From a mountain high in Chile’s Atacama Desert, astronomers with the National Science Foundation’s Atacama Cosmology Telescope (ACT), including astronomer Mark Devlin, 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 new estimate matches the standard model of the universe, as well as measurements taken of the same light by the Planck satellite, an agreement that adds a fresh twist to an ongoing debate in the astrophysics community, says Simone Aiola, postdoctoral research associate at the Flatiron Institute’s Center for Computational Astrophysics and first author of one of two new pre-print articles posted to arXiv. In 2019, a team of researchers 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 brought up concerns that one of the sets of measurements might be incorrect. “Now we’ve come up with an answer where Planck and ACT agree,” says Aiola. “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 that an object 1 megaparsec, around 3.26 million light-years, is moving away from Earth 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 kilometers per second by the Planck team and slower than the 74 kilometers per second constant inferred from the measurements of galaxies. “I didn’t have a particular preference for any specific value. It was going to be interesting one way or another,” says Steve Choi, a postdoc at Cornell University and lead author of the second other pre-print article posted to arXiv. “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.”
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 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, but because judging cosmic distances is hard scientists instead measure the angle in the sky between two distant objects, with Earth and the two objects forming a cosmic triangle. If scientists also know the physical separation between those objects, they can use geometry to estimate the distance of the objects from Earth.
Subtle variations in temperature and polarization in the CMB’s glow, resulting from quantum fluctuations in the early universe that were then amplified during expansion into regions of varying density, offer anchor points to form the other two points of the triangle. 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 in temperature and half that in polarization. For comparison, the 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,” says Suzanne Staggs, ACT’s principal investigator and the Henry deWolf Smyth Professor of Physics at Princeton University.
As ACT continues making observations, astronomers will have an even clearer picture of the CMB and a more exact 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, which could resolve inconsistencies between measurements of the CMB and the motions of galaxies.
“We’re continuing to observe half the sky from Chile with our telescope,” says Devlin, who is ACT’s deputy director and Penn’s Reese W. Flower Professor of Astronomy and Astrophysics. “As the precision of both techniques increases, the pressure to resolve the conflict will only grow.”
The two pre-print articles highlighted in this release are available on the open-access arXiv.org and have been submitted to the Journal of Cosmology and Astroparticle Physics.
The Atacama Cosmology Telescope is supported by the U.S. National Science Foundation by grants AST-0408698, AST-0965625 and AST-1440226.