Artist’s impression of two white dwarf stars merging and creating a Type Ia supernova. Credit: ESO/L. road
An analysis of more than two decades of supernova explosions convincingly advances modern cosmological theories and reinvigorates efforts to answer fundamental questions.
Astrophysicists have carried out a powerful new analysis that puts the most precise limits ever on the composition and evolution of the universe. With this analysis, called Pantheon+, cosmologists are at a crossroads.
Pantheon+ convincingly finds that the cosmos is about two-thirds dark energy and one-third matter, predominantly in the form of dark matter, and has been expanding at an accelerated rate for the past billion years. However, Pantheon+ also cements a major disagreement over the pace of this expansion that has yet to be resolved.
By placing the prevailing modern cosmological theories, known as the Standard Model of Cosmology, on an even firmer basis of evidence and statistics, Pantheon+ further closes the door to alternative frameworks representing dark energy and dark matter. Both are foundations of the Standard Model of cosmology, but have not yet been directly detected. They are among the model’s greatest mysteries. Following the Pantheon+ results, researchers can now conduct more precise observational tests and refine explanations for the observable cosmos.
G299 was left over by a particular class of supernova called Type Ia. Credit: NASA/CXC/U.Texas
“With these Pantheon+ results, we are able to put the most precise constraints on the dynamics and history of the universe to date,” says Dillon Brout, Einstein Fellow at the Center for Astrophysics | Harvard and Smithsonian. “We’ve combed through the data and can now say with more confidence than ever how the universe has evolved over the eons and that the best current theories about dark energy and dark matter hold strong.”
Brout is the lead author of a series of papers describing the new Pantheon+ analysis, published jointly on October 19 in a special issue of The Astrophysical Journal.
Pantheon+ is based on the largest dataset of its kind, including more than 1,500 stellar explosions called Type Ia supernovae. These bright explosions occur when white dwarf stars, remnants of stars like our Sun, accumulate too much mass and undergo a runaway thermonuclear reaction. Because Type Ia supernovae eclipse entire galaxies, stellar explosions can be seen at distances exceeding 10 billion light-years, or across roughly three-quarters of the total age of the universe. Since supernovae shine at nearly uniform intrinsic luminosities, scientists can use the explosions’ apparent brightness, which decreases with distance, along with redshift measurements as markers of time and space. This information, in turn, reveals how fast the universe is expanding during different epochs, which is then used to test theories of the universe’s fundamental components.
The big discovery in 1998 of the acceleration of the growth of the universe was thanks to a study of type Ia supernovae in this way. Scientists attribute the expansion to an invisible energy, hence called dark energy, inherent in the fabric of the universe itself. Subsequent decades of work have continued to collect ever larger datasets, revealing supernovae over an even wider range of space and time, and Pantheon+ has brought them together in the most statistically robust analysis to date.
“In many ways, this latest Pantheon+ analysis is the culmination of more than two decades of diligent efforts by observers and theorists around the world to decipher the essence of the cosmos,” says Adam Riess, one of the winners of the 2011 Nobel Prize in Physics for the discovery of the accelerating expansion of the universe and Bloomberg Distinguished Professor at Johns Hopkins University (JHU) and the Space Telescope Science Institute in Baltimore, Maryland. Riess is also an alumnus of Harvard University and holds a PhD in astrophysics.
“With this combined Pantheon+ dataset, we get a precise view of the universe from when it was dominated by dark matter to when the universe became dominated by dark energy.” —Dillon Brout
Brout’s own career in cosmology dates back to his undergraduate years at JHU, where he was taught and mentored by Riess. There Brout worked with Dan Scolnic, then a doctoral student and Riess’s advisor, who is now an assistant professor of physics at Duke University and another co-author of the new series of papers.
Several years ago, Scolnic developed the original Pantheon analysis of approximately 1,000 supernovae.
Now, Brout and Scolnic and their new Pantheon+ team have added 50% more supernova data points to Pantheon+, along with improvements in analysis techniques and addressing potential sources of error, ultimately yielding twice as many accuracy than the original Pantheon.
“This leap in both the quality of the dataset and our understanding of the physics that underpins it would not have been possible without a stellar team of students and collaborators working diligently to improve all facets of the ‘analysis,’ says Brout.
Taking the data as a whole, the new analysis holds that 66.2% of the universe is manifested as dark energy, and the remaining 33.8% is a combination of dark matter and matter. To reach an even more comprehensive understanding of the constituents of the universe at different epochs, Brout and colleagues combined Pantheon+ with other strongly evidenced, independent and complementary measurements of the large-scale structure of the universe and with measurements from the world’s oldest light. the universe, the cosmic microwave background.
“With these results from Pantheon+, we are able to put the most precise constraints on the dynamics and history of the universe to date.” —Dillon Brout
Another key result of Pantheon+ relates to one of the primary goals of modern cosmology: determining the current expansion rate of the universe, known as the Hubble constant. Pooling the Pantheon+ sample with data from the SH0ES (Supernova H0 equation of state) collaboration, led by Riess, results in the tightest local measurement of the current expansion rate of the universe
Pantheon+ and SH0ES together find a Hubble constant of 73.4 kilometers per second per megaparsec with only 1.3% uncertainty. Put another way, for every megaparsec, or 3.26 million light years, the analysis estimates that in the nearby universe, space itself is expanding at more than 160,000 miles per hour.
However, observations from a completely different time in the universe’s history predict a different story. Measurements of the earliest light in the universe, the cosmic microwave background, when combined with the current Standard Model of cosmology, consistently set the Hubble constant at a rate significantly lower than observations made using Type Ia supernovae and other astrophysical markers. This major discrepancy between the two methodologies has been called the Hubble tension.
The new Pantheon+ and SH0ES datasets increase this Hubble tension. Indeed, the strain has now passed the important 5-sigma threshold (about a one in a million chance of arising due to chance) that physicists use to distinguish between possible statistical flukes and something that must ‘understand accordingly. Reaching this new statistical level highlights the challenge both theorists and astrophysicists face in trying to explain Hubble’s constant discrepancy.
“We thought it might be possible to find clues to a new solution to these problems in our data set, but instead we’re finding that our data rules out many of these options, and that deep discrepancies remain as stubborn as ever.” says Brout. .
The Pantheon+ results could help indicate where the solution to the Hubble strain lies. “Many recent theories have begun to point towards exotic new physics in the very early universe, but these unverified theories must withstand the scientific process and the Hubble strain remains a major challenge,” says Brout.
Overall, Pantheon+ gives scientists a comprehensive look through much of cosmic history. The first, most distant supernovae in the dataset shine 10.7 billion light-years away, when the universe was about a quarter of its current age. In that earlier epoch, dark matter and its associated gravity kept the expansion rate of the universe in check. This state of affairs changed dramatically over the next billion years as the influence of dark energy overwhelmed that of dark matter. Since then, dark energy has hurled the contents of the cosmos farther and farther and at an ever greater rate.
“With this combined Pantheon + dataset, we get a precise view of the universe from the time it was dominated by dark matter to when the universe became dominated by dark energy,” says Brout. “This data set is a unique opportunity to see how dark energy ignites and drives the evolution of the cosmos on the largest scales to date.”
Studying this change now with even stronger statistical evidence will hopefully lead to new insights into the enigmatic nature of dark energy.
“Pantheon+ is giving us our best chance yet to constrain dark energy, its origins, and its evolution,” says Brout.
Reference: “The Pantheon+ Analysis: Cosmological Constraints” by Dillon Brout, Dan Scolnic, Brodie Popovic, Adam G. Riess, Anthony Carr, Joe Zuntz, Rick Kessler, Tamara M. Davis, Samuel Hinton, David Jones, W. D’Arcy Kenworthy, Erik R. Peterson, Khaled Said, Georgie Taylor, Noor Ali, Patrick Armstrong, Pranav Charvu, Arianna Dwomoh, Cole Meldorf, Antonella Palmese, Helen Qu, Benjamin M. Rose, Bruno Sanchez, Christopher W. Stubbs, Maria Vincenzi, Charlotte M. Wood, Peter J. Brown, Rebecca Chen, Ken Chambers, David A. Coulter, Mi Dai,…