Science advances by proposing ideas and then trying to prove them wrong. This process becomes especially challenging when dealing with the Universe on its largest scales. Dark energy and dark matter are among the most difficult concepts to test. Observations across vast regions of space clearly suggest that something is influencing gravity in ways Einstein’s theory does not fully explain. Yet within our own solar system, everything appears to behave exactly as expected.
A new study by Slava Turyshev, a physicist at NASA’s Jet Propulsion Laboratory, explores how researchers might address this mismatch. His work suggests that the key may lie in being extremely precise and selective in how experiments are designed to search for signs of dark energy and dark matter closer to home.
The “Great Disconnect” Between Cosmic and Local Physics
At the center of the problem is what scientists call the “Great Disconnect.” The laws of physics seem to operate differently depending on the scale being observed. In regions with very little matter (i.e. no gravitational force), the effects linked to dark energy or modified gravity become much more noticeable. In contrast, in dense environments filled with matter and strong gravity, those same effects seem to vanish, at least based on current instruments.
Within the solar system, everything aligns with traditional physics. Planets follow their expected orbits. Measurements of spacetime around the Sun, including data from spacecraft signals, match predictions precisely. Every probe sent through the solar system behaves as if only standard gravity is at work. There are no clear signs of anything unusual.
Strong Evidence From the Distant Universe
The situation changes dramatically when looking far beyond our local neighborhood. On the scale of galaxies and beyond, the Universe appears to be expanding. While scientists continue to debate the exact rate of this expansion, there is strong evidence that something is influencing gravity or spacetime in ways not fully captured by current theories.
At present, dark energy is the best explanation for this behavior, even though its true nature remains unknown.
Screening Effects and the Hidden “Fifth Force”
One possible explanation involves a phenomenon known as “screening.” In this idea, whatever is causing the discrepancy changes how it behaves depending on the surrounding environment. As density increases, its effects become weaker or harder to detect.
There are two main types of screening models. The first is called the “chameleon” model. In this scenario, a hypothetical fifth force of nature (other than gravity, electromagnetism, and the two nuclear forces) adjusts its strength based on the amount of nearby matter. In low density regions, it becomes strong and produces effects associated with dark energy. In dense areas, it weakens so much that current instruments cannot detect it, even though it still exists. Around objects like the Sun, it might only appear in a thin outer layer, but in principle it could still be measured there.
Vainshtein Screening and Suppressed Forces
Another explanation is the Vainshtein screening model. Here, the force itself does not change. Instead, the surrounding gravity effectively suppresses its influence, making it appear weak. The model introduces the concept of a Vainshtein Radius, which marks the distance where the force regains its normal strength.
For the Sun, this radius is estimated to extend about 400 light years. That region includes many stars, meaning the force would remain suppressed well beyond the solar system and even across large parts of the galaxy.
Why New Solar System Missions May Be Needed
Both screening models could leave subtle traces in large-scale observations collected by missions such as Euclid and The Dark Energy Spectroscopic Instrument (DESI). However, these surveys focus on distant galaxies and cannot directly reveal how such forces behave within the solar system.
To test these ideas locally, scientists would need a dedicated mission designed specifically for that purpose. Even more important, researchers would need a falsifiable theory that predicts what such a mission should detect.
The Importance of Testable Predictions
Dr. Turyshev emphasizes that without a clear, testable prediction, additional experiments in the solar system are unlikely to yield new results. So far, observations have consistently confirmed general relativity. Continuing to run similar experiments without new theoretical guidance may not provide useful insights.
However, if scientists can use data from large cosmological surveys to develop precise hypotheses that apply to the solar system, then it becomes possible to design targeted experiments to test them.
Looking Ahead: Building Better Instruments
It may take time to develop instruments sensitive enough to detect these subtle effects. In the meantime, incremental progress will be important, with missions focused on improving measurement capabilities step by step.
If a well-defined and testable prediction emerges from current data, and if an experiment can realistically be built to test it, pursuing that opportunity could lead to a major breakthrough. Such a discovery has the potential to reshape our understanding of gravity, dark energy, and the fundamental workings of the Universe.
