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Emergent gravity puts galaxies' phantom halo at 170 solar masses per square parsec
Youngsub Yoon of Sejong University asked what dark halo an astronomer would infer in a universe with no dark matter. His answer, 170 solar masses per square parsec, falls inside the observed band of 90 to 220.
The Scientist · Science desk
What happened
- A researcher at Sejong University used Erik Verlinde's theory of gravity to predict the observed central surface density of dark matter, in a study published in Physics of the Dark Universe on Sept. 20.
- Observations show that quantity holding nearly constant between 90 and 220 solar masses per square parsec across galaxies whose luminosities differ by a factor of about 400,000.
- Youngsub Yoon calculated the central surface density that would be inferred if dark matter were assumed to exist, and got 170 solar masses per square parsec.
- Verlinde proposed emergent gravity in 2016 to account for gravitational phenomena on galactic scales without assuming that dark matter exists.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint The comparison band is a factor of 2.4 wide, so agreement inside it limits emergent gravity only loosely; a narrower observational determination of the scale is what would make the prediction a real constraint.
- exposure The prediction inherits the fate of the relation Yoon proposed in 2024 to patch Verlinde's missing superposition principle. If that relation is wrong, the 170 goes with it.
- decision Dark matter fits reach this quantity by fitting a halo; anyone comparing the two pictures now has a specific figure from the no-halo side to check against the same observations.
- precedent Yoon ties his result to Chae's wide-binary work, which asks the weak-gravity programme to be judged on more than galaxy rotation speeds.
The quantity here is a surface density. Verlinde-inspired emergent gravity contains no dark matter, so what Yoon calculated is the central surface density an astronomer would infer by fitting a halo anyway [3]. His paper calls that a phantom-halo scale [11]. The observed version of the quantity is the product of a central dark matter density and the size of the central region [2]; in emergent gravity both factors follow from the modified law, and there is no halo profile to adjust. Modifying gravity in weak fields is an old move, proposed by Mordehai Milgrom in 1983 [5].
Getting a number out took a repair first. Newtonian gravity lets you add the pull of several masses; Verlinde's does not obey that superposition principle [6]. Yoon flagged the problem in 2024 and proposed a relation to handle multiple masses, and this calculation runs on it [6].
"In my 2024 study, I pointed out a technical issue in Verlinde's theory of gravity and proposed a relation to address it. It is meaningful that, in this study, the relation led to a result consistent with the actual observational value," Yoon said [7].
The observed scale runs from 90 to 220 solar masses per square parsec, a factor of 2.4 from bottom to top [2][12]. The midpoint of that band is 155 and its geometric centre about 141, so the prediction lands in the upper half, roughly 1.2 times the geometric centre [13][14]. A single number falling anywhere inside a window that wide is consistent with the data without being tightly constrained by it. What makes the regularity striking is the other figure in the same sample: luminosity varies by about 400,000 while the surface density varies by 2.4 [2][12]. The published account does not include an uncertainty on the 170 or the number of galaxies behind the observed band.
The result does not show whether the same relation survives contact with other galactic-scale tests. It is a single-author theory paper [11], and the relation that makes Verlinde's gravity computable for many masses is that same author's [6].
Yoon added, "Professor Kyu-Hyun Chae of Sejong University has reported results from observations of wide binary stars that deviate from the predictions of Newtonian gravity in the very weak-gravity regime. I hope that this study, together with other research reexamining conventional theories of gravity in the weak-gravity regime, will contribute to testing new theories of gravity" [9].
What to watch
- Whether an independent group reproduces 170 solar masses per square parsec from Verlinde's theory, with or without Yoon's 2024 relation.
- Any tighter observational determination of the central surface density scale that narrows the 90 to 220 range.
- Whether Chae's reported wide-binary deviations from Newtonian gravity hold up in independent samples.