How much dark matter is there in the universe?

Astrophysicists have made a powerful new analysis that is the most accurate measure yet of the universe’s composition and evolution.

With this analysis, called Pantheon+, cosmologists find themselves at a crossroads. Pantheon+ convincingly concludes that the cosmos It’s made up of about two-thirds dark energy and one-third matter – mostly in the form of dark matter. and that it is expanding at an accelerated rate during the last billions of years.

However, Pantheon+ also consolidates a major disagreement over the pace of this expansion that has yet to be resolved. By placing the prevailing modern cosmological theories, known as the Standard Model of Cosmology, on an even firmer statistical and evidence base, Pantheon+ further closes the door to alternative frameworks that account for dark matter and energy.

Both are pillars of the Standard Model of Cosmology, but they have yet to be directly detected and are one of the biggest mysteries of the model. Thanks to the Pantheon+ results, researchers can now perform more accurate observational tests and refine explanations of the ostensible cosmos.

“With these Pantheon+ results, we are able to place 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 and Smithsonian.

“We’ve scoured the data and can now say with more confidence than ever before how the universe evolved over the eons and that the best current theories about dark energy and dark matter hold up.”

66.2% of the universe manifests as dark energy, with the remaining 33.8% being a combination of dark matter and matter.

Brout is the lead author of a series of articles describing the new Pantheon+ analysis, jointly published in a special issue of The Astrophysical Journal. Pantheon+ is based on the largest dataset of its kind, comprising more than 1,500 explosive stars called 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.

As type Ia supernovas outrun entire galaxies, stellar detonations can be glimpsed at distances greater than 10 billion light-years, or about three-quarters of the total age of the universe.

As supernovae burn with nearly uniform intrinsic brightness, scientists can use the apparent brightness of the explosions, which diminishes with distance, along with redshift measurements as time and space markers. This information, in turn, reveals the expansion rate of the universe during different epochs, which is used to test theories of the fundamental building blocks of the universe.

The big discovery in 1998 of the accelerated growth of the universe was thanks to the study of type Ia supernovae in this way. Scientists attribute the expansion to an invisible energy, hence called dark energy.inherent in the very fabric of the universe.

Subsequent decades of work continued to collect ever-larger datasets, revealing supernovae across an even wider swath of space and time, and Pantheon+ has now assembled them in the most statistically robust analysis to date.

“In many ways, this latest analysis of Pantheon+ is the culmination of more than two decades of diligent efforts by observers and theorists around the world to unravel the essence of the cosmos,” said Adam Riess, one of the Nobel Prize winners. 2011 for the discovery of the accelerated 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 an alumnus of Harvard University with a Ph.D. in astrophysics.

Brout’s career in the field of cosmology dates back to his undergraduate years at JHU, where he was mentored and mentored by Riess. There Brout worked with then-doctoral student and Riess advisor Dan Scolnic, who is now an associate professor of physics at Duke University and another co-author of the new series of articles.

Several years ago, Scolnic developed Pantheon’s original analysis of approximately 1,000 supernovae. Now, Brout and Scolnic and their new Pantheon+ team have added about 50% more supernova data points to Pantheon+, along with improvements in analysis techniques and addressing potential sources of error, yielding twice the accuracy of the original Pantheon.

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“This leap in the quality of the dataset and in our understanding of the physics behind it would not have been possible without a stellar team of students and collaborators who worked diligently to improve every facet of the analysis,” says Brout. Taking the data as a whole, The new analysis maintains 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 building blocks of the universe at different times, Brout and his colleagues combined Pantheon+ with other independent and complementary measurements of the large-scale structure of the universe and with measurements of the oldest light in the universe, the background. cosmic microwave.

Another important result of Pantheon+ is related to one of the main goals of modern cosmology: to determine the current rate of expansion of the universe, known as the Hubble constant. Combining the Pantheon+ sample with data from the Supernova H0 for the Equation of State (SH0ES) collaboration, led by Riess, results in the most accurate local measurement of the universe’s current expansion rate.

Pantheon+ and SH0ES together 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 in the nearby universe, space itself is expanding at over 160,000 kilometers per hour.

However, observations from a completely different time in the history of the universe predict a different story. Measurements of the oldest light in the universe, the cosmic microwave background, when combined with the current Standard Model of Cosmology, consistently fix the Hubble constant at a significantly slower rate than observations made through Type Ia supernovae. and other astrophysical markers. This considerable discrepancy between the two methodologies was called hubble voltage.

The new data sets Pantheon+ and SH0ES augment this Hubble strain. In fact, the strain surpassed the important threshold of 5 sigma (about a one in a million chance of occurring by chance) that physicists use to distinguish between possible statistical flukes and something to be understood accordingly.

Reaching this new statistical level highlights the challenge for theorists and astrophysicists trying to explain the Hubble constant discrepancy. “We thought it would be possible to find clues to a new solution to these problems in our dataset, but we are finding that our data excludes many of these options and that the deep discrepancies remain as stubborn as ever,” says Brout. . .

The Pantheon+ results could help identify where the solution for the Hubble strain lies. “Many recent theories have begun to point towards exotic new physics in the early universe, however, these unverified theories must stand the scientific process and the Hubble strain remains a major challenge,” says Brout.

Overall, Pantheon+ gives scientists a comprehensive view of much of cosmic history. The oldest and most distant supernovae in the dataset shine at 10.7 billion light-years away, that is, when the universe was about a quarter of its current age.

At that earlier time, dark matter and its associated gravity kept the universe’s expansion rate in check. This state of affairs changed dramatically over the next billions of years, as the influence of dark energy overtook that of dark matter. Since then, dark energy has pushed further and further away the contents of the cosmos, and it has done so at an ever-increasing rate.

“With this Pantheon+ dataset, we get an accurate view of the universe from the time it was dominated by dark matter to the time the universe was dominated by dark energy,” says Brout. “This dataset is a unique opportunity to see how dark energy ignites and drives the evolution of the cosmos on the largest scales to date.”

Studying this shift now, with even stronger statistical evidence, will lead to new insights into the enigmatic nature of dark energy. “Pantheon+ is giving us our best chance to constrain dark energy, its origins and its evolution,” says Brout.


The Pantheon+ Analysis: Cosmological Constraints

Source: Harvard and Smithsonian.

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