March 17, 20254 min readBest-Yet Baby Pictures of the Universe UnveiledThe final results from the Atacama Cosmology Telescope offer the sharpest, most sensitive view of the early cosmos that anyone has ever seenBy Lee Billings edited by Dean VisserThe Atacama Cosmology Telescope, seen here on the Chilean peak of Cerro Toco, mapped the big bangs afterglow from 2007 to 2022. Mark Devlin, Deputy Director of the Atacama Cosmology Telescope and the Reese Flower Professor of Astronomy at the University of PennsylvaniaSometimes, a picture can be worth much more than a thousand words. For instance, one measure associated with the pictures belownew high-definition snapshots of the cosmos in its infancyis 1,900 zetta-suns.Two views of the cosmic microwave background (CMB), the afterglow of the big bang, as seen by the Atacama Cosmology Telescope (ACT). The upper image shows ACTs measurements of CMB temperature augmenting earlier measurements from the Planck satellite, while the lower image shows ACTs measurements of CMB polarization. Blue and orange denote variations in temperature and polarization. Each images zoomed-in portion is 10 degrees across, or twenty times the Moon s width seen from Earth.ACT Collaboration; ESA/Planck CollaborationOn supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.Equivalent to almost two trillion trillion suns, thats the amount of mass (or its counterpart as energy) that these images show to exist in the entire observable universe, which extends almost 50 billion light-years in all directions. The images, released today, are among the final results from the Atacama Cosmology Telescope (ACT), a National Science Foundationfunded observatory that operated on a mountaintop in Chile from 2007 until 2022. The researchers will present their results, which have not yet been peer-reviewed, tomorrow at a meeting of the American Physical Society.What I love about these new images is how they bring the whole history of the universe to life, says Jo Dunkley, a Princeton University cosmologist, who led the ACT analysis group. The fact that you can just look out into the sky to see the whole sweep of cosmic time is beautiful. And with ACT, weve been able to see this better than ever before.What Did ACT See?ACT observed the big bangs afterglow, the cosmic microwave background (CMB), which was emitted when the universe was just 380,000 years old. Back then the cosmos was essentially a fireball, an expanding bubble of plasma as hotand as opaqueas the surface of the sun. This opacity makes the CMBs light the oldest that anyone can ever see. Building on past CMB surveys (such as that of the European Space Agencys Planck satellite), ACT measured the intensity and polarization of the light emitted by this material with unprecedented sharpness and sensitivity. These values were then translated into estimates of the temperature, density and velocity of the swirling primordial stuff from which galaxies and larger cosmic structures would eventually coalesce. Those estimates in turn let ACTs researchers effectively gauge the sum of all things and the way they came together.How Much Stuff Is in the Universe?Of the staggering 1,900 zetta-sun quantity the researchers came up with, only 100 zetta-suns come from normal matter: hydrogen makes up three quarters of the latter figure, and helium comprises the rest of it. Both of these elements emerged in the immediate aftermath of the big bang. All the rest of the elementsthe carbon in your cells, the calcium in your bones, the oxygen you breathe and even the gold in your jewelryshowed up much later, after the ignition of the first stars, and are akin to a rounding error, just a meager sheen upon the greater cosmic ocean.Of the remaining 1,800 zetta-suns worth of material out there, 500 zetta-suns represent dark matter, the invisible substance that serves as gravitational glue that holds galaxies together. But the bulk, some 1,300 zetta-suns, comes from the density of dark energy, the mysterious force thats powering an acceleration in the rate of cosmic expansion. So most of the universes stuff is actually in the form of things for which we only have placeholder namesand a very limited understanding.Why This MattersDespite how little we seem to know, the most salient figure to assign to this all-encompassing baby picture of the cosmos arguably isnt two trillion trillion suns (or a thousand words, for that matter). It is, in fact, six: the total number of core parameters that plug into the standard model of cosmology, called Lambda-CDM (with Lambda being a shorthand for dark energy and CDM referring to a sluggishly cold type of dark matter inferred from observations). Just six numbers, if properly arranged, seem to quite thoroughly explain the curious patterns imprinted on the CMBand how they led to the cosmos we dwell in today.Plancks results had already suggested a similar conclusion. But with ACTs five-times-higher resolution and three-times-greater sensitivity to polarization, researchers hoped to see signs of new physics beyond Lambda-CDM that its predecessor might have missed. We came into this thinking the detailed patterns wed see in ACTs polarization data would reveal something about alternative cosmic models, Dunkley says.And it didbut not exactly as hoped.Whats Next?Rather than discovering telltale quirks that signpost a path to unraveling the nature of dark matter, dark energy and other cosmic mysteries, ACTs results instead reinforced the soundness of the standard model of cosmology. The hoped-for breakthroughs may have to wait for fresh results from a new generation of CMB surveys, such as the Simons Observatory thats now being built on the very same Chilean mountaintop that previously hosted ACT.The [Lambda-CDM] model just matches perfectly with all our data, which is pretty amazing, actuallythat were able to look back to this earliest observable time, and this simple model is still working, Dunkley says. Somethings still missing from our understanding; we dont know what dark matter and dark energy are, for example. But this result is important because its showing us that a lot of other things that couldve made the universe more complicated arent happening. The early universe doesnt seem to be where our problem lies.