The universe has undergone significant changes. Examining the contrasts between the universe as we perceive it today and its origin nearly 14 billion years ago is a crucial area of study for astrophysicists and cosmologists. Their focus is primarily on the first billion years following the Big Bang, when the first stars and galaxies began to emerge, marking the dawn of the universe. This was the initial phase when celestial objects began to emit light on their own rather than merely reflecting the remnants of the Big Bang, and it was also the first occurrence when elements heavier than helium started forming via nuclear fusion in stars.

In a recent study, a group of scientists utilized computer simulations to explore what star clusters looked like during the dawn of the universe. Their objective was to create models of star and galaxy formation that could be confirmed by new observations made by the JWST. This approach will enhance astronomers’ understanding of galaxy formation in the early universe, particularly the influence of galaxies on dark matter, which remains enigmatic, during the birth of the first stars from cosmic dust.

The research employed a cosmological simulation code called Arepo to recreate the dawn of the universe within a three-dimensional box measuring 1.9 megaparsecs on each side. This size converts to 60 quintillion kilometers or 40 quintillion miles. Within this box, the simulation contained 450 million particles representing early elemental matter, including hydrogen, helium, various isotopes, ions, and molecules that formed together. Additionally, it incorporated particles simulating known dark matter, which is affected by gravity but does not interact with other forces. When these aggregates of particles coalesced and surpassed a specific mass threshold known as jeans mass, the code indicated the formation of a star.

The team aimed to identify where the simulated stars and particles formed structures like star clusters, galaxies, and galaxy clusters. They implemented a method to group particles that were sufficiently adjacent to be considered connected, utilizing a friend of friends algorithm. By executing multiple iterations of this algorithm in the simulated universe—some focused on dark matter and others on ordinary matter such as stars, dust, and gas—the researchers sought to ascertain the arrangement of matter in the early universe.

The resulting simulated clusters were found to have dimensions comparable to actual clusters observed by astronomers in the early universe. However, no real clusters with metal-rich stars matching those in the simulations have yet been identified. Furthermore, the number of stars present in the simulated cluster was consistent with previous observations of distant star clusters recorded by the JWST. Many simulated star clusters were unstable, indicating they were not fully bound by their internal gravity. The team also found that as stable star clusters began merging into larger structures, such as galaxies, they became unstable once more.

An unexpected finding emerged from the study. The friend-of-a-friend algorithm produced varying results when assessing dark matter versus ordinary matter. The discrepancy reached up to 50%, implying that an algorithm targeting dark matter might detect only half the objects identified by an algorithm focused on regular matter. This variance depended on the mass of the identified star clusters or galaxies, particularly evident for objects within a moderate size range of 10,000 to 100,000 solar masses and very low masses around 1,000 solar masses.

The researchers could not ascertain the reasons behind this phenomenon, suggesting their simulations might be overly simplistic for accurately representing all conditions present during the universe’s dawn. Notably, they mentioned the absence of newly formed stars ejecting materials into space in their simulations. Consequently, they proposed treating their discovery as an upper limit on the frequency of star-like and, by extension, star-containing objects forming in the early universe. Their results might illustrate instances in nature where star formation occurs extremely efficiently, yet sorting out the roles of all involved processes remains necessary.

The conclusion drawn was that cosmic dawn clusters could have coalesced to create the foundations of modern galaxies or possibly evolved into the luminous cores of later galaxies. Additionally, the simulated clusters appeared to be strong candidates for forming medium-sized black holes, the remnants of which may be detectable with deep-space telescopes.

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