Artist’s impression of two white dwarf stars merging and creating a type Ia supernova. Credit: ESO/L. Calcada
An analysis of more than two decades of supernova explosions convincingly bolsters modern cosmological theories and reinvigorates efforts to answer fundamental questions.
A powerful new analysis has been performed by astrophysicists that places the most precise limits ever known on the composition and evolution of the universe. With this analysis, dubbed Panthéon+, cosmologists find themselves at a crossroads.
Pantheon+ convincingly concludes that the cosmos is roughly two-thirds dark energy and one-third matter – mostly in the form of dark matter – and is growing at an accelerating rate over the past billion years. However, Pantheon+ also cements a major disagreement over the pacing of this expansion that has yet to be resolved.
By placing dominant modern cosmological theories, known as the Standard Model of Cosmology, on an even stronger evidential and statistical foundation, Pantheon+ further closes the door to alternative frameworks taking into account dark energy and black matter. Both are the foundations of the standard model of cosmology but have not yet been directly detected. They are among the biggest mysteries of the model. Following the results of Pantheon+, researchers can now pursue more precise observational tests and refine explanations of the ostensible cosmos.
G299 was left behind by a particular class of supernova called Type Ia. Credit: NASA/CXC/U.Texas
“With these results from Pantheon+, we are able to impose 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’ve combed through the data and can now say with more confidence than ever how the universe has evolved over the eons and that today’s best theories of dark energy and dark matter hold up.”
Brout is the lead author of a series of articles describing the new Pantheon+ analysispublished jointly on October 19 in a special issue of The Astrophysical Journal.
Pantheon+ is based on the largest dataset of its kind, including over 1,500 stellar explosions called Type Ia supernovae. These luminous 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 groundbreaking discovery in 1998 of the accelerated growth of the universe was through a study of Type Ia supernovae in this way. Scientists attribute the expansion to invisible energy, hence dubbed dark energy, inherent in the fabric of the universe itself. The following decades of work continued to compile ever larger datasets, revealing supernovae in an even wider range of space and time, and Pantheon+ has now brought them together in the most statistically robust analysis to date. day.
“In many ways, this latest Pantheon+ analysis is the culmination of more than two decades of diligent effort by observers and theorists around the world to decipher the essence of the cosmos,” says Adam Riess, one of the award winners. of the 2011 Nobel Prize in Physics for the discovery of the accelerating expansion of the universe and the Bloomberg Distinguished Professor at Johns Hopkins University (JHU) and the Space Telescope Science Institute in Baltimore, Maryland. Riess is also a Harvard University alumnus, with a doctorate in astrophysics.
“With this combined Pantheon+ dataset, we get an accurate view of the universe from when it was dominated by dark matter to when the universe became dominated by dark energy.” — Dillon Bread
Brout’s own career in cosmology dates back to his undergraduate years at JHU, where he was taught and advised by Riess. There, Brout worked with Dan Scolnic, then a doctoral student and adviser to Riess, who is now an assistant professor of physics at Duke University and another co-author on the new series of papers.
Several years ago, Scolnic developed the original Pantheon analysis of about 1,000 supernovae.
Today, Brout and Scolnic and their new Pantheon+ team have added approximately 50% more supernova data points to Pantheon+, coupled with improvements in analysis techniques and resolution of potential error sources, which has ultimately achieved twice the accuracy of the original Pantheon.
“This leap in the quality of the dataset and in our understanding of the physics behind it would not have been possible without an outstanding team of students and collaborators working diligently to improve every facet of the analysis,” says Brout.
Taking the data as a whole, the new analysis indicates that 66.2% of the universe manifests as dark energy, with the remaining 33.8% 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 times, Brout and his colleagues combined Pantheon+ with other strongly demonstrated, independent and complementary measurements of the large-scale structure of the universe and with measurements of the first light in the universe, the cosmic microwave background.
“Thanks to these Pantheon+ results, we are able to pose the most precise constraints on the dynamics and history of the universe to date.” — Dillon Bread
Another key result from 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. Pooling the Pantheon+ sample with data from the SH0ES collaboration (Supernova H0 for the state equation), 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. In other words, for every megaparsec, or 3.26 million light-years, the analysis estimates that in the near universe, space itself is expanding at over 160,000 miles per hour.
However, observations from an entirely different time in the history of the universe predict a different story. Measurements of the universe’s first light, the cosmic microwave background, when combined with the current standard model of cosmology, consistently peg the Hubble constant at a significantly lower rate than observations taken via supernovae of the type Ia and other astrophysical markers. This huge discrepancy between the two methodologies has been called the Hubble tension.
The new Pantheon+ and SH0ES datasets increase this Hubble tension. In fact, the voltage has now passed the important 5 sigma threshold (about one in a million chance of occurring due to chance) that physicists use to distinguish between possible statistical chance and something that therefore needs to be understood. Reaching this new statistical level highlights the challenge for theorists and astrophysicists trying to explain the constant Hubble gap.
“We thought it would be possible to find clues to a new solution to these problems in our data set, but instead we find that our data rules out many of these options and that deep divergences remain just as stubborn. than ever,” says Brout. .
The Pantheon+ results could help point to where the solution to the Hubble tension 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 continues to be a major challenge,” says Brout.
Overall, Pantheon+ offers scientists a comprehensive look at much of cosmic history. The oldest and most distant supernovae in the data set shine 10.7 billion light-years away, which is from when the universe was about a quarter of its current age. . At that earlier time, dark matter and its associated gravity controlled the rate of expansion of the universe. Such a state of affairs changed dramatically over the ensuing billions of years, as the influence of dark energy overtook that of dark matter. Dark energy has since scattered the contents of the cosmos further and further and at an ever increasing rate.
“With this combined Pantheon+ dataset, we get an accurate view of the universe from when it was dominated by dark matter to when the universe became dominated by dark energy,” says Brout. . “This dataset is a unique opportunity to see dark energy activate and drive the evolution of the cosmos on larger scales right up to the present time.”
Studying this shift now with even stronger statistical evidence will hopefully lead to new insights into the enigmatic nature of dark energy.
“Pantheon+ gives us our best chance yet to constrain dark energy, its origins and 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, 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