Endings and beginnings: ACT releases its final data, shaping the future of cosmology

(Press-News.org) 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.

The three papers published in JCAP are:

Sigurd Naess, Yilun Guan, Adriaan J. Duivenvoorden, Matthew Hasselfield, Yuhan Wang and the Atacama Cosmology Telescope collaboration - The Atacama Cosmology Telescope: DR6 maps

Thibaut Louis, Adrien La Posta, Zachary Atkins, Hidde T. Jense and the Atacama Cosmology Telescope collaboration - The Atacama Cosmology Telescope: DR6 power spectra, likelihoods and ΛCDM parameters

Erminia Calabrese, J. Colin Hill, Hidde T. Jense, Adrien La Posta and the Atacama Cosmology Telescope collaboration - The Atacama Cosmology Telescope: DR6 constraints on extended cosmological models

The sixth ACT data release was made available in March as a preprint, and the collaboration’s three official papers have now been published in JCAP following peer review. ACT is operated by the namesake collaboration, an international consortium of more than 100 researchers from universities and research institutes around the world, who have jointly authored these new studies.

Planck and ACT: “Sibling” and Complementary Telescopes

“It’s the first time that a new experiment has reached the same level of observational capability as Planck,” explains Thibaut Louis (Université Paris-Saclay and CNRS/IN2P3), first author of one of the three newly published papers.

The Planck satellite, operated by the European Space Agency (ESA), was launched in 2009 with the goal of mapping the cosmic microwave background (CMB) with extremely high precision. Cosmologists often describe this radiation as the Universe’s “fossil light,” emitted during the earliest stages of cosmic evolution. Its observations allowed scientists to reconstruct the composition, age, and geometry of the primordial Universe.

Planck was a landmark mission, but it left some gaps—several of which have now been filled thanks to ACT’s work. Unlike Planck, which was a satellite in orbit, ACT is a ground-based telescope located at about 5,000 meters of altitude in Chile’s Atacama Desert. While Planck focused mainly on measuring the temperature of the CMB, ACT also observed its polarization—especially in this latest data release.

Cosmological tension confirmed

One of ACT’s most important results so far is confirming one of today’s biggest headaches in cosmology: the so-called Hubble tension.

Put simply: we know the Universe is expanding, and we can estimate its current expansion rate (we’ve also learned this expansion is accelerating) from observations. The snag is that the value inferred using data from extremely distant epochs like the CMB differs from the value obtained by observing much closer astronomical objects. “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," explains Colin Hill, cosmologist at Columbia University and co-lead of one of the papers This may sound like an even bigger problem, but it’s actually a crucial finding: scientists now know there really is an issue with the model we use to describe the Universe (see box: the ΛCDM model), assuming the expansion rate measured from nearby objects stays unchanged.

Extended models “fail the test”

If that weren’t enough, ACT goes further in telling us we “have a problem.” Over recent decades, precisely because of the Hubble tension, many “extended” versions of the standard model have been proposed to try to resolve the discrepancy.

In one of the three new papers, led by Calabrese, the main extended models (“there are about thirty,” she notes) were tested against the new data.

The result? “They’re gone,” she admits, adding: “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.”

Again, sweeping away a set of theories that aimed to fix a problem might not sound exciting. “It does shrink the theoretical ‘playground’ a bit,” Calabrese concedes. But it’s good news: it means cleaning house, narrowing the viable paths forward, and no longer spending energy on what are evidently dead ends.

Sharper maps

With this latest ACT release, scientists have obtained a much sharper image of the “infant” Universe than the one provided by Planck. “This is mainly because ACT  has a larger diameter — six meters compared to Planck’s one and a half meters — and sharpness increases with mirror size,” explains Sigurd Naess of the University of Oslo, one of the paper leads. “But it's also because ACT’s images of the polarized light are much more sensitive than Planck's.’ One of the three new papers presents this new map, while another compresses these maps into angular power spectra, crucial for cosmological studies.

This doesn’t mean that Planck’s results are now obsolete. On the contrary, as Louis, points out, “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.”

Far from being the end of a project, ACT’s sixth and final data release marks a new beginning — one that will hopefully bring us closer to understanding our Universe and solving some of its biggest remaining mysteries. “We want the community to keep using and exploring these data,” says Erminia Calabrese. “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.”

To learn more:

The Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is the oldest light in the Universe — a faint glow filling the entire sky, originating about 380,000 years after the Big Bang. It is the remnant of the moment when the Universe cooled enough to become transparent, allowing light to travel freely through space. Studying it helps scientists understand what the cosmos was like and how it has evolved from its origins to the present day. Instruments that have observed it are COBE, WMAP, Planck and, more recently, ACT and the South Pole Telescope.

The ΛCDM Model

The ΛCDM model is the standard theoretical framework of cosmology — the one that best describes how the Universe is structured and how it has evolved. Λ (Lambda) stands for dark energy, a mysterious force driving the accelerated expansion of the Universe, while CDM stands for Cold Dark Matter, an invisible form of matter that does not emit light but whose gravity holds galaxies together. Along with a small amount of “normal” matter (made of atoms, like us and the stars), these components explain nearly everything we observe in the cosmos.

 

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