Late-time growth weakly affects the significance of high-redshift massive galaxies

Recent observations by the James Webb Space Telescope have revealed massive galaxies at very high redshift ($z\simeq 7-15$). The question of whether the existence of such galaxies is expected in the corresponding JWST surveys has received a lot of attention, though the answer straddles areas of cosmology and complex astrophysical details of high-redshift galaxy formation. The growth rate of density fluctuations determines the amplitude of overdensities that collapse to form galaxies. Late-time modifications of growth, combined with measurements at both $z\sim 1$ from large-scale structure and $z\sim 1000$ from the cosmic microwave background, affect the predictions for the abundance of first galaxies in the universe. In this paper, we point out that the late-time growth rate of structure affects the statistical significance of high-redshift, high-mass objects very weakly. Consequently, if the existence and abundance of these objects are confirmed to be unexpected, the variations in the late-time growth history are unlikely to explain these anomalies.

💡 Research Summary

The deployment of the James Webb Space Telescope (JWST) has introduced a profound tension into the standard $\Lambda$CDM cosmological paradigm. Observations of massive galaxies at very high redshifts ($z \simeq 7-15$) have revealed a population of objects that appear significantly more massive than what is predicted by current models of early galaxy formation. This “impossibly early galaxy” problem has sparked intense debate regarding whether our understanding of cosmic structure formation or the underlying cosmological parameters requires fundamental revision.

One prominent hypothesis proposed to resolve this tension is the modification of the late-time growth rate of cosmic structures. The logic follows that if the rate at which density fluctuations grow was higher than previously assumed, the probability of forming massive galaxies at such early epochs would increase, potentially reconciling JWST observations with theoretical predictions. This paper rigorously investigates whether such modifications to the late-time growth history can effectively explain the statistical significance of these high-redshift, high-mass galaxies.

The study utilizes a comprehensive approach by integrating two critical cosmological benchmarks: the Cosmic Microwave Background (CMB) data from $z \sim 1000$, which defines the initial amplitude of density fluctuations, and Large-Scale Structure (LSS) measurements from $z \sim 1$, which constrain the subsequent evolution of the universe. By analyzing the interplay between these two epochs, the authors demonstrate that any variation in the late-time growth rate that remains consistent with both CMB and LSS observations has a remarkably weak effect on the predicted abundance of galaxies at $z \simeq 7-15$.

The findings are significant: the statistical significance of these high-redshift massive galaxies is largely insensitive to changes in the late-time growth history. Consequently, if the existence of these galaxies is indeed confirmed to be a statistical anomaly, simply adjusting the late-time growth of structure will not suffice to resolve the discrepancy. This conclusion narrows the scope of potential solutions, suggesting that the origin of this anomaly likely lies elsewhere—either in the primordial physics of the early universe, such as the initial power spectrum of density fluctuations, or in the complex astrophysical processes of galaxy formation, such as an unexpectedly high star formation efficiency in the early universe. Ultimately, this paper directs the scientific community to look beyond late-time cosmic evolution and toward the fundamental physics of the early universe and high-redshift galaxy assembly.