A controversy over one of modern cosmology’s foundational ideas has been renewed, with new research that points to critical issues in a recent analysis defending the view that our universe is expanding.
Late last year, a research team at Yonsei University in South Korea presented new findings that challenged the longstanding view about our expanding universe, following new analysis of Type Ia supernovae. These stellar explosions hold a significant place in the modern debate since measurements of their brightness have helped measure the apparent expansion of the cosmos.
Based on their luminosity, the team argued that Type Ia supernovae are affected by their age, a factor that resulted in biases in past cosmological measurements. After correcting for this, the team’s new results shook the world of cosmology, as they implied that the cosmic expansion scientists have long observed may have already transitioned from acceleration to deceleration.
If this were true, it could also imply that dark energy, the mysterious nonluminous energy that currently explains the universe’s expansion, could be evolving, instead of behaving as a cosmological constant—a finding that struck a significant blow to assumptions in the standard Lambda-CDM model.
Pushback from Proponents of the Standard Model
However, in June, an international team led by University of Southampton researchers pushed back on those findings. According to the new analysis, the Southampton team said there appears to be a far weaker relationship between the ages of supernovae and luminosity, arguing now that the ages of the progenitors of these catastrophic stellar explosions may not change as dramatically across cosmic history as those of their host galaxies.
If true, it would seem to indicate that the proposed age correction was too small to upend the current evidence in support of an accelerating universe.
However, the research published in June wasn’t the final word on this brewing controversy.
A Cosmological Debate Continues
Now, the original Yonsei research team has fired back in Monthly Notices of the Royal Astronomical Society with an additional study, in which they point to methodological problems in both central arguments of the Southampton team’s recent rebuttal.
Once again, at the heart of the controversy is the debate over the relationship between the age of supernovae and the standardization of their luminosity. In short, the Yonsei team argues that the analysis presented in June combined supernovae across too broad a range of redshifts. The result is that the apparent correlations between their brightness are thereby weakened.
Dr. Chul Chung of Yonsei University, who led the recent research, says the issues with the new analysis are clear.
“The shallow age-bias slope reported in the rebuttal is therefore the result of a clear methodological problem in the analysis,” Chung said in a statement. “When the comparison is restricted to an appropriate redshift interval, the strong age dependence reappears.”
Chung and his colleagues also raise issues with how the June rebuttal addresses changes in supernova progenitor ages, noting that while reducing the estimated age difference across redshift would appear to weaken the required correction, doing so also results in steeper age luminosity relationships.
Correcting for such effects, Chung and his colleagues argue, produces results that remain largely unchanged based on their initial controversial findings.
It is important to acknowledge that while the new findings do not offer irrefutable evidence that cosmic expansion has ended, they do present strong new findings that revitalize the dispute over the effects of Type Ia supernovae and whether misinterpretations about them could be a driving force behind one of cosmology’s most important measurements.
Further, Chung and the Yonsei research team say their corrected supernova results align with independent findings based on baryon acoustic oscillation measurements obtained with the Dark Energy Spectroscopic Instrument (DESI). This could also be significant, if verified, since the team says this would appear to suggest that dark energy is indeed evolving.
New Tests and Ongoing Debate
In the coming months, observations by the Vera C. Rubin Observatory could provide additional clues, and the Yonsei team suggests that comparing supernovae observations solely from within young host galaxies could potentially eliminate the suspected evolutionary biases that they say are present in current interpretations.
Thanks to the far greater collection of supernova samples from Rubin, researchers may soon be armed with enough new data to determine whether the controversy does indeed reflect past overlooked errors or if the evidence confirms that current views about the universe’s expansion may require a fundamental reset.
The recent study by Chung and his colleagues, “Still non-accelerating: age-bias correction in supernova cosmology is robust to host-progenitor age mapping,” appeared in Monthly Notices of the Royal Astronomical Society.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.