The universe's 'missing matter' problem has finally been solved
Half of the normal matter in the universe has been hiding since the Big Bang. Astronomers have just detected it for the first time.
When Jean-Pierre Macquart arrived home from work one night in 2019, he was buzzing with excitement. He'd just helped solve a decades-old cosmic mystery with the help of a team of international astronomers. He couldn't wait to tell his wife.
Macquart had successfully weighed the universe for the first time, finally discovering where half of all the normal matter was hiding. But as he stepped through the door, ready to explain his monumental find, the ethereal secrets of thehe'd uncovered were quickly replaced by the practicalities of existence.
Within minutes, he was wrangling two children, ages two and four, and taking to the kitchen, helping his wife with the cooking. In reflecting on the evening he says he likely helped with the meal, but it wasn't all that memorable. His head was "still up in the sky." The discovery he'd made earlier in the day, which he says "put to bed" the mystery of the universe's missing matter problem, was still playing on his mind.
In a new study, published in the journal Nature on Wednesday, Macquart and a team of international astronomers detail their discovery for the first time. They reveal how a stream of helped solve a lingering mystery about the normal matter in the universe -- and how their technique has provided a whole new way to look at the cosmos.
The 'missing matter' problem
Macquart, an astronomer at the International Centre for Radio Astronomy Research in Australia, and his team have been searching the cosmos for fast radio bursts, or FRBs, using a huge telescope array in the Australian outback known as the Australian Square Kilometre Array Pathfinder. The extremely energetic bursts travel through the vast emptiness of space and are detected by ASKAP's 36 dish antennas stationed in the radio-quiet desert of Western Australia.
Macquart and a quantum of collaborators from institutions across the globe, part of the the Commensal Real-time ASKAP Fast Transients Survey investigative team, realized the bursts could also be used to detect the "missing matter" of the universe.
The universe is made up of "ordinary matter," dark matter and dark energy. The latter constituents make up around 95% of the known universe and are incredibly mysterious. We know they exist but we've never been able to detect them.
On the other hand, you have ordinary matter. Macquart explains that ordinary matter, or baryonic matter, is all the "stuff" you and I are made of, as well as what makes up the planets, stars and galaxies. "It's anything you can think of on the periodic table," he says. Early calculations in the 1990s showed this type of matter makes up the other measly 5% of all matter in the universe, and scientists went searching for it.
"When they looked ... a few decades ago, they could only account for about half of that," says Macquart. Counting up all the matter they could see -- the galaxies, stars, planets, gases -- scientists fell woefully short of their 5% target. The matter was missing from their measurements.
But astronomers had an inkling as to where they might find it. Over the years, a number of different methods have been used to try to detect the missing matter, but researchers weren't able to adequately detect all the normal matter across the universe, mostly because they focused in on specific regions of space. Macquart likens this to trying to tell how big a dog is just "by looking at the size of its tail."
But the new technique pioneered by the team -- using FRBs -- lets you look at the whole dog.
"What FRBs do is go way out into the stretches [of space] where [other] techniques simply fall over," he says.
Blast from the past
are mysterious and intriguing cosmic phenomena. They were first identified in 2007, but their origins continue to elude scientists. They are still quite rare, but we are . New telescopes and radio arrays, like ASKAP, allow astronomers to pinpoint the source of these radio wave bursts from deep space.
ASKAP is a key piece of the new study because it is basically always watching a large patch of the sky, like a cosmic Big Brother. Every second it takes 10 trillion measurements and then averages out to around 1 billion measurements per second, looking for signs of FRBs.
To ping an ASKAP antenna on Earth, the radio waves travel from distant galaxies, enduring a long journey that takes them through the vast nothingness of space between galaxies. While we might traditionally see this region of space as empty, it's actually full of particles like electrons that can bump into the wave as it zips through the universe from as far as 3 billion light-years away.
"As the radio waves travel across the cosmos, they interact with the free electrons, smearing the radio signal," says Geraint Lewis, an astrophysicist at the University of Sydney who was not affiliated with the study. It's this smearing of the radio signal that was key to finding the missing matter.
The astronomers counted "the number of electrons lying along our line of sight" back to the FRB sources, according to Lewis, providing a measure of the hidden matter in the cosmos. After studying five different FRBs, from five different locations, the team found their measurements lined up almost perfectly with predictions of how much normal matter should exist in the universe.
The puzzle was finally solved, and cosmologists could breathe a little easier -- their models for understanding the universe weren't incorrect.
"It puts to rest what could have been a real cosmic embarrassment," says Xavier Prochaska, an astronomer at the University of California, Santa Cruz and co-author on the new paper, during a media briefing. "We all expected to detect it, eventually, but until we did, it was an embarrassment."
Mapping the cosmic web
With the mystery of the missing matter solved, the team believe they can use FRBs as a new tool to probe the cosmos.
The FRB detection method is super sensitive compared to previous methods and allows researchers to detect the ordinary matter locked in the vast gas-filled space between galaxies. This means astronomers might be able to map out the so-called cosmic web, the filaments that link the universe together.
"The technique ... is going to be a technique that allows us to map out where the gas is," says Prochaska.
"As of today, we can mainly show you this image from a computer simulation of the cosmic web, but give us five years and at least 100 more of these FRBs and we should be able to show you a more high-fidelity map of the real universe."
The team will continue looking for FRBs with ASKAP, and Macquart notes they are building a "ginormous machine" that will be able to find more of the bursts, increasing the rate of detection 20-fold. Such a leap could enable the team to pick up 100 of the signals within a year and help reshape how we view the universe, back to its earliest days.
"We might even be able to say something about the Epoch of Reionization, when the universe was turned from neutral matter to ionized matter," he says.
Of course, the missing matter only makes up a very small percentage of all the matter in the universe, and there are big cosmological questions that still need answering,
"Whilst we know where all of the normal material is spread throughout the universe, we have still only tied down less than 5% of the cosmos," says Lewis. "Dark matter and dark energy remain the next nut to crack."
On the other hand, there's another pressing puzzle for Macquart to turn his attention to now that his discovery is out in the world. Unlike the missing matter problem, it's one that many of us down on Earth can easily understand. One that speaks to the practicalities of existence a little more.
What on Earth should I cook for dinner?