Astrophysicists have been trying to understand the composition of cosmic objects and phenomena in the universe for decades. Previous theoretical studies suggest that quantum fluctuations in the early universe, known as primordial quantum scattering, could have given rise to so-called primordial black holes.
In a paper published in Physical review lettersRecently, researchers at the Niels Bohr Institute, the Autonomous University of Madrid and the University of Paris CNRS have explored the possibility that these fluctuations may also influence the formation of larger cosmic structures, such as heavy galaxy clusters such as El Gordo. El Gordo is the largest distant galaxy cluster ever observed with current telescopes, captured for the first time more than 10 years ago.
“The question of how structure forms in the universe may be one of the oldest, but since the early 1980s, it has acquired a new dimension,” José María Izquiaga, one of the researchers who conducted the study, told Phys.org. . “It was then that scientists realized the amazing relationship between the smallest and largest scales, in which quantum fluctuations in the early universe are stretched by cosmic inflation to seed the formation of galaxies and large-scale structures in the universe.”
After physicists began to learn more about the connections between the early and late universe, the idea that black holes could form in the early universe began to emerge. In 2015, the first observations of black hole mergers via gravitational waves renewed interest in the field, sparking new theoretical studies focusing on the primordial origin of black holes.
“Juan, Vincent, and I were investigating the formation of primordial black holes in the early universe,” said Izquiaga. “Our primary contribution was the realization that when quantum fluctuations dominate the dynamics of cosmological inflation, it leads to a spectrum of non-Gaussian intensity fluctuations, with heavy exponential tails. In other words, quantum propagation facilitates the generation of large fluctuations that would collapse into a primordial black hole.”
After studying primordial black holes in the early universe, Izquiaga and his colleagues Vincent Vennin and Juan García Pelido began to wonder if the same mechanism underlying their formation, namely the enhanced non-Gaussian tail in the distribution of primordial turbulence, could also lead to the formation of other very large cosmic structures. In their latest work, they specifically explored the possibility of this mechanism affecting the collapse of larger objects such as dark matter halos, which would later host galaxies and galaxy clusters.
“The formation of larger objects early in the history of the universe can help mitigate some of the tensions between observations and our standard cosmological model,” explained Izquiaga. “For example, under standard assumptions, massive clusters like El Gordo might appear to be external, while quantum scattering would make them normal.”
As part of their latest study, Ezquiaga and colleagues calculated the halo mass function and mass abundance as a function of redshift in the presence of heavy exponential tails. This allowed them to determine whether quantum scattering could increase the number of large galaxy clusters, depleting dark matter halos.
“Since gravity is always attractive,” Izquiaga said, “the inhomogeneity will only grow because more densities will attract mass to their surroundings and they will become more hollow under densities.” “The question is whether the inhomogeneities in the early universe were large and frequent enough to lead to the gravitational collapse necessary to explain the observed structures in the universe. Given the initial distribution of perturbations, one need only press ‘play’ and allow the system to evolve in terms of gravity, in our case. , we had previous understanding to the distribution of elemental perturbations when quantitative diffusivity is included, so our task in this work was to quantify this spectrum in an appropriate manner and analyze the results for the number of massive clusters as a function of redshift.”
The researchers’ paper suggests that quantum fluctuations in the early universe may underlie not only the formation of mesoscale galaxies and primordial black holes, but also of massive galaxy clusters, such as the fantastic “El Gordo” and Pandora clusters. This means that existing observations of galaxy clusters can be explained using existing theories, without the need to incorporate new physics into the Standard Model.
“Another very exciting finding of our work is that it predicts unique signatures that could be tested in the near future,” Izquiaga said. “In particular, we show that not only does quantum diffusion make heavy clusters easier to form early on, but also that the amount of superstructure should be lower than expected.”
The simultaneous enhancement of massive cosmic structures and depletion of substructures (i.e., halos) is not predicted by other theoretical models. However, this possible theoretical explanation for the formation of large galaxy clusters appears to be consistent with recent cosmological observations and could also solve other shortcomings of the Standard Model.
In their next studies, Izquiaga and his colleagues would like to paint a more complete picture of the structures in the universe and their composition. This could eventually also help fully investigate predictions of quantum diffusivity.
“Next for us is a full test of this model’s predictions against observations,” Izquiaga added. “Fortunately, there are many new observations we can use. In particular, very recent observations by the James Webb Space Telescope seem to indicate that there are many massive, high redshift galaxies, and some things are naturally consistent with our expectations, but we are waiting to be understood.” Astronomers are quite systematic and confirm this “unexpected” group. Other observations that may be interesting to us are the number of dwarf galaxies with surveys of galaxies such as the Dark Energy Survey and the constraints of the subhalos from the powerful lensing.”
José María Izquiaga et al., Massive galaxy clusters like El Gordo hint at primordial quantum propagation, Physical review letters (2023). DOI: 10.1103/PhysRevLett.130.121003
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