The most precise accounting of dark energy and dark matter to date

Pantheon+ convincingly states that the cosmos is about two-thirds dark energy and one-third matter – mostly in the form of dark matter – and has been expanding at an accelerating rate over 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 dominant modern cosmological theories known as the Standard Model of Cosmology on an even more solid evidence base and statistical basis, Pantheon+ closes the door to alternative frameworks that take dark energy and dark matter into account. Both are cornerstones of the Standard Model of cosmology, but have yet to be discovered directly and are among the model’s greatest mysteries. Building on the results of Pantheon+, researchers can now conduct more accurate observational tests and refine explanations for the alleged cosmos.

“With these Pantheon+ results, we are able to constrain the dynamics and history of the Universe to date as precisely as possible,” says Dillon Brout, Einstein Fellow at the Center for Astrophysics | Harvard & Smithsonian. “We’ve combed through the data and now we can say with more confidence than ever before how the universe evolved over the eons and that the current best theories of dark energy and dark matter hold.”

Brout is the lead author of a series of articles describing the new Pantheon+ analysis, published jointly today in a special issue of The Astrophysical Journal.

Pantheon+ is based on the largest dataset of its kind, which includes more than 1,500 stellar explosions known as 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 outshine entire galaxies, the stellar detonations can be seen at distances more than 10 billion light-years, or about three-quarters the total age of the Universe. Because the supernovae glow with nearly uniform intrinsic brightness, scientists can use the apparent brightness of the explosions, which decreases with distance, along with redshift measurements, as markers in time and space. This information, in turn, shows how rapidly the universe is expanding at different epochs, which is then used to test theories about the fundamental components of the universe.

The groundbreaking discovery of the accelerated growth of the Universe in 1998 was due to studying Type Ia supernovae in this way. Scientists attribute the expansion to an invisible energy, hence called dark energy, inherent in the structure of the universe itself. In the decades that followed, larger and larger datasets were compiled, revealing supernovae over an even larger space and time range, and Pantheon+ has now brought them together in the most statistically robust analysis yet.

“In many ways, this latest Pantheon+ analysis is the culmination of more than two decades of diligent efforts by observers and theorists worldwide to unravel the essence of the cosmos,” says Adam Riess, one of the winners of the 2011 Nobel Prize in Physics for the discovery of the accelerated 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 a graduate of Harvard University and has a PhD in astrophysics.

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 graduate student and Riess advisor Dan Scolnic, who is now an assistant professor of physics at Duke University and another co-author of the new series of articles.

A few years ago, Scolnic developed the original pantheon analysis of about 1,000 supernovae.

Now, Brout and Scolnic and their new Pantheon+ team have added around 50 percent more supernovae data points to Pantheon+, coupled with improvements in analysis techniques and addressing potential sources of error, ultimately resulting in twice the accuracy of the original Pantheon.

“This leap in both dataset quality and our understanding of the underlying physics would not have been possible without an outstanding team of students and collaborators who worked diligently to improve every facet of the analysis,” says Brout.

Looking at the data as a whole, the new analysis says 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 comprehensive understanding of the constituents of the Universe at different epochs, Brout and colleagues combined Pantheon+ with other strongly supported, independent and complementary measurements of the large-scale structure of the Universe and with measurements from the earliest light of the Universe, the cosmic microwave background.

Another important Pantheon+ result 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 von Riess-led SH0ES (Supernova H0 for the Equation of State) collaboration results in the most rigorous local measurement of the current expansion rate of the Universe.

Together, Pantheon+ and SH0ES 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 space in the nearby Universe is expanding by more than 160,000 miles per hour.

However, observations from a very different epoch in the history of the universe predict a different story. Measurements of the earliest light in the Universe, the cosmic microwave background, combined with the current standard model of cosmology, consistently fix the Hubble constant at a rate significantly slower than observations made over Type Ia supernovae and other astrophysical markers . This significant discrepancy between the two methods has been dubbed the Hubble voltage.

The new datasets Pantheon+ and SH0ES amplify this Hubble tension. In fact, the voltage has now crossed the important 5 sigma threshold (roughly a one in a million chance of it occurring by chance) that physicists use to distinguish between possible statistical fluke and something that needs to be understood accordingly. Reaching this new statistical level underscores the challenge for theorists and astrophysicists to explain the discrepancy in the Hubble constant.

“We thought it might be possible to find evidence of a novel solution to these problems in our data set, but instead we find that our data excludes many of these options and that the profound discrepancies remain as persistent as ever,” says bread .

The Pantheon+ results could help pinpoint where the solution to the Hubble voltage lies. “Many recent theories point to exotic new physics in the very early Universe, but such unconfirmed theories must withstand the scientific process, and the Hubble voltage remains a major challenge,” says Brout.

Overall, Pantheon+ offers scientists a comprehensive look back at much of cosmic history. The earliest and most distant supernovae in the dataset shine from a distance of 10.7 billion light-years, when the Universe was about a quarter of its current age. In this earlier era, 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 apart at ever greater speeds.

“With this combined Pantheon+ dataset, we get an accurate 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 dataset is a unique opportunity to see how dark energy is turning on and fueling the evolution of the cosmos on the grandest scales to the present day.”

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.