How did the first galaxies form in the cosmos? And what factors determined their further development? So far, astronomers using telescopes can only observe snapshots of this cosmic evolution – and some early phenomena remain mysterious. A new cosmological simulation now sheds more light on the darkness. Because it reproduces the development of our universe from the Big Bang to today with greater resolution and more comprehensiveness than before. The resulting structures, from the cosmic network to details of individual galaxies, agree well with astronomical observations and the laws of the cosmological Standard Model. However, even these simulations cannot explain some puzzling phenomena.
When the first stars and galaxies formed in the universe, the cosmic dawn broke: the energetic radiation from the young stars brought light into the cosmos and ionized the previously neutral hydrogen. This epoch of reionization created important conditions for the development and growth of the first galaxies. But how exactly galaxies formed back then and what factors led to the Milky Way and other galaxies growing has only been partially clarified. The cosmological standard model does provide the “guard rails” for this development. But some observations from the James Webb Telescope from shortly after the Big Bang raise questions. Some early galaxies appear surprisingly massive, and the images also show phenomena such as “red dots”, which have so far been difficult to explain.
COLIBRE: cold gases and dust included
In order to clarify these and other questions about cosmic development, cosmological simulations play an important role. With their help, astronomers can test whether the parameters specified by the Standard Model are sufficient to reconstruct the path of the universe from the Big Bang to the present day – or whether as yet unrecognized, additional influences could have been at work. The problem, however, is that high-resolution simulations that take all influencing factors into account require enormous computing power. For a long time, they were therefore only able to recreate small sections of the universe and its development. However, simulations that cover larger spatial sections were coarser in order to limit the computing time. In addition, they often ignored certain factors that were difficult to model. So they only simulated hot gases and not the more complex interactions of cold gases in interstellar bar clouds. “Most gas in galaxies is cold and dusty, but previous large simulations had to ignore this,” explains project leader Joop Schaye from Leiden University.
But that has now changed: “With COLIBRE, we are now bringing these essential components into the picture,” explains Schaye. COLIBRE is a cosmological hydrodynamic simulation that, thanks to recent advances in algorithms and supercomputers, allows a more comprehensive reconstruction of the chemical and physical conditions during galaxy formation and evolution. The underlying models were developed by an international research team over the last ten years. This allows COLIBRE to understand cold gases and dust grains and their influence on events. The simulation reproduces the processes in three different volumes of the cosmos and resolutions and includes a total of around 20 times more particles than previous simulations of comparable size, as Schaye and his colleagues explain. The computational effort was significant: the largest simulation required 72 million CPU hours.
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COLIBER Simulations)
Agreement with astronomical observations
The first results are now available. “It is exciting to see how galaxies are created in the computer that are hardly distinguishable from their real counterparts,” says co-author Carlos Frenk from Durham University. The COLIBRE simulations show a cosmic evolution that fits well with the observational data, for example in terms of the number, sizes, luminosity or colors of the galaxies formed. “I even jokingly asked my colleagues: Which galaxy catalog do these images come from?” says Frenk. “COLIBRE demonstrates that models match what we see more closely when we also model the underlying physical processes more realistically.” This is particularly true for the integration of cold gas, dust and the outflows caused by stars and black holes.
The new COLIBRE simulation also demonstrates that the galaxies and phenomena observed in the cosmos can be explained by the standard cosmological model – including some of the early, supposedly too massive galaxies. “Some results from the Webb telescope seemed to contradict the standard model,” says co-author Evgenii Chaikin from Leiden University. But they could also be reproduced using the COLIBRE simulation. The new simulation also proves to be a helpful tool for testing hypotheses and theories, as the researchers explain.
However, even the COLIBRE simulations cannot explain all the mysterious phenomena of the early cosmos. For example, the “Little Red Dots” detected by the Webb telescope – red points of light from a few hundred million years after the Big Bang – do not appear in the simulation. What these objects are is still unclear. However, astronomers suspect that they are the precursors of later supermassive black holes. Schaye and his team hope that COLIBRE can also solve this mystery. Because the simulations with the highest resolution are still running. Their calculation will not be completed until autumn 2026.
Source: Joop Schaye (Leiden University) et al., Monthly Notices of the Royal Astronomical Society, doi: 10.1093/mnras/stag375