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Supernova peekaboo could provide clues to our universe's age

Gravitational lensing technique offers hope in cracking cosmological conundrum

Unable to cut it open and count the rings, scientists have hit upon another method to figure out the age of the universe.

In 2014 and 2015, a supernova popped up in view of the Hubble Space Telescope, exhibiting a phenomenon which, since the 1960s, cosmologists have theorized might offer insight into the nature of dark matter and shed light on the age of the universe.

The supernova did not appear just once, but four times as its image was bent and distorted by the strong gravitational lens created by the intervening MACS J1149 galaxy cluster field. Light has energy and therefore relativistic mass – according to Einstein's famous equation – which can be bent in gravitational fields, hence the other name for the lensing phenomenon: an Einstein cross.

Measures of the age of the universe have relied on two approaches. Firstly, by looking at evidence from the early universe via microwave background radiation emitted shortly after the Big Bang, which scientists can still detect today. The other involves measuring the brightnesses of distant Type-Ia supernovae and their redshift, which indicates the speed at which they travel away from us.

The problem is that these two approaches vary by about half a billion years, suggesting a fundamental problem with one of the techniques or, more worryingly, our understanding of the universe.

To try to crack this problem, a team of physicists encompassing the University of Minnesota and the UK's University of Portsmouth have released results from a third technique, also involving supernovae and the gravitational lensing discoveries from 2014 and 2015.

With new analysis of the time delays between the recurring images, the researchers were able to measure the age of the universe through the Hubble Constant – actually a rate of expansion – using a theory developed in 1964 by Norwegian astronomer Sjur Refsdal.

Or Graur, a reader in astrophysics at the University of Portsmouth's Institute of Cosmology and Gravitation, helped discover the supernova in 2014 using the Hubble Space Telescope.

However, this being complex cosmology, that result has failed to settle the debate over the age of the universe – around 13 billion years.

Patrick Kelly, lead author of the research and an assistant professor at the University of Minnesota School of Physics and Astronomy, said the latest result would provide more insight into the problem.

"Our measurement favors the value from the cosmic microwave background, although it cannot exclude the supernova value. 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 our understanding of galaxy-cluster dark matter," he said.

The results are published in the journals Science and The Astrophysical Journal. Funding came from NASA through the Space Telescope Science Institute and the National Science Foundation. ®

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