In September 2014, the discovery of Laniakea (“immeasurable paradise” or “immense celestial horizon” in Hawaiian) was announced, a supercluster (supercluster, in English) of galaxies comprising the Virgo supercluster, of which the Milky Way is a part of the Milky Way . We owe this to an international team of astronomers consisting of R. Brent Tully from the University of Hawaii at Mānoa, Hélène Courtois from the University of Lyon I, Yehuda Hoffman from the Hebrew University of Jerusalem and Daniel Pomarède from the CEA.
Richard Brent Tully had already made a name for himself by discovering, together with J. Richard Fisher, a connection for determining the distance of a spiral galaxy, an empirical method now commonly known as “Tully-Fisher’s law”. The American researcher, together with Daniel Pomarède and his Australian colleague Cullan Howlett, publishes a new article in the Astrophysical Journal today.
Nine years later, the three men announce the discovery of Ho’oleilana, a term from the Hawaiian song of creation Kumulipo, which evokes the origin of the world. It is one of the large structures of the observable universe. The universe collects clusters of galaxies. In this case, it is a kind of spherical shell about a billion light-years in diameter, in which there is a greater density of galaxy clusters, with the Bouvier Supercluster (Boötes in English) at its center, a distance of 820 million light-years The Milky Way.
A prediction half a century old
This is the first time we have directly detected a sphere of this type by measuring the distances and velocities of galaxies to map the observable cosmoscosm. However, it is an example of a structure that has already been discovered indirectly through scientific analysis of what we call statistical correlations in samples of galaxy populations and galaxy clusters. These spherical structures are manifestations of what are called baryonic acoustic waves or BAO (Baryon Acoustic Oscillations).
As Daniel Pomarède explains on Twitter, these galaxy bubbles were predicted almost 50 years ago by physics Nobel laureate James Peebles. In fact, however, they had been foreseen as early as the mid-1960s, a few years before Peebles, as part of what Andrei Sakharov called the cold Big Bang theory. Today we only refer to the work of the Nobel Prize winner with one of his colleagues because it was about the theory of the Big Bang. Big Bang, a definitive achievement of the standard model of cosmology, which “maybe” is not the case with dark matter, dark matter, or dark energy, dark energy.
BAOs are precisely one of the phenomena that cosmologists have been using for years to measure parameters of the standard model of cosmology, such as the Hubble-Lemaître constant, the curvature of the universe, and dark energy. Ho’oleilana can be considered a kind of Big Bang fossil fossil, although it somehow arose about 380,000 years later, at the time when the fossil radiationfossil radiation was emitted.
At this point the reader must have been wondering for some time what these BAOs are, these baryonic acoustic oscillations as they are sometimes called.
According to the Standard Model, during the Big Bang quantum fluctuations in the density of matter are generated, which rapidly collapse concentrations of dark matter. These concentrations attract normal baryonic matter, which consists of protons and neutrons. Unlike dark matter, however, these baryons are sensitive to the electromagnetic force, so the pressure of the photon gas that envelops all matter opposes the baryon matter. This creates spherical acoustic waves that propagate, much like the waves would travel around the impact points of raindrops in a pond and as shown in this animation. These waves overlap, but when the atoms form at the time of recombination, the radiation pressure is gone and the bubbles of matter freeze. Well, almost, because the expansion of space will then expand them. © CAASTRO
BAOs, sound waves traveling at half the speed of light
After the beginning of the observable universe and at least since the time of the original nucleosynthesis, a few minutes after the mythical Planck time, the universe is a mixture of baryons coupled with photons and photons already bathed in dark matter.
Fluctuations in dark matter density then produce spherical sound waves that travel at almost half the speed of light from areas of overdense dark matter. At the point of recombination, when the first neutral atoms appear 380,000 years after the Big Bang, the light decouples from the baryonic matter and the photon flux-driven front of these sound waves temporarily freezes.
As a result, zones of overdensity of normal matter form in the observable cosmos, forming shells (the diameter of which is determined by the speed of sound waves generated by acoustic oscillations). These areas will be privileged places for the formation of galaxies and their accumulation in the form of clusters. Later, the increasingly dominant presence of dark energy (which was not the case in the first billion years) will affect the growth rate of galaxy clusters.
Especially if we look at a large sample of galaxies on the surface of a sphere centered on the terrestrial observer (so at the same time in the history of the cosmos and at the same distance from us for each galaxy) and if we measure at the Distances between two pairs appear an excess of those pairs for a distance value related to that of the shells of material we talked about earlier, as shown in the video above.
An excellent presentation from BAO. To get a reasonably accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then click on “Subtitles” and finally “Auto-translate”. Select “French”. © PBS Spacetime
A key to studying dark energy
We then have a kind of standard meter whose intrinsic length is known at some point in cosmic history. By measuring the apparent value of this length to us, we can infer an absolute distance. And if we measure different spectral shifts, we can draw a curve connecting cosmological distance and spectral shift. Measurements of distance and time make it possible to estimate the expansion rates of space at a given point in time. By playing this game, but with the apparent luminosity of the luminosity standards – the SupernovaeSupernovae SN Ia – we discovered the accelerated expansion of space over the last few billion years.
However, depending on the cosmological model we are considering, with or without dark energy, the latter being a cosmological constant or not, we do not get the same curve. Likewise, the growth rate of galaxy clusters is not the same. Supernovae are therefore a second way to prove the existence of dark energy and to explore its nature.
The BAO and Ho’oleilana. © Daniel Pomarède
So much for Ho’oleilana theory, but for practice, this discovery is the latest result of a program called “Cosmicflows” started in the late 2000s by Hélène Courtois and Brent Tully. The first was based on a catalog of the velocities of 1,700 galaxies, but over the years we went from Cosmicflows-1 to Cosmicflows-4 with 30,000 galaxies. Daniel Pomarède has been “responsible for the mapping of the Cosmicflows catalogs and their 3D and interactive visualization since 2010”, according to the CEA’s press release, which ends with the following statements:
“Ho’oleilana has the geometric properties of an acoustic baryon oscillation predicted by theory, including highlighting a rich supercluster at its center, but it stands out more than expected. Ho’oleilana is slightly larger than expected and the Hubble constant derived from its size is consistent with measurements in the local Universe (supernovae, Cepheid-Skepheids…) and in conflict with measurements in the distant Universe (Planck with the CMB, SDSS and QuasareQuasare…).
Future, more detailed data, such as that from the Dark Energy Spectroscopic Instrument (DESI) or the 4MOST Hemisphere Survey, could make it possible to discover similar structures elsewhere in the nearby Universe. Researchers will use this data to study and confirm more details about Ho’oleilana, BAO, and the rate of expansion of the universe. »
Remember that in cosmology there is a tension between the two main measurements of the Hubble-Lemaître constant.
In February 2023, Daniel Pomarède, cosmographer at CEA, came to CentraleSupélec to talk about his research work on galaxy superclusters. © CosmiCS, the CentraleSupélec astronomy club