Fig. 2.

‘Hubble’ cellular flow. a Plot of azimuthally-averaged growth rate against distance from the colony centroid, averaged over 32 colonies 10 min prior to buckling. The growth rate is approximately constant spatially. Error bars are standard errors of the mean. b Plot of the azimuthually-averaged radial velocity of bacteria as a function of distance from the centroid of the colony, averaged over all colonies 10 min prior to buckling (see Supplementary Note 3 for details). Solid line: best fit. Error bars are standard errors of the mean. c Plot of the ‘Hubble constant’ for the individual colonies, plotted against the bacterial growth rate. Dashed line: best fit; solid line: H = 0.5Λ