Galaxies are accelerating away from each other much faster than previously thought, scientists claim.
New measurements from NASA's Hubble Space Telescope confirm that the universe is expanding about 9% faster than expected, based on its trajectory seen shortly after the Big Bang.
The findings back up previous Hubble measurements published in 2016, reducing the chances that the disparity is an accident from 1 in 3,000 to only 1 in 100,000.
Astronomers admit that new physics may be needed to better understand the cosmos.
"This is not what we expected," said Adam Riess, Bloomberg Distinguished Professor of Physics and Astronomy at The Johns Hopkins University, Nobel Laureate and the project's leader.
"This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke."
In the study, Riess and his team used a new method to analyse light from 70 stars in our neighbouring galaxy, the Large Magellanic Cloud.
The stars, called Cepheid variables, brighten and dim at predictable rates that are used to measure the distances between nearby galaxies.
The usual method for measuring the stars is incredibly time-consuming - the Hubble can only observe one star for every 90-minute orbit around Earth.
However, using their new method, the researchers were able to observe a dozen Cepheids in the same amount of time it would normally take to observe just one.
With this new data, Riess and the team were able to strengthen the foundation of the "cosmic distance ladder", which is used to determine distances within the Universe.
They were also able to calculate the "Hubble constant" - a value of how fast the cosmos expands over time.
Ancient star could be one of the oldest in the universe
The team combined their Hubble measurements with another set of observations, made by the Araucaria Project, a collaboration between astronomers from institutions in Chile, the US, and Europe.
The combined measurements produced a more accurate result, allowing the team to "tighten the bolts" of the rest of the distance ladder.
However, as the team's measurements have become more precise, their calculation of the Hubble constant has remained at odds with the expected value.
The expected value is derived from observations of the early universe's expansion by the European Space Agency's Planck satellite, based on conditions Planck observed 380,000 years after the Big Bang.
"This is not just two experiments disagreeing. We are measuring something fundamentally different," Riess explained.
"One is a measurement of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and on measurements of how fast it ought to be expanding.
"If these values don't agree, there becomes a very strong likelihood that we're missing something in the cosmological model that connects the two eras."
While Riess doesn't have an answer as to exactly why the discrepancy exists, he and his team will continue to fine-tune the Hubble constant, with the goal of reducing the uncertainty to 1%.
These most recent measurements, published today in the Astrophysical Journal Letters, bring the uncertainty in the rate of expansion down from 10% in 2001 to 1.9% in the present study.
