The James Webb Space Telescope's view of the infant universe has delivered genuine surprises: bright galaxies, rapid star formation, and evidence of accreting black holes only a few hundred million years after the Big Bang. These findings have ignited debates about whether our best cosmological theories can survive new data. In this article, astrophysicist Sandro Tacchella argues the crisis lies in the simplified assumptions long used to model early galaxy and black hole growth. The tension exposed by the JWST, he suggests, is a productive one—forcing us to rethink how complexity emerged in the early universe. However, we shouldn't completely throw out our theories just yet!

In its first four years of operation, the James Webb Space Telescope (JWST) has transformed our view of the early universe. With unprecedented infrared sensitivity and spectroscopic capability, it allows us to observe galaxies and black holes as they were when the cosmos was a few hundred million years old. These observations have revealed unexpected richness and complexity: bright galaxies, vigorous star formation, and evidence for actively accreting black holes at times earlier than many theoretical models anticipated. But while these findings are surprising in detail, they do not necessarily indicate that cosmology itself is in crisis. Rather, JWST is exposing the limits of our models of galaxy formation and giving us new clues about the physics of galactic birth, growth, and the wider cosmic context in which those processes unfold.

The foundations of modern cosmology

Our understanding of the cosmos rests on a conceptual framework that has been extraordinarily successful at describing the large-scale evolution of the universe. General relativity links the geometry of spacetime to its energy content, and when combined with the cosmological principle—the assumption that the universe is homogeneous and isotropic on large scales—it leads to a simple set of equations governing cosmic expansion. The resulting picture is the so-called ΛCDM model, where Λ represents dark energy, the mysterious driver of accelerated expansion, and CDM denotes cold dark matter, an invisible form of matter that seeds structure formation.

ΛCDM is supported by a wealth of independent observations. The detailed pattern of fluctuations in the cosmic microwave background (CMB) radiation, emitted when the universe was just 380,000 years old, provides a precise snapshot of primordial density variations. Measurements of the large-scale distribution of galaxies, gravitational lensing, and the abundances of light elements produced in the early universe all align remarkably well with the ΛCDM prediction.

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JSWT's near-infrared imaging and spectroscopy have opened a window into the first few hundred million years of cosmic history.

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These successes demonstrate that the architecture of ΛCDM is sound, even if its foundations remain incomplete. ΛCDM should be understood as a phenomenological model: it encodes, with remarkable accuracy, how the universe behaves on large scales, without yet specifying the fundamental nature of dark matter or dark energy. This is not a weakness but a familiar stage in scientific progress. History offers many examples in which effective descriptions preceded deeper understanding. When the microphysics of dark matter and dark energy are eventually uncovered, ΛCDM can be upgraded into a fully physical theory rather than replaced. A true crisis would arise only if observations forced us to abandon its core predictions about cosmic expansion or structure formation; for example through a contradiction between the cosmic microwave background and the late-time universe that could not be resolved by any plausible extension of the model. That situation has not emerged.