1 10 100 r 0.01 0.1 1 C (r, t) t = 5 t = 10 t = 20 t = 40 t = 80 t = 160 t = 320 t = 640 t = 1280 1 10 100 r 0.01 0.1 1 C P (r, t) 1 10 100 1000 t 10 100 L(t) L(t)  L(t) OP z = 2 FIG. 6: (color online) The coarsening length L ( t ) as a function of time for the density (circles) and order parameter (squares) correlations for v 0 = 0 . 1, ρ 0 = 0 . 7 and L = 1024. For this density v$_{c}$ ≃ 0 . 42 and the system was started in a disordered state. The straight line has slope 2. Although no single scal- ing exponents can be extracted for the two length scales L ( t ), it is clear that the growth with time is faster than would be obtained for z = 2. FIG. 5: (color online) Early time, two-point density (top panel) and order parameter (bottom panel) correlation function for v 0 = 0 . 1 < v$_{c}$ = 0 . 42 mean density ρ 0 = 0 . 7, and L = 1024. The dashed horizontal line indicates the value of the correlation function at which we extract the coarsening length scale L ( t ), shown in Fig. 6. has been termed “giant number fluctuations” in these ac- tive systems [20] and is the underlying mechanism for the large fluctuations in the density in the flocking state of our system. The top inset of Fig. 1 shows a real space snapshot of the density obtained for ρ$_{0}$ > ρ$_{c}$ and v$_{0}$ > v$_{c}$ ( ρ$_{0}$ ) In this region the systems consists of well defined stripes of the high density ordered phase alternating with stripes of the low density disordered phase. In the ordered region the polarization is always normal to the long direction of the stripes and the stripes travel at a fixed speed in the di- rection of polar order. Panels (a) and (b) of Fig. 9 show histograms of the density and magnitude of the order pa- rameter for a fixed density ρ$_{0}$ = 0 . 7 > ρ$_{c}$ and different self propulsion speeds, v$_{0}$ . For small v$_{0}$ , the histograms are unimodal (fitted with a gaussian peaked at mean den- sity ρ$_{0}$ = 0 . 7), signalling a uniform state. Above a char- acteristic value of v$_{0}$ the histograms acquire a bimodal structure (fitted with two overlapping Gaussians peaked at low and high densities), corresponding to the striped phase. The boundary v$_{c}$ ( ρ$_{0}$ ) corresponding to the onset of the striped phase and shown in Fig. 1 is determined as the value of v$_{0}$ corresponding to the onset of this bimodal structure. The error bars on these data point are sim- ply the step size of our increments in v$_{0}$ . Within these error bars, the same values of v$_{c}$ ( ρ$_{0}$ ) are obtained from the onset of a bimodal structure in both the density and B. Striped Phase 10