The Atacama Cosmology Telescope (ACT) in Chile spent nearly two decades studying how the universe began, what it’s made of, and how it evolved to its current state. The observatory was decommissioned in 2022, but its last batch of data is still sending shockwaves through the cosmological community.

A recent study published in the Journal of Cosmology and Astroparticle Physics used this data to test about 30 “extended” models of the universe’s evolution—alternatives to the standard model of cosmology. These alternatives attempt to explain certain cosmological phenomena that the standard model can’t, such as the Hubble tension, a discrepancy between different measurements of the universe’s expansion rate.

The researchers ruled out every extended model they tested. Alongside another JCAP-published study that used ACT’s final data to confirm the Hubble tension, the findings deepen this cosmological mystery, leaving experts with even more questions about what’s driving the expansion of the universe.

“We assessed them completely independently,” Erminia Calabrese, a Cardiff University cosmologist and co-author of the study that tested the extended models, said in a statement. “We weren’t trying to knock them down, only to study them. And the result is clear: The new observations, at new scales and in polarization, have virtually removed the scope for this kind of exercise. It does shrink the theoretical ‘playground’ a bit.”

A deepening cosmological mystery

There are two main ways to measure the universe’s expansion rate, also known as the Hubble constant. One method involves looking at the radiation leftover from the Big Bang (the cosmic microwave background), and the other looks at galaxies and supernovae in the local universe.

According to the standard model of cosmology, both methods should produce the same value. The problem is, they don’t. This, in essence, is the Hubble tension.

Researchers have spent years trying to explain this discrepancy, and they’ve come up with many interesting hypotheses. Others have found evidence to suggest the Hubble tension may not actually exist. The latest ACT data strengthen the case for this cosmological conundrum but bring experts no closer to resolving it.

More questions, but a clearer path to answers

So, “What’s the big deal?” you ask. Well, for one thing, confirming the Hubble tension with ACT’s observations means we can be pretty darn sure this problem is real.

The telescope measured the cosmic microwave background with unprecedented precision, producing polarization maps that complement the temperature maps created by the European Space Agency’s Planck spacecraft. By comparison, the ACT maps—published in a third JCAP study—have much higher resolution than Planck’s. This is largely owed to the fact that ACT’s primary mirror is much larger than Planck’s, with a diameter of about 20 feet (6 meters).

“When we compare [the maps], it’s a bit like cleaning your glasses,” Calabrese said. As such, the ACT data fill several gaps in our understanding of the CMB.

“Our new results demonstrate that the Hubble constant inferred from the ACT CMB data agrees with that from Planck—not only from the temperature data, but also from the polarization, making the Hubble discrepancy even more robust,” Colin Hill, a cosmologist at Columbia University and co-author of the study that used ACT data to confirm the Hubble tension, said in the statement.

ACT’s observations also allowed Calabrese and her colleagues to rule out many of the extended models that attempt to explain the Hubble tension. While this narrows the range of possibilities, it provides a clear path forward. If these models are dead ends, it’s time to stop pursuing them and look for answers elsewhere.

ACT’s operational lifespan may be over, but its final data release marks a new beginning for cosmologists working to resolve the Hubble tension. Experts will continue using the data for years to come, inching their way closer to a better understanding of our expanding universe.