After decades of exploring the universe, the Atacama Cosmology Telescope (ACT) in northern Chile concluded its mission in 2022. Its final data release has revealed important insights, particularly regarding the universe’s expansion.
ACT first opened its eyes to the cosmos in October 2007. However, it wasn’t gazing at stars or galaxies. Instead, it focused on microwaves that originated from the early universe, specifically the cosmic microwave background (CMB). This “fossil” light was emitted when the universe was only about 380,000 years old, providing a unique glimpse into its infancy.
The CMB is crucial for cosmologists because it offers a clear view of the universe’s early state. While the European Space Agency’s Planck satellite provided widespread CMB data, ACT could zoom into small regions at high resolution. This allowed it to analyze the polarization of the CMB, which sheds light on vital aspects like dark matter and the distribution of neutrons. Changes in these elements affect how the CMB appears.
In November, the ACT team released their sixth and final public dataset, along with three articles detailing their findings. These results highlighted a puzzling issue known as the Hubble tension — a disagreement between measurements of the universe’s current expansion rate. Current estimates of the Hubble constant, derived from early universe probes like ACT and Planck, are significantly lower than those based on nearby observations, such as supernova brightness.
Expert opinions suggest that the Hubble tension could lead to a scientific breakthrough. Dr. Sarah M. Deustua, an astronomer at the American Astronomical Society, notes, “The tension compels us to consider new physics. It could change our understanding of cosmic expansion and what lies beyond.”
Historically, the Hubble constant has been a cornerstone of cosmology, yet this tension has emerged as one of today’s most significant mysteries. ACT confirmed this tension and ruled out several theoretical models intended to resolve it. More than 30 “extended” cosmological models were tested, all proving inadequate.
While these negative results might seem discouraging, they are valuable. As scientists, each failed hypothesis helps narrow down the search for answers. In the end, understanding what doesn’t fit is just as crucial as finding what does.
As we move forward, the legacy of the ACT and its data will continue to influence cosmology. With ongoing analysis, researchers hope to uncover more truths about our universe. For further insights, check out the Journal of Cosmology and Astroparticle Physics for the final ACT publications.
