The gravitationally lensed Supernova (SN) Refsdal was discovered by University of Minnesota astronomer Patrick Kelly in 2014 in the field of MACS J1149.6+2223, a galaxy cluster 5 billion light-years away in the constellation of Leo. This event occurred 9.3 billion years ago, and represents the first known example of a strongly lensed supernova with multiple images. It is named after astronomer Sjur Refsdal, who created a theory in 1964 on how to measure the Hubble constant (also known as Hubble’s law), which describes that galaxies are moving away from Earth at speeds proportional to their distance, so the further they are the faster the move away from Earth. SN Refsdal is the first supernova in which this measurement theory was put into practice.
The enlarged inset view reveals four images of SN Refsdal, spotted in 2014, arranged around a giant elliptical galaxy within the cluster. The light from the supernova passes so closely to the galaxy’s dense core that several light paths are redirected and focused toward Earth. The result is that astronomers see four images that form an Einstein Cross. The blue streaks wrapping around the galaxy are the stretched images of SN Refsdal’s parent spiral galaxy, which has been distorted by the warping of space. Computer models of the cluster predict that another image of the stellar blast will appear within 5 years. The red circle marks the possible location of the next supernova image. Astronomers may have missed an earlier appearance of the supernova in 1995, as marked by the blue circle. These multiple appearances of the exploding star are due to the various paths its light is taking through the maze of clumpy dark matter in the galactic grouping. Each image takes a different route through the cluster and arrives at a different time, due, in part, to differences in the length of the pathways the light follows to reach Earth. Image credit: NASA / ESA / S. Rodney, JHU / FrontierSN team / T. Treu, UCLA / P. Kelly, UC Berkeley / GLASS team / J. Lotz, STScI / Frontier Fields team / M. Postman, STScI / CLASH team / Z. Levay, STScI.
There are two precise measurements of the expansion of the Universe, or Hubble constant: calculations from nearby observations of supernovae, and using the Cosmic Microwave Background that began to steam freely shortly after the Big Bang.
However, these two measurements differ by approximately 9%, which is the point of debate on current theories about the makeup and age of the Universe.
“If new, independent measurements confirm this disagreement between the two measurements of the Hubble constant, it would become a chink in the armor of our understanding of the cosmos,” Dr. Kelly said.
“The big question is if there is a possible issue with one or both of the measurements.”
“Our research addresses that by using an independent, completely different way to measure the expansion rate of the Universe.”
In their new research, Dr. Kelly and colleagues calculated the expansion rate of the Universe by using data from four different images of SN Refsdal in 2014.
Astronomers worldwide had correctly predicted that the supernova would appear at a new position in 2015, and the NASA/ESA Hubble Space Telescope then captured a fifth image.
These multiple images appeared because SN Refsdal was gravitationally lensed by the MACS J1149.6+2223 galaxy cluster.
By using the time delays between the appearances of the images the authors were able to measure the Hubble constant.
The measurement favors the value from the Cosmic Microwave Background, although it is not in strong disagreement with the supernova value.
“The measurement of the expansion rate of the Universe is a rollercoaster,” said Stony Brook University’s Professor Simon Birrer.
“While a few years ago most strong lensing measurements yielded higher values in tension with the Cosmic Microwave Background estimates, more recent estimates and revised methodology has resulted in lower values.”
“Our research corroborates a trend, yet does not provide the last word on the expansion rate.”
“If observations of future supernovae that are also gravitationally lensed by clusters yield a similar result, then it would identify an issue with the current supernova value, or with our understanding of galaxy-cluster dark matter,” Dr. Kelly said.
Using the same data, the astronomers found that some current models of galaxy-cluster dark matter were able to explain their observations of the supernovae.
This allowed them to determine the most accurate models for the locations of dark matter in the galaxy cluster, a question that has long plagued astronomers.
“The prediction and subsequent observation of the fifth image of SN Refsdal was a great success of our cosmological model based on general relativity and the mysterious dark matter,” said Stony Brook University’s Professor Anja von der Linden.
“Now, these data have allowed multiple teams to further refine their models of how dark matter is distributed in galaxy clusters, yielding a precise measurement of the Hubble constant from a lensed supernova.”
The results appear in two papers (paper #1 & paper #2) in the journal Science and the Astrophysical Journal.
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Patrick L. Kelly et al. Constraints on the Hubble constant from Supernova Refsdal’s reappearance. Science, published online May 11, 2023; doi: 10.1126/science.abh1322
Patrick L. Kelly et al. The Magnificent Five Images of Supernova Refsdal: Time Delay and Magnification Measurements. ApJ 948, 93; doi: 10.3847/1538-4357/ac4ccb
