When it comes to understanding the structure of the universe, most of what scientists believe exists belongs to a dark, gloomy realm. Ordinary matter, that is, what we can see and touch, only 5% of the universe. Cosmologists say that all that remains is dark energy and dark matter, which are mystical substances labeled "dark," in part to reflect our ignorance of their true nature.
While there isn't a single idea that explains everything we want to know about the universe, an idea that came up two years ago could answer several big questions. It's called the Dark Dimension Scenario, and it offers a specific recipe for dark matter, which hints at a close connection between dark matter and dark energy. This scene may also tell us why gravity – carving the universe at its greatest scale – is so weak compared to other forces.
The scheme proposes an unseen dimension that exists in the already complex field of string theory, which attempts to unify quantum mechanics and Einstein's theory of gravity. In addition to the four familiar dimensions—three infinite spatial dimensions plus one temporal dimension—string theory suggests that there are six extremely small spatial dimensions.
In the universe of the dark dimension, one of the extra dimensions is much larger than the others. It's not 100 million trillion times smaller than the diameter of a proton, but about 1 micron wide — minutes by everyday standards, but huge compared to others. Massive particles that carry gravity are created in this dark dimension, and they make up what scientists believe is about 25% of our universe of dark matter and form the glue that holds galaxies together. (Current estimates put the remaining 70% to be made up of dark energy, which is driving the expansion of the universe.)
This scenario "allows us to make connections between string theory, quantum gravity, particle physics, and cosmology, [while at the same time] solving some of the mysteries associated with them," said Ignatios Antoniadis, a physicist at the Sorbonne who is actively working on the proposal for a dark dimension.
While there is no evidence of the existence of a dark dimension yet, the scenario does make a testable ** for cosmological observations and desktop physics. This means that we may not have to wait too long to see if this hypothesis will stand up to empirical scrutiny, or be relegated to the list of tantalizing ideas that never live up to its original promises.
Rajesh Gopakumar, a physicist and director of the International Center for Theoretical Sciences in Bangalore, said: "The dark dimension envisioned here has the advantage of being easily excluded as upcoming experiments become more acute.
The dark dimension was inspired by a long-standing mystery about cosmological constants – a term specified by the Greek letter lambda, which Albert Einstein introduced into his gravitational equation in 1917. Einstein, like many of his peers, believed in a static universe, and he added the term to prevent the equation from describing an expanding universe. But in the 1920s, astronomers discovered that the universe was indeed expanding, and in 1998 it was observed that it was growing at an accelerated pace, fueled by what is now commonly referred to as dark energy – which can also be expressed in the lambda equation.
Cumrun Vafa (left), Irene Valenzuela, and Miguel Montero have crafted a dark dimension scene in which giant gravitons inhabit a huge extra dimension.
Since then, scientists have been grappling with one striking feature: its estimate of 10,122 in Planck is "the smallest measurement parameter in physics," says Cumrun Vafa, a physicist at Harvard University. In 2022, when he contemplated this almost unfathomable insignificance with two members of his research team – Miguel Montero now at the Institute of Theoretical Physics in Madrid and Irene Valenzuela currently at CERN, VAFA had an insight: such a tiny lambda is a truly extreme parameter, which means that it can be considered within the framework of the work on string theory that preceded VAFA.
Earlier, he and others came up with a conjecture that explained what happens when an important physical parameter reaches an extreme value. It's called distance conjecture, and it refers to "distance" in the abstract sense: when one parameter moves towards the far edge of possibility, and thus assumes an extremum, it has an effect on other parameters.
Thus, in string theory equations, key values (e.g., particle mass, or the coupling constants that determine the strength of the interaction) are not fixed. Changing one will inevitably affect the other.
For example, as has already been observed, a very small should be accompanied by a lighter, weakly interacting particle, the mass of which is directly related to the value of . "What will they be? Wafa wondered.
As he and his colleagues pondered the question, they realized that distance conjecture chord theory, combined with another key insight: in order for these light particles to appear at almost zero, an extra dimension of string theory would have to be much larger than the others—perhaps large enough for us to detect its presence and even measure it. They have reached the dark dimension.
In order to understand the origin of the inferred particles of light, we need to rewind the cosmological history back to the first microsecond after the big **. At this time, the universe is dominated by radiation – photons and other particles move at speeds close to the speed of light. These particles have already been described by the Standard Model of particle physics, but in a dark-dimensional scenario, when familiar particles collide together, a family of particles that are not part of the Standard Model may appear.
From time to time, these radiation particles collide with each other to produce what we are talking about'Dark graviton', "said Georges Obied, a physicist at the University of Oxford, who helped develop the theory of dark gravitons.
Generally, physicists define gravitons as massless particles that travel at the speed of light and transmit gravitational pull, similar to massless photons that transmit electromagnetic forces. But in this case, as Orbide explains, these early collisions produced a different type of graviton—something with mass. Not only that, but they also produced a range of different gravitons.
There's a massless graviton, which is the usual graviton as we know it," Obiid said. "And then there are infinite copies of dark graviton, and all of them are huge. Roughly speaking, the hypothetical mass of the dark graviton is an integer multiplied by a constant m, the value of which is related to the cosmological constant. They have a whole "tower" with a wide range of mass and energy levels.
To understand how it all works, imagine our four-dimensional world as the surface of a sphere. We can never leave that surface—for better or worse—and so can every particle in the Standard Model.
However, gravitons can be everywhere for the same reason that gravity is everywhere. That's where the dark dimension comes in.
To depict this dimension, Vafa says, think of every point on the imaginary surface of our four-dimensional world and attach a small ring to it. This cycle (at least schematically) is an additional dimension. If two Standard Model particles collide and produce gravitons, gravitons "can leak into that extradimensional circle and propagate around it like waves," Vafa said. (Quantum mechanics tells us that every particle, including gravitons and photons, can behave like particles and waves — a 100-year-old concept known as wave-particle duality.)
When gravitons leak into the dark dimension, the waves they produce can have different frequencies, each corresponding to a different energy level. And those huge gravitons, which move around the extradimensional loop, have a significant gravitational effect where the loop attaches to the sphere.
Maybe this is dark matter? Vafa mused. After all, the gravitons they concocted were weakly interacting but able to gather some gravitational pull. He noted that one advantage of the idea is that gravitons have been part of physics for 90 years and were first proposed as carriers of gravity. (It should be noted that gravitons are hypothetical particles that are not directly detected.) To explain dark matter, "we don't have to introduce a new particle," he said.
Gravitons, which can leak into the extradimensional domain, are "natural candidates for dark matter," says Georgi Dvali, director of the Max Planck Institute for Physics, who has no idea to study dark dimensions directly.
A large dimension, such as the hypothetical dark dimension, will have space with long wavelengths, which means low-frequency, low-energy, low-mass particles. However, if a dark graviton leaks into the tiny dimension of string theory, its wavelength will be very short, and its mass and energy will be very high. Supermassive particles like this are unstable and have a short lifespan. They "have long since disappeared, and there is no possibility of acting as dark matter in the current universe," Dewally said.
Gravitational force and its carrier, gravitons, permeate all dimensions of string theory. But the dark dimension is so much larger than the other extra dimensions – many orders of magnitude higher – that the strength of gravity is diluted, and if it seeps significantly into the more spacious dark dimension, it appears weak in our four-dimensional world. "This explains the extraordinary difference between gravity and other forces," Devalli said, noting that the same effect is seen in other extradimensional scenes.
Since the dark dimension scenario can be used for things such as dark matter, it can be empirically tested. "If I give you some correlations that you can never test, you will never be able to prove me wrong," said Valenzuela, co-author of the Original Dark Dimension**. "* Something you can actually prove or refute is much more interesting.
Astronomers have known about the existence of dark matter – at least in some form – since 1978 – when astronomer Vera Rubin determined that galaxies were spinning so fast that their outermost edge stars would have been thrown into the distance if it weren't for some huge reservoir of unseen material blocking them. However, it turned out to be very difficult to determine this substance. Although experiments have been carried out for nearly 40 years to detect dark matter, this particle has not yet been discovered.
If dark matter turns out to be dark gravitons, their interaction is very weak, vafa says, and this will not change. "They will never be found directly. ”
But there may be an opportunity to indirectly discover the characteristics of these gravitons.
One strategy that Vafa and his collaborators are employing is to use large-scale cosmological surveys to map the distribution of galaxies and matter. Within these distributions, there may be "small differences in clustering behavior," Obied says, which would indicate the presence of dark gravitons.
When heavier dark gravitons decay, they produce a pair of lighter dark gravitons with a total mass slightly less than the mass of their parent particle. The missing mass is converted into kinetic energy (consistent with Einstein's formula, e = mc2), which gives the newly created graviton a bit of a boost – the "kick speed" is estimated to be about 1/10,000th of the speed of light.
These kick speeds, in turn, may affect how galaxies are formed. According to standard cosmological models, galaxies begin as a mass of matter, and their gravitational pull attracts more matter. But gravitons with sufficient kicking speed can escape this gravitational grip. If they do, the resulting galaxies will have slightly less mass than the standard cosmological model**. Astronomers can look for this difference.
So far, the 1,000-degree survey's observations of the structure of the universe are consistent with those of the dark dimension: analysis of the survey data puts the upper limit of the kick speed very close to the value of Obiid and his co-authors**. The Euclid Space Telescope, launched in July last year, will undergo more rigorous testing.
At the same time, physicists also plan to test the idea of a dark dimension in the laboratory. If gravity leaks into a dark dimension with a diameter of 1 micron, any deviation from the expected gravitational force between two objects separated by the same distance can be looked for in principle. This is not an easy experiment to conduct, says Armin Shayeghi, a physicist at the Austrian Academy of Sciences. But "there's a simple reason why we had to do this experiment," he added: we won't know how gravity behaves at such close distances until we look at it.
The closest measurement to date – conducted at the University of Washington in 2020 – involved a 52-micron separation between the two testing facilities. The Austrian team hopes to eventually reach the 1 micron range of the dark dimension**.
While physicists find the proposal of Dark Dimension interesting, some are skeptical that it will succeed. "It's very interesting to look for extra dimensions through more precise experiments," said Juan Maldacena, a physicist at the Institute for Advanced Study, "although I think the probability of finding them is low."
Joseph Conlon, a physicist at the University of Oxford, is also skeptical: "There are a lot of ideas that would be important if true, but probably not. This is one of them. The conjectures on which it is based are somewhat ambitious, and I think the current evidence is rather weak.
Of course, the weight of the evidence can be changed, which is why we did the experiment in the first place. The proposal of the dark dimension, if supported by upcoming tests, has the potential to bring us closer to understanding what dark matter is, how it is linked to dark energy and gravity, and why gravity appears weak compared to other known forces. "Theorists are always trying to put this kind of'Together'。The dark dimension is one of the most promising ideas I've heard in this direction," Gopakumar said.
But the irony of one thing that the Dark Dimension Theory can't explain is why the cosmological constant is so small – a puzzling fact that basically kicks off the entire line of research. "It's true that this program doesn't explain this fact," Wafa admitted. "But what we can say from this situation is that if the lambda is small – and you detail the consequences of that – a whole set of amazing things might be in place.