[비즈한국] Gravity is everywhere. It is the most familiar force we know. Yet, at the same time, it is the most suspicious and incomprehensible one. When looking at the universe on a large scale, gravity cannot be explained solely by the matter we are familiar with. Stars at the edges of galaxies rotate far too quickly than expected. The same applies to galaxies within galaxy clusters. If they were rotating at such high speeds, the stars and galaxies should have scattered in all directions long ago, yet the structure of the universe remains stable. It appears as though the universe is held together by something heavier and more gravitationally potent than what meets the eye.
From Aristotle to Galileo, Newton, and Einstein, the great physicists and philosophers of history have all challenged the secrets of gravity. And yet, we still do not understand gravity with 100% perfection.
This is precisely where the greatest debate in modern cosmology takes place. For a long time, astronomers devised the concept of dark matter to explain a "universe heavier than it looks." Dark matter contributes to gravity but does not emit any light. It is not merely dark; it neither emits nor absorbs light. Because it does not interact with light in any way, it cannot be seen with conventional telescopes. We can only sense it vaguely through the gravitational effects created by the dark matter filling space. Dark matter acts like an invisible hand, a skeleton that holds galaxies and galaxy clusters together.
However, we have yet to determine exactly what dark matter is. Consequently, an alternative hypothesis emerged: MOND, or Modified Newtonian Dynamics. Physicists who advocate for MOND argue that there is no need to assume the existence of invisible matter in the first place. Instead, they attempt a more dramatic assumption: that we simply need to correct the law of gravity itself. They explain that in regions where gravitational acceleration is extremely weak, or on a very large distance scale, gravity may operate in a way entirely different from what Newton and Einstein described.
The reason MOND has long been considered an attractive alternative is clear. It has shown remarkable results, particularly with galaxy rotation curves. The outer stars of spiral galaxies rotate too fast if we consider only the visible matter—baryons. Instead of adding dark matter, MOND suggests that the law of gravity operates differently at the outskirts of galaxies where the gravitational scale is very weak.
Gravity weakens in inverse proportion to the square of the distance as you move further away. If you double the distance, it becomes four times weaker; triple the distance, and it becomes nine times weaker. If we call the distance 'r', the strength of gravity decreases in proportion to approximately 1/r². The exponent of 2 is crucial here. MOND suggests that this number might not be 2. For instance, what if it were 1? Gravity would weaken much more slowly as the distance increases. If so, even objects very far away could be held by a stronger gravitational pull than expected. This is how MOND explains the movement of rapidly moving stars and galaxies without dark matter.
Is that truly the case? On the massive scale of tens or hundreds of millions of light-years, does gravity weaken more gently than 1/r²? To find this answer, looking at just a single galaxy is insufficient. We must examine how gravity operates on a scale larger than galaxy rotation curves—in fact, on the scale of the entire universe. There is an excellent tool for this: the Cosmic Microwave Background (CMB), the oldest light in the universe that has traveled from the furthest reaches of space.
Recently, astronomers utilized the Atacama Cosmology Telescope (ACT) in the Atacama Desert of Chile to complete a map of the CMB across the entire universe over several years. They published a variety of analytical results, including a surprising finding that meticulously analyzed the possibility of MOND across the entire universe. So, what was the conclusion?
The CMB is light that has traveled from very far away, the very edge of the universe. As this light travels, it passes through various galaxy clusters. These clusters contain many hot, rapidly moving electrons. The photons of the CMB undergo a scattering effect when they collide with free electrons within these galaxy clusters.
The pattern of this scattering changes depending on the direction in which the galaxy cluster is moving relative to us. If a galaxy cluster is approaching us or moving away from us relative to the CMB, the electrons within it move as well. As a result, the CMB photons scattered by these electrons contain a minute Doppler effect reflecting the movement of the galaxy cluster. Consequently, CMB light passing through a galaxy cluster approaching us appears slightly hotter, while light passing through a receding galaxy cluster appears colder. This effect is known as the Kinematic Sunyaev-Zel'dovich (SZ) effect.
Of course, this signal is so faint that if we were to consider only a single galaxy cluster, it would essentially be buried in background noise. However, by aggregating and statistically analyzing hundreds of thousands of galaxies and galaxy clusters across the universe, we can determine how these clusters are moving toward us on average. Consider two galaxy clusters in the universe. They do not move completely at random. The two clusters pull on each other with their gravity. They have a tendency to move closer to one another. That velocity contains clues as to how the gravity exchanged between the two clusters functions.
Based on the latest CMB map data completed via ACT, astronomers incorporated vast amounts of galaxy map data from the SDSS collected over several years. The galaxies used in this study have redshifts between 0.44 and 0.66. This range is significant because, in this interval, the spatial distribution of galaxies does not change drastically. Therefore, we can exclude the effects of the evolution of the cosmic large-scale structure over time and cleanly analyze only how gently or steeply gravity weakens according to distance.
The average distance between the two galaxy clusters used in this analysis was about 30–230 Mpc. Given that the diameter of our Milky Way, which is 100,000 light-years across, is only 0.03 Mpc, this study did not just look at gravity within a single galaxy; it tested gravity on a cosmological scale that far exceeds any single galaxy. So, what was the value of the exponent regarding how gravity weakens with distance as confirmed through this extensive survey? The result is approximately 2.1±0.3. In the standard Newtonian law and today’s ΛCDM cosmology based on Einstein's theory of relativity, 'n' should be 2. On the other hand, assuming the simplest model of MOND, 'n' would have to be 1.
However, the actual observation yielded a value close to 2. It is certainly not 1. It seems that MOND does not function at all on the scale of the entire universe. Of course, this discovery alone may not lead physicists who believe in MOND to abandon their hopes entirely. MOND involves many complex factors, such as effects from the gravity of other surrounding galaxies and clusters, known as the External Field Effect (EFE). Even if an object is placed in a weak gravitational field on its own, if a larger external gravitational field exists in the background, it can influence the object's motion. However, this study did not systematically analyze EFE.
Nevertheless, the fact that gravity was tested on a cosmological scale spanning tens to hundreds of Mpc is significant. Therefore, even when taking the external field effect into account, it seems difficult to strongly refute these results.
Reference
https://www.science.org/content/article/newton-s-law-gravity-passes-its-biggest-test-ever
https://iopscience.iop.org/article/10.1088/1475-7516/2025/11/061
https://iopscience.iop.org/article/10.1088/1475-7516/2025/11/062