The latest ACT measurements confirm the value of the Hubble constant - a critical parameter indicating the present rate of cosmic expansion - by observing distant cosmological epochs. This confirmed rate differs from estimates derived using nearby astronomical objects, reinforcing the ongoing "Hubble tension" and challenging the standard cosmological framework.

ACT's expanded analyses decisively rule out many extended theoretical models proposed to address this tension. These independent tests, led by Erminia Calabrese of Cardiff University, demonstrate that about thirty alternative models do not fit the newly available data.

Calabrese stated, "We assessed them completely independently. We weren't trying to knock them down, only to study them. And the result is clear: the new observations, at new scales and in polarization, have virtually removed the scope for this kind of exercise." The result is a streamlined theoretical landscape, directing future efforts toward new explanations.

A key achievement of this release is ACT's provision of high-resolution polarization maps of the cosmic microwave background (CMB), the Universe's oldest observable light. These maps complement the temperature data from the Planck mission at substantially greater detail.

Thibaut Louis of Universite Paris-Saclay and CNRS/IN2P3 stated, "It's the first time that a new experiment has reached the same level of observational capability as Planck." Sigurd Naess of the University of Oslo explained, "ACT has a larger diameter - six meters compared to Planck's one and a half meters - and sharpness increases with mirror size. But it's also because ACT's images of the polarized light are much more sensitive than Planck's."

Although Planck's measurements remain crucial, ACT's new data fill gaps and help build a more complete view of the early Universe. As Louis noted, "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."

ACT's final data release serves as a resource for ongoing cosmological studies. Calabrese emphasized, "We want the community to keep using and exploring these data. We've provided the first interpretation, in which we have great expertise after years of work on this instrument. Now we're delighted to hand the data over to the community for future and ongoing explorations."

Research Report: The Atacama Cosmology Telescope: DR6 maps

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