The standard model for how galaxies formed in the early universe predicted that the James Webb Space Telescope (JWST) would see dim signals from small, primitive galaxies. But data are not confirming the popular hypothesis that invisible dark matter helped the earliest stars and galaxies clump together.
Instead, the oldest galaxies are large and bright, in agreement with an alternate theory of gravity, according to new research from Case Western Reserve University published November 12 in The Astrophysical Journal. The results challenge astronomers’ understanding of the early universe.
“What the theory of dark matter predicted is not what we see,” said Case Western Reserve astrophysicist Stacy McGaugh, whose paper describes structure formation in the early universe.
McGaugh, professor and director of astronomy at Case Western Reserve, said instead of dark matter, modified gravity might have played a role. He says a theory known as MOND, for Modified Newtonian Dynamics, predicted in 1998 that structure formation in the early universe would have happened very quickly—much faster than the theory of Cold Dark Matter, known as lambda-CDM, predicted.
JWST was designed to answer some of the biggest questions in the universe, such as how and when did stars and galaxies form? Until it was launched in 2021, no telescope was able to see that deeply into the universe and far back in time.
Lambda-CDM predicts that galaxies were formed by gradual accretion of matter from small to larger structures, due to the extra gravity provided by the mass of dark matter.
“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 said.
The small pieces assembled in larger and larger structures until galaxies formed. JWST should be able to see these small galaxy precursors as dim light.
“The expectation was that every big galaxy we see in the nearby universe would have started from these itty-bitty pieces,” he said.
But even at higher and higher redshift—looking earlier and earlier into the evolution of the universe—the signals are larger and brighter than expected.
MOND predicted that the mass that becomes a galaxy assembled rapidly and initially expands outward with the rest of the universe. The stronger force of gravity slows, then reverses, the expansion, and the material collapses on itself to form a galaxy. In this theory, there is no dark matter at all.
The large and bright structures seen by JWST very early in the universe were predicted by MOND over a quarter century ago, McGaugh said. He co-authored the paper with former Case Western Reserve postdoctoral researcher Federico Lelli, now at INAF—Arcetri Astrophysical Observatory in Italy, and former graduate student Jay Franck. The fourth co-author is James Schombert from the University of Oregon.
“The bottom line is, ‘I told you so,'” McGaugh said. “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.” He added that finding a theory compatible with both MOND and General Relativity is still a great challenge.
Reference: Stacy S. McGaugh et al, Accelerated Structure Formation: the Early Emergence of Massive Galaxies and Clusters of Galaxies, The Astrophysical Journal (2024). DOI 10.3847/1538-4357/ad834d, On arXiv: DOI: 10.48550/arxiv.2406.17930
