Ho’oleilana: a fossil dating from a time close to the birth of the Universe

Illustration of Hoʻoleilana. Red region (left) shows the enclosed shell with individual galaxies depicted as luminous tiny specks. Photo credit: Frédéric Durillon, Animea Studio; Daniel Pomarède, IRFU, CEA University Paris-Saclay. This work benefited from a government funding by France 2030 (P2I-Graduate School of Physics) under reference ANR-11-IDEX-0003.

The question of our place in the Universe: where are we in the Universe? is a fascinating question, at the origin of one of the oldest sciences: Cosmography, the cartography of the Cosmos. Thanks to a method based on the study of the velocity fields of galaxies measured by different telescopes, the cosmographers, made up of a team of researchers from the CEA, the University of Hawaii and the University of Queensland, reconstruct the web cosmic of our nearby Universe and reveals its architecture revealing large structures of superclusters of galaxies, but also large voids, shaped by gravitation [1].

Daniel Pomarède, from Irfu, has been in charge of mapping the Cosmicflows catalogs and their 3D and interactive visualization since 2010. Between 2014 and today, their catalog has increased from 8,000 to 30,000 galaxies and the map is growing. with the inclusion of a galaxy catalog measured by the SDSS collaboration [2].

With their latest Cosmicflows-4 catalog, they have just discovered an immense bubble of galaxies, a new fossil from the beginnings of our Universe, dating from the same time as the fossil radiation of the cosmic microwave background. This is the first time that a remnant of a wave, witness to the primordial universe, has been measured individually in the distribution of galaxies.

These results have just been published in The Astrophysical Journal.

This entity received the name Ho’oleilana, a term coming from the Hawaiian song of creation, the Kumulipo, which evokes the origin of the world.

This has the shape of a spherical shell where superclusters of already known galaxies are located (see figure above). This would be the first observation of the imprint left by a baryonic acoustic wave BAO (Baryon Acoustic Oscillation) [3]. These oscillations find their origin in the opposition between gravitation and plasma pressure, two antagonistic phenomena, exacerbated in regions of higher density, such as that identified at the Bouvier Supercluster in the center of Ho’oleilana. The measurement of its diameter is one billion light years, and its center is at a distance of 820 million light years from us. It has the size predicted for a BAO which would have grown with the expansion of the Universe since the moment the wave was frozen, at the time of the formation of the first hydrogen atoms 380,000 years after the Big Bang.

The implications of Ho’oleilana
By comparing their results with simulations, the researchers demonstrated that the shell structure identified as Ho’oleilana has less than a 1% probability of being an accident. Ho’oleilana has the geometric properties of a theoretically predicted acoustic baryon oscillation, including the prominence at its center of a rich supercluster, but it stands out more strongly than expected. Ho’oleilana is slightly larger than expected and the Hubble constant that can be deduced from its size is in agreement with measurements in the local Universe (supernovae, Cepheids, etc.) and in tension with the measurements made in the distant universe (Planck with the CMB, SDSS and quasars, etc.)

Future, more in-depth data, such as those from the Dark Energy Spectroscopic Instrument (DESI) or the 4MOST Hemisphere Survey, could make it possible to detect similar structures elsewhere in the nearby Universe. Researchers will use this data to study and confirm other details regarding Ho’oleilana, BAO, and the expansion rate of the Universe.