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Astrophysicists have been trying for decades to understand the formation of cosmological objects and phenomena in the universe. Previous theoretical studies suggest that quantum fluctuations in the early universe, known as primordial quantum scattering, may have given rise to so-called primordial black holes.
In an article published in Physical Review Letters, researchers from the Niels Bohr Institute, Universidad Autnoma de Madrid and CNRS Universit de Paris recently explored the possibility that these fluctuations could also influence the creation of even larger cosmological structures, such as heavy galaxy clusters like “El Gordo” . El Gordo is the largest cluster of distant galaxies ever observed using existing telescopes, which was first captured more than 10 years ago.
“The question of how structure formed in the universe may be one of the oldest, but since the early 1980s it has acquired a new dimension,” Jose Mara Ezquiaga, one of the researchers who conducted the study. . “At the time, scientists realized the incredible connection between the smallest and largest scales, where quantum fluctuations in the early universe are stretched by cosmic inflation to seed the formation of galaxies and large-scale structures ladder in the universe”.
As physicists began to learn more about the connections between the early and late universes, 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 this area, sparking new theoretical studies focused on the primordial origin of black holes.
“Juan, Vincent and I were studying the formation of primordial black holes in the early universe,” Ezquiaga said. “Our key contribution was to understand that when quantum fluctuations dominate the dynamics of cosmic inflation, this leads to a spectrum of non-Gaussian density fluctuations, with heavy exponential tails. In other words, quantum diffusion makes it easier to generate large fluctuations that would collapse into a primordial black hole”.
After studying primordial black holes in the early universe, Ezquiaga and his colleagues Vincent Vennin and Juan Garcia-Bellido began to wonder whether the same mechanism underlying their formation, namely an enhanced non-Gaussian tail in the distribution of primordial perturbations , could also lead to the formation of other very large cosmological structures. In their recent work, they specifically explored whether this mechanism affects the collapse of larger objects such as dark matter halos, which will later host galaxies and clusters of galaxies.
“The formation of larger objects early in the universe’s history could help ease some tensions between the observations and our standard cosmological model,” Ezquiaga explained. “For example, massive clusters like El Gordo may look like outliers under standard assumptions, while quantum diffusion makes them look natural.”
As part of their recent study, Ezquiaga and his colleagues computed the mass function of the halo and the abundance of clusters 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 by depleting dark matter halos.
“Because gravity is always attractive, inhomogeneities will only grow as excessive densities attract mass to their surroundings and lower densities become more emptier,” Ezquiaga said. “The question is whether the inhomogeneities in the early universe are large and frequent enough to lead to the gravitational collapse needed to explain the observed structures in the cosmos. Given an initial distribution of perturbations just hit ‘play’ and let the system gravitationally evolve, in our case, we had a previous understanding of the distribution of initial perturbations including quantum scattering, so our task in this work was to properly parameterize this spectrum 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 not only underlie the formation of medium-sized galaxies and primordial black holes, but also that of massive galaxy clusters, such as the fascinating ‘El Gordo’ and Pandora clusters. . This would mean that current observations of galaxy clusters could be explained using existing theories, without the need to incorporate new physics into the Standard Model.
“The other very interesting result of our work is that it predicts unique signatures that could be tested in the near future,” said Ezquiaga. “In particular, we show that quantum scattering not only makes the initial formation of heavy clusters easier, but also that the amount of substructure should be less than expected.”
The simultaneous enhancement of massive cosmological structures and the depletion of substructures (i.e. halos) is not predicted by other theoretical models. However, this potential theoretical explanation for the formation of large galaxy clusters appears to align with recent cosmological observations and could potentially address other shortcomings of the Standard Model as well.
In their forthcoming studies, Ezquiaga and his colleagues would like to paint a more complete picture of the universe’s structures and their formation. This could eventually also help fully probe predictions of quantum diffusion.
“The next step for us is to fully test the predictions of this model against the observations,” Ezquiaga added. “Fortunately, there are many new observations we can use. In particular, the very recent observations from the James Webb Space Telescope seem to indicate that there are many more massive, high-redshift galaxies, something that naturally aligns with our predictions, but we are waiting for astronomers fully understand their systematics and confirm this “unexpected” population.Other observations that might be of interest to us are numerical counts of dwarf galaxies with galaxy surveys such as the Dark Energy Survey and constraints on subhalos due to strong lensing.”
Jose Mara Ezquiaga et al, Massive Galaxy Clusters Like El Gordo Hint at Primordial Quantum Diffusion, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.121003
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