Gravitational-Susceptibility Dip Behind DESI's results: Addressing the Phantom Crossing by Local limit of Nonlocal Gravity

The universe is expanding, and for decades, astronomers have believed this expansion is driven by a mysterious force called dark energy. In the standard model of cosmology, this force is thought to be a constant, unchanging energy inherent to empty space itself, pushing galaxies apart at a steady, predictable rate. This model has worked well for many observations, but it leaves physicists with deep questions about why the universe behaves this way and what dark energy actually is. Recently, a massive new survey of the cosmos has begun to challenge this simple picture. By mapping the positions of millions of galaxies and quasars, scientists have found hints that dark energy might not be constant after all. Instead, it appears to be changing over time, and in a way that defies the current rules of physics as we understand them. This new data suggests that the force driving the universe's expansion is crossing a critical threshold, shifting from one type of behavior to another in a way that the standard theory struggles to explain without invoking unstable or impossible forms of matter.

A team of researchers from Sharif University of Technology in Iran has proposed a new way to understand these puzzling results without breaking the laws of physics. They suggest that the strange behavior seen in the data is not caused by a new, exotic type of energy, but by a subtle change in how gravity itself works on the largest scales. Their work focuses on a specific theory called nonlocal gravity, which imagines that the force of gravity is not just a simple interaction between two points in space, but is influenced by the history of the field around it, much like how a material's response to an electric field can depend on its past state. In this framework, the strength of gravity is governed by a factor called gravitational susceptibility, which can vary as the universe evolves. The researchers found that if this susceptibility changes over time, it can create an illusion of dark energy behaving strangely, even if the dark energy itself remains perfectly normal and constant.

The key to their discovery lies in the latest data from the Dark Energy Spectroscopic Instrument, or DESI. This instrument has measured the positions of over fourteen million galaxies and quasars, creating the most detailed map of the universe's large-scale structure ever made. When the DESI team combined their measurements with data from the cosmic microwave background and supernovae, the results pointed toward a dark energy that changes its nature. Specifically, the data suggests that the equation of state for dark energy—a number that describes how its pressure relates to its density—has crossed a critical line known as the "phantom divide." In the standard view, crossing this line would require dark energy to become a "phantom" fluid, a type of matter that violates fundamental energy conditions and is generally considered unstable or impossible in standard physics. The DESI data indicates this crossing happened roughly 4 billion years ago, when the universe was about two-thirds of its current age.

The researchers showed that this phantom crossing does not require the existence of such impossible matter. Instead, they demonstrated that a changing gravitational susceptibility can mimic this effect perfectly. In their model, the dark energy fluid remains constant and well-behaved at all times, with a fixed relationship between its pressure and density. However, because the gravitational susceptibility changes as the universe expands, an observer measuring the expansion rate would calculate a different, changing value for the dark energy's behavior. It is as if the observer is looking at a steady object through a lens that is slowly changing its shape; the object hasn't moved, but the view of it has shifted. The researchers calculated exactly how this susceptibility must change to match the DESI observations. They found that the susceptibility must dip slightly at a specific time in the universe's history, reaching a minimum around the same time the phantom crossing was observed, before returning to its previous state.

To test this idea, the team used powerful computer simulations to compare their model against the actual data. They fed the latest measurements from the cosmic microwave background, the DESI galaxy survey, and three different catalogs of supernovae into their equations. The results were striking: the model with the changing gravitational susceptibility fit the data just as well as, and in some cases better than, the standard model of cosmology. The analysis revealed that the best fit for the data required the gravitational susceptibility to have a dip with a depth of about minus 0.025 at a redshift of roughly 0.4, which corresponds to the time when the phantom crossing was detected. This dip is a small but significant feature in the fabric of spacetime's response to matter, and its presence explains the observed shift in the dark energy behavior without needing to introduce any new, exotic particles or fields.

The study also explored how this new model affects our understanding of the universe's age and expansion rate, known as the Hubble constant. The researchers found that the depth of the susceptibility dip is linked to the value of this expansion rate. When the model accounts for the dip, it suggests a slightly different expansion rate than the standard model, helping to resolve some of the tensions that have appeared in recent years between different methods of measuring the universe's growth. Importantly, the model remains consistent with all the major cosmological probes, including the detailed patterns of light from the early universe. The researchers concluded that the phantom crossing seen by DESI is likely an effective phenomenon, a natural consequence of a modified gravitational framework rather than a sign of unstable dark energy.

This work offers a compelling alternative to the idea that the universe is filled with exotic, unstable matter. By showing that a simple change in the rules of gravity can reproduce the complex behavior seen in the data, the researchers provide a path forward that keeps the laws of physics intact. The model suggests that the universe's expansion history is more dynamic than previously thought, not because the energy driving it is changing, but because the way gravity responds to that energy is evolving. The findings do not prove that this is the final answer, but they demonstrate that the strange signals detected by DESI can be explained within a coherent, modified theory of gravity. As more data comes in from future surveys, this idea of a changing gravitational susceptibility will be further tested, potentially reshaping our understanding of the invisible forces that govern the cosmos.