  Giant number fluctuations (GNF) are an anomaly universally observed in active
fluids with polar or nematic order. In this paper, we show that GNF arise in
the fluid phase of active Brownian particles (ABP), where the polar order is
absent. GNF in ABP extends over a large but finite length which characterizes
the growing velocity correlations. To suppress unwanted phase separation and
allow ones to explore the disordered fluid phase at large activities, we impart
the inertia, or the mass, to the ABP. A linearized hydrodynamic theory captures
our findings, but only qualitatively. We find numerically a nontrivial scaling
relation for the density correlation function, which the linearized theory
cannot explain. The results suggest ubiquitousness of the anomalous
fluctuations even in the disordered homogeneous fluid phase in the absence of
the directional order.
