Ripples in the Universal Hubble Flow

Large streaming velocities indicate bulk motions on scales as large as superclusters—clusters of clusters of galaxies—in addition to a smooth cosmological Hubble expansion. The existence of such motion has long been controversial. “None of the standard models can easily explain streaming,” says Jeremiah Ostriker (Princeton). “One might say that the velocities are not just Doppler shifts, but that would be iconoclastic.”