Astronomers may have spotted strands in the ‘cosmic web’ linking galaxies
The universe’s biggest parts are also some of the hardest to see.
If you could zoom all the way out from Earth, past the sun, past our galaxy, beyond the local group of galaxies the Milky Way belongs to, all the way out to gaze upon the whole universe… it might look something like an extremely unsatisfying serving of cotton candy.
You’d see massive holes mostly devoid of galaxies, stars, and even atoms, and between those dead spots you’d see countless delicate strands of all different lengths and sizes. The Milky Way, Andromeda galaxy, and our entire backyard cluster amount to little more than a few pixels in this incomprehensibly vast structure, which astronomers have come to call the “cosmic web.”
Most of our evidence of the web comes from simulations and inferences. But now an international group of researchers has caught the first direct glimpse of two strings of gas, which they call “filaments,” connecting multiple galaxies in the void. Although their map covers just a tiny spot of the massive web, it helps confirm that the models are on the right track—and that the web plays a major role in how galaxies grow.
“The cosmic web acts as [a series of] funnels through which matter flow under gravity, so its properties are of profound importance to how galaxies form,” says says Khee-Gan Lee, a cosmologist at the Kavli Institute for the Physics and Mathematics of the Universe, who was not involved in the research. He calls this first observation of filaments linking galaxies “a big deal.”
In recent decades, researchers have come to suspect that the cosmic web is more than just a passive clustering of galaxies. Stretched by the expansion of the universe while simultaneously squeezed by gravity, simulations suggest, the connective filaments thin out over time. More recently astronomers have also noticed that star-forming galaxies don’t quite shine as brightly as they should, as if something was getting in the way. This apparent interference indirectly proved that the cosmic web exists as a network of trace amounts of hydrogen gas.
Together, these theories describe a universe where gas streams along the thinning filaments into the nodes—crossroads where different strands flow together. There, hydrogen piles up and fuels the growth of huge galaxies and supermassive black holes. The new map, published Thursday in Science, represents the first direct imaging of intergalactic filaments in one such node.
“It’s been a race over the last [few] years for this detection of intergalactic emission,” says Lutz Wisotzki, a cosmologist at the University of Potsdam in Germany who works on similar observations. “Everyone wants to see things, not just infer that something is there.”
But it’s hard to imagine a more tenuous target than the hydrogen that holds the universe together. Photogenic favorites like the Orion Nebula are already emptier than the best laboratory vacuum-chambers on Earth, Wisotzki says, and the web’s strands should be about a million times thinner still, with each liter containing perhaps one atom. Our instruments can only detect those atoms only if passing light beams happen to make them glow in just the right way, so scouring the universe for its dim filaments is a bit like searching for clouds at night.
To see the nearly invisible the team zoomed in on a particular cluster of very young galaxies, peering so deeply into space that they measured light that left the violent region during the so-called “cosmic noon,” an era a few billion years after the Big Bang when stars were forming fast and furiously. Following up on hints from an earlier survey, they trained the European Southern Observatory’s Very Large Telescope in Chile on a part of the sky about the size of a small lunar crater. After thirty hours of observation spanning several nights, they had collected enough light to trace the outlines of two faint filaments threading their way from galaxy to galaxy, each more than three million light-years long, or 30-times the length of the entire Milky Way.
The strands were only visible, says co-author Hideki Umehata, an astronomer at the University of Tokyo, because the baby galaxies lit up the wispy gas like the glow of a city illuminating passing clouds. “We identified the relatively bright part,” he says. “We fortunately found a very good place to first see the cosmic web.”
Wisotzki, a fellow intergalactic hydrogen hunter who helped build the instrument that took the shot, says the research sets a new record for the largest pile of hydrogen ever detected. Previous searches had spotted hydrogen around brilliant galactic centers trailing off into the darkness, but never before has anyone discerned strings of the stuff linking galaxies. He adds, however, that a cosmic web filament is only one interpretation. Since seeing the gas requires illumination and the team is looking in a bright place, he isn’t completely sure whether they’ve really pinned down one of the threads that knits the universe together. Perhaps they’ve just found some generic cosmic fog, or debris tossed off by intergalactic fender benders. “Are we seeing the gases,” he asks, “or are we seeing the torches that light up the gas?”
Umehata, for his part, acknowledges that there’s probably some galactic flotsam in the mix, and that it’s hard to separate one type of gas from another. He hopes that future observations will reveal the extent of the filaments, and perhaps detect them flowing into the galaxies they connect.
Further exploration could turn the web into a tool for scrutinizing the cosmos on the largest and smallest scales. In addition to its ongoing role in galaxy making, the web is also a relic of the Big Bang, each filament the end result of some microscopic ripple in the universe’s first moments getting stretched to cosmic proportions. More thorough web measurements could, for instance, tell us how gravity pushed matter around when the universe was very small, hot, and young.
But until then, this new map of colossal filaments from bygone eras helps us better grasp our immediate surroundings. We sit here, on a planet orbiting a star, whirling around a galaxy nourished by a cosmic river of gas that leads to somewhere else.