An artistic impression of two white dwarf stars merging and creating a Type Ia supernova. Credit: ESO/L. Calsada
Analysis of supernova explosions that have lasted for more than two decades convincingly bolsters modern cosmological theories and revitalizes efforts to answer fundamental questions.
Astrophysicists have conducted a powerful new analysis that places the most precise limits ever on the formation and evolution of the universe. With this analysis, nicknamed Pantheon+, cosmologists find themselves at a crossroads.
Pantheon+ convincingly argues that the universe is made up of about two-thirds of dark energy and one-third of matter – mostly in the form of dark matter – and has been expanding at an accelerating pace over the past billions of years. However, Pantheon+ also solidifies a major controversy over the pace of this unresolved expansion.
By placing the dominant modern cosmological theories, known as the Standard Model of cosmology, on firmer evidential and statistical foundations, Pantheon+ also closes the door on alternative frameworks that account for dark energy And the dark matter. Both are cornerstones of the Standard Model of cosmology but have not yet been directly discovered. They are among the model’s biggest puzzles. Following on from Pantheon+’s results, researchers can now perform more accurate observational tests and refine explanations for the apparent universe.
G299 was left behind by a particular class of supernovae called Type Ia. Credit: NASA/CXC/U.Texas
“With these Pantheon+ results, we can place the most precise constraints on the dynamics and history of the universe to date,” says Dillon Prout, Einstein’s Fellow at the Center for Astrophysics. Harvard and Smithsonian. “We’ve combed through the data and can now say with more confidence than ever before how the universe has evolved over the ages and that the current best theories of dark energy and dark matter are powerful.”
Pruitt is the lead author of a series of papers describing the new Pantheon + Analysiswas jointly published on October 19 in a special issue of Astrophysical Journal.
Pantheon+ is based on the largest data set of its kind, featuring more than 1,500 starbursts 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 breakthrough discovery of the accelerating growth of the universe in 1998 was thanks to the study of Type Ia supernovae in this way. Scientists attribute this expansion to invisible energy, and therefore it is called dark energy, which is inherent in the fabric of the universe itself. Subsequent decades of work have continued to assemble ever larger data sets, revealing supernovae across a wider range of space and time, and Pantheon+ has now brought them together in the most powerful statistical analysis yet.
Says Adam Rees, one of the winners of the 2011 Nobel Prize in Physics for discovering the accelerating expansion of the universe and Bloomberg Distinguished Professor of Johns Hopkins University (JHU) and Space Telescope Science Institute in Baltimore, Maryland. Reese is also a Harvard graduate, and holds a Ph.D. in astrophysics.
“With this combined Pantheon+ data set, we get an accurate view of the universe from the time when it was dominated by dark matter to the time when dark energy became dominant in the universe.” – Dillon Brut
Pruitt’s career in cosmology dates back to his undergraduate years at Johns Hopkins University, where he was tutored and advised by Reese. There Pruitt worked with Dan Skolnick, then a doctoral student and Reiss advisor, who is now an assistant professor of physics at Duke University and another co-author on the new paper series.
Several years ago, Skolnik developed the original pantheon analysis of about 1,000 supernovae.
Now, Brout, Scolnic and their new team have added Pantheon+ about 50 percent more supernova data points to the Pantheon+, along with improvements in analysis techniques and handling potential sources of error, which ultimately resulted in poor accuracy of the original Pantheon.
“This leap in data set quality and our understanding of the physics that underpins it would not have been possible without an excellent team of students and collaborators working hard to improve every aspect of the analysis,” says Pruitt.
Looking at the data as a whole, the new analysis sees 66.2% of the universe appearing as dark energy, with the remaining 33.8% being a mixture of matter and dark matter. To gain a more comprehensive understanding of the constituent components of the universe at different eras, Brout and colleagues combined Pantheon+ with other strongly proven, independent, complementary scales to the large-scale structure of the universe and with measurements from the universe’s closest light, the Cosmic Microwave Background.
“With these Pantheon+ results, we can place the most precise constraints on the dynamics and history of the universe to date.” – Dillon Brut
Another major result of Pantheon+ relates to one of the main goals of modern cosmology: determining the current rate of expansion of the universe, known as the Hubble constant. Assembling the Pantheon+ sample with data from SH0ES (Supernova H0 for the Equation of State), led by Reese, results in the most stringent local measurement of the universe’s current expansion rate.
Allanthion+ and SH0ES together find the 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 era in the history of the universe predict a different story. Measurements of the universe’s oldest light, the cosmic microwave background, when combined with the current Standard Model of cosmology, consistently corroborate the Hubble constant at a rate much lower than observations made via Type Ia supernovae and other astrophysical markers. This great discrepancy between the two methodologies is called the Hubble tension.
The new Pantheon+ and SH0ES datasets amplify Hubble’s tension. In fact, the tension has now crossed the important 5-sigma threshold (the probability of one occurring in a million due to random chance) that physicists use to distinguish between possible statistical chance and something to understand accordingly. Reaching this new statistical level highlights the challenge both theorists and astrophysicists face in trying to explain the inconsistency of the Hubble constant.
“We thought it would be possible to find evidence for a new solution to these problems in our data set, but instead we found that our data exclude many of these options and that deep discrepancies remain as intractable as ever,” says Brout.
The Pantheon+ results can help indicate where the Hubble tension resolves. “Many modern theories are beginning to point to strange new physics in the early universe, however, such unverified theories have to withstand the scientific process, and the Hubble tension remains a huge challenge,” says Pruitt.
Overall, Pantheon+ offers scientists a comprehensive look back through much of cosmic history. The oldest and most distant supernova in the dataset gleams from 10.7 billion light-years away, since the universe was roughly a quarter of its current age. In that earlier era, dark matter and its associated gravity were in control of the expansion rate of the universe. Such a situation changed dramatically over the following several billion years as the influence of dark energy overshadowed the effect of dark matter. Since then, dark energy has pushed the contents of the universe apart and at an ever-increasing rate.
“With this combined Pantheon+ data set, we get an accurate view of the universe from a time when dark matter was dominated by dark energy,” says Pruitt. “This data set is a unique opportunity to see dark energy at work and drive the evolution of the universe at the highest levels during the present time.”
We hope that studying this change now with stronger statistical evidence will lead to new insights into the nature of mysterious dark energy.
“Pantheon+ gives us our best opportunity yet to constrain dark energy, its origins, and its evolution,” says Pruitt.
Reference: “Pantheon + Analysis: Cosmic Constraints” by Dillon Pruitt, Dan Skolnick, Brody Popovich, Adam J. Reese, Anthony Carr, Joe Zontz, Rick Kessler, Tamara M. Davies, Samuel Hinton, David Jones, W. Darcy Kenworthy, Eric R. Peterson, Khaled Saeed, Georgie Taylor, Noor Ali, Patrick Armstrong, Pranav Scharvaux, Ariana Dumoh, Cole Mulldorf, Antonella Palmes, Helen Coe, Benjamin M. Rose, Bruno Sanchez, Christopher W. Stubbs, Maria Vincenzi, Charlotte M. Wood, Peter J. Brown, Rebecca Chin, Ken Chambers, David A. Coulter, May Day, Georgios Demetriadis, Alexei F. Filipenko, Ryan J. Foley, Saurabh Jha, Lisa Kelsey, Robert B Kirchner, Anis Muller, Jesse Muir, Seshadri Nadthor, Yen Chin Pan, Armin Rist, Cesar Rojas Bravo, Masao Sacco, Matthew Seibert, Matt Smith, Benjamin E. Stahl and Phil Wiseman, October 19, 2022, Astrophysical Journal.
DOI: 10.3847 / 1538-4357 / ac8e04