High up in Chile’s Atacama Desert, miles away from the dull glow of light pollution, the secluded Atacama Cosmology Telescope is in prime position to search the sky for answers. The question most recently on its mind? The age of the universe, a cosmic quandary that can be answered in different ways, depending on how you measure the universe’s accelerating expansion.
A paper recently published in the Journal of Cosmology and Astroparticle Physics has measured the rate of that expansion, called the Hubble constant, using the National Science Foundation’s telescope in Chile.
The team found the Hubble constant to be 68 km per second per megaparsec — meaning for every megaparsec, or 3.26 million light-years, the speed of the universe’s expansion increases by 68 km per second. The number the international team of astronomers and physicists found, after 730 days of observation spanning from 2013 to 2016, was nearly the same rate as was previously reported by the European Space Agency’s Planck satellite in 2013.
“Now we’ve come up with an answer where Planck and [Atacama Cosmology Telescope] agree,” Simone Aiola, a researcher at the Flatiron Institute’s Centre for Computational Astrophysics and a co-author of the paper, said in a press release. “It speaks to the fact that these difficult measurements are reliable.”
There’s a pretty big reason it was worth recalculating the constant: There are a few ways to measure the rate of the universe’s expansion, from which the age of the universe can be deduced. You can measure the rate based on stellar things near to us, like pulsating Cepheid stars. You can also measure the expansion by looking at the polarised light of the universe’s cosmic microwave background, the most distant detectable radiation from the Big Bang, which is what the Atacama team did here. This fuzzy light has variation in its polarisation, enabling scientists to measure how far the light has travelled and how long that travel took. That’s why understanding the rate of the universe’s expansion matters: It changes how far the light went, and thus, the age of everything.
Here’s the rub: Those two ways of calculating Hubble’s constant have come up with pretty different rates — one 2019 study came up with nearly 74 km per second per megaparsec, while another from the same year found a number that split the difference between the other two. Though the differences may sound small, the varying estimates mean a range of hundreds of millions of years in determining how old our universe is. (The higher the constant, the younger the universe).
The Atacama team’s findings place the universe’s age at around 13.77 billion years. Our solar system, for comparison, is about 4.57 billion years old, and Homo sapiens emerged somewhere around 300,000 years ago.
The differing numbers so far don’t mean that any party is necessarily wrong (though the team behind the new paper, working with better-resolution imagery of the cosmic microwave background than their predecessors at Planck, affirmed that the earlier team’s maths was solid). What it all definitely means is that we’re missing something when it comes to how the universe’s expansion works.
The disparity between local and distant measurements of the Hubble constant could mean that “there’s a problem with one of the types of measurements that we’re not interpreting correctly, and therefore there’s some kind of systematic problem with one measurement or the other,” said Michael Niemack, an astrophysicist at Cornell University and a co-author of the recent paper. “The more exciting possibility is that there’s something missing from our cosmological model.”
The best may be yet to come for the Atacama telescope, which had its first light in 2007 and has the benefit of being on the ground, making it easier to manage than a space-based telescope.
“We have yet to extract all the information from the data we collected already with the Atacama Cosmology Telescope,” Steve Choi, an astrophysicist at Cornell University and lead author of the paper, said in an email. “I am hopeful that we will learn even more exciting physics about our universe with the upcoming experiments at the Atacama, like CCAT-prime and Simons Observatory,” referring to two upcoming high-altitude observatories in the desert. The telescope in CCAT-prime was renamed the Fred Young Submillimeter Telescope this September, and it will look at a slew of cosmological features, while the Simons Observatory will focus its observational capacities on the cosmic microwave background.
Perhaps one party assessing the universe’s age is overlooking something in their maths — with the many known unknowns of space science, and the stuff that’s wholly unknown, it’s possible. But according to Niemack, there just as easily could be something else in the mix that would explain the different numbers.
“This might be a hint that we’re just on the verge of discovering something new and exciting that we haven’t known before about how our universe works,” he said.