Our knowledge about the universe is growing and expanding at an increasing rate, just like the universe itself. Europa Press reports that new observations from the James Webb Space Telescope analyzed by Johns Hopkins University (USA) suggest that a new feature of the cosmos may be behind the mystery that has intrigued scientists for a decade: why the universe is expanding faster today than in its infancy billions of years ago.
Published in 'The Astrophysical Journal', the new data confirm the measurements from the Hubble Space Telescope of distances between stars and nearby galaxies, providing a crucial verification to address the discrepancy in measurements of the mysterious expansion of the universe. Known as the Hubble tension, the discrepancy remains unexplained even by the best cosmological models.
The research is based on the discovery by Adam Riess, Nobel Prize winner and professor of Physics and Astronomy at Johns Hopkins University, that the universe's expansion is accelerating due to a mysterious "dark energy" that permeates vast expanses of space between stars and galaxies.
As the author explains: "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. Now that two flagship telescopes from NASA mutually confirm their findings, we must take this Hubble tension problem very seriously: it is a challenge, but also an incredible opportunity to learn more about our universe."
Riess's team used the largest sample of Webb data collected during its first two years in space to verify the Hubble telescope's measurement of the universe's expansion rate, a number known as the Hubble constant. They used three different methods to measure distances to galaxies hosting supernovas, focusing on distances previously measured by the Hubble telescope known to produce the most precise "local" measurements of this number. The observations from both telescopes closely aligned, revealing that the Hubble measurements are accurate and ruling out an inaccuracy large enough to attribute the tension to a Hubble error.
However, the Hubble constant remains an enigma because measurements based on telescopic observations of the current universe consistently yield higher values compared to projections made using the "standard cosmology model," a widely accepted framework of how the universe operates calibrated with data from the 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, observations-based measurements with telescopes regularly yield a higher value, from 70 to 76, with an average of 73 km/s/Mpc. This mismatch has puzzled cosmologists for over a decade because a difference of 5-6 km/s/Mpc is too large to be explained simply by measurement or observational technique failures. (Megaparsecs are huge distances. Each one is equivalent to 3.26 million light-years, and a light-year is the distance light travels in a year: 9.4 trillion kilometers, or 5.8 trillion miles).
Since the new Webb data rule out significant biases in the Hubble measurements, the Hubble tension may be due to unknown factors or gaps in cosmologists' understanding of physics yet to be discovered, Riess's team reports.
"The Webb data is like looking at the universe in high definition for the first time and really enhances the signal-to-noise ratio of the measurements," comments Siyang Li, a graduate student working at Johns Hopkins University on the study.
The new study covered approximately one-third of the complete Hubble galaxy sample, using the known distance to a galaxy called NGC 4258 as a reference point. Despite the smaller data set, the team achieved impressive precision, showing differences between measurements of less than 2%, much smaller than the approximate 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 compared measurements based on carbon-rich stars and the brightest red giants in the same galaxies. All galaxies observed by Webb along with their supernovas yielded a Hubble constant of 72.6 km/s/Mpc, almost identical to the value of 72.8 km/s/Mpc found by Hubble for the same galaxies.
The study included Webb data samples from two groups working independently to refine the Hubble constant, one from Riess's SH0ES team (Supernova, H 0 , for the dark energy equation of state) and another from the Carnegie-Chicago Hubble Program, as well as from other teams. The combined measurements allow for the most precise determination to date of the accuracy of distances measured with Hubble's Cepheid stars, which are crucial for determining the Hubble constant.
Although the Hubble constant has no practical effect on the solar system, Earth, or daily life, it reveals the evolution of the universe on extremely large scales, with vast areas of space stretching and pushing distant galaxies away from each other like raisins in a fermenting mass. It is a key value that 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 further discrepancies with the standard cosmological model that have emerged in recent years, said Marc Kamionkowski, a cosmologist at Johns Hopkins who helped calculate the Hubble constant and recently helped develop a possible new explanation for the tension.
The standard model explains the evolution of galaxies, the cosmic microwave background from the Big Bang, the abundance of chemical elements in the universe, and many other key observations based on 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 account for 96% of its composition and its accelerated expansion.
"One possible explanation for the Hubble tension would be that something is missing in our understanding of the early universe, like a new component of matter (primitive dark energy) that gave the universe an unexpected boost after the Big Bang," points out Kamionkowski, who did not participate in the new study. "And there are other ideas, like the strange properties of dark matter, exotic particles, changing electron masses, or primordial magnetic fields that could work. Theorists have quite a bit of leeway to be creative."