Unknown factors are accelerating the expansion of the universe

Observations from the James Webb Space Telescope (JWST) have added a new layer of understanding to the enigma of why the universe is expanding more quickly now than it did billions of years ago. 

Known as the Hubble tension, this discrepancy in measurements of the universe’s expansion rate has baffled scientists for over a decade. 

The latest data confirms the accuracy of the Hubble Space Telescope’s measurements, ruling out significant errors in its observations and suggesting that the answer lies in unexplored aspects of the universe’s physics.

Observations and predictions of the expansion rate

The research was led by Nobel laureate Adam Riess of Johns Hopkins University. “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,” Riess said. 

“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.”

Riess and his team used data from the James Webb telescope to cross-verify Hubble’s measure of the Hubble constant, a value describing the universe’s rate of expansion. 

The team focused on distances to galaxies hosting supernovae that had previously been measured by Hubble, using three independent methods to confirm the precision of Hubble’s results. 

The findings showed that Webb’s measurements aligned closely with those of Hubble, ruling out any observational inaccuracies large enough to account for the tension.

The persistent Hubble tension

The Hubble constant measured through telescope observations consistently yields a value of 70 to 76 kilometers per second per megaparsec, with a mean of 73 km/s/Mpc. 

In contrast, the standard cosmological model predicts a lower value of about 67-68 km/s/Mpc. This 5-6 km/s/Mpc difference has puzzled cosmologists, as it is too significant to attribute to simple measurement errors.

“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 study co-author Siyang Li, a graduate student at Johns Hopkins University. 

The new observations covered roughly a third of Hubble’s full galaxy sample, providing results with an impressive precision difference of under 2% – far smaller than the Hubble tension discrepancy, which stands at around 8-9%.

Measuring the expansion of the universe 

The study employed JWST’s advanced technology to measure distances using Cepheid variables, a gold-standard method for gauging cosmic distances. The experts also incorporated measurements of carbon-rich stars and the brightest red giants. 

Combined observations of these galaxies and their supernovae produced a Hubble constant of 72.6 km/s/Mpc, closely matching Hubble’s previous finding of 72.8 km/s/Mpc for the same galaxies.

These findings further confirm that the Hubble tension is unlikely to stem from flaws in measurement techniques. Instead, the discrepancy hints at new, unknown factors influencing the universe’s behavior. 

As the researchers noted, this might indicate gaps in the current understanding of fundamental physics.

Possible explanations for the Hubble tension

Marc Kamionkowski, a cosmologist at Johns Hopkins who was not involved in the study, speculated on what might account for the Hubble tension. 

“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. 

He also suggested other possibilities, including “funny dark matter properties, exotic particles, changing electron mass, or primordial magnetic fields that may do the trick.”

Although the standard model of cosmology has successfully explained many aspects of the universe, including the cosmic microwave background and the evolution of galaxies, it does not fully account for the nature of dark matter and dark energy. These mysterious components make up approximately 96% of the universe and drive its accelerated expansion.

The structure and history of the universe 

The Hubble constant may not have practical implications for everyday life on Earth, but it plays a vital role in mapping the structure and history of the universe. It helps cosmologists trace the universe’s state 13-14 billion years after the Big Bang and refine models of its large-scale behavior.

Resolving the Hubble tension could unlock new insights into other inconsistencies in the standard cosmological model that have come to light in recent years. 

Riess and his team emphasized the importance of this challenge, noting that continued investigation could yield groundbreaking discoveries about the cosmos.

“The Webb data is providing unprecedented clarity, and as we delve deeper, we have the opportunity to reveal fundamental truths about the universe. The Hubble tension isn’t just a problem to solve – it’s a doorway to deeper understanding,” Riess concluded.

The study is published in The Astrophysical Journal,

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