New
observations from the James Webb Space
Telescope suggest that a new
feature in the universe—not a flaw in telescope measurements—may be behind the
decadelong mystery of why the universe is expanding faster today than it did in
its infancy billions of years ago.
The new data confirms Hubble Space Telescope measurements of distances between nearby stars
and galaxies, offering a crucial cross-check to address the mismatch in
measurements of the universe’s mysterious expansion. Known as the Hubble
tension, the discrepancy remains unexplained even by the best cosmology models.
“The discrepancy between the observed
expansion rate of the universe and the predictions of the standard model
suggests that our understanding of the universe may be incomplete,” said Nobel
laureate and lead author Adam Riess, a Bloomberg Distinguished Professor and professor of
Physics and Astronomy at Johns Hopkins University. “With two NASA flagship
telescopes now confirming each other’s findings, we must take this [Hubble
tension] problem very seriously—it’s a challenge but also an incredible
opportunity to learn more about our universe.”
Published in The Astrophysical
Journal, the research
builds on Riess’ Nobel Prize–winning discovery that the universe’s expansion is
accelerating owing to a mysterious “dark energy” permeating vast stretches of
space between stars and galaxies.
Riess’ team used the largest sample of
Webb data collected over its first two years in space to verify the Hubble
telescope’s measure of the expansion rate of the universe, a number known as
the Hubble constant. They used three different methods to measure distances to
galaxies that hosted supernovae, focusing on distances previously gauged by the
Hubble telescope and known to produce the most precise “local” measurements of
this number. Observations from both telescopes aligned closely, revealing that Hubble’s
measurements are accurate and ruling out an inaccuracy large enough to
attribute the tension to an error by Hubble.
Still, the Hubble constant remains a
puzzle because measurements based on telescope observations of the present
universe produce higher values compared to projections made using the “standard
model of cosmology,” a widely accepted framework of how the universe works
calibrated with data of cosmic microwave background, the faint radiation left
over from the big bang.
While the standard model yields a Hubble
constant of about 67-68 kilometers per second per megaparsec, measurements
based on telescope observations regularly give a higher value of 70 to 76, with
a mean of 73 km/s/Mpc. This mismatch has perplexed cosmologists for more than a
decade because a 5-6 km/s/Mpc difference is too large to be explained simply by
flaws in measurement or observational technique. (Megaparsecs are huge
distances. Each is 3.26 million light-years, and a light-year is the distance
light travels in one year: 9.4 trillion kilometers, or 5.8 trillion miles.)
Since Webb’s new data rules out
significant biases in Hubble’s measurements, the Hubble tension may stem from
unknown factors or gaps in cosmologists’ understanding of physics yet to be
discovered, Riess’ team reports.
“The Webb data is like looking at the
universe in high definition for the first time and really improves the
signal-to-noise of the measurements,” said Siyang Li, a graduate student
working at Johns Hopkins University on the study.
The new study covered roughly a third of
Hubble’s full galaxy sample, using the known distance to a galaxy called NGC
4258 as a reference point. Despite the smaller dataset, the team achieved
impressive precision, showing differences between measurements of under 2%—far
smaller than the approximately 8-9% size of the Hubble tension discrepancy.
In addition to their analysis of
pulsating stars called Cepheid variables, the gold standard for measuring
cosmic distances, the team cross-checked measurements based on carbon-rich
stars and the brightest red giants across the same galaxies. All galaxies
observed by Webb together with their supernovae yielded a Hubble constant of
72.6 km/s/Mpc, nearly identical to the value of 72.8 km/s/Mpc found by Hubble
for the very same galaxies.
The study included samples of Webb data
from two groups that work independently to refine the Hubble constant, one from
Riess’ SH0ES team (Supernova, H0, for the Equation of State of Dark Energy) and
one from the Carnegie-Chicago Hubble Program, as well as from other teams. The
combined measurements make for the most precise determination yet about the
accuracy of the distances measured using the Hubble Telescope Cepheid stars,
which are fundamental for determining the Hubble constant.
Although the Hubble constant does not
have a practical effect on the solar system, Earth, or daily life, it reveals
the evolution of the universe at extremely large scales, with vast areas of
space itself stretching and pushing distant galaxies away from one another like
raisins in rising dough. It is a key value scientists use to map the structure
of the universe, deepen their understanding of its state 13-14 billion years
after the big bang, and calculate other fundamental aspects of the cosmos.
Resolving the Hubble tension could
reveal new insights into more discrepancies with the standard cosmological
model that have come to light in recent years, said Marc Kamionkowski, a Johns Hopkins cosmologist who helped calculate the
Hubble constant and has recently helped develop a possible new explanation for
the tension.
The standard model explains the
evolution of galaxies, cosmic microwave background from the big bang, the
abundances of chemical elements in the universe, and many other key
observations based on the known laws of physics. However, it does not fully
explain the nature of dark matter and dark energy, mysterious components of the
universe estimated to be responsible for 96% of its makeup and accelerated
expansion.
“One possible explanation for the Hubble
tension would be if there was something missing in our understanding of the
early universe, such as a new component of matter—early dark energy—that gave
the universe an unexpected kick after the big bang,” said Kamionkowski, who was
not involved in the new study. “And there are other ideas, like funny dark
matter properties, exotic particles, changing electron mass, or primordial
magnetic fields that may do the trick. Theorists have license to get pretty
creative.”
Source: https://hub.jhu.edu/2024/12/09/webb-telescope-hubble-tension-universe-expansion/preview/
Image: A representation of the evolution
of the universe over 13.77 billion years. The far left depicts the earliest
moment we can now probe, when a period of “inflation” produced a burst of
exponential growth in the universe. (Size is depicted by the vertical extent of
the grid in this graphic.) For the next several billion years, the expansion of
the universe gradually slowed down as the matter in the universe pulled on
itself via gravity. More recently, the expansion has begun to speed up again as
the repulsive effects of dark energy have come to dominate the expansion of the
universe.
Image credit: NASA’s Goddard Space Flight Center
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