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The cosmic web, the secret infrastructure of the universe

Our ability to see and map this intergalactic infrastructure could eventually unlock the mysteries of how our universe formed.

(CN) — Outer space may seem dark and impossibly vast, a mostly empty place punctuated seemingly at random by enormous, bright galaxies. 

But space isn't quite as dark or empty as one might imagine. Instead, there exists a sprawling infrastructure of gas that connects and feeds galaxies, like a massive network of twisting pipelines or a spider web. Astronomers call it the "cosmic web."

"It is the basic architecture of the universe, and the skeleton of the universe, that determines where all the matter is," Christopher Martin, a professor of physics at Caltech University, said in a Zoom interview last week. "The galaxies are all connected by these filaments, like pipelines bringing gas into galaxies, which can then be converted into stars."

While astronomers have theorized about the cosmic web for decades, Martin and his team of researchers recently managed for the first time to create an image of an actual filament — a strand of the cosmic web — from a relatively empty pocket of space.

The image, a short snippet of 3D animation, was first published last month in the journal Nature Astronomy.

The image comes courtesy of the Keck Cosmic Web Imager, an imaging spectrometer designed by Martin and his team at Caltech, the first telescope ever built for such a purpose.

"It basically takes a picture at every wavelength," says Martin. "And so the cube is basically an image at every wavelength."

Astronomers have known about the cosmic web for decades. A famous paper published in 1986, "A Slice of the Universe," produced the first observational evidence of filaments — bands of gas connecting different galaxies. 

The paper started with a theoretical problem: Theories about gravity and the vast stretches of nothingness between them contradicted each other. There had to be other stuff out there, other matter. 

In 1989, a team of American scientists discovered a piece of the cosmic web: a filament measuring roughly 500 million light years in length that would later come to be known by the not-so-catchy name “CfA2 Great Wall.” The term "cosmic web" was first coined by astrophysicist Richard Bond in a paper published in 1996, which posited a web of such filaments connecting the entire universe. 

In the past, researchers have been able to glimpse parts of these filaments that were illuminated by quasars, extremely bright galactic nuclei. But while before “we saw the filamentary structures under the equivalent of a lamppost," Martin said, “now we can see them without a lamp."

It took Martin and his team three years to come up with this new image of the cosmic web, including at least 30 to 40 nights of pointing the spectrometer into space. 

"I thought, 'This might work,'" Martin recalled. Using the Cosmic Web Imager, ”we targeted a region which we expected to be very dense and [have] the brightest emissions.”

"In the first 20-minute exposure, we started detecting the brightest emissions,” he said. Then, “after the first few nights, we started detecting emissions [of light] in the fainter regions."

As the researchers had hoped, the Imager had started picking up light from dimmer galaxies, too. “The pervasiveness of it — the fact that it correlated with all the galaxies in any given regions [and] not just the most dense regions — was a big surprise."

"It was wonderful,” Martin added of the discovery. “In my career, it is my most important scientific result."

The filament observed by Martin's team would have been roughly 1.5 million light years long — but that was eight or nine billion years ago, when the light was first being emitted from the depths of space. Today, the universe is around three times as large, and the filament is likely around 5 million light years in length. 

Much is still being learned about these filaments and their makeup. While researchers know they’re mostly made up of hydrogen gas, whether they also include traces of other and heavier elements is an open question — one that Martin hopes to answer some day.

Most matter in the universe is dark matter, a hypothetical form of matter that we don’t entirely understand, but that we think makes up the majority of all mass and energy in the universe. 

Of the non-dark matter (or normal matter), the vast majority of it exists within these gas filaments in the cosmic web. Only a small fraction of it is contained within galaxies themselves. Think of galaxies as cities and the cosmic web as a network of highways, or perhaps electrical wiring, that connects them all. 

"Most of the atoms are in this intergalactic medium, in this gas between galaxies," Martin said — and yet "we've never been able to make maps of it.” With these new images, “we're now seeing most of the normal matter in the universe.” That makes the Keck Cosmic Web Imager “a new observational tool to figure out how the universe is structured."

Not only that, the Imager allows astronomers to look at the web over time, to begin to understand how galaxies form and even how the universe, as a structure, formed. "It's essentially giving us a much better insight into the history of the formation of everything,” Martin said. 

In other words: While the science might be complex, the implications are broad, extending into some of life’s biggest questions.

The cosmic web is “one of the first chapters in the history of the formation of everything," an excited Martin explained. Understanding the Cosmic Web could help explain not only the origin of galaxies and stars but maybe even of life itself, including the living creatures that ultimately found a home on a planet called Earth in the Milky Way Galaxy. "It's applicable to many, many outstanding questions in astrophysics and cosmology."

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