Two decades' worth of observations of supernova explosions and a powerful new analysis tool has provided the most accurate accounting of dark energy and dark matter to date.
Dark energy and dark matter — often collectively known as the "dark universe" — are mysterious because despite making up at least 95% of the universe's energy and matter content, they can't be observed directly. The existence of dark energy is inferred from the fact it drives the accelerating expansion of the universe. Dark matter, which does not interact with light and is thus "invisible" is indirectly detected due to its gravitational influence, which literally prevents galaxies from flying apart as they rotate.
The new analysis, dubbed Pantheon+, confirms that the matter-energy content of the universe is made up by around two-thirds dark energy and one-third matter, most of which is in the form of dark matter. The analysis also confirms that the universe has been expanding at an accelerating rate for the last few billion years, and leaves a key disagreement in the rate of this expansion still unresolved.
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The results obtained by the astrophysicists behind the Pantheon+ analysis provide further statistical evidence that the modern cosmological theories that make up the Standard Model provide the best explanation for dark energy and dark matter. This means that the Pantheon+ analysis could help close the door on alternative frameworks that attempt to explain these mysterious aspects of the cosmos and could help scientists hone their understanding of the dark universe.
"With these Pantheon+ results, we are able to put the most precise constraints on the dynamics and history of the universe to date," Dillon Brout, an astrophysicist at the Center for Astrophysics at Harvard & Smithsonian, said in a statement. "We've combed over the data and can now say with more confidence than ever before how the universe has evolved over the eons and that the current best theories for dark energy and dark matter hold strong."
Shedding light on the dark universe with cosmic candles
Pantheon was first developed by Dan Scolnic, a physicist at Duke University in North Carolina and co-author on the new research, several years ago, when the analysis relied on data from 1,000 supernovas, or stellar explosions.
In the new study, Brout, Scolnic and their colleagues brought this dataset up to more than 1,500 supernovas. Additionally, they improved analysis techniques as well as addressing potential sources of error. The upgrades have granted Pantheon+ twice the precision of the original Pantheon.
The technique relies on what astronomers call Type Ia supernovas, which are a type of cosmic explosion that occurs when stellar remnants called white dwarfs accumulate matter from a companion star at a rapid rate, triggering runaway thermonuclear reactions.
This variety of cosmic explosions can be so bright that they outshine the light output from every star in their galaxy combined, so astronomers have spotted this type of supernova as much as 10 billion light-years away. Because light takes a finite time to travel to Earth, astronomers are looking back in time as well — in the case of 10 billion light-years, back to when the universe was just one-quarter of its current age.
Every Type Ia supernova releases the same amount of light, so astronomers call them "standard candles" and use these events to measure cosmic distances and the expansion of the universe.
Distance measurements are possible because the brightness of Type Ia supernova light diminishes as it travels. Calculating the expansion of the universe is more complicated; it relies on determining how much the light has been stretched out — or "redshifted," as astronomers call it — as it travels for billions of years across an expanding universe.
The redshift from supernovas at varying distances and thus at different periods in cosmic history reveals how fast the universe was expanding during its different epochs. These measurements can then be used to test theories of the fundamental components of the universe, including dark energy and dark matter.
Two separate teams of scientists in 1998 used observations of distant Type Ia supernovas to calculate that the expansion of the universe was in fact accelerating. This came as a major shock to physicists, who had assumed that whatever had triggered the initial rapid expansion of the universe — commonly called "the Big Bang" — had dissipated and the expansion rate of the universe had slowed.
The accelerating expansion of the universe is analogous to pushing a child on a swing, watching them slow almost to a stop when suddenly they begin swinging more rapidly and to greater heights without any further push.
Dark energy became a placeholder name for the cosmic push that is stretching out the very fabric of the universe faster and faster. Dark matter is almost the flip side of this coin, with its gravitational influence helping to hold galaxies together internally as dark energy pushed them apart from each other.
The expansion of the universe began speeding up when dark energy began to dominate over the influence of matter — predominantly dark matter — and began to drive the universe apart at an ever-increasing rate.
"With this combined Pantheon+ dataset, 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 said. "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."
The "Hubble tension" remains
In addition, the Pantheon+ analysis exacerbates a lingering problem in cosmology.
Scientists call the rate of expansion of the universe the Hubble constant, which the Pantheon+ analysis calculates at 45.6 miles (73.4 kilometers) per second per megaparsec with only 1.3% uncertainty. This means for every megaparsec, which equals 3.26 million light-years, the analysis estimates that in the nearby universe space itself is expanding at more than 160,000 mph (260,000 kph).
Another way of measuring the Hubble constant uses the cosmic microwave background (CMB) radiation, a fossil light left over from an event shortly after the Big Bang. However, this approach and the Type Ia supernova approach suggest different values for the Hubble constant. This difference has become known as the "Hubble tension," and the Pantheon+ results further highlight this lingering discrepancy, since the analysis is precise enough that astronomers calculate that the odds of the tension arising due to random chance is just one in 1 million.
Despite this, these results could help scientists pin down the source of the disparity between these two different measurement techniques.
"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 rule out many of these options and that the profound discrepancies remain as stubborn as ever," Brout said. "Many recent theories have begun pointing to exotic new physics in the very early universe, however, such unverified theories must withstand the scientific process and the Hubble tension continues to be a major challenge."
Pantheon+ brings together observations of Type Ia supernovas made since the groundbreaking discovery of dark energy in 1998 to better constrain this element of the universe.
Adam Riess, a cosmologist at Johns Hopkins University and the Space Telescope Science Institute in Maryland and a co-author on the new research, was one of the members of the teams behind the 1998 revelation. He shared the 2011 Nobel Prize in Physics for the discovery of the accelerating expansion of the universe.
"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," Riess said.
The team's research is described in a series of papers published Wednesday (Oct. 19) in The Astrophysical Journal.
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