"We are seeing the first steps towards making the earliest stars and galaxies," says Suzanne Staggs, director of ACT and Henry deWolf Smyth Professor of Physics at Princeton University. "And we're not just seeing light and dark, we're seeing the polarization of light in high resolution. That is a defining factor distinguishing ACT from Planck and other, earlier telescopes."
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 has five times the resolution of Planck, and greater sensitivity," says Sigurd Naess, a researcher at the University of Oslo and a lead author of one of several papers related to the project. "This means the faint polarization signal is now directly visible."
The polarization image reveals the detailed movement of the 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 Staggs. "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 a majority of competing alternatives, says the research team. The work has not yet gone through peer review, but the researchers will present their results at the American Physical Society annual conference on March 19.
The new images give a remarkably clear view of very, very subtle variations in the density and velocity of the gases that filled the young universe. "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 helping scientists to answer 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, the Joseph Henry Professor of Physics and Astrophysical Sciences at Princeton University and the ACT analysis leader.
"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 1,900 'zetta-suns', or almost 2 trillion trillion Suns," says Erminia Calabrese, professor of astrophysics at the University of Cardiff and a lead author on one of the new papers. 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 are mysterious dark matter, and the equivalent of 1,300 are the dominating vacuum energy (also called dark energy) of empty space.
Tiny neutrino particles make up at most four zetta-suns of mass. Of the normal matter, three-quarters of the 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 IJCLab, University Paris-Saclay and one of the lead authors of the new papers. "Our new measurements of its abundance agree very well with theoretical models and with observations in galaxies." The elements that we humans are made of - 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", explains Mark Devlin, the Reese W. Flower Professor of Astronomy at the University of Pennsylvania, and ACT's deputy director. "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%.
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, an assistant professor at Columbia University and one of the lead authors of the new papers. Alternates 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 evidence for explanations of the tension, and they just weren't there in the data."
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."
ACT completed its observations in 2022, and attention is now turning to the new, more capable, Simons Observatory at the same location in Chile. The new ACT data are shared publicly on NASA's LAMBDA archive.
Related Links
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