The cosmos is not turning out quite the way astronomers had imagined. When the James Webb Space Telescope (JWST) began peering back through time, into the depths of the universe, scientists anticipated it would detect faint, fledgling galaxies — small and dim signals from the earliest moments of cosmic history.
But instead of hazy glimmers of primordial galaxies slowly coming together, JWST has delivered an astonishing surprise: ancient galaxies that are large, bright, and fully formed. This discovery is now challenging one of the most widely accepted theories in astrophysics — one that relies on invisible dark matter to explain how galaxies first emerged in the young universe.
A Bold Challenge to Dark Matter
The recent findings, published by a team from Case Western Reserve University in The Astrophysical Journal, cast doubt on the conventional model of galaxy formation, known as Lambda Cold Dark Matter (λ-CDM). This model has been the backbone of cosmology for decades. It posits that the gravitational pull of dark matter allowed early stars and galaxies to coalesce out of a smooth, primordial soup of ordinary matter.
“What the theory of dark matter predicted is not what we see,” said Stacy McGaugh, an astrophysicist and professor at Case Western Reserve University. McGaugh, a longtime skeptic of the dark matter hypothesis, believes the latest data might lend support to a much less popular idea: that gravity itself may work differently in the farthest reaches of space and time.
Instead of dark matter guiding the formation of galaxies, McGaugh suggests a theory known as Modified Newtonian Dynamics, or MOND. According to MOND, galaxies could have formed more rapidly under a different gravitational framework — one that doesn’t require the existence of dark matter at all. The evidence from JWST, McGaugh argues, fits more comfortably with this theory than with the established model.
A Quarter-Century-Old Prediction Vindicated?
The MOND theory was first proposed in 1983 by the Israeli physicist Mordehai Milgrom, but it never gained widespread acceptance. In the late 1990s, McGaugh and his colleagues predicted that if MOND were correct, the universe’s first galaxies would appear large and bright in their earliest stages. Now, over 25 years later, that prediction seems to align with the data streaming back from JWST.
“Astronomers invented dark matter to explain how you get from a very smooth early universe to big galaxies with lots of empty space between them that we see today,” McGaugh explained. In the dark matter model, galaxies formed gradually. Bit by bit, small structures would merge to become larger ones. By that logic, JWST should have spotted faint traces of these early building blocks. But that’s not what it found.
“The expectation was that every big galaxy we see in the nearby universe would have started from these itty-bitty pieces,” McGaugh said. Yet even at higher redshifts — meaning deeper into the past — JWST continues to detect fully-formed galaxies that defy the expectations set by the dark matter model.
In the MOND framework, however, galaxies formed in a rapid burst, initially expanding with the universe’s growth, only to collapse inward under a modified force of gravity. McGaugh’s recent study, co-authored with Federico Lelli, now at INAF Arcetri Astrophysical Observatory, and James Schombert from the University of Oregon, supports this interpretation. “The large and bright structures seen by JWST very early in the universe were predicted by MOND over a quarter century ago,” McGaugh emphasized.
What’s Next for Our Understanding of the Universe?
The implications of this research are profound. If MOND continues to align with observational data, it could reshape our understanding of fundamental physics. Yet McGaugh remains cautious. “Finding a theory compatible with both MOND and General Relativity is still a great challenge,” he noted.
Indeed, this is what makes modern cosmology so challenging. The Standard Model might not explain an observation, so scientists turn to a different model. They find this model neatly fits the anomalous observation, only to discover that it fails in other instances. So, a complete model of how the universe works is still remarkably far out of reach.
The revelations from JWST come at a time when cosmology is already grappling with mysteries like dark energy and the rapid expansion of the universe. As more data from the telescope arrives, scientists may find themselves reevaluating not just how galaxies form, but also the very nature of gravity and the forces that shaped our universe. In what way exactly remains to be seen.
McGaugh’s excitement is tempered by a sense of vindication. “The bottom line is, ‘I told you so,’” he said jokingly. “I was raised to think that saying that was rude, but that’s the whole point of the scientific method: Make predictions and then check which come true.”
For now, astronomers are left with more questions than answers, as they seek to understand how the oldest galaxies in the cosmos came to shine so brightly.
