*A curved and stretched sheet of graphene laying over another curved sheet creates a new pattern that impacts how electricity moves through the sheets. A new model suggests that similar physics might emerge if two adjacent universes are able to interact. Credit: Alireza Parhizkar, JQI*

Sometimes physicists come up with bizarre tales that sound like science fiction. Einstein’s description of the curvature of space and time was subsequently confirmed by astronomical measurements, for example. Others persist as mathematical curiosities or mere possibilities.

In a new work published in Physical Review Research, JQI Fellow Victor Galitski and JQI graduate student Alireza Parhizkar investigate the speculative notion that our universe is merely one half of a pair of interacting universes. Their mathematical model may offer a new viewpoint on fundamental aspects of reality, such as why our universe expands as it does and how this connects to the smallest lengths permitted by quantum physics. These subjects are essential to comprehending our universe and comprise one of the greatest mysteries of contemporary science.

The two scientists discovered this new perspective while doing research on graphene sheets, which consist of carbon atoms arranged in a hexagonal arrangement. They found that research on the electrical characteristics of stacked graphene sheets yielded findings that resembled miniature universes and that the underlying phenomenon may be applicable to other fields of physics. In stacks of graphene, novel electrical behaviors emerge from interactions between the individual sheets, so it is possible that distinct physics could emerge from interacting layers in other contexts, such as in cosmological models of the entire universe.

According to Galitski, who is also a Chesapeake Chair Professor of Theoretical Physics at the Department of Physics, this is an interesting and ambitious concept. “It is almost suspicious that it works so well by naturally ‘predicting’ basic properties of our universe, such as inflation and the Higgs particle, as we discussed in a subsequent preprint.”

The extraordinary electrical capabilities of stacked graphene and its putative connection to our world having a twin are a result of the unique physics caused by moiré patterns. Moiré patterns are created when two overlapping repeating patterns, such as the hexagons of atoms in graphene sheets or the grids of window screens, are twisted, offset, or stretched.

The emergent patterns can repeat over large distances compared to the underlying patterns. In graphene stacks, the new patterns alter the physics that occurs in the sheets, namely the behavior of the electrons. In the special case called “magic angle graphene,” the moiré pattern repeats over a length that is about 52 times longer than the pattern length of the individual sheets, and the energy level that governs the behaviors of the electrons drops precipitously, allowing new behaviors, including superconductivity.

Galitski and Parhizkar discovered that the physics of two sheets of graphene may be viewed as the physics of two two-dimensional universes in which electrons periodically jump between universes. This prompted the duo to generalize the mathematics to apply to universes with any number of dimensions, including our own four-dimensional universe, and to investigate whether comparable phenomena originating from moiré patterns could occur in other fields of physics. This initiated a line of inquiry that brought them face to face with one of cosmology’s most pressing issues.

Parhizkar explains, “We questioned if we can experience moiré physics when two real universes merge into one.” “What are you looking for when you pose this question? You must first determine the length scale of each universe.”

A length scale, or a scale of a physical value in general, specifies the level of precision applicable to whatever is being viewed. If you’re estimating the size of an atom, a ten-billionth of a meter is relevant, but it’s meaningless if you’re measuring a football field, which uses a different unit of measurement. In our equations, some of the tiniest and greatest sizes are fundamentally constrained by physics ideas.

The universe scale that Galitski and Parhizkar were concerned with is known as the Planck length, and it determines the shortest length consistent with quantum mechanics. The Planck length is intimately related to a constant referred to as the cosmological constant, which is contained in Einstein’s field equations ,the constant determines whether the universe tends to expand or contract in the absence of gravitational forces.

This constant is vital to the existence of the cosmos. Theoretically, scientists can ascertain the value of the constant by observing the cosmos, measuring numerous aspects, such as the rate at which galaxies are receding, plugging the data into the equations, and calculating the constant’s value.

Because the universe contains both relativistic and quantum effects, this basic strategy encounters an obstacle. Even at cosmological sizes, the effect of quantum fluctuations over the immense vacuum of space should influence behavior. But when scientists attempt to merge Einstein’s relativistic understanding of the universe with quantum vacuum theories, they run into difficulties.

One of these issues is that whenever researchers attempt to utilize observations to approximate the cosmological constant, the calculated value is substantially smaller than would be predicted by other portions of the theory. Moreover, rather than settling on a consistent value, the value fluctuates wildly depending on how much detail they include in the approximation. This persistent issue is referred to as the cosmological constant problem, or at times the “vacuum disaster.”

This is by far the widest discrepancy between measurements and theoretical predictions, according to Parhizkar. It indicates that something is amiss.

Since moiré patterns can produce significant changes in scale, moiré effects seemed to be a good lens through which to examine the issue. Galitski and Parhizkar developed a mathematical model (which they refer to as moiré gravity) by taking two copies of Einstein’s theory of how the world evolves through time and incorporating new mathematical variables that allow the two copies to interact. Instead of studying the energy and length scales of graphene, they studied the cosmic constants and lengths of universes.

Galitski claims that this concept emerged spontaneously when they were working on a seemingly unrelated project financed by the John Templeton Foundation to replicate astronomical occurrences by investigating hydrodynamic flows in graphene and other materials.

By manipulating their model, they demonstrated that two interacting universes with enormous cosmic constants might override the behavior predicted by the separate cosmological constants. The interactions generate behaviors regulated by a significantly smaller effective shared cosmic constant than the individual constants. The computation for the effective cosmological constant circumvents the problem of researchers’ approximations fluctuating in value, as the effects of the two worlds in the model cancel one other out over time.

“We never assert that this solves the cosmological constant problem,” explains Parhizkar. “Honestly, that is a rather arrogant claim. This is merely a beautiful discovery that if you merge two universes with cosmological constants that are 120 orders of magnitude higher than what we experience, there is still a probability that you will obtain a cosmological constant that is extremely little.”

In preliminary follow-up work, Galitski and Parhizkar have begun to expand on this new perspective by exploring a more thorough model of two interacting worlds, which they term “bi-worlds.” Each of these worlds is, by our typical criteria, a full world in its own right, with identical sets of all matter and fields. Since the mathematics permitted it, they also added fields that existed simultaneously in both realms, which they termed “amphibian fields.”

The new model yielded additional noteworthy outcomes, as determined by the researchers. As they worked out the equations, they discovered that a portion of the model resembled crucial elements of reality. The more thorough model continues to suggest that two worlds may explain a modest cosmological constant and provides specifics on how such a bi-world might leave a distinct mark on the cosmic background radiation — the light that lingers from the universe’s infancy.

Therefore, future tests could establish whether this distinct perspective generated by graphene merits greater consideration or is only a fascinating novelty among physicists’ playthings.

The idea is falsifiable by experimentation, which is a positive thing, according to Parhizkar. “We haven’t investigated all the impacts; that’s difficult to accomplish.” “If it is not falsified, it is tremendously interesting since it addresses the problem of the cosmological constant and describes many other crucial aspects of physics. I don’t hold out much hope for that; I believe it is actually too good to be true.”

**Further information:** Alireza Parhizkar et al, Strained bilayer graphene, emergent energy scales, and moiré gravity, *Physical Review Research* (2022). DOI: 10.1103/PhysRevResearch.4.L022027

Alireza Parhizkar, Victor Galitski, Moiré Gravity and Cosmology. arXiv:2204.06574v1 [hep-th], arxiv.org/abs/2204.06574

**Source:** Joint Quantum Institute