Although the big bang happened long ago, it's still with us because photons from the universe's hot birth pervade space and constitute the cosmic microwave background (shown). Now these ancient photons have revealed the motions of groups and clusters of galaxies born long after the big bang. As the photons zip through hot gas in galaxy groups and clusters, the gas boosts the photons' energy and shortens their wavelength, producing the so-called Sunyaev-Zel'dovich effect, named for two Russians who predicted it before astronomers observed it. But Rashid Sunyaev and Yakov Zel'dovich also predicted that the motions of galaxy groups should affect the photons, something no one ever saw. Now, in the 20 July issue of Physical Review Letters, astronomers will report that they have summed up weak signals from thousands of galaxies and detected the so-called kinematic Sunyaev-Zel'dovich effect, finding to no one's surprise that galaxy groups tend to move toward one another under the influence of their gravity. Future observations could be more revelatory: They may help pin down the nature of the mysterious force that is accelerating the universe's expansion.
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