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The hypothesis of a fifth force emerges amidst the so-called “Great Disconnect,” a problem that separates the behavior of gravity on cosmic scales and within the solar system, where planets, probes, and measurements around the Sun still precisely follow traditional physics predictions.
A study led by Slava Turyshev, a physicist at NASA’s Jet Propulsion Laboratory, proposes a new way to investigate whether a mysterious fifth force might be hidden in the solar system. The research stems from an unresolved difference: the Universe on large scales shows signs of effects not fully explained by known gravity, while everything in the vicinity of the Sun remains compatible with traditional physics.
The discussion involves concepts difficult to test, such as dark energy and dark matter, used to explain observations made in very vast regions of space. Even so, within the solar system, planets, probes, and spacetime measurements around the Sun follow expected predictions, with no clear signs of unusual behavior.
Fifth force may explain difference between cosmic and local scales
The central point of the research is the so-called “Great Disconnect,” an expression used to describe the apparent difference between physics observed on large scales and physics measured near Earth. In regions with little matter, effects linked to dark energy or modified gravity may become more noticeable, while in dense environments these signs seem to disappear.
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On the scales of galaxies and beyond, there is strong evidence that something influences gravity or spacetime in ways not yet fully explained by current theories. Dark energy remains the primary explanation for this behavior, although its true nature remains unknown.
Within the solar system, the situation is different. The orbits of planets, spacecraft signals, and measurements around the Sun remain aligned with general relativity, which makes it more difficult to find any direct clue of a possible fifth force.
Screening models attempt to explain invisible force
One of the explanations analyzed involves the phenomenon called screening, in which the effect responsible for the difference changes behavior according to the surrounding environment. When density increases, this effect becomes weaker or harder to detect with current instruments.
The first model is known as chameleon. In this scenario, a hypothetical fifth force, different from gravity, electromagnetism, and the two nuclear forces, would adjust its intensity according to the amount of nearby matter.
In low-density regions, this force would become stronger and produce effects associated with dark energy. In dense areas, it would weaken to the point of not being detected, although it would continue to exist.
Near objects like the Sun, this fifth force might only appear in a thin outer layer. Even so, the research indicates that, in principle, it could still be measured in this region.
Vainshtein model extends challenge for solar measurements
Another hypothesis is the Vainshtein screening model. In this case, the force does not change intensity by itself, but the surrounding gravity suppresses its influence and makes it appear weak.
This model introduces the concept of the Vainshtein Radius, which marks the distance at which the force would regain its normal intensity. For the Sun, this radius is estimated at about 400 light-years, a region that includes many stars.
The consequence is that the force would remain suppressed far beyond the solar system. This also means that its effects could remain hidden even in large parts of the galaxy.
Current missions are not enough to test the hypothesis
Missions like Euclid and the Dark Energy Spectroscopic Instrument, known as DESI, can record subtle traces of these models in large-scale observations. However, these surveys observe distant galaxies and do not directly show how these forces would behave within the solar system.
To test the hypothesis locally, scientists would need a mission dedicated to that goal. More than that, it would be necessary to formulate a falsifiable theory, with clear predictions about what such a mission should detect.
Turyshev emphasizes the importance of testable predictions before new experiments in the solar system. Without a precise hypothesis, repeating measurements similar to those already made tends to reconfirm general relativity, without offering relevant advances.
New instruments can search for signs of the **fifth force**
Research indicates that data from cosmological surveys can help create more precise hypotheses applicable to the solar system. With well-defined predictions, it would be possible to design targeted experiments to look for signs of the **fifth force**.
Developing instruments sensitive enough to detect such subtle effects may take time. Meanwhile, missions focused on improving measurement capabilities, step by step, can pave the way for more ambitious tests.
Should a clear prediction emerge from current data and a viable experiment be built to test it, the search could lead to an important discovery. Concrete evidence of a **fifth force** would have the potential to change our understanding of gravity, dark energy, and the fundamental workings of the Universe.
With information from **Universe Today**
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