UV Luminosity Functions from HST and JWST: A Possible Resolution to the High-Redshift Galaxy Abundance Puzzle and Implications for Cosmic Strings

TL;DR

This paper tests whether cosmic strings can resolve the excess of bright high-redshift galaxies observed by JWST and earlier HST measurements by embedding a cosmic-string seeded halo mass function into the Zeus21 semi-analytic model. Through two inference frameworks—Conservative (flexible, redshift-specific SFEs) and Fiducial (smooth, redshift-evolving SFEs)—the authors explore degeneracies between star-formation physics and cosmic-string phenomenology and derive new upper bounds on the string tension $G\mu$, improving upon CMB constraints. In the fiducial scenario, a relative reduction of parameter degeneracies allows a joint HST+JWST fit with a possible peak near $G\mu \approx 5\times10^{-9}$ under certain priors, though this is sensitive to parameterization; overall UVLFs constrain $G\mu$ to be $\lesssim \mathcal{O}(10^{-8})$ with current data. The work highlights UVLFs as a promising probe of cosmic-string physics, while noting the need for better high-redshift star-formation efficiency modeling and more robust halo mass function calibrations to draw definitive conclusions. It also outlines strategies, such as exploiting galaxy clustering, to break degeneracies and tighten future constraints in concert with other cosmological probes.

Abstract

Recent observations of high redshift galaxies by the James Webb Space Telescope suggest the presence of a bright population of galaxies that is more abundant than predicted by most galaxy formation models. These observations have led to a rethinking of these models, and numerous astrophysical and cosmological solutions have been proposed, including cosmic strings, topological defects that may be remnants of a specific phase transition in the very early moments of the Universe. In this paper, we integrate cosmic strings, a source of nonlinear and non-Gaussian perturbations, into the semi analytical code Zeus21, allowing us to efficiently predict the ultraviolet luminosity function (UVLF). We conduct a precise study of parameter degeneracies between star-formation astrophysics and cosmic-string phenomenology. Our results suggest that cosmic strings can boost the early-galaxy abundance enough to explain the measured UVLFs from the James Webb and Hubble Space Telescopes from redshift z = 4 to z = 17 without modifying the star-formation physics. In addition, we set a new upper bound on the string tension of $Gμ\lessapprox 10^{-8}$ ($95\%$ credibility), improving upon previous limits from the cosmic microwave background. Although with current data there is some level of model and prior dependence to this limit, it suggests that UVLFs are a promising avenue for future observational constraints on cosmic-string physics.

Figures (10)

• Figure 1: Comparison between predictions of UVLFs with (dashed line) and without (solid line) the presence of cosmic strings with a string tension corresponding to the $95\%$ credibility limits (CL) from Planck 2014. HST observations from Ref. Bouwens_2021 are shown in comparison in black with their error bar corresponding to a $68\%$ confidence interval. In both cases, the astrophysical parameters are standard for galaxy formation models and are the same in both predictions; only the presence of strings changes. These results highlight the very high sensitivity of UVLFs to the presence of cosmic strings for string tensions close to the limits obtained by Planck data. This sensitivity is greater at higher redshifts.
• Figure 2: Comparison of the cosmic string loops halo mass function for different string tensions and the Sheth $\&$ Tormen (ST) halo mass function as a function of redshift. The factor $N$ in Equation \ref{['eq:halomassloop']} is set equal to $N=570$. The vertical axis corresponds to the halo mass function and the horizontal axis to the mass range. As shown in Section \ref{['sec:hmffromstrings']}, the cosmic string halo mass function becomes subdominant for most string tensions compared to the ST halo mass function. As redshift increases, the string halo mass function gradually tends to dominate the dark matter haloes high mass regime over the ST halo mass function. This figure highlights that cosmic strings can explain certain anomalies by forming dark matter haloes at high redshift while remaining consistent with observations at lower redshift when their contributions become subdominant.
• Figure 3: Comparison of the impact of the five star formation parameters on final UVLF at redshift $z=8$. The first four parameters of the SFE and the last $G\mu$ related to cosmic strings (and $M_c$ in units of $M_{\odot}$). Each parameter is varied individually while keeping all others constant. The factor $N$ in Equation \ref{['eq:halomassloop']} is set equal to $N=570$. The impact of cosmic strings tends to diminish and become completely negligible as $G\mu$ decreases.
• Figure 4: Marginalized two-dimensional posterior distribution of $\epsilon_\star$ assuming the conservative scenario at redshift $z=7$ and assuming log-uniform prior on $G\mu$. The color ranges from darkest to lightest correspond to the regions containing $68\%$ percent, $95\%$ percent, and $99\%$ percent of the samples. These results suggest a strong degeneracy between the SFE and the cosmic string tension. Without more information on the SFE, this degeneracy remains unbroken and results in some sensitivity to whether one assumes a log-uniform (left) or uniform (right) prior on $\epsilon_\star$.
• Figure 5: Redshift-by-redshift marginal posterior distributions of $\log_{10}[(N/570)^{2/3} G\mu]$ for the conservative scenario with HST UVLFs assuming a log-uniform prior and a prior range on $G\mu$ of $[10^{-11},10^{-6}]$ at redshifts $z=4$ to $z=8$ (top); and with JWST UVLFs for redshifts $z=9$ to $z=17$ (bottom). The vertical lines indicate the upper limits of $\log_{10}[(N/570)^{2/3} G\mu]$ in which $95\%$ percent of the samples are contained contain. The most competitive bound is at redshift 9 and with $N=570$ gives $G\mu\le 1.24\times10^{-8}$. The bounds at $z=11$ and $z=14$ are very similar.