The James Webb Space Telescope (JWST) has been making surprising discoveries since its launch, with the detection of galaxies that seem to defy our current understanding of the early universe. These galaxies, found at redshifts above 10, are much brighter and more numerous than predicted by our current models, leaving astronomers puzzled. The question of what could be causing this surplus has sparked a variety of hypotheses, including the possibility of cosmic strings, which are one-dimensional defects in spacetime that could have formed during the universe's early phase transitions.
The authors of this paper propose that cosmic strings, which are a generic prediction of many Grand Unified Theories, could be the answer to the JWST's observations. These strings, if they exist, can seed dark matter halos at any epoch, including the very early universe, where they would have a significant impact on the formation of galaxies. The key to this idea is the UV luminosity function (UVLF), which is a count of galaxies at different brightness levels over time. JWST has pushed the boundaries of UVLF measurements to redshift 17, revealing a surplus of bright galaxies that standard models struggle to explain.
The challenge lies in the fact that any cosmological explanation must not only account for the observed surplus but also be consistent with previous measurements. Enhancing the matter power spectrum to match JWST's findings would conflict with Hubble's observations at lower redshifts. Therefore, the solution must be subtle and gradual, allowing early help without disrupting later cosmic structures. This is where cosmic strings come into play, as they can provide the necessary boost in structure formation at early times while fading away as the universe evolves.
To test this hypothesis, the authors developed a semi-analytic code called Zeus21, which can predict UVLFs across a wide range of assumptions in milliseconds. By comparing these predictions with the observed UVLFs, they found that cosmic strings can account for the surplus of bright galaxies at redshifts 4 to 17 without requiring an abrupt jump in star-formation efficiency or extreme stochasticity. At lower redshifts, where Hubble measures UVLFs, the string contribution is negligible, and the models with and without strings are nearly indistinguishable. However, at higher redshifts, the string-free model fails to explain the observed abundance of bright galaxies, while the string model continues to align with the data.
This result has significant implications for our understanding of galaxy formation. It suggests that the extra galaxies detected by JWST do not necessarily have to be strange or behave differently from later galaxies. Instead, they could be a result of a few additional places for galaxy formation during the early universe. This idea is supported by the fact that UVLFs are highly sensitive to cosmic strings, allowing for a new upper limit on the string tension, Gμ ≲ 10⁻⁸, which is a significant improvement over previous constraints.
However, the authors are cautious about their findings, acknowledging that the limit depends on the chosen model and priors. They also highlight the uncertainty in early galaxy star-formation efficiency, particularly in JWST samples. Additionally, they address a potential wrinkle where accounting for string loop velocities weakens the bound, though it still improves on previous observations.
Looking forward, the authors suggest that the key to resolving this degeneracy lies in studying galaxy clustering. Galaxies born in massive halos are expected to cluster more strongly, and measurements of this clustering are becoming feasible at redshifts 10 and beyond. By examining how galaxies cluster, astronomers may be able to distinguish between the effects of cosmic strings and more efficient star formation, providing a more comprehensive understanding of the early universe and the role of these mysterious strings.
