From a mountain high in Chile’s Atacama Desert, astronomers with the National Science Foundation’s Atacama Cosmology Telescope (ACT) have taken a fresh look at the oldest light in the universe. Their new observations plus a bit of cosmic geometry suggest that the universe is 13.77 billion years old, give or take 40 million years.
The newly calculated estimate precisely matches the one provided by the “standard model” of the universe and measurements of the same light made by the Planck satellite.
“The standard model, the one behind Jim Peebles’ Nobel Prize, comes through with flying colors,” said Lyman Page, Princeton’s James S. McDonnell Distinguished University Professor in Physics, who was the ACT’s principal investigator from 2004 to 2014.
This adds a fresh twist to an ongoing debate in the astrophysics community, said Simone Aiola, a visiting research collaborator (and former postdoctoral researcher) at Princeton who is the first author of one of two new papers released today on the findings. Along with the papers, the researchers are also sharing years of ACT data and sophisticated software tools on NASA’s LAMBDA archive.
The controversy: In 2019, a research team measuring the movements of galaxies calculated that the universe is hundreds of millions of years younger than the Planck team predicted. That discrepancy suggested that a new model for the universe might be needed and sparked concerns that one of the sets of measurements might be incorrect.
“Now we’ve come up with an answer where Planck and ACT agree,” said Aiola, who is also a researcher at the Flatiron Institute’s Center for Computational Astrophysics in New York City. “It speaks to the fact that these difficult measurements are reliable.”
The age of the universe also reveals how fast the cosmos is expanding, a number quantified by the Hubble constant. The ACT measurements suggest a Hubble constant of 67.6 kilometers per second per megaparsec. That means an object 1 megaparsec (around 3.26 million light-years) from Earth is moving away from us at 67.6 kilometers per second due to the expansion of the universe. This result agrees almost exactly with the previous estimate of 67.4 km/s per Mpc by the Planck satellite team, but it’s slower than the 74 km/s per Mpc inferred from the measurements of galaxies.
“We don’t know if the tension is due to systematic effects or to something new that we have not figured out,” said Page. “Cosmology is as exciting as ever.”
“I didn’t have a particular preference for any specific value — it was going to be interesting one way or another,” said Steve Choi, a 2018 Ph.D. alumnus of Princeton and the first author of the other new paper, which comes largely from work he did with Page. “We find an expansion rate that is right on the estimate by the Planck satellite team. This gives us more confidence in measurements of the universe’s oldest light.”
The close agreement between the ACT and Planck results and the standard cosmological model is bittersweet, Aiola said. “It’s good to know that our model right now is robust,” he said, “but it would have been nice to see a hint of something new.” Still, the disagreement with the 2019 study of the motions of galaxies maintains the possibility that unknown physics may be at play, he said.
This new picture of the oldest light in the universe was taken by the Atacama Cosmology Telescope. This covers a swath of the sky 50 times as wide as the moon, representing a region of space 20 billion light-years across. The light, emitted just 380,000 years after the Big Bang, varies in polarization (represented here by redder or bluer colors). An international team of astrophysicists used the spacing between these variations to calculate a new estimate for the universe’s age: 13.77 billion years.
Like the Planck satellite, ACT peers at the afterglow of the Big Bang. This light, known as the cosmic microwave background (CMB), marks a time just 380,000 years after the universe’s birth, when protons and electrons joined to form the first atoms. Before that time, the cosmos was opaque to light.
If scientists can estimate how far light from the CMB traveled to reach Earth, they can calculate the universe’s age. That’s easier said than done, though. Judging cosmic distances from Earth is hard. So instead, scientists measure the angle in the sky between two distant objects, with Earth and the two objects forming an enormous triangle. If scientists also know the physical separation between those objects, they can use high school geometry to estimate the distance of the objects from Earth.
Subtle variations in the CMB’s glow offer anchor points to form the other two vertices of the triangle. Those variations in temperature and polarization resulted from quantum fluctuations in the early universe that got amplified by the expanding universe into regions of varying density. (The denser patches would go on to form galaxy clusters.) Scientists have a strong enough understanding of the universe’s early years to know that these variations in the CMB should typically be spaced out every billion light-years for temperature, and half that for polarization. (For scale, our Milky Way galaxy is about 200,000 light-years in diameter.)
ACT measured the CMB fluctuations with unprecedented resolution, taking a closer look at the polarization of the light. “The Planck satellite measured the same light, but by measuring its polarization in higher fidelity, the new picture from ACT reveals more of the oldest patterns we’ve ever seen,” said Suzanne Staggs, Princeton’s Henry deWolf Smyth Professor of Physics, who took over as ACT’s principal investigator in 2014.
As ACT continues making observations, astronomers will have an even clearer picture of the CMB and an even more precise idea of how long ago the cosmos began. The ACT team will also scour those observations for signs of physics that doesn’t fit the standard cosmological model. Such strange physics could resolve the disagreement between the predictions of the age and expansion rate of the universe arising from the measurements of the CMB and the motions of galaxies.
“If [the galaxy-based measurement] is real, and we have to come up with something new in the universe to explain it, then signs of that should show up in our data, and we are not seeing any,” said Jo Dunkley, a professor of physics and astrophysical sciences at Princeton who was Aiola’s mentor. “That doesn’t mean to say they’re not there. But our new look at the CMB has showed up features that Planck wasn’t able to see before, and the fact is, everything that we see looks bang-on to the standard model.”
“We’re continuing to observe half the sky from Chile with our telescope,” said Mark Devlin, ACT’s deputy director and the Reese W. Flower Professor of Astronomy and Astrophysics at the University of Pennsylvania. “As the precision of both techniques increases, the pressure to resolve the conflict will only grow.”
“We only have the one celestial sphere to look at,” said Staggs. “We would really like to measure every single pixel of that sphere — to plumb all the information out — and we’re getting closer and closer to doing that.”
