Hoʻoleilana, a billion-light-year-wide bubble of galaxies, astounds astronomers

For the first few hundred thousand years after the big bang, the entire universe was a blistering and dense plasma similar to the sun’s interior, with spots of heat that emanated pressure waves. But once the expanding cosmos turned 380,000 years old, the plasma cooled and thinned out, leaving those oscillations with no medium to travel through.
Last Updated : 27 September 2023, 14:41 IST

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By Adam Mann for Scientific American

Cosmic cartographer Brent Tully was inspecting his team’s latest high-precision maps of the positions and motions of 56,000 galaxies in the local universe when he noticed a colossal ringlike structure.

“It was one billion light-years in diameter,” recalls his colleague Daniel Pomarède of Paris-Saclay University. “This is exactly what you’d expect for a BAO shell. I’ve been working with Brent for 13 years, and we’ve never talked about the possibility of uncovering a BAO.”

BAO stands for “baryon acoustic oscillation,” a sort of frozen sound wave created by processes near the dawn of time. For the first few hundred thousand years after the big bang, the entire universe was a blistering and dense plasma similar to the sun’s interior, with spots of heat that emanated pressure waves. But once the expanding cosmos turned 380,000 years old, the plasma cooled and thinned out, leaving those oscillations with no medium to travel through.

This left behind titanic remnant bubbles centered on those former heat spots, each with slightly more baryons—building blocks of matter such as neutrons and protons—in them. Over billions of years, gravity pulled additional material into those baryon-dense regions, and galaxies and galactic clusters preferentially formed along their boundaries in thin shells like dust settling on a soap bubble. Astronomers have glimpsed these large-scale patterns in surveys of hundreds of thousands of galaxies across huge swaths of sky. But nobody had ever spotted an individual BAO until Tully and Pomarède’s finding—that is, if it’s real.

According to theoretical predictions, that formation—which the researchers named Hoʻoleilana, a term that means “sent murmurs of awakening” in Hawaiian—isn’t quite the right size to be a BAO. This discrepancy could either imply that conditions in the early universe weren’t quite what astronomers have expected or that the structure is a chance alignment of galaxies masquerading as a BAO. Tully and Pomarède think their discovery could be used to probe fundamental properties of the cosmos. But in order to do so, they need to convince the rest of the community that the result is what they believe it to be.

Pomarède considers serendipity to have played a large part in his career. At a conference in Ouagadougou, Burkina Faso, many years ago, he was showing off a computer program he’d created for visualizing astronomical data when Tully approached him. Tully “looked at me and said, ‘All my life, I’ve dreamt of having this software,’” Pomarède says.

The two have since worked together to map our cosmic surroundings in detail. In 2014 they co-discovered the Laniakea Supercluster, a collection of around 100,000 nearby galaxies, including the Milky Way, that stretches over half a billion light-years. Their most recent dataset uses information from multiple telescopes to produce distance measurements to celestial objects with a precision level as small as 0.001 percent, says Cullan Howlett of the University of Queensland in Australia, who is also a co-author of the Hoʻoleilana finding.

It was while looking through this dataset, which maps things farther afield than the team’s previous catalogs, that Tully spied Hoʻoleilana. The spherical collection of galaxies is situated roughly 820 million light-years from Earth. In its center sit the Bootes superclusters, two collections of about a dozen galaxy clusters, while the bubble’s edges include other monumental cosmic structures, such as the Sloan Great Wall, the CfA2 Great Wall and the Hercules Supercluster. The team’s findings appeared in two recent papers in the Astrophysical Journal.

The size of any individual BAO is set by the speed of sound in the early universe’s primordial plasma—which was roughly half the speed of light. This created pressure waves with particular amplitudes, which were stretched out by later cosmic expansion to a bit less than half a billion light-years. But Hoʻoleilana’s radius is actually about 10 percent greater than would be expected with such processes. To Tully and his colleagues, this could indicate something important about the nascent universe.

According to the standard model of cosmology, those early hot spots should have been scattered randomly throughout space. “But it may be that there was some intrinsic pattern that the basic model of cosmology doesn’t predict,” Howlett says, and this pattern could have caused things near us to be larger than naively presumed.

The hotspots themselves are thought to have arisen from a bizarre hypothetical epoch a split second after the big bang known as inflation, during which the entire universe greatly ballooned in size. Subatomic quantum fluctuations in the primordial cosmos would have been magnified to a macroscopic level, generating the presumably random distribution of hot and cold patches that later formed BAOs.

Inflation as a concept has been around since the 1980s, Howlett says, “but there’s a million different theories for the exact details of how it occurred.” In principle, crafting explanations for a slightly oversized BAO could help physicists narrow down these myriad theories, ideally to a single one.

Of course, the aberrant size of the team’s BAO could also lead to other conclusions. The outlines of the billionish-light-year bubbles are extremely faint and only become apparent when one examines an enormous number of objects over great distances, says Kyle Dawson of the University of Utah, co-spokesperson for the Dark Energy Spectroscopic Instrument (DESI). He’s more inclined to believe that this latest finding is something of a coincidence, a chance alignment that simply looks like a sphere with a radius around what you’d expect for a BAO.

Seeking to determine how often such statistical flukes might occur, Howlett created computer simulations that modeled universes that he artificially smoothed out to prevent the initial hotspots’ oscillations from turning into large-scale structures. Of the 256 simulations he ran, only two produced features that resembled BAOs—and even then, they weren’t as BAO-like as Hoʻoleilana. This suggests, Howlett says, that the chance of accidentally creating such structures is less than 1 percent.

Dawson isn’t entirely convinced. “One percent still happens,” he says.

Nathalie Palanque-Delabrouille, the other co-spokesperson for DESI, is more persuaded by the team’s arguments. After the putative BAO’s formation, subsequent gravitational interactions among the galaxies and clusters could have caused distortions to its size, she says. “It’s actually pretty close,” Palanque-Delabrouille adds. “The fact that it doesn’t match exactly could just be because, in this particular case, the motions of the galaxies were such that the feature is no longer exactly how we would expect it.” She suggests that future observations could try to determine how much variation there is among individual BAOs, perhaps explaining why this one is slightly off.

DESI in particular is poised to help weigh in on this matter and could be the key to finding additional structures like Hoʻoleilana. Mounted on the four-meter Mayall Telescope atop Kitt Peak in the desert outside Tucson, Ariz., the instrument is making a detailed three-dimensional map of 40 million galaxies in both the local and the distant universe. That surveyed slice of the cosmos should be big enough to allow DESI to find and study BAOs in detail. Additional information from the European Space Agency’s recently launched Euclid satellite, as well as ground-based telescopes such as the Square Kilometer Array, will help cosmographers produce even better plots of galaxies and galactic clusters, Pomarède says. Given such work, he often feels like he’s partaking in a long lineage of humans mapping their surroundings in order to better understand their place in the universe.

Tully agrees. “As details are filled in, we see richer complexity,” he says, “and we increasingly appreciate the place we call home.”

Published 27 September 2023, 14:41 IST

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