Giant 'bubble' of galaxies a remnant of the early universe

An international team of astronomers has discovered a giant ‘bubble’ of galaxies, measuring one billion light-years across, which they believe to be a fossil-like remnant of the birth of the universe 13 billion years ago. Structures of this type are predicted by the Big Bang Theory as the result of 3D ripples found in the material of the early universe, known as baryon acoustic oscillations (BAO).

The discovery was led by the University of Hawaiʻi (UH) Institute of Astronomy and so named Ho’oleilana, after a Hawaiian creation chant evoking the origin of structure. It has been described in The Astrophysical Journal.

As per the Big Bang Theory, during the first 400,000 years, the universe was a cauldron of hot plasma similar to the interior of the Sun. Within this plasma, electrons were separated from the atomic nuclei. During this period, regions with slightly higher density began to collapse under gravity, even as the intense bath of radiation attempted to push matter apart. This struggle between gravity and radiation made the plasma oscillate or ripple and spread outward.

The largest ripples in the early universe depended on the distance a sound wave could travel. Set by the speed of sound in the plasma, this distance was almost 500 million light-years, and was fixed once the universe cooled and stopped being plasma, leaving vast three-dimensional ripples. Throughout the eons, galaxies formed at the density peaks, in enormous bubble-like structures. Patterns in the distribution of galaxies, properly discerned, could reveal the properties of these ancient messengers.

Astronomers located Ho’oleilana, which is centred approximately 820 million light-years from our own galaxy, using data from Cosmicflows-4 — the largest known compilation of precise distances to galaxies. As noted by study leader Brent Tully, “We were not looking for it … [but] it is so huge that it spills to the edges of the sector of the sky that we were analysing.”

Tully’s team discovered that Ho’oleilana had been noted in a 2016 research paper as the most prominent of several shell-like structures seen in the Sloan Digital Sky Survey. However, the earlier work did not reveal the full extent of the structure, and that team did not conclude they had found a BAO.

Using the Cosmicflows-4 catalogue, Tully’s team were able to see a full spherical shell of galaxies, identify its centre, and show that there is a statistical enhancement in the density of galaxies in all directions from that centre. Ho’oleilana also encompasses many well-known structures previously found by astronomers, such as the Harvard/Smithsonian Great Wall containing the Coma Cluster, the Hercules Cluster and the Sloan Great Wall.

“Mapping Ho’oleilana in three dimensions helps us understand its content and relationship with its surroundings,” said team member Daniel Pomarède of CEA Paris-Saclay University. “It was an amazing process to construct this map and see how the giant shell structure of Ho’oleilana is composed of elements that were identified in the past as being themselves some of the largest structures of the universe.”

BAO provide an independent way to measure the expansion rate of the universe and how this rate has changed throughout cosmic history. The team believes this may be the first time astronomers have successfully identified an individual structure associated with a BAO, and that it could help bolster our knowledge of the effects of galaxy evolution as well as the expansion rate of the universe.

“Our analysis suggests because this bubble is larger than expected, the universe has expanded further than originally predicted,” said team member Dr Cullan Howlett from The University of Queensland (UQ), who noted that discrepancies in measurements of the expansion rate of the universe are “probably the biggest issue in cosmology right now”.

“The very large diameter of one billion light years is beyond theoretical expectations,” Tully added. “If its formation and evolution are in accordance with theory, this BAO is closer than anticipated, implying a high value for the expansion rate of the universe.”

Researchers are set to use data from the next generation of galaxy surveys, including from the Dark Energy Spectroscopic Instrument, to study and confirm more details about Ho’oleilana, BAO and the expansion rate of the universe. According to Howlett, “We’re now a step closer to a great change in the field of cosmology, where the whole model of the universe might need re-evaluating.”

Image caption: Artist’s impression of the giant bubble of galaxies discovered by an international team of astronomers. Image credit: Frédéric Durillon.