Astrophysicists have performed a powerful new analysis that places the most precise limits yet 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 mainstream modern cosmological theories, known as the Standard Model of Cosmology, on even stronger evidence and statistical foundations, Pantheon+ further closes the door to alternative frameworks that consider dark energy and dark matter. Both are the foundations of the Standard Model of Cosmology but have yet to be directly detected and rank among the model’s greatest mysteries. Following the results of Pantheon+, researchers can now refine explanations of the ostensible cosmos.
“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 lead author on a series of papers describing the new Pantheon+ analysis, co-published today 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 bright bursts 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 eclipse entire galaxies, stellar detonations can be seen at distances greater than 10 billion light-years, or as far back as about three-quarters of the total age of the universe. Since supernovae flare up with nearly uniform intrinsic luminosities, scientists can use the apparent brightness of the explosions, which decreases with distance, along with redshift measurements as markers of time and space. This information, in turn, reveals how fast the universe is expanding at different times, 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 of Johns Hopkins University and the Space Telescope Science Institute in Baltimore, Maryland. Riess is also a Harvard University alumnus, with a Ph.D. in astrophysics.
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 advisor 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.
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 achieve an even more complete understanding of the constituent components of the universe at different times, Brout and his colleagues combined Pantheon+ with other strongly evidenced, independent and complementary measurements of the large-scale structure of the universe. and with measurements of the first light of the universe. universe, cosmic microwave background.
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, but these 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 overview of 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. This state of affairs changed drastically 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.