An international team including two Nobel laureates says the universe's expansion is still accelerating, directly rebutting a study published last November that claimed cosmic expansion had begun to slow. [1] The rebuttal, first previewed in June and formally published this week in Monthly Notices of the Royal Astronomical Society, concludes that the earlier claim rested on a measurement error rather than evidence that dark energy is weakening. [1][2]

The November study, from a South Korean research team, argued that Type Ia supernovae — the exploding white dwarf stars whose brightness first revealed cosmic acceleration in the 1990s — did not all reach the same peak brightness as the universe aged. If true, that would mean decades of supernova-based distance measurements had been systematically misread, and that the accelerating expansion widely credited to dark energy might be an illusion. [1]

The new analysis, led by Dr. Phil Wiseman of the University of Southampton and co-authored by Professors Adam Riess and Brian Schmidt — who shared the 2011 Nobel Prize in Physics with Saul Perlmutter for the original 1998 discovery of cosmic acceleration — identifies two specific problems in the South Korean team's method. First, the earlier analysis treated the age of a supernova's host galaxy as equivalent to the age of the individual star that exploded, rather than accounting for the difference. Second, it failed to correct for host galaxy mass, a standard step in modern cosmological measurement. [1] "Extraordinary claims require especially careful testing," Riess said. "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." [1]

Wiseman put the outcome more bluntly: "The previous and well-accepted measurements were, in fact, fine and our current understanding of the fate of the universe remains robust. 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." [1]

That closing point is the one worth sitting with. The team's third co-author, Professor Mark Sullivan, framed the episode as science functioning as intended rather than as a false alarm to be forgotten: "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." [1] Co-author Dr. Brodie Popovic described the process similarly — a forced re-examination of assumptions that, in this case, held up. "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." [1]

Nothing about this rebuttal makes dark energy less mysterious. Nearly three decades after its discovery, physicists still cannot say what the force actually is, only that its effects on the universe's expansion remain measurable and, per this week's analysis, real. The value of the episode is not that a contested claim was proven wrong — contested claims often are — but that the correction arrived through the same open, checkable process that produced the original error, rather than through authority or consensus alone.

-- KENJI NAKAMURA, Tokyo