MSGID: 1:317/3 64a0fd6c   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Astrophysicists propose a new way of measuring cosmic expansion: Lensed   
   gravitational waves    
      
     Date:   
         July 1, 2023   
     Source:   
         University of California - Santa Barbara   
     Summary:   
         The universe is expanding; we've had evidence of that for about a   
         century. But just how quickly celestial objects are receding from   
         each other is still up for debate.   
      
      
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   FULL STORY   
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   The universe is expanding; we've had evidence of that for about a   
   century. But just how quickly celestial objects are receding from each   
   other is still up for debate.   
      
   It's no small feat to measure the rate at which objects move away from   
   each other across vast distances. Since the discovery of cosmic expansion,   
   its rate has been measured and re-measured with increasing precision,   
   with some of the latest values ranging from 67.4 up to 76.5 kilometers   
   per second per megaparsec, which relates the recession velocity (in   
   kilometers per second) to the distance (in megaparsecs).   
      
   The discrepancy between different measurements of cosmic expansion   
   is called the "Hubble tension." Some have called it a crisis in   
   cosmology. But for UC Santa Barbara theoretical astrophysicist Tejaswi   
   Venumadhav Nerella and colleagues at the Tata Institute of Fundamental   
   Research in Bangalore, India, and the Inter-University Center for   
   Astronomy and Astrophysics in Pune, India, it is an exciting time.   
      
   Since the first detection of gravitational waves in 2015, detectors have   
   been significantly improved and are poised to yield a rich haul of signals   
   in the coming years. Nerella and his colleagues have come up with a method   
   to use these signals to measure the universe's expansion, and perhaps   
   help to settle the debate once and for all. "A major scientific goal of   
   future detectors is to deliver a comprehensive catalog of gravitational   
   wave events, and this will be a completely novel use of the remarkable   
   dataset," said Nerella, co-author of a paper published in Physical   
   Review Letters.   
      
   Measurements of the cosmic expansion rate boil down to velocity and   
   distance.   
      
   Astronomers use two kinds of methods to measure distances: the first   
   start with objects with a known length ("standard rulers") and look at   
   how big they appear in the sky. These "objects" are features in cosmic   
   background radiation, or in the distribution of galaxies in the universe.   
      
   A second class of methods starts with objects of known luminosity   
   ("standard candles") and measures their distances from Earth using   
   their apparent brightness. These distances are connected to those of   
   farther bright objects and so on, which builds up a chain of measurement   
   schemes that is often called the "cosmic distance ladder." Incidentally,   
   gravitational waves themselves can also help measure cosmic expansion,   
   since the energy released by the collision of neutron stars or black   
   holes can be used to estimate the distance to these objects.   
      
   The method that Nerella and his co-authors propose belongs to the second   
   class but uses gravitational lensing. This is a phenomenon that occurs   
   when massive objects warp spacetime, and bend waves of all kinds that   
   travel near the objects. In rare cases, lensing can produce multiple   
   copies of the same gravitational wave signal that reach Earth at different   
   times -- the delays between the signals for a population of multiple   
   imaged events can be used to calculate the universe's expansion rate,   
   according to the researchers.   
      
   "We understand very well just how sensitive gravitational wave detectors   
   are, and there are no astrophysical sources of confusion, so we can   
   properly account for what gets into our catalog of events," Nerella   
   said. "The new method has sources of error that are complementary   
   to those of existing methods, which makes it a good discriminator."   
   The sources of these signals would be binary black holes: systems of   
   two black holes that orbit each other and ultimately merge, releasing   
   massive amounts of energy in the form of gravitational waves. We haven't   
   yet detected strongly lensed examples of these signals, but the upcoming   
   generation of ground-based detectors is expected to have the necessary   
   level of sensitivity.   
      
   "We expect the first observation of lensed gravitational waves in the   
   next few years," said study co-author Parameswaran Ajith. Additionally,   
   these future detectors should be able to see farther into space and   
   detect weaker signals.   
      
   The authors expect these advanced detectors to start their search   
   for merging black holes in the next decade. They anticipate recording   
   signals from a few million black hole pairs, a small fraction (about   
   10,000) of which will appear multiple times in the same detector due   
   to gravitational lensing. The distribution of the delays between these   
   repeat appearances encodes the Hubble expansion rate.   
      
   According to lead author Souvik Jana, unlike other methods of   
   measurement, this method does not rely on knowing the exact locations   
   of, or the distances to, these binary black holes. The only requirement   
   is to accurately identify a sufficiently large number of these lensed   
   signals. The researchers add that observations of lensed gravitational   
   waves can even provide clues on other cosmological questions, such as   
   the nature of the invisible dark matter that makes up much of the energy   
   content of the universe.   
      
       * RELATED_TOPICS   
             o Space_&_Time   
                   # Black_Holes # Astrophysics # Cosmology # Astronomy #   
                   Cosmic_Rays # Asteroids,_Comets_and_Meteors # Big_Bang   
                   # Galaxies   
       * RELATED_TERMS   
             o Physical_cosmology o Ultimate_fate_of_the_universe o   
             Astronomy o Shape_of_the_Universe o Radio_telescope o Big_Bang   
             o Cosmic_microwave_background_radiation o Extraterrestrial_life   
      
   ==========================================================================   
   Story Source: Materials provided by   
   University_of_California_-_Santa_Barbara. Original written by Sonia   
   Fernandez. Note: Content may be edited for style and length.   
      
      
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   Journal Reference:   
      1. Souvik Jana, Shasvath J. Kapadia, Tejaswi Venumadhav, Parameswaran   
      Ajith.   
      
         Cosmography Using Strongly Lensed Gravitational Waves from Binary   
         Black Holes. Physical Review Letters, 2023; 130 (26) DOI: 10.1103/   
         PhysRevLett.130.261401   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230701135624.htm   
      
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