숨겨진 '암흑 차원'이 우주의 비밀을 풀다

Scientists have proposed the existence of a fifth “dark dimension” that governs the behavior of both dark energy and dark matter—and which may be observationally detected in the near future, according to a recent preprint study . The dark dimension hypothesis could potentially provide an elegant solution to some of the biggest mysteries in science, including the nature of dark matter, an unidentified substance that makes up most mass in the universe, and dark energy, the term for whatever is causing the universe to expand at an accelerating rate. The idea arose from string theory, a decades-old framework that suggests the universe contains many dimensions beyond the four dimensions that we can perceive as humans—time, and three-dimensional space. In a series of studies published over the past several years, a group of string theory experts have suggested that one of these dimensions could simultaneously govern dark matter and dark energy, and that this dimension changes over time. “The question was whether or not this extra dimension is frozen or can evolve,” said Cumrun Vafa, the Timken University Professor in the physics department at Harvard University and an author on these studies, including the newest preprint, in a call with 404 Media. “The fifth dimension is somehow related to the value of dark energy, so it's natural that when the value of the extra dimension changes, the value of dark energy also changes.” When Vafa and his colleagues started to develop this idea, the conventional wisdom was that dark energy had a fixed value, according to the standard model of cosmology, a well-corroborated set of assumptions that explains most laws and phenomena in the universe. “We were scrutinized by our colleagues for predicting that string theory requires dark energy to change, whereas dark energy looks constant,” said Vafa. “This is not good for our field because you are predicting string theory is inconsistent with observation.” But the assumption that dark energy has a fixed value, a central tenet of the standard model, has been challenged by the Dark Energy Spectroscopic Instrument (DESI), a major survey of the universe based at the Kitt Peak National Observatory in Arizona, which released its first results in 2025. To the surprise of many scientists, the DESI release hinted that the value of dark energy may shift over time, a discovery that was in line with predictions of a dark dimension put forward by Vafa’s team. “We didn't write a model to fit the data, but rather we had a model based on theory,” said Vafa. “Now that fits the data automatically. That is why I believe in it. It's easy to write a model after you know the data to try to fit it somehow. This is not what we did.” “Our model is still the best model, as far as I know, that fits the DESI results,” he added. In their newest preprint, which is currently in the process of peer review, Vafa and his colleagues expand on the workings of this possible dark dimension in light of the DESI data. According to string theory, the universe has many dimensions that are “compactified” into subatomic scales that are not detectable by current instruments, which is why we have not discovered them. The hypothetical dark dimension, however, may be enlarged to a micron scale range. This would allow gravitons, which are the hypothetical particles that mediate the force of gravity, to spill over into the dark dimension, where they would become dark matter particles. In this view, dark matter is not made of some new form of exotic particle. Instead, it’s made up of gravitons that also exist in our familiar universe and have traversed into the dark dimension. “We are using the extra dimension to our benefit to explain what dark matter is,” Vafa said. “We don't need to introduce a new particle or particles. We are just saying it's the usual gravity.” As this dark dimension evolves, so does the mass of these dark matter particles in that realm; Vafa’s team predicts they are becoming lighter. Likewise, the value of dark energy weakens over time. In this way, dark matter and dark energy—long thought to be likely unrelated—arise from the properties of the same extra dark dimension. It’s a trippy and esoteric concept, and it may soon be put to the test. If a dark dimension does exist, then it should manifest as tiny deviations in Newton's gravitational force at micrometer scales. A team at the Institute for Quantum Optics and Quantum Information in Vienna is currently designing an experiment to test this idea, which could yield results in the coming years. Moreover, if dark matter particles are just gravitons that wandered into a dark dimension, then there should be some subtle observational hints of “a fifth force” in the dark sector. For example, scientists plan to test the hypothesis by closely watching the movements of stars in interacting galaxies, which are held together by dark matter halos, to search for a dark fifth force. When the galaxies come near each other, “the visible matter attracts the visible matter of the other galaxy, and dark matter of one also attracts the dark matter of the other galaxy as well,” Vafa explained. “But there's extra force between the dark matter compared to the visible matter because of this fifth force, so you get a more attractive force pulling the dark matter towards each other more than the visible part.” “There are multiple fronts where things can come together,” he concluded. “Of course, the most exciting would be the confluence of all of these together, that somehow they agree. That would be really remarkable.”