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Euclid calling: downloading the Universe


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      This Hubble Space Telescope image shows the powerful gravity of a galaxy embedded in a massive cluster of galaxies producing multiple images of a single distant supernova far behind it. The image shows the galaxy’s location within a large cluster of galaxies called MACS J1149.6+2223, located more than 5 billion light-years away. In the enlarged inset view of the galaxy, the arrows point to the multiple copies of an exploding star, named Supernova Refsdal, located 9.3 billion light-years from Earth.Credit: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley), and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI) Astronomers investigating one of the most pressing mysteries of the cosmos – the rate at which the universe is expanding – are readying themselves to study this puzzle in a new way using NASA’s Nancy Grace Roman Space Telescope. Once it launches by May 2027, astronomers will mine Roman’s wide swaths of images for gravitationally lensed supernovae, which can be used to measure the expansion rate of the universe.
      There are multiple independent ways astronomers can measure the present expansion rate of the universe, known as the Hubble constant.  Different techniques have yielded different values, referred to as the Hubble tension. Much of Roman’s cosmological investigations will be into elusive dark energy, which affects how the universe is expanding over time. One primary tool for these investigations is a fairly traditional method, which compares the intrinsic brightness of objects like type Ia supernovae to their perceived brightness to determine distances. Alternatively, astronomers could use Roman to examine gravitationally lensed supernovae. This method of exploring the Hubble constant is unique from traditional methods because it’s based on geometric methods, and not brightness.
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      Using various observatories like NASA’s Hubble Space Telescope and James Webb Space Telescope, astronomers have discovered just eight gravitationally lensed supernovae in the universe. However, only two of those eight have been viable candidates to measure the Hubble constant due to the type of supernovae they are and the duration of their time-delayed imaging.
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      “Rather than gathering several pictures of trees, this new telescope will allow us to see the entire forest in a single snapshot,” Pierel explained.
      In particular, the High Latitude Time Domain Survey will observe the same area of sky repeatedly, which will allow astronomers to study targets that change over time. This means there will be an extraordinary amount of data – over 5 billion pixels each time – to sift through in order to find these very rare events.
      A team led by Strolger and Pierel at STScI is laying the groundwork for finding gravitationally lensed supernovae in Roman data through a project funded by NASA’s Research Opportunities in Space and Earth Science (ROSES) Nancy Grace Roman Space Telescope Research and Support Participation Opportunities program.
      “Because these are rare, leveraging the full potential of gravitationally lensed supernovae depends on a high level of preparation,” said Pierel. “We want to make all the tools for finding these supernovae ready upfront so we don’t waste any time sifting through terabytes of data when it arrives.”
      The project will be carried out by a team of researchers from various NASA centers and universities around the country.
      The preparation will occur in several stages. The team will create data reduction pipelines designed to automatically detect gravitationally lensed supernovae in Roman imaging. To train those pipelines, the researchers will also create simulated imaging: 50,000 simulated lenses are needed, and there are only 10,000 actual lenses currently known.
      The data reduction pipelines created by Strolger and Pierel’s team will complement pipelines being created to study dark energy with Type Ia supernovae.
      “Roman is truly the first opportunity to create a gold-standard sample of gravitationally lensed supernovae,” concluded Strolger. “All our preparations now will produce all the components needed to ensure we can effectively leverage the enormous potential for cosmology.”
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      By Hannah Braun
      Space Telescope Science Institute, Baltimore, Md.
      ​​Media Contact:
      Claire Andreoli
      NASA’s Goddard Space Flight Center, Greenbelt, Md.
      301-286-1940
      Christine Pulliam
      Space Telescope Science Institute, Baltimore, Md.
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