New measurement confirms cosmos is expanding too fast for current models, turning the Hubble tension into a crisis

The cosmos really seems to be expanding fast. A new measurement confirms what highly debated previous results had shown: The cosmos is expanding faster than predicted by theoretical models, and faster than our current understanding of physics can explain. This discrepancy between data and model became known as the Hubble tension. Now, results published in ApJL (The Astrophysical Journal Letters) provide even stronger support for the faster rate of expansion. “The tension now turns into a crisis,” said Dan Scolnic (leader of the research team).

Determining the expansion rate of the cosmos—branded as the Hubble constant—has been a key scientific pursuit ever since 1929, when Edwin Hubble first discovered that the cosmos was expanding. Dan Scolnic (an associate professor of physics at Duke University) explains it as trying to build the cosmos’s growth chart: we know what size it was at the time of the Big Bang, but how did it get to the current size is the question to be answered. In Scolnic’s analogy, the cosmos’s baby picture represents the distant cosmos, the primordial seeds of galaxies. The cosmos’s current headshot represents the local cosmos, which contains the Milky Way and its neighbours. The standard model of cosmology is the growth curve linking the two. The issue is: things don’t actually connect. “This is saying, to some respect, that our model of cosmology might be broken,” said Dan Scolnic.

Measuring the cosmos requires a cosmic ladder, which is a succession of methods used by astronomers to measure the distances to celestial objects, with each method, or “rung,” dependent on the previous for calibration. The ladder utilised by Scolnic was created by a separate team using data from the Dark Energy Spectroscopic Instrument (DESI), which is observing over 100,000 galaxies every night from its vantage point at KPNO (the Kitt Peak National Observatory). Scolnic recognized that cosmic ladder could be anchored nearer to Earth with a more accurate distance to the Coma Cluster (one of the galaxy clusters nearest to us). “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.”

To get a accurate distance to the Coma cluster, Scolnic and his collaborators utilised the light curves from 12 Type Ia supernovae within the cluster. Just like candles lighting a dark trail, Type Ia supernovae have a predictable luminosity correlated to their distance, making them dependable objects for distance calculations. The team arrived at a distance of roughly 320 million light-years, almost in the center of the range of distances reported across 40 years of prior studies—a reassuring sign of its accuracy. “This measurement isn’t biased by how we think the Hubble tension story will end,” said Dan Scolnic. “This cluster is in our backyard, it has been measured long before anyone knew how important it was going to be.”

Utilising this high-precision measurement as a first rung, the team attuned the rest of the cosmic distance ladder. They arrived at a value for the Hubble constant of 76.5 kilometers per second per megaparsec, which basically means that the local cosmos is expanding 76.5 kilometers per second faster every 3.26 million light-years. This value matches current measurements of the expansion rate of the local cosmos. However, like all of those measurements, it conflicts with measurements of the Hubble constant utilising predictions from the distant cosmos. In other words: it matches the cosmos’s expansion rate as other teams have lately measured it, but not as our present understanding of physics predicts it. The longstanding question is: is the flaw in the models or in the measurements?

Scolnic’s team’s new results add support to the emerging picture that the root of the Hubble tension lies in the models. “Over the last decade or so, there’s been a lot of re-analysis from the community to see if my team’s original results were correct,” said Scolnic, whose research has unswervingly challenged the Hubble constant predicted using the standard model of physics. “Ultimately, even though we’re swapping out so many of the pieces, we all still get a very similar number. So, for me, this is as good of a confirmation as it’s ever gotten.” “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 said. “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?”