Supernova explosions reveal the precise details of dark energy and dark matter

Supernova explosions reveal the precise details of dark energy and dark matter

Type Ia Supernova

Artist’s impression of two white dwarf stars merging to produce a Type Ia supernova. Credit: ESO/L. Calçada

Analysis of more than two decades’ worth of supernova explosions convincingly informs modern cosmological theories and fuels efforts to answer fundamental questions.

Astrophysicists have performed a powerful new analysis that places the most precise constraints ever on the composition and evolution of the universe. With this analysis, called Pantheon+, cosmologists have found themselves at a crossroads.

Pantheon+ conclusively reveals that the cosmos consists of about two-thirds dark energy and one-third matter—predominantly in the form of dark matter—and that it has been expanding at an accelerated rate for the past several billion years. However, Pantheon+ also cements a major disagreement over the pace of that 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 evidentiary and statistical foundation, Pantheon+ further closes the door to alternative frameworks that take into account dark energy and Black matter. Both are fundamental to the Standard Model of cosmology, but have yet to be directly discovered. They are among the biggest mysteries of the model. Following the Pantheon+ results, researchers can now pursue more precise observational tests and refine explanations for the apparent cosmos.

G299 Type Ia Supernova

G299 was left behind by a special class of supernova called Type Ia. Credit: NASA/CXC/U.Texas

“With these Pantheon+ results, we are able to place 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 & Smithsonian. “We combed through the data and can now say with more confidence than ever before how the universe has evolved over eons and that the current best theories for dark energy and dark matter are strong.”

Brout is the lead author of a series of papers describing the new Pantheon+ analysispublished together on October 19 in special edition no The Astrophysical Journal.

Pantheon+ is based on the largest dataset of its kind, consisting of more than 1,500 stellar explosions called Type Ia supernovae. These brilliant 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 accelerated growth of the universe in 1998 was due to the study of a Type Ia supernova in this way. Scientists attribute the expansion to invisible energy, therefore called dark energy, inherent in the fabric of the universe itself. The following decades of work continued to compile ever larger datasets, revealing supernovae over an even wider range of space and time, and Pantheon+ has now combined them into the most statistically robust analysis to date.

“In many ways, this latest Pantheon+ analysis represents the culmination of more than two decades of effort by observers and theorists around the world to decipher the essence of the cosmos,” said Adam Riess, one of the 2011 Nobel Prize winners in Physics for the discovery of the accelerating expansion of the universe and Bloomberg Distinguished Professor on Johns Hopkins University (JHU) and Space Telescope Science Institute in Baltimore, Maryland. Riess is also a graduate of Harvard University, with a doctorate in astrophysics.

“With this combined Pantheon+ dataset, we get a precise look at the universe from when it was dominated by dark matter to when the universe was dominated by dark energy.” — Dillon Bread

Brout’s career in cosmology dates back to his undergraduate years at JHU, where he was taught and advised by Riess. There, Brout worked with then-doctoral student and Riess advisor Dan Scolnik, 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 about 50 percent more supernova data points to Pantheon+, along with improvements in analysis techniques and addressing potential sources of error, ultimately yielding twice the accuracy of the original Pantheon.

“This leap in the quality of the dataset and in our understanding of the underlying physics would not have been possible without a stellar team of students and collaborators working diligently to improve every aspect of the analysis,” says Brout.

Taking the data as a whole, the new analysis says that 66.2 percent of the universe manifests as dark energy, while the remaining 33.8 percent is a combination of dark matter and matter. To gain an even more comprehensive understanding of the compositional components of the universe at different epochs, Brout and colleagues combined Pantheon+ with other strongly established, independent and complementary measures of the large-scale structure of the universe and measurements from the earliest light to the cosmic microwave background.

“With these Pantheon+ results, we are able to place the most precise constraints on the dynamics and history of the universe to date.” — Dillon Bread

Another key result of Pantheon+ relates to one of the most important goals of modern cosmology: nailing down 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 tightest local measurement of the universe’s current expansion rate.

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 a rate of 160,000 miles per hour.

However, observations from a completely different era of the universe’s history predict a different story. Measurements of the universe’s earliest light, the cosmic microwave background, when combined with the current Standard Model of cosmology, consistently determine the Hubble constant at a rate significantly slower than observations from Type Ia supernovae and other astrophysical markers. This significant discrepancy between the two methodologies is called the Hubble tension.

The new Pantheon+ and SH0ES datasets amplify this Hubble tension. In fact, the tension has now passed the important 5-sigma threshold (about a one in a million chance of occurring due to random chance) that physicists use to distinguish between possible statistical cases and something that must be understood accordingly. Achieving this new statistical level highlights the challenge for both theorists and astrophysicists to try to explain Hubble’s constant drift.

“We thought it might be possible to find clues to a new solution to these problems in our data set, but instead we find that our data rule out many of these options and that the deep outliers remain as stubborn as ever,” says Brout.

The Pantheon+ results could help indicate where the solution to the Hubble tensions lies. “Many recent theories have begun to point to exotic new physics in the very early universe, however, such untested theories must withstand the scientific process and the Hubble tension remains a major challenge,” says Brout.

Overall, Pantheon+ offers scientists a comprehensive look back through much of cosmic history. The earliest, most distant supernovae in the dataset shine from 10.7 billion light-years away, which means from when the universe was about a quarter of its current age. In that earlier era, dark matter and its associated gravity kept the rate of expansion of the universe in check. Such a state changed dramatically in the next few billion years as the influence of dark energy overcame the influence of dark matter. Since then, dark energy has been throwing the contents of the cosmos farther and farther and faster.

“With this combined Pantheon+ dataset, we get a precise view of the universe from the time when it was dominated by dark matter to the time when the universe was dominated by dark energy,” says Brout. “This data set is a unique opportunity to see how dark energy turns on and drives the evolution of the cosmos on the largest scales through the present time.”

Studying this change now with even stronger statistical evidence will hopefully lead to new insights into the mysterious nature of dark energy.

“Pantheon+ gives us the best chance yet to constrain dark energy, its origin and its evolution,” says Brout.

Reference: “The Pantheon+ Analysis: Cosmological Constraints” 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 Moller, Jessie Muir, Seshadri Nadathur, Yen-Chen Pan, Armin Rest, Cesar Rojas-Bravo, Masao Sako, Matthew R. Siebert, Mat Smith, Benjamin E. Stahl, and Phil Wiseman, October 19, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ac8e04

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