Atacama Cosmology Telescope Concludes Mission, Unveils New Cosmic Insights

The Atacama Cosmology Telescope (ACT) has officially concluded its nearly two-decade-long observational campaign, culminating in a significant final data release that enhances our understanding of the early Universe. Three papers published in the Journal of Cosmology and Astroparticle Physics detail the sixth and final major data release from ACT, which includes refined measurements of the Hubble constant—often referred to as the Universe's "speedometer." This release confirms the ongoing "Hubble tension," a discrepancy between the expansion rate of the Universe derived from observations of distant galaxies and that obtained from nearby galaxies.

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The Hubble tension has emerged as a central issue in modern cosmology, with ACT's latest findings reinforcing the existence of this discrepancy. Colin Hill, a cosmologist at Columbia University, noted that the Hubble constant inferred from ACT's cosmic microwave background (CMB) data aligns with that from the European Space Agency's Planck satellite, further solidifying the tension. This persistent mismatch suggests potential shortcomings in the ?CDM model, the prevailing framework for understanding cosmological phenomena.

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In response to the Hubble tension, researchers have proposed numerous extended cosmological models to reconcile the differing measurements. However, ACT's recent data release has systematically tested around thirty of these models, with Erminia Calabrese, a cosmologist at Cardiff University, stating, “They’re gone.” This decisive outcome eliminates many theoretical possibilities, allowing researchers to focus on more viable explanations for the observed cosmic phenomena.

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ACT's observations complement those of the Planck satellite, which mapped the CMB with high precision. While Planck primarily focused on temperature variations, ACT's latest release emphasizes detailed polarization measurements, providing a sharper view of the early Universe. Sigurd Naess from the University of Oslo explained that ACT's larger mirror size contributes to its enhanced resolution, allowing for more sensitive imaging of polarized light.

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Although ACT's observational phase has concluded, the final data release marks the beginning of a new chapter for the scientific community. Researchers are encouraged to utilize and explore the newly available data, which promises to yield further insights into the Universe's evolution. Calabrese expressed optimism about the community's engagement with the data, stating, “We’ve provided the first interpretation... Now we’re delighted to hand the data over to the community for future and ongoing explorations.”

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• “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, Cosmologist, Columbia University

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• “They’re gone,” — Erminia Calabrese, Cosmologist, Cardiff University

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• “the real importance of the new data is that they are complementary to the previous ones and together contribute to an extremely rich composite picture.” — Thibaut Louis, Université Paris-Saclay and CNRS/IN2P3

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The conclusion of the ACT's mission and the release of its final data represent a pivotal moment in cosmology, providing critical insights into the Universe's expansion and the ongoing Hubble tension. As researchers delve into the new data, the potential for groundbreaking discoveries remains vast, ensuring that the scientific narrative surrounding the cosmos continues to evolve.

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