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 Pantheon+, cosmologists find themselves at a
crossroads.
Pantheon+ convincingly finds that the cosmos is composed of about two-thirds
dark energy and one-third matter—mostly in the form of dark matter—and is
expanding at an accelerating pace over the last several billion years.
However, Pantheon+ also cements a major disagreement over the pace of that
expansion that has yet to be solved.
By putting prevailing modern cosmological theories, known as the Standard
Model of Cosmology, on even firmer evidentiary and statistical footing,
Pantheon+ further closes the door on alternative frameworks accounting for
dark energy and dark matter. Both are bedrocks of the Standard Model of
Cosmology but have yet to be directly detected and rank among the model's
biggest mysteries. Following through on the results of Pantheon+,
researchers can now pursue more precise observational tests and hone
explanations for the ostensible cosmos.
"With these Pantheon+ results, we are able to put the most precise
constraints on the dynamics and history of the universe to date," says
Dillon Brout, an Einstein Fellow at the Center for Astrophysics | Harvard
& Smithsonian. "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."
Brout is the lead author of a series of papers 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, comprising more than
1,500 stellar explosions called Type Ia supernovae. These bright blasts
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 glimpsed at distances exceeding 10 billion light years, or back
through about three-quarters of the universe's total age. Given that the
supernovae blaze with nearly uniform intrinsic brightnesses, scientists can
use the explosions' apparent brightness, which diminishes with distance,
along with redshift measurements as markers of time and space.
That information, in turn, reveals how fast the universe expands during
different epochs, which is then used to test theories of the fundamental
components of the universe.
The breakthrough discovery in 1998 of the universe's accelerating growth was
thanks to a study of Type Ia supernovae in this manner. Scientists attribute
the expansion to an invisible energy, therefore monikered dark energy,
inherent to the fabric of the universe itself. Subsequent decades of work
have continued to compile ever-larger datasets, revealing supernovae across
an even wider range of space and time, and Pantheon+ has now brought them
together into the most statistically robust analysis to date.
"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 Adam Riess, one of the
winners of the 2011 Nobel Prize in Physics for the discovery of the
accelerating 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 alum of Harvard
University, holding a Ph.D. in astrophysics.
Brout's own career in cosmology traces back to his undergraduate years at
JHU, where he was taught and advised by Riess. There Brout worked with
then-Ph.D.-student and Riess-advisee Dan Scolnic, 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
approximately 1,000 supernovae.
Now, Brout and Scolnic and their new Pantheon+ team have added some 50
percent more supernovae data points in Pantheon+, coupled with improvements
in analysis techniques and addressing potential sources of error, which
ultimately has yielded twice the precision of the original Pantheon.
"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.
Taking the data as a whole, the new analysis holds that 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 even more comprehensive understanding of the constituent
components of the universe at different epochs, Brout and colleagues
combined Pantheon+ with other strongly evidenced, independent and
complementary measures of the large-scale structure of the universe and with
measurements from the earliest light in the universe, the cosmic microwave
background.
Another key Pantheon+ result relates to one of the paramount goals of modern
cosmology: nailing down the current expansion rate of the universe, known as
the Hubble constant. Pooling the Pantheon+ sample with data from the SH0ES
(Supernova H0 for the Equation of State) collaboration, led by Riess,
results in the most stringent 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. Stated another way, for
every megaparsec, or 3.26 million light years, the analysis estimates that
in the nearby universe, space itself is expanding at more than 160,000 miles
per hour.
However, observations from an entirely different epoch of the universe's
history predict a different story. Measurements of the universe's earliest
light, the cosmic microwave background, when combined with the current
Standard Model of Cosmology, consistently peg the Hubble constant at a rate
that is significantly less than observations taken via Type Ia supernovae
and other astrophysical markers. This sizable discrepancy between the two
methodologies has been termed the Hubble tension.
The new Pantheon+ and SH0ES datasets heighten this Hubble tension. In fact,
the tension has now passed the important 5-sigma threshold (about
one-in-a-million odds of arising due to random chance) that physicists use
to distinguish between possible statistical flukes and something that must
accordingly be understood. Reaching this new statistical level highlights
the challenge for both theorists and astrophysicists to try and explain the
Hubble constant discrepancy.
"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.
The Pantheon+ results could help point to where the solution to the Hubble
tension lies. "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," says Brout.
Overall, Pantheon+ offers scientists a comprehensive lookback through much
of cosmic history. The earliest, most distant supernovae in the dataset
gleam forth from 10.7 billion light years away, meaning from when the
universe was roughly a quarter of its current age. In that earlier era, dark
matter and its associated gravity held the universe's expansion rate in
check.
Such state of affairs changed dramatically over the next several billion
years as the influence of dark energy overwhelmed that of dark matter. Dark
energy has since flung the contents of the cosmos ever-farther apart and 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," says Brout. "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 changeover now with even stronger statistical evidence will
hopefully lead to new insights into dark energy's enigmatic nature.
"Pantheon+ is giving us our best chance to date of constraining dark energy,
its origins, and its evolution," says Brout.
Reference:
Dillon Brout et al, The Pantheon+ Analysis: Cosmological Constraints, The
Astrophysical Journal (2022).
DOI: 10.3847/1538-4357/ac8e04
Tags:
Space & Astrophysics