An international team of astrophysicists says the universe is still expanding at an accelerating rate, rejecting recent claims that cosmic expansion may have begun to slow.
The researchers challenged a study published last year claiming the growth of the universe is slowing. They argue that the standard interpretation remains intact: a mysterious force known as dark energy is continuing to drive the universe’s accelerating expansion.
The team includes two Nobel Laureates and scientists from institutions around the world. According to the researchers, the controversy sparked by the findings reported last November was caused by a misunderstanding in the analysis rather than evidence that scientists’ basic picture of the universe had suddenly broken down.
The new paper was published in Monthly Notices of the Royal Astronomical Society.
New Analysis Challenges a Cosmic Slowdown
The study directly responds to research from a South Korean team that claimed the universe may have entered a period of decelerating expansion. That earlier work suggested dark energy, which acts somewhat like anti-gravity, could be weakening with time.
Lead author Dr. Phil Wiseman, from the University of Southampton, said the latest analysis supports the measurements that astronomers have relied on for years.
“The previous and well-accepted measurements were, in fact, fine and our current understanding of the fate of the universe remains robust,” said lead author Dr. Phil Wiseman, from the University of Southampton.
“Thankfully we have averted this crisis, but the mystery about why the rate of expansion of the universe is still accelerating remains.
“By proving our measurements are correct, we can get back to trying to understand what this dark energy actually is, rather than wondering if it exists at all.”
Supernovae and the Discovery of Cosmic Acceleration
The international research group included Professor Adam Riess and Professor Brian Schmidt, who shared the 2011 Nobel Prize in Physics with Professor Saul Perlmutter.
Their Nobel-winning work relied on observations of Type Ia supernovae, extremely bright explosions produced by white dwarf stars. By studying these events at different distances, the researchers found that more distant objects appeared to be receding faster. That evidence led to the conclusion that the expansion of the universe is accelerating.
That interpretation has remained widely accepted ever since. However, the South Korean study published last year challenged it by arguing that Type Ia supernovae did not all reach the same peak brightness as the universe aged.
If that were true, astronomers could have been misreading the supernova data and incorrectly concluding that cosmic expansion was speeding up when it was actually slowing.
Researchers Identify Problems With the Earlier Analysis
The University of Southampton-led team says it found a key problem in the way the earlier study estimated the ages of the exploding stars. According to the researchers, the previous analysis treated the age of a galaxy as though it were the same as the age of the individual star that later exploded as a supernova.
The team also said the South Korean study did not properly account for the mass of the galaxies hosting the supernovae. Correcting for host galaxy mass is a standard step in modern cosmology that helps scientists make more accurate measurements.
Professor Riess added: “Extraordinary claims require especially careful testing.
“What we find is that when we calibrate these supernovae, accounting for different host environments and populations, the evidence for cosmic acceleration remains remarkably consistent.”
Dark Energy Mystery Remains
Professor Mark Sullivan, also from the University of Southampton, said questioning established ideas and observations remains an essential part of the scientific process.
“This is how progress is made. Although this idea did not turn out to be correct, it has opened up new ways of thinking about how supernovae explode and how we can measure dark energy more accurately,” he added.
Fellow co-author Dr. Brodie Popovic said the debate also provided researchers with an opportunity to examine the astrophysics of supernova explosions more closely and reconsider the assumptions built into cosmological measurements.
“We’ve recently been really focused on astrophysics of the explosions and how they impact cosmology,” he said.
“This was a good opportunity to go back and go over all of our assumptions – it turns out, yes, we do understand this stuff and we’re accounting for it in our cosmology measurement.”
