowered, it can be a signal of the
existence of the QCD critical point.
