# New study challenges Einstein and Newton's theories of gravity

Tod Strohmayer (GSFC), CXC, NASA - Illustration: Dana Berry (CXC)

Gravity is the force that attracts objects toward the Earth and maintains the orbital motion of planets around the Sun.

Our scientific understanding of gravity was established by Isaac Newton in 1687. Newton's theory of gravity stood the test of time for two centuries until Albert Einstein proposed his '*General Theory of Relativity*,' filling in the gaps left by Newton's theory of gravity.

Despite the many successes of Einstein's theory of gravity, many phenomena, such as gravity inside a black hole and gravitational waves, can't be explained.

Recently, a new study has found direct evidence for a modified theory of gravity at low acceleration. Prof. Kyu-Hyun Chae carried out the study from Sejong University in Seoul, Korea. Chae observed the orbital behaviors of cosmic structures called wide binary stars from data collected by the European Space Agency's Gaia space telescope.

These findings are significant as they point towards a new theory of gravity, different from the Newton-Einstein theory.

### Newton's & Einstein's theories of gravity

Newton's theory of gravity was revolutionary at the time. It successfully explained the attraction between bodies on Earth and beyond, granting us a deeper understanding of planetary motion.

However, as the scope of technology expanded, Newton's framework revealed gaps in its ability to account for complex gravitational phenomena. Anomalies in Mercury's orbit were one of them, which puzzled astronomers and showed that the theory falls apart in explaining extreme gravitational conditions.

Then, in 1915, Einstein published his opus magnum, "The General Theory of Relativity." This transformative theory reimagined gravity as the curvature of spacetime itself, unifying mass and energy in a cosmic dance.

Einstein's theory bridged Mercury's orbit and explained the bending of starlight around massive bodies during a solar eclipse. However, even Einstein's visionary insights fell short in the face of the cosmic abyss—black holes, where gravity becomes infinitely intense.

To fill in these gaps, scientists proposed the concept of *dark matter*. This elusive form of matter is invisible as it doesn't interact with light, but its effects can be seen through its gravitational pull. It was postulated to explain the discrepancies between observed gravitational effects and predictions.

But we don't know what form dark matter takes and if it even exists.

### MOND: Modified Newtonian dynamics

Although dark matter could potentially explain discrepancies, many scientists have been skeptical because of the lack of evidence. This has led to alternate theories.

Modified Newtonian dynamics, or MOND, was first proposed by Israeli scientist Mordehai Milgrom in 1983 and could explain these galactic anomalies, including the ones observed by Chae.

The basic premise of MOND is that Newtonian gravity, which works well for most everyday situations, might behave differently at extremely low accelerations.

This deviation from Newtonian physics is suggested to occur when the gravitational fields are weak. MOND suggests that at these low accelerations, the force of gravity no longer follows the familiar inverse square law but exhibits a different functional form.

The idea behind MOND is to modify Newtonian gravity to explain gravitational anomalies, like galaxy orbital velocities, without requiring dark matter.

It suggests that acceleration depends on masses and a scale-dependent function, which means that how gravity operates changes based on the size or scale of the system being studied, distinct from traditional gravity.

### Wide binary star systems

Chae analyzed 26,500 comprehensive binary star systems within 650 lightyears from the data collected by the Gaia telescope.

Wide binary star systems consist of two stars in relatively distant orbits around each other. Chae's investigation into these systems revealed that at ultra-low accelerations, the observed accelerations were 30-40% higher than traditional predictions, suggesting a potential breakdown of standard gravity.

This unexpected acceleration boost is explained by A Quadratic Lagrangian (AQUAL), a MOND-influenced theory of gravity co-authored by Milgrom, marking direct evidence of standard gravity breakdown at weak acceleration.

In a press release, Chae explained why he chose to study these systems, "From the start, it seemed clear to me that gravity could be most directly and efficiently tested by calculating accelerations because the gravitational field itself is an acceleration."

"My recent research experiences with galactic rotation curves led me to this idea. Galactic disks and wide binaries share some similarity in their orbits, though wide binaries follow highly elongated orbits while hydrogen gas particles in a galactic disk follow nearly circular orbits," he said.

Chae's study does more than challenge the status quo; it lays the foundation for a broader exploration of gravity's mysteries. He hopes his results will be confirmed and refined using larger and better data sets.

The findings of the study are published in *The Astrophysics Journal*.

**Study abstract:**

A gravitational anomaly is found at weak gravitational acceleration g_{N} ≲ 10^{−9} m s^{−2} from analyses of the dynamics of wide binary stars selected from the Gaia DR3 database that have accurate distances, proper motions, and reliably inferred stellar masses. Implicit high-order multiplicities are required and the multiplicity fraction is calibrated so that binary internal motions agree statistically with Newtonian dynamics at a high enough acceleration of ≈ 10^{−8} m s^{−2}. The observed sky-projected motions and separation are deprojected to the 3D relative velocity v and separation r through a Monte Carlo method, and a statistical relation between the Newtonian acceleration g_{N} ≡ GM/r^{2} (where M is the total mass of the binary system) and a kinematic acceleration g ≡ v^{2}/r is compared with the corresponding relation predicted by Newtonian dynamics. The empirical acceleration relation at ≲10^{−9} m s^{−2} systematically deviates from the Newtonian expectation. A gravitational anomaly parameter δ_{obs−newt} between the observed acceleration at g_{N} and the Newtonian prediction is measured to be: δ_{obs−newt} = 0.034 ± 0.007 and 0.109 ± 0.013 at g_{N} ≈ 10^{−8.91} and 10^{−10.15} m s^{−2}, from the main sample of 26,615 wide binaries within 200 pc. These two deviations in the same direction represent a 10σ significance. The deviation represents a direct evidence for the breakdown of standard gravity at weak acceleration. At g_{N} = 10^{−10.15} m s^{−2}, the observed to Newton-predicted acceleration ratio is g_{obs}/g_{pred} = 10^{√2 δ}^{obs-newt} = 1.43 ± 0.03. This systematic deviation agrees with the boost factor that the AQUAL theory predicts for kinematic accelerations in circular orbits under the Galactic external field