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                           Astronomy Picture of the Day   
      
       Discover the cosmos! Each day a different image or photograph of our   
         fascinating universe is featured, along with a brief explanation   
                       written by a professional astronomer.   
      
                                   2023 June 29   
       The illustration shows the beams from pulsars around the image and a   
        pair of merging black holes on the upper left. A grid depicting the   
      warping of spacetime by passing gravitational waves spreads across the   
      image center. Please see the explanation for more detailed information.   
      
                     A Message from the Gravitational Universe   
       Illustration Credit: NANOGrav Physics Frontier Center; Text: Natalia   
                             Lewandowska (SUNY Oswego)   
      
      Explanation: Monitoring 68 pulsars with very large radio telescopes,   
      the North American Nanohertz Observatory for Gravitational Waves   
      (NANOGrav) has uncovered evidence for the gravitational wave (GW)   
      background by carefully measuring slight shifts in the arrival times of   
      pulses. These shifts are correlated between different pulsars in a way   
      that indicates that they are caused by GWs. This GW background is   
      likely due to hundreds of thousands or even millions of supermassive   
      black hole binaries. Teams in Europe, Asia and Australia have also   
      independently reported their results today. Previously, the LIGO and   
      Virgo detectors have detected higher-frequency GWs from the merging of   
      individual pairs of massive orbiting objects, such as stellar-mass   
      black holes. The featured illustration highlights this   
      spacetime-shaking result by depicting two orbiting supermassive black   
      holes and several of the pulsars that would appear to have slight   
      timing shifts. The imprint these GWs make on spacetime itself is   
      illustrated by a distorted grid.   
      
         Open Science: Browse 3,000+ codes in the Astrophysics Source Code   
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                         Tomorrow's picture: asteroid day   
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