Evrim Yazgin
Cosmos science journalist
Tensions among astrophysicists continue to mount over Edwin Hubble’s 100-year old calculations about the speed which the universe is expanding.
“The tension now turns into a crisis,” says Dan Scolnic, an associate professor of physics at Duke University in the US.
Scolnic is part of a team who have measured the rate of expansion of the universe, called the Hubble constant. Their research confirms what has been suggested before – that the universe is expanding faster than predicted by theoretical models. In fact, it is expanding faster than can be explained by physics.
This discrepancy has become known as the Hubble tension.
It has been an area of debate and confusion for cosmologists for nearly 100 years. In 1929, US astronomer Edwin Hubble was the first person to discover that the universe was expanding.
Physicists can tell what the universe was like in the past by observing distant light. The cosmic speed limit set by light means that more distant objects observed on Earth are actually from an earlier period in the universe’s history.
By looking at the difference between the local current universe and the early universe, cosmologists can see how much the universe has grown since the Big Bang.
The problem is this doesn’t fit with the standard model of cosmology – the theoretical model meant to predict how the universe evolves.
“This is saying, to some respect, that our model of cosmology might be broken,” Scolnic says.
Scolnic likens the method of measuring distances to to rungs on a ladder. Each rung relies on the previous one to calibrate and calculate its distance. Scolnic’s team used a ladder produced by a separate team using data from the Dark Energy Spectroscopic Instrument (DESI).
DESI observes more than 100,000 galaxies every night from the Kitt Peak National Observatory in Arizona, USA.
Scolnic’s team suggested that the ladder could have its first rung closer to Earth – the Coma Cluster, one of the nearest galaxy clusters. This would provide a more precise distance and, therefore, calibration for higher rungs.
“The DESI collaboration did the really hard part, their ladder was missing the first rung,” said Scolnic. “I knew how to get it, and I knew that that would give us one of the most precise measurements of the Hubble constant we could get, so when their paper came out, I dropped absolutely everything and worked on this non-stop.”
The team worked out that the Coma Cluster is 320 million light-years from Earth. This is more than 3,000 times the diameter of the Milky Way.
From this they found the Hubble constant is 76.5 km per second per megaparsec. This means the local Universe is expanding 76.5 km per second faster every 3.26 million light-years.
This value matches other measurements, but not theoretical predictions.
“We’re at a point where we’re pressing really hard against the models we’ve been using for two and a half decades, and we’re seeing that things aren’t matching up,” Scolnic says. “This may be reshaping how we think about the Universe, and it’s exciting! There are still surprises left in cosmology, and who knows what discoveries will come next?”
The research is published in the Astrophysical Journal Letters.
