After a multi-decade-year mission to understand the nature of the universe, a telescope perched in the mountain plateaus of northern Chile said goodbye in 2022. Now, its final data release is revealing the telescope’s legacy: a field in tension.
In October 2007, the Atacama Cosmology Telescope (ACT) saw its first light. But it was not light from a star, or even a distant galaxy. Instead, ACT was designed to hunt for microwaves, especially the kind of microwaves left over from some of the earliest epochs of the universe. This “fossil” light, known as the cosmic microwave background (CMB), was emitted when the universe was just 380,000 years old.
ACT was especially good at looking at the CMB’s polarization, which tells us a lot about the state of the early universe. If you change the amount of dark matter in the cosmos, how it’s distributed, how many neutrinos there are, or any of another dozen or so properties of the cosmos, you change what the CMB’s light looks like.
Final ACT
In November, the ACT team released their sixth and final public dataset as three articles published in the Journal of Cosmology and Astroparticle Physics. While cosmologists will continue to mine the data for many years to come, the core team also provided their final suite of analyses and studies before saying farewell for good.
Their findings matched what surveys like Planck had already identified: that something funny is going on with the expansion of the universe. Measurements of the present-day expansion rate, known as the Hubble rate or Hubble constant, taken with early-universe probes like Planck and ACT, reveal a number that is quite a bit slower than estimates based on nearby measurements, like supernova dimming.
This discrepancy has come to be known as the Hubble tension, and it is perhaps the greatest unsolved mystery in modern cosmology. But ACT didn’t just confirm the existence of the tension; it also destroyed some very good ideas.
ACT axes 30 cosmic models
Cosmologists have been busy concocting many theoretical explanations for the Hubble tension. Many of these are called “extended” cosmological models, since they take the standard cosmological picture and add a few extra ingredients or forces to the universe.
But these ingredients and forces don’t just exist today; they also must have existed when the CMB was first emitted. So ACT’s exquisite view of the CMB allowed the team to put many of these models — around 30, in fact — to the test.
All of them failed.
But in science, you only lose if you don’t learn anything, and ACT’s negative results help cosmologists in their search. In other words, you can only know the right answer once you’ve crossed off all the wrong answers.