Oldest surviving light reveals the universe's true age

Ancient light from the Big Bang has revealed a precise new estimate for the universe's age: 13.77 billion years, give or take 40 million years.

The new estimate, based on data from an array of telescopes in the Chilean Atacama Desert, also weighs in on one of the most important disagreements in astrophysics: How fast is the universe expanding? Described in two scientific papers, the new result gives a significant boost to one side of the disagreement, though the physicists couldn't prove the other side of the dispute wrong.

Here's the problem: Physicists need to understand the universe's expansion rate to make any sense of cosmology — the science of our whole universe's past, present and future. They know that a mysterious substance called dark energy is causing the universe to expand (at an ever-increasing rate) in all directions.. But when astronomers point their telescopes into space to measure the Hubble constant (H0) — the number that describes how fast the universe is expanding at different distances from us or another point — they come up with numbers that disagree with each other, depending on the method they use.

One method, based on measurements of how fast nearby galaxies are moving away from the Milky Way, produces one H0. Another method, based on studying the oldest light in space, or cosmic microwave background (CMB), produces another H0. This disagreement has left scientists wondering whether there's some important blind spot in their measurements or theories, as Live Science previously reported. These new results seem to show that there weren't any measurement errors on the CMB side.

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"We find an expansion rate that is right on the estimate by the Planck satellite team," which is another study of the CMB, Cornell University astrophysicist Steve Choi, lead author of one of two new papers, said in a statement. "This gives us more confidence in measurements of the universe's oldest light."

The data from the Planck satellite, released in 2018, were the most important measurements of the CMB before now. With an unprecedented level of precision, they showed how sharply CMB measurements of H0 disagree with measurements based on the movement of nearby galaxies.

These new results recalculated the CMB measurement from scratch using an entirely different set of telescope data and calculations, and came up with very similar results. That doesn't prove that the CMB measurement of H0 is correct — there could still be some problem with the physics theories used to make the calculation — but it does suggest that there aren't any measurement errors on that side of the disagreement.

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Relying on data from the Atacama Cosmology Telescope (ACT) in Chile's Atacama Desert, the researchers tracked faint differences between different parts of the CMB -- which appears to have different energy levels in different parts of the sky. The CMB, which formed as the universe cooled after the Big Bang, is detectable in every direction in space as a microwave glow. It's more than 13 billion light-years in the distance, a relic of a time before stars and galaxies formed. 

By combining  theories on how the CMB formed with precise measurements of its fluctuations, physicists can determine how fast the universe was expanding at that moment in time. That data can then be used to calculate H0.

The ACT methodically scanned half the sky between 2013 and 2016, looking particularly at microwave light. Then researchers spent years cleaning up and analyzing the data with the aid of supercomputers, removing other microwave sources that are not part of the CMB, to stitch together a full map of the CMB. The whole time, they "blinded" themselves to the implications of their work, they wrote in their papers, meaning they didn't look at how their choices affected estimates of H0 until the very end. Only when the full CMB map was complete did the researchers use it to calculate H0.

The new CMB map also offered a new measure for the distance between Earth and the CMB. That distance, combined with a new measurement of how fast the universe has expanded over time, allowed a precise calculation of the age of the universe.

"I didn't have a particular preference for any specific value — it was going to be interesting one way or another," Choi said.

 It's still possible, as Live Science has previously reported, that some error in those theories is messing up the  calculation. But it's not clear what the error would be.

The other approach to calculating H0 relies on pulsing stars known as cepheids, which reside in distant galaxies and pulse regularly. That timed pulsing allows researchers to perform precise calculations of their motion and distances from Earth.

With those direct speed measurements, it's fairly straightforward to come up with a measurement of H0. There are no complicated cosmological theories involved. But it's possible, some scientists have proposed, that our region of the universe is just weirdly empty, and not representative of the whole universe. It's even possible that there are measurement issues with the cepheids, and that these cosmic measuring sticks don't work quite the way physicists expect.

For now, the true H0 remains a mystery. But CMB researchers have more ammunition for their side of the disagreement.

Both new papers describing the new analysis have been published July 14 to the preprint database arXiv and submitted for formal peer review.

Originally published on Live Science.