There is a hidden sector in physics. It’s a precarious area of science, dealing with undetected quantum fields and their particles which, if we could observe just once, would explain phenomena like dark matter, particle anomalies, and aspects of string theory. Some of the members of the hidden sector may even sound familiar — axions, sterile neutrinos, and dark photons. And while they would be illuminating, they’re not susceptible to the strong, weak, or electromagnetic forces, making them less able to interact with everyday visible matter. The hidden sector, then, is made up of hypothetical particles that could flood the space around us in their own secretive dance, revealing in that choreography answers to questions we’ve been studying for decades. And yet throughout the decades every carefully constructed and expensive experiment has failed to produce evidence that they exist.
The alternative solution is not to look for new particles, but to modify the laws of gravity.
We evoke dark matter to explain the motions of stars and galaxy clusters. With the amounts of normal matter in our universe and with our current laws of physics, stars at the fringes of their galaxies should move slower because they’re further away from the galaxy’s center of mass. And yet they move at the same rate as the stars towards the center. Similarly, galaxies come together in huge, thick clusters which we call the cosmic web. Simulations reveal a sticky, textured network with regions of empty space adorning the opaque points where galaxies accumulate. If normal matter can’t account for these observations — nor can it account for gravitational lensing and temperature fluctuations in the CMB — then there must be a more mysterious counterpart of matter in the universe. Dark matter says particles surround a galaxy’s stars, explaining why they move the same rate at the edges as they do closer to their center of mass. But the alternative, modified gravity theory, doesn’t suggest there’s anything more to the cosmos than the matter we already see. Instead, it says the particles of our standard model exert a larger force on one another than we’ve so far assumed.
With modified gravity, there is a change to Newton’s assumption that gravity weakens according to the inverse square law. The inverse square law is an equation which can also be seen with light and sound. It says that the intensity of an effect, like a light source, diminishes in proportion to the distance from the source squared. For example, at 2 feet from the source, the light will be 1/4th as intense. At 4 feet, it will be 1/16th as intense, at 5 feet it will be 1/25th as intense. We see this beautifully exemplified in our Solar System.
There is a difference, however, in the Newtonian view of gravity and the one Einstein gave us with his theory of relativity. With relativity, gravity is not really a force but instead the product of curved space and time. But modified gravity and relativity can be compatible using different kinds of fields that explain how attraction comes from mass. Modified gravity is not one theory but rather a collection of about ten theories — not as simple and straightforward as particle dark matter. This lack of simplicity and beauty makes modified gravity a more unsavory option to dark matter. But what it is is as important as what it isn’t. Modified gravity is not a pseudoscience or a desperate attempt to explain this persistent mystery. It’s clumsier and cannot offer the allure of exciting new particles ruminating somewhere out in the pathways of the cosmos, but it is as valid an explanation as any other. In fact, in many cases its predictions are more accurate than those of its counterparts.
Measurements of over 150 galaxies found a correlation between the pull of normal matter and the pull of normal matter with dark matter combined. That is, there seems to be a direct relation between the amount of normal matter and the amount of dark matter in the galaxies. But this is a strange result considering that there should be a different amount of dark matter in each spiraling, twinkling galactic body in space. Each galaxy is, after all, different from every other both in birth and in shape. To reproduce these results from computer simulations requires added parameters not needed with modified gravity. In modified gravity, normal matter is all there is so that a relationship between total gravity and visible gravity is expected, not forced.
Galaxies with less stars, aptly named low-surface-brightness galaxies (LSB’s, they’re sometimes only slightly brighter than the night sky), should not only have less matter but also less dark matter. The lower amount of matter — and thus gravity — would mean that stars further from the center should be orbiting at slower speeds. And yet the stars followed speeds similar to those in normal galaxies, like the Milky Way. The relation between dark matter and normal matter in these fainter, dimmer galaxies must be closer than anticipated, a problem which researchers solved by adding the caveat that amounts of dark matter depend on star brightness — a dimmer galaxy has more dark matter. But arriving at these calculations from natural phenomena asks that natural events (supernovae sweeping matter out of galaxies) have an incredibly high level of efficiency, one some physicists consider to be implausible. In contrast, no such feats are needed with modified gravity.
Modified gravity successfully predicted dwarf galaxies traveling around Andromeda as the gravitational pull from our sister galaxy outweighed the internal gravity of the dwarfs. To have achieved this same prediction with computer simulations of dark matter would have called for added parameters to the data. There is a dynamic between the two theories — while modified gravity is extremely successful on the scale of galaxies, it cannot describe galactic clusters and their gravitational lensing. It’s here, in the enormous accumulations of these cosmic bodies, that dark matter is able to make the most accurate predictions regarding their motion and distribution. Dark matter particles also give us a reason why gas clouds from colliding galaxy clusters will stick together while the stars of the galaxies will slide between each other. Modified gravity does not.
Because each theory is successful in its own way, some scientists believe the key may be to find mutual ground between the two ideas — dark matter particles that can disguise themselves as modified gravity. If dark matter particles can become superfluids, for example, once collected in galaxies they can have similar effects to those proposed by modified gravity. This superfluid wouldn’t manifest in galaxy clusters because there’s not enough gravity, meaning that in this theory galactic clusters would only show as dark matter whereas in individual galaxies they would show as modified gravity.
Ongoing projects like the Dark Energy Survey, along with upcoming technology (the Large Synoptic Survey Telescope that aims to produce the widest and deepest image of the cosmos and the James Webb Space Telescope going online in 2021) will further study galaxies formed soon after the Big Bang and gather more information on the LSB galaxies which seem to be at odds with dark matter.
Like so many theories of physics, the most recognizable of which are quantum mechanics and general relativity, there is no one simple equation that can describe everything we see. We may be able to define one observation and attribute it to certain laws but being able to define all of our measurements takes balance and a level of open-mindedness to accept little known ideas like modified gravity — giving them value even if they challenge our beliefs in ways that aren’t very beautiful.