There’s always a touch of melancholy when a chapter that has absorbed years of work comes to an end. In the case of the Atacama Cosmology Telescope (ACT), those years amount to nearly twenty — and now the telescope has completed its mission. Yet some endings are also important beginnings, opening new paths for the entire scientific community.
The three papers just published in the Journal of Cosmology and Astroparticle Physics (JCAP) by the ACT Collaboration describe and contextualize in detail the sixth and final major ACT data release — perhaps the most important one — marking significant advances in our understanding of the Universe’s evolution and its current state.
ACT’s data clarify several key points: the measurement of the Hubble constant (the number that indicates the current rate of cosmic expansion — the Universe’s “speedometer”) obtained from observations at very large cosmological distances is confirmed, and it remains markedly different from the value derived from the nearby Universe. This is both a problem and a remarkable discovery: it confirms the so-called “Hubble tension,” which challenges the model we use to describe the cosmos.
ACT’s observations also rule out many of the so-called extended models — theoretical alternatives to the standard cosmological model. That’s another “problem,” since it narrows the range of possibilities, but it also represents a new, cleaner starting point: time to stop pursuing these models and look elsewhere.
Last but not least, ACT provides new polarization maps of the cosmic microwave background — the Universe’s “fossil light” — which complement Planck’s temperature maps, but with much higher resolution. “When we compare them, it’s a bit like cleaning your glasses,” says Erminia Calabrese, cosmologist at Cardiff University, ACT collaboration member and coordinator of one of the three papers.
