New high-definition pictures of the early universe

New research by the Atacama Cosmology Telescope (ACT) collaboration has produced the clearest images yet of the universe’s infancy—the earliest cosmic time currently accessible to humans. Measuring light that travelled for more than 13 billion years to reach a telescope high in the Chilean Andes, the telescope’s images reveal the universe when it was about 380,000 years old—the equivalent of hours-old baby pictures of a now middle-aged cosmos.

This achievement marks ACT’s sixth and final data release (DR6), offering the clearest images yet of the baby universe. 

“Our collaboration built the telescope over 15 years ago with several upgrades to the cameras to take advantage of new technologies.” says Mark Devlin, the Reese W. Flower Professor of Astronomy at the University of Pennsylvania and ACT deputy director. “We completed the project on a high note, and we could not be more pleased with the results.”

The new pictures of this background radiation, known as the cosmic microwave background (CMB), add higher definition to those observed more than a decade ago by the Planck space-based telescope. “ACT measures the intensity and polarization of the light at five times the resolution of Planck and with greater sensitivity,” says Sigurd Naess, a researcher at the University of Oslo and a lead author of one of the new batch of papers. “This means the faint polarization signal is now directly visible.”

The team’s measurements now independently weigh in with comparable precision to Planck on many anomalies the community had noted. “In particular,” says Mathew Madhavacheril, assistant professor in the Department of Physics and Astronomy at Penn and a coauthor on the paper, “it seems to be the case that the amount by which light bends around dark matter structures is just as predicted by Einstein’s theory of gravity.” Their data, he says, also clearly prefer a universe that is “flat” in shape, rather than curved like the surface of a sphere.

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The polarization image reveals the detailed movement of hydrogen and helium gas in the cosmic infancy. “Before, we got to see where things were, and now we also see how they're moving,” says Suzanne Staggs, director of ACT and Henry deWolf Smyth Professor of Physics at Princeton University. “Like using tides to infer the presence of the moon, the movement tracked by the light’s polarization tells us how strong the pull of gravity was in different parts of space.” 

The new results confirm a simple model of the universe and have ruled out most competing alternatives, says the research team. The work has not yet gone through peer review, but the researchers have submitted a suite of papers to the Journal of Cosmology and Astroparticle Physics and are presenting their results at the American Physical Society annual meeting on March 19. 

Measuring the universe’s infancy

In the first several hundred thousand years after the Big Bang, the primordial plasma was so hot that light couldn’t propagate freely, making the universe effectively opaque. The CMB represents the first stage in the universe's history that we can see—effectively, the universe’s baby pictures. 

The images give a remarkably clear view of very, very subtle variations in the density and velocity of the gases. “There are other contemporary telescopes measuring the polarization with low noise, but none of them cover as much of the sky as ACT does,” says Naess. What look like hazy clouds in the light’s intensity are more and less dense regions in a sea of hydrogen and helium—hills and valleys that extend millions of light years across. Over the following millions to billions of years, gravity pulled the denser regions of gas inwards to build stars and galaxies.

These detailed images of the newborn universe are revealing answers to longstanding questions about the universe’s origins. 

“By looking back to that time—when things were much simpler—we can piece together the story of how our universe evolved to the rich and complex place we find ourselves in today, ” says Jo Dunkley, Joseph Henry Professor of Physics and Astrophysical Sciences at Princeton and the ACT analysis leader. 

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“We’ve measured more precisely that the observable universe extends almost 50 billion light years in all directions from us and contains as much mass as 1900 ‘zetta-Suns’ or almost 2 trillion trillion Suns,” says Erminia Calabrese, professor of astrophysics at the University of Cardiff and a lead author. Of those 1,900 zetta-Suns, the mass of normal matter—the kind we can see and measure—makes up only 100. Another 500 zetta-Suns of mass is the invisible dark matter of an as-yet unknown nature, and the equivalent of 1,300 is the dominating vacuum energy [dark energy] of empty space.

Tiny subatomic particles known as neutrinos make up at most four zetta-Suns of mass. Of normal matter, three quarters of its mass is hydrogen and a quarter helium. “Almost all of the helium in the universe was produced in the first three minutes of cosmic time,” says Thibaut Louis, CNRS researcher at the University of Paris-Saclay, and one of the lead authors. “Our new measurements of its abundance agree very well with theoretical models and with observations in galaxies.” 

The elements that comprise humans—mostly carbon, with oxygen and nitrogen and iron and even traces of gold—were formed later in stars and are just a sprinkling on top of this cosmic stew.

ACT’s new measurements have also refined estimates for the age of the universe and how fast it is growing today. The infall of matter in the early universe sent out sound waves through space, like ripples spreading out in circles on a pond. 

“A younger universe would have had to expand more quickly to reach its current size, and the images we measure would appear to be reaching us from closer by,” says Devlin. “The apparent extent of ripples in the images would be larger in that case—in the same way that a ruler held closer to your face appears larger than one held at arm’s length.”  

The new data confirm that the age of the universe is 13.8 billion years, with an uncertainty of only 0.1%. “After more than a decade of work, it’s incredible to see a ground-based experiment like ACT reach the level of precision once thought to be exclusive to space-based missions,” says Adriaan Duivenvoorden, a lead author and research fellow at the Max Planck Institute for Astrophysics.

The Hubble tension

In recent years, cosmologists have disagreed about the Hubble constant, which is the rate at which space is expanding today. Measurements derived from the CMB have consistently shown an expansion rate of 67-68 kilometers per second per megaparsec (Mpc), while measurements derived from the movement of nearby galaxies indicate a Hubble constant as high as 73-74 km/s/Mpc. Using their newly released data, the ACT team confirmed the lower value for the Hubble constant and with increased precision. 

“We took this entirely new measurement of the sky, giving us an independent check of the cosmological model, and our results show that it holds up,” says Duivenvoorden. 

A major goal of the work was to investigate alternative models for the universe that would explain the disagreement. “We wanted to see if we could find a cosmological model that matched our data and also predicted a faster expansion rate,” says Colin Hill, assistant professor at Columbia University and one of the lead authors. 

Alternate models include changing the way neutrinos and the invisible dark matter behave, adding a period of accelerated expansion in the early universe, or changing fundamental constants of nature. 

“We have used the CMB as a detector for new particles or fields in the early universe, exploring previously uncharted terrain,” says Hill. ‘The ACT data show no evidence of such new signals. With our new results, the standard model of cosmology has passed an extraordinarily precise test.” 

“It was slightly surprising to us that we didn't find even partial evidence to support the higher value,” says Staggs. “There were a few areas where we thought we might see some partial evidence for explanations of the tension, but they just weren’t there in the data.”

The background radiation measured by ACT is extremely faint. “To make this new measurement, we needed a five-year exposure with a sensitive telescope tuned to see millimeter-wavelength light,” says Devlin. “Our colleagues at National Institute Standards Technology provided detectors with cutting-edge sensitivity, and the National Science Foundation supported ACT’s mission for more than two decades to get us here.” 

In surveying the sky, ACT has also seen light emitted from other objects in space. “We can see right back through cosmic history,” says Dunkley, “From our own Milky Way, out past distant galaxies hosting vast black holes and huge galaxy clusters, all the way to that time of infancy.”

The new ACT data are shared publicly on NASA’s LAMBDA archive. “And we’re not just publishing results and data—we’re making the code available so anyone can dive in and redo the analysis,” says Louis. 

“To me, these new findings underscore just how rich the cosmic microwave background dataset can be,” says Madhavacheril. “It lets us explore an incredible range of physics—from mere fractions of a second after the Big Bang to the more recent universe we see today. It’s been especially interesting to look for ‘new physics’ that might resolve the Hubble tension, but so far, none of the models we tested reconciles our measurements of the expansion rate with the value some astronomers get from studying closer galaxies. Our data’s high precision means we can deeply probe these new models and test them more thoroughly than ever before.”

In with the new

ACT completed its observations in 2022, and the team’s attention is now turning to the new, more capable, telescope at the same location in Chile. “It’s not like we just packed up and stopped after ACT. We followed it up with the Simons Observatory, a much, much bigger and more capable instrument,” says Devlin.

Devlin, who recently returned from Chile after leading a team of researchers and engineers for several weeks, was there to witness the first successful operation, or ‘first light,’ of the Simons Observatory (SO) Large Aperture Telescope, marking a significant milestone. 

Devlin serves as co-director of SO and is the principal investigator of the National Science Foundation-funded Advanced Simons Observatory. “This work, supported by a tremendous partnership between public and private organizations, is the culmination of eight years of effort by dozens of SO researchers to make the world’s most capable ground-based cosmology telescope. The moment the second mirror went in, we moved to make the first observations with the telescope and all initial indications pointed to a huge success.”

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The team doesn’t have any scientific results just yet, Devlin clarifies, but they have the start of observations. “Here's out with the old—let’s celebrate what we did and the new stuff we achieved—and then let’s look forward to what we have coming in the future, which is the Simons Observatory.”

Liz Fuller-Wright from Princeton University contributed to this story.

The research by the Atacama Cosmology Telescope collaboration was supported by the U.S. National Science Foundation (AST-0408698, AST-0965625 and AST-1440226 for the ACT project, as well as awards PHY-0355328, PHY-0855887 and PHY-1214379), the University of Pennsylvania, Princeton University, and a Canada Foundation for Innovation award. The project is led by the University of Pennsylvania and Princeton University and, with 160 collaborators at 65 institutions. ACT operated from 2007-2022.

The pre-peer review articles highlighted in this release are available on https://act.princeton.edu/ and will appear on the open access arXiv.org. In addition to the authors mentioned above, lead authors of these papers include Zachary Atkins of Princeton University, Yilun Guan of University of Toronto, Hidde Jense of Cardiff University), Adrien La Posta of University of Oxford, Matthew Hasselfield of Flatiron Institute and Yuhan Wang of Cornell University. 

The Simons Observatory research is supported by the Simons Foundation (Award #457687), the Heising-Simons Foundation, the National Science Foundation, the governments of the United Kingdom and Japan, the University of Pennsylvania, Princeton University, University of Chicago, University of California at San Diego, the University of California at Berkeley, Lawrence Berkeley National Laboratory, the Ministry of Science, Technology, Knowledge, and Innovation, the Parque Astronomico de Atacama, and other participating institutions.