Artist’s impression of two white dwarf stars merging and creating a Type Ia supernova. Credit: ESO/L. Calçada
An analysis of more than two decades of supernova explosions convincingly propels modern cosmological theories and reinvigorates efforts to answer fundamental questions.
Astrophysicists have performed a powerful new analysis that sets the most precise limits ever seen on the composition and evolution of the universe. With this analysis, called Pantheon+, cosmologists find themselves at a crossroads.
Pantheon+ convincingly finds that the cosmos is made up of roughly two-thirds dark energy and one-third matter, predominantly in the form of dark matter, and is expanding at an accelerating rate over the last few billion years. However, Pantheon+ also cements a major disagreement over the pace of that expansion that has yet to be resolved.
By placing prevailing modern cosmological theories, known as the Standard Model of cosmology, on an even firmer statistical and evidential basis, Pantheon+ further closes the door on alternative frameworks that explain dark energy Y dark matter. Both are the building blocks of the Standard Model of Cosmology, but have yet to be directly detected. They are among the biggest mysteries of the model. Following the Pantheon+ results, researchers can now perform more precise observational tests and refine explanations of the apparent cosmos.
G299 was left behind by a particular class of supernovae called Type Ia. Credit: NASA/CXC/U.Texas
“With these Pantheon+ results, we can put the most precise constraints on the dynamics and history of the universe to date,” says Dillon Brout, an Einstein Fellow in the Center for Astrophysics. Harvard and Smithsonian. “We have reviewed the data and can now say with more confidence than ever before how the universe has evolved over the eons and that the best current theories about dark energy and dark matter remain sound.”
Brout is the lead author of a series of articles describing the new Pantheon+ Analysisjointly published on October 19 in a special issue of the astrophysical journal.
Pantheon+ is based on the largest dataset of its kind, comprising more than 1,500 exploding stars called Type Ia supernovae. These bright explosions occur when[{” attribute=””>white dwarf stars — remnants of stars like our Sun — accumulate too much mass and undergo a runaway thermonuclear reaction. Because Type Ia supernovae outshine entire galaxies, the stellar detonations can be glimpsed at distances exceeding 10 billion light years, or back through about three-quarters of the universe’s total age. Given that the supernovae blaze with nearly uniform intrinsic brightnesses, scientists can use the explosions’ apparent brightness, which diminishes with distance, along with redshift measurements as markers of time and space. That information, in turn, reveals how fast the universe expands during different epochs, which is then used to test theories of the fundamental components of the universe.
The great discovery in 1998 of the accelerated 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 continued to collect larger and larger data sets, revealing supernovae over an even wider range of space and time, and Pantheon+ has now 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 unravel the essence of the cosmos,” says Adam Riess, one of the Nobel Prize winners. 2011 in Physics for the discovery of the accelerating expansion of the universe and the Bloomberg Distinguished Professor in Johns Hopkins University (JHU) and the Space Telescope Science Institute in Baltimore, Maryland. Riess is also an alumnus of Harvard University and has a Ph.D. in astrophysics.
“With this combined Pantheon+ dataset, we get an accurate view of the universe from the time it was dominated by dark matter until the universe became dominated by dark energy.” — Dillon Brout
Brout’s own career in cosmology dates back to his student years at JHU, where he was taught and advised by Riess. There, Brout worked with then-doctoral student and Riess adviser Dan Scolnic, who is now an assistant professor of physics at Duke University and another co-author of the new paper series.
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 about 50 percent more supernova data points to Pantheon+, along with improvements to analysis techniques and addressing potential sources of error, ultimately yielding twice as many of precision than the original Pantheon.
“This leap in both the quality of the dataset and our understanding of the physics behind it would not have been possible without a stellar team of students and collaborators who worked diligently to improve every facet of the analysis,” says Brout.
Taking the data as a whole, the new analysis holds that 66.2 percent of the universe manifests as dark energy, with the remaining 33.8 percent being a combination of dark matter and matter. To arrive at an even more complete understanding of the constituent components of the universe at different epochs, Brout and his colleagues combined Pantheon+ with other complementary, independent, and strongly evidenced measurements of the large-scale structure of the universe and with measurements of the earliest light in the universe. the universe, the cosmic microwave background.
“With these Pantheon+ results, we can 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 rate of expansion of the universe, known as the Hubble constant. Combining the Pantheon+ sample with data from the SH0ES (Supernova H0 for Equation of State) collaboration, led by Riess, results in the most accurate 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 history of the universe 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 fix the Hubble constant at a rate that is significantly slower than observations taken via Type 1 supernovae. Ia and other astrophysical markers. This considerable discrepancy between the two methodologies has been called the Hubble strain.
The new Pantheon+ and SH0ES data sets add to this Hubble tension. In fact, the strain has now passed the all-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 therefore be understood. Reaching this new statistical level highlights the challenge for both theorists and astrophysicists in trying to explain the constant Hubble discrepancy.
“We thought it would be possible to find clues to a novel solution to these problems in our data set, but instead we found that our data rules out many of these options and that the deep discrepancies remain as stubborn as ever,” says Brout. . .
The Pantheon+ results could help pinpoint where the solution to the Hubble strain lies. “Many recent theories have begun to point to exotic new physics in the very early universe, however such unverified theories must withstand the scientific process and the Hubble strain remains a major challenge,” says Brout.
Overall, Pantheon+ offers scientists a comprehensive look back across much of cosmic history. The oldest and most distant supernovae in the dataset shine 10.7 billion light-years away, that is, when the universe was about a quarter of its current age. In that earlier era, dark matter and its associated gravity kept the expansion rate of the universe in check. Such a state of affairs changed dramatically over the next several billion years as the influence of dark energy overtook that of dark matter. Since then, dark energy has thrown the contents of the cosmos further and further apart and at an ever-increasing rate.
“With this combined Pantheon+ dataset, we get an accurate view of the universe from the time it was dominated by dark matter until the universe became dominated by dark energy,” says Brout. “This dataset is a unique opportunity to see how dark energy ignites and drives the evolution of the cosmos on the largest scales up to the present.”
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 to date to constrain dark energy, its origins, and its evolution,” says Brout.
Reference: “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, Georgios Dimitriadis, Alexei V. Filippenko, Ryan J. Foley, Saurabh W. Jha, Lisa Kelsey, Robert P. Kirshner, Anais Möller, Jessie Muir, Seshadri Nadathur, Yen-Chen Pan, Armin Rest, Cesar Rojas-Bravo, Masao Sako, Matthew R. Siebert, Matt Smith, Benjamin E. Stahl, and Phil Wiseman, October 19, 2022, the astrophysical journal.
DOI: 10.3847/1538-4357/ac8e04
