Evrim Yazgin
Cosmos science journalist
Astrophysicists have completed the most accurate calculations of the amount of dark matter and dark energy in the universe to date.
The powerful analysis, called Pantheon+, provides the most precise limits yet on the composition and evolution of the universe, but also heightens some discrepancies.
A paper on the Pantheon+ study, which was led by researchers at the Harvard & Smithsonian Center for Astrophysics (CfA) in the US, was published in a special issue of the Astrophysical Journal.
Pantheon+ finds that the universe is about two-thirds (66.2%) dark energy and one-third (33.8%) matter. Of that matter, most is dark matter, with ‘ordinary matter’ – that we can see and touch – making up less than five percent of the cosmos.
The universe has been expanding at an accelerating pace over the last several billion years according to the study.
Pantheon+ provides supporting evidence for the prevailing theories of cosmology, known as the ‘Standard Model of Cosmology,’ thereby putting to rest other alternative frameworks that try to understand the strange phenomena known as dark energy and dark matter. Both are critical pieces of the Standard Model of Cosmology but have yet to be directly observed.
Experiments searching for dark matter are currently being built or already underway. One such lab, called the “Genius Lair” is in a gold mine under the Victorian town of Stawell.
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“With these Pantheon+ results, we are able to put the most precise constraints on the dynamics and history of the universe to date,” says lead author of the Astrophysical Journal paper, Dillon Brout, an Einstein Fellow at the CfA. “We’ve combed over the data and can now say, with more confidence than ever before, how the universe has evolved and that the current best theories for dark energy and dark matter hold strong.”
Pantheon+ is based on analysis of 1,500 star explosions called Type Ia supernovae.
Type Ia supernovae occur when white dwarf stars – remnants of medium-sized stars like our sun – undergo a runaway thermonuclear reaction after having accumulated too much mass.
Because these explosions are so bright, they can be seen at distances of more than 10 billion light years – that’s about three-quarters of the universe’s total age. By measuring the apparent brightness of the supernovae (which dim as they get further away), cosmologists can use time and space markers, called “redshift” measurements, to calculate how fast the universe is expanding at different points in its history.
It was a 1998 analysis of these Type Ia supernovae that first revealed the universe’s expanding growth.
“In many ways, this latest Pantheon+ analysis is a culmination of more than two decades’ worth of diligent efforts by observers and theorists worldwide in deciphering the essence of the cosmos,” says Dr Adam Riess, who did not work on the Pantheon+ paper, but was one of the winners of the 2011 Nobel Prize in Physics for the discovery of the accelerating expansion of the universe.
“This leap in both the dataset quality and in our understanding of the physics that underpin it, would not have been possible without a stellar team of students and collaborators working diligently to improve every facet of the analysis,” says Brout.
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Pantheon+ provides a high-precision result for the current expansion rate of the universe, known as the Hubble constant. Pantheon+ results were combined with the Supernova H0 for the Equation of State (SH0ES) collaboration, led by Riess in calculating the Hubble constant.
Pantheon+ and SH0ES found a Hubble constant of 73.4 kilometres per second per megaparsec with only 1.3% uncertainty in their result. In other words, for every 3.26 million light years (one megaparsec), the universe in our part of space is expanding at more than 260,000 kilometres per hour!
But it’s not all good news coming out of Pantheon+.
Measurements of earliest light in the universe, the cosmic microwave background, and the Standard Model of Cosmology predict a Hubble constant in an earlier epoch of the universe which is far lower than that which is calculated from observations of Type Ia supernovae.
Far from resolving this issue, dubbed the “Hubble tension”, the new Pantheon+ and SH0ES data makes things worse. In fact, the tension has now passed the important 5-sigma threshold, which is used by physicists to distinguish between possible statistical flukes and something which is the result of real physics.
“We thought it would be possible to find clues to a novel solution to these problems in our dataset, but instead we’re finding that our data rules out many of these options and that the profound discrepancies remain as stubborn as ever,” says Brout.
Notwithstanding this supernova-sized headache for cosmologists, Pantheon+ does provide a comprehensive view of the evolution of the universe for much of its history.
“With this we get a precise view of the universe from the time when it was dominated by dark matter to when the universe became dominated by dark energy,” Brout explains. “This dataset is a unique opportunity to see dark energy turn on and drive the evolution of the cosmos on the grandest scales up through present time.”
Studying this transfer with greater statistical evidence will hopefully lead to a clearer understanding of how dark energy works.
“Pantheon+ is giving us our best chance to date of constraining dark energy, its origins, and its evolution,” Brout adds.