NASA scientist says a mysterious “fifth force” may be hiding in our solar system

Bridging the “Great Disconnect”: The Future of Hunting Dark Energy in Our Own Backyard

For decades, astrophysicists have faced a frustrating paradox. When we look at the vast reaches of the cosmos, everything suggests that Einstein’s theory of gravity is incomplete. The expansion of the universe and the behavior of galaxies point toward the existence of dark energy and dark matter—mysterious forces that reshape the fabric of space on a massive scale.

However, when we look inside our own solar system, the mystery vanishes. Every planetary orbit, every spacecraft trajectory, and every measurement of spacetime around the Sun aligns perfectly with standard general relativity. This gap is what NASA physicist Slava Turyshev calls the “Great Disconnect.”

How “Screening” Hides a Fifth Force

If dark energy is influencing the entire universe, why can’t we find it here? The answer may lie in a phenomenon known as “screening.” This theory suggests that certain forces change their behavior based on the density of the surrounding environment.

The Chameleon Model

In the chameleon model, a hypothetical fifth force of nature adjusts its strength based on the amount of nearby matter. In the low-density voids of intergalactic space, this force becomes strong and drives the expansion of the cosmos. But in high-density regions—like our solar system—the force weakens so significantly that it becomes nearly invisible to current instruments.

Vainshtein Screening

Another possibility is Vainshtein screening. In this model, the force doesn’t change its nature, but the surrounding gravity effectively suppresses its influence. This creates a “Vainshtein Radius,” a boundary within which the force remains hidden. For our Sun, this radius is estimated to extend roughly 400 light years, meaning the force would be suppressed across a significant portion of our galactic neighborhood.

A New Strategy: Detection and Guardrails

To overcome these screening effects, researchers are moving away from general searches and toward a more selective, high-precision strategy. According to recent research, this involves a two-branch approach to discovery.

The first is the detection-first branch. This focuses on finding a specific “local anomaly”—such as a violation of the Einstein equivalence principle (EEP) or a Shapiro-delay signal—which would then trigger a joint analysis of both cosmological and solar system data.

The second is the guardrail branch. Here, solar system tests act as boundaries, enforcing strict constraints on parameters like the PPN parameters $gamma$ and $beta$, and the variation of the gravitational constant ($dot G/G$), effectively closing the gaps where modified gravity might be hiding.

The Next Generation of Precision Instruments

Bridging the disconnect requires a leap in measurement precision. Future trends in solar system exploration are focusing on several key technological targets to uncover these subtle residuals:

• Atom Interferometers in Space (AIS): These “drag-free” systems could potentially detect Einstein equivalence principle (EEP) violations with a precision of $eta_{EEP} sim (1text{–}10) times 10^{-17}$.
• Advanced Lunar Laser Ranging (LLR): Using sub-mm-class LLR, scientists aim to constrain the variation of the gravitational constant to $|dot G/G| sim (3-5) times 10^{-15} text{ yr}^{-1}$.
• Optical Shapiro Delay: By refining measurements of spacetime curvature, researchers forecast a reach of $|gamma-1| lesssim (2-5) times 10^{-6}$.
• Atomic Clocks: New developments in space-based clocks could improve the reach for ultralight dark matter (ULDM) coupling by 3 to 10 times.

These local efforts will perform in tandem with large-scale surveys like Euclid and the Dark Energy Spectroscopic Instrument (DESI), mapping the bridge from cosmic linear responses to local screened residuals.

FAQ: Understanding the Search for Dark Energy