Astronomers spy cosmic dust, with a twist, from the very early universe

The universe is a dusty place. Cosmic particles can range from the size of a single large molecule up to a bit larger than a grain of terrestrial sand, and these can accumulate in billowing clouds light-years wide. The general scientific understanding was that dust piles up gradually, produced by stars and supernovae over hundreds of millions of years. Dust is usually a fixture of mature galaxies, or so astronomers thought. 

But in a new paper published Wednesday in the journal Nature, astronomers found a specific type of cosmic dust, high in carbon, in young distant galaxies just 800 million years after the Big Bang. That accumulation happened far earlier than current theories of dust formation suggest is possible. It’s a finding that could change how astronomers understand the creation of stars and evolution of galaxies in the early universe, and ultimately, how that young universe grew into the cosmos we know today. 

For a long time, astronomers treated the cosmic stuff the way we might view a dust bunny under a sofa: as a nuisance. Scientists tried to look beyond large clouds of cosmic dust, treated more like obstacles than subjects of study in their own right. “The way most astronomers interact with it is that [dust] actually absorbs a lot of the light that we’re trying to observe,” says lead study author Joris Witstok, a post-doctoral researcher with the Kavli Institute for Cosmology at Cambridge, in the UK. 

But that’s changed in recent years, thanks to observatories such as NASA’s James Webb Space Telescope, which uses infrared light to see through the clouds. Scientists have also come to appreciate the dust itself, realizing these tiny flecks of carbon, silicon, and other matter are responsible for large-scale processes in the universe, such as new star formation. 

”For example, in the Milky Way, we have these sites where new stars are forming, and they’re very dusty,” Witstok says. “There’s big clouds of gas and dust and the dust really helps to allow the gas to cool and contract and therefore form new stars.”

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It’s not that the early universe was dustless. Previous studies had found large quantities of dust in galaxies in the very early universe, according to Witstok. Astronomers are interested in this early dust because it represents when stars began to produce some of the first elements heavier than hydrogen.

“The first stars that started to convert hydrogen into helium, which was the only thing that was around all the way at the beginning, into the heavier elements like carbon, oxygen,” Witstok says. 

Large primordial stars may have expelled vast quantities of dust, made of these heavier elements, toward the end of their life cycles, or during supernovae explosions as they died. 

But previous studies hadn’t been able to detect carbonaceous dust—meaning it’s rich in carbon—at such early times. 

“The thing that is really a new discovery here is that we’re able to pinpoint the type of dust grains that we’re seeing,” Witstok says. ”What we’re actually able to tell is that there’s something producing, specifically, these carbon dust grains on a very short timescale. And that’s where the surprise lies.”

Spectrographic observations of dust nearer to Earth, within the Milky Way galaxy, made this discovery possible. Spectroscopy breaks light into a spectrum and looks for telltale signs of absorbed light at certain wavelengths associated with different elements and compounds—sort of like reading a unique rainbow. 

Carbonaceous dust produces a spectroscopic “bump” at a wavelength of 217.5 nanometers, a wavelength that places it in the ultraviolet portion of the spectrum. At least, that’s the wavelength of the light as it left its home galaxy billions of years ago. 

“Since it’s been traveling over roughly 13 billion years, while the universe is expanding, the light really gets stretched with that expansion,” Witstok says, a phenomenon known as redshift. Light that was ultraviolet gets stretched longer, so that the wavelength—about 1.5 to 2 micrometers—is now in the infrared, the part of the spectrum JWST is fine-tuned to measure. 

“That’s exactly why we couldn’t do this before,” Witstok says. “Because with JWST, we’re now for the first time able to look and make these very precise measurements in the infrared.”

[Related: Physicists figured out a recipe to make titanium stardust on Earth] 

Now that researchers have measured this carbonaceous dust at an earlier time in the universe than expected, they’re left trying to figure out what process could be producing it. There are two theories, Witstok says, though neither are perfect. 

The first is that supernovae in early galaxies make the dust, with dying stars expelling the material before their final fiery death throes. But the problem there, he says, is that violent forces unleashed by the supernovae might also destroy much of that dust.

Another source of the dust could be Wolf-Rayet stars, massive, hot, and fast-burning stars that can expel a large portion of their mass into space in less than a million years’ time. “But again, it’s the question of how much can they actually produce?” Witstok says. “Is it enough to explain what we’re seeing in the early universe?”

Witstok and his colleagues hope to answer those questions with computer simulations. Theorists can try to tweak models of supernovae and Wolf-Rayet stars to try to find the conditions that produce the carbonaceous dust seen in the JWST observations. 

And further observations of early galaxies may net answers as well, he says. “We could start to look at what might be hints of an unusual number of Wolf-Rayet stars within those galaxies, for example.”

Whatever is driving carbonaceous dust creation in the early universe may hold clues for understanding how galaxies in the more recent universe evolved, and how stars and planets form, too. ”Dust is this really key component of how galaxies evolve,” Witstok says. ”That we’re now starting to see more and more evidence of it forming very early on is telling us that perhaps this evolution is taking place more quickly than we previously thought. That then has a knock-on effect, down the line, as to how we get to the present.”