MAPPING THE UNIVERSE by Noam I. Libeskind and Yehuda Hoffman Coma NGC 5846 Cluster Abell 3574 Abell S753 Abell 3565 Virgo The Great Attractor Centaurus Hydra Antlia Cosmic Mariners Bulk flow toward Antlia-Centaurus "T here's no such thing as a gravitational time machine!" belted out a voice across the hall. "You can't just reverse gravity!" One of us (NIL) was lecturing on work that we and our collaborators had been developing, in which we tried to answer two simple questions: How did the Milky Way form, and why does it look the way it does? Our approach was rooted in observations of what's termed the local neighborhood - the immediate (relatively speaking) vicinity of our galaxy, out to some 100 million light-years. But instead of 14 OCTOBE R 2 019 * SK Y & TELESCOPE using telescopes, we were building galaxies in computers and then running time forwards and backwards to see how their appearance changed. The lecture was in Tallinn, Estonia, at a meeting dedicated to the 100th birthday of Yakov Zeldovich, a towering figure in 20th-century physics and astrophysics. Besides helping to develop the Katyusha rocket and being an instrumental player in the Soviet Union's atomic bomb project, Zeldovich left an indelible mark on our understanding of the universe on its largest scales, including on how to map the cosmos. H. M. COURTOIS E T A L. / ASTR ONOMICAL JOUR NAL 146:69 (2013) Astronomers chart the vastness of space by tracking the currents that galaxies swim in.