Bold claim: The last data release from a long-running telescope just overturned nearly three dozen cosmology ideas.
The Atacama Cosmology Telescope (ACT) in Chile spent almost twenty years investigating the origins, composition, and evolution of the universe. Although the facility was retired in 2022, its final data batch continues to reverberate through the field of cosmology.
A recent paper in the Journal of Cosmology and Astroparticle Physics used ACT’s final data to test roughly 30 “extended” models of cosmic evolution—alternatives to the standard cosmology framework. These models aim to address phenomena that the conventional model struggles to explain, notably the Hubble tension, which is the disagreement between different measurements of the universe’s expansion rate.
The study ruled out every extended model it examined. In parallel with another JCAP paper that used ACT’s final data to reaffirm the Hubble tension, these results intensify the mystery surrounding what drives cosmic expansion and what the correct description of the universe should be.
“We evaluated them completely independently,” said Erminia Calabrese, a cosmologist at Cardiff University and co-author of the extended-model analysis. “We weren’t aiming to debunk them; we aimed to test them. The outcome is clear: new observations, at new scales and in polarization, have dramatically narrowed the feasible space for this kind of approach. It narrows the theoretical ‘playground’ a bit.”
A deepening cosmological puzzle
There are two principal methods to measure the universe’s expansion rate, also known as the Hubble constant. One method uses the afterglow of the Big Bang—the cosmic microwave background (CMB). The other relies on observations of nearby galaxies and supernovae. According to the standard cosmology model, both paths should converge on the same value. They do not, which is the essence of the Hubble tension.
Researchers have proposed many explanations for this discrepancy, with a variety of intriguing ideas. Some analyses even suggest the tension could disappear under certain interpretations. The latest ACT findings strengthen the case that the tension is real while leaving scientists without a definitive resolution.
More questions, but a clearer route to answers
So, why does this matter? First, confirming the Hubble tension with ACT’s data strengthens the belief that the problem is genuine. ACT measured the CMB with exceptional precision and produced polarization maps that complement the temperature maps created by ESA’s Planck mission. In fact, ACT’s maps—described in a companion JCAP paper—feature higher resolution than Planck’s, thanks largely to ACT’s larger primary mirror, about 6 meters in diameter.
As Calabrese put it, comparing the maps is like cleaning a set of glasses: the new data fill important gaps in our understanding of the CMB.
Colin Hill, a cosmologist at Columbia University and co-author of the study confirming the Hubble tension via ACT data, noted that the Hubble constant inferred from ACT’s CMB measurements agrees with Planck’s value—not only from temperature data but also from polarization. This strengthens the case that the discrepancy is robust.
Beyond narrowing the range of viable extended models, ACT’s final data set provides a clearer path forward. If many proposed extensions are effectively dead ends, it may be time to pivot toward alternative explanations and new ideas.
While ACT’s hardware is no longer in operation, its final data release marks a new beginning for cosmologists. The data will continue to inform research for years to come as scientists refine their understanding of the universe’s ongoing expansion.
