Ancient Supernova may hold key to dark energy

Astronomers are a step closer to cracking one of the secrets of dark energy - the mysterious force believed to be causing the universe's accelerated expansion.

Dark energy makes up approximately 68% of the universe, but its exact nature is unknown as is its influence on our ballooning cosmos.

Now scientists have been handed a clue by the discovery of a unique supernova in the early Universe. Although the light from the cosmic explosion has been travelling for over 10 billion years, it was incredibly luminous and its light was magnified by the gravity of a galaxy to make it appear even brighter.

“No one has found a supernova like this before, and the nature of the system means it may be able to help solve some big problems in astrophysics such as the nature of the force that drives the expansion of the universe,” explains Dr Daniel Perley, a reader in astrophysics at Liverpool John Moores University.

The galaxy is in direct line of sight between the supernova and the Earth and the manner in which its gravity effects the light as it travels towards our viewpoint is the key to the puzzle.

“We are seeing the light from this distant supernova split into multiple images, what we call ‘gravitationally lensed’,” explains Jacob Wise, a PhD student at the Astrophysics Research Institute, who first realised its significance.

Light's varying paths to Earth

“When light is ‘lensed’, the different paths the light follows to get to Earth don't all have the same length, so light moving along different paths takes variable amounts of time to reach us.”

In the case of a supernova that shines for months, this means that we can see the different images of the same source all together at the same time, but each one is probing a different time in the supernova's evolution.

“What's exciting about that is that the amount of time difference between different images depends on the expansion rate of the universe,” added Dr Perley. 

The team, which is working with Caltech, Stockholm University, and several other institutes worldwide on the study, plans to measure these time differences precisely in order to determine how fast the universe is expanding.  To do that would tell us about the force (dark energy) that is causing the universe's expansion to accelerate.

New calculation on expansion of Universe

With various numbers posited by astronomers, Perley suspects the supernova calculation will help cast the "tiebreaking vote".

He said: “Studies of afterglow of the Big Bang give one number for the so-called Hubble constant - the measurement of the expansion speed of the universe - while studies of nearby galaxies give a different number.  Astronomers are calling this the Hubble Tension.  Hence, studies of lensed supernovae could indicate which of these two numbers we should really believe.”

Such was the brightness of the ‘magnified’ supernova, that despite its immense distance, it was spotted with medium-sized ground-based telescopes, including the Zwicky Transient Facility in California (the first telescope to detect the supernova was the Zwicky Transient Facility in California, but it couldn't see the multiple images.  The Liverpool Telescope was the first facility to see the multiple images - and thereby prove it was gravitationally lensed) and the Liverpool Telescope in La Palma.  It was later followed up by the Keck Telescopes in Hawaii, the Hubble Space Telescope, and the James Webb Space Telescope.

Added Jacob: “Our colleagues in Stockholm first noticed the supernova but it was us who spotted that the light had been bent into multiple images.

“All the major observatories in the Northern Hemisphere plus the space-based telescopes have been looking at this but it was the Liverpool Telescope, run from LJMU, that got there first,” beamed Wise.

The study - Discovery of SN 2025wny: A Strongly Gravitationally Lensed Superluminous Supernova at z = 2.01 - is published in the journal Astrophysical Letters and authored by Joel Johansson, Daniel A. Perley, Ariel Goobar, Jacob L. Wise, Yu-Jing Qin, Zoë McGrath, Steve Schulze, Cameron Lemon, Anjasha Gangopadhyay, Konstantinos Tsalapatas, and 24 other co-authors.