Universe's Expansion Is Still Accelerating, International Team Finds

An international team of astrophysicists says the universe is still expanding at an accelerating rate, rejecting a study published last year that claimed cosmic expansion may be slowing down. The new analysis, published in Monthly Notices of the Royal Astronomical Society and led by Dr. Phil Wiseman of the University of Southampton, argues that the standard picture remains intact: a mysterious force called dark energy, which acts somewhat like anti-gravity, is continuing to drive the universe's accelerating expansion.

The team, which includes 2011 Physics Nobel laureates Adam Riess and Brian Schmidt, was responding to a South Korean-led study published last year that suggested the universe may have entered a period of decelerating expansion, implying dark energy could be weakening over time. Riess and Schmidt shared the 2011 Nobel Prize in Physics with Saul Perlmutter for showing, through observations of Type Ia supernovae — extremely bright explosions from white dwarf stars — that more distant supernovae appeared to be receding faster, evidence that the universe's expansion is accelerating.

The Southampton-led team says the earlier study contained two key problems: it treated the age of a galaxy as though it were the same as the age of the individual star that exploded as a supernova within it, and it did not properly account for the mass of the galaxies hosting the supernovae — a standard step in modern cosmology. 'The previous and well-accepted measurements were, in fact, fine, and our current understanding of the fate of the universe remains robust,' Wiseman said.

Riess said that 'extraordinary claims require especially careful testing,' adding that when the team calibrated the supernovae while accounting for different host environments and populations, the evidence for cosmic acceleration remained consistent. Co-author Mark Sullivan of Southampton said the episode opened up new ways of thinking about how supernovae explode and how dark energy is measured, even though the original idea did not hold up.