U.S.-German Team Searches for Dark Matter with Superconducting Gravimeters

Physicists from Indiana University (IU) and the Black Forest Observatory (BFO) have utilized superconducting gravimeters in an effort to detect dark matter within the earth (doi.org/10.1103/PhysRevLett.124.051102). Their work demonstrates a method as to how gravimeters might be employed to probe for dark matter.

Since dark matter does not emit, absorb, or reflect light, it has thus far not been detected or observed. Nevertheless, physicists believe dark matter makes up 85% of matter in the universe. Its discovery would likely lead to major advances in physics and revolutionize our understanding of the universe.

CDO Interactions May Be Detectible

Physicists believe dark matter to have a strong influence on the structure and evolution of our universe. Calculations suggest that many galaxies would fly apart or not have formed as they did without large amounts of unseen matter at play. Some scientists have hypothesized that dark matter is made up of smaller components, called compact dark objects, or CDOs, which exhibit small non-gravitational interactions with normal matter.

“Dark matter is assumed to have the same gravitational interactions as normal matter,” IU professor Charles Horowitz explained. “In addition to gravity, dark matter could have very feeble electromagnetic or nuclear interactions with normal matter. These additional non-gravitational interactions must be very weak, or else we would have already detected dark matter because of them.
“There have been many searches for dark matter made of new kinds of particles that so far have been unsuccessful. CDOs provide an alternative, and much more massive, possibility for dark matter.”

Differences in Gravitational Forces Between Two Gravimeters Examined

In their search to detect the undetectable, Horowitz and Rudolf Widmer-Schnidrig of BFO sought to detect the presence of CDOs by utilizing superconducting gravimeters to probe the earth for gravitational changes. Due to weak interaction with normal matter, dark matter is believed to move about inside normal bodies in ways conventional matter cannot. Since dark matter is believed to interact with normal matter gravitationally, the researchers looked for differences in gravitational forces between locations in order to identify CDOs, incorporating 10 years of data from the BFO and Canberra gravimeters.

The tool they used is a spring type gravimeter in which the mechanical spring is replaced by magnetic levitation of a small (2.54 cm diameter, or 17.2 g) niobium sphere in a field of superconducting persistent coils, which utilizes the perfect stability of supercurrents to create a perfectly stable spring. Magnetic levitation provides independent adjustment of the total levitation force and force gradient, so it is able to support the weight of the sphere while yielding a large displacement for even the slightest changes in gravity.

The magnetic field is generated by two niobium wire coils, cooled with liquid helium to <9.2 K, that carry persistent currents to provide an extremely stable magnetic field. The niobium coils are axially aligned, with one just below the center of the sphere and another offset 2.5cm below the sphere. The sensor operates in liquid helium cooled to 4 K in a vacuum insulated Dewar.

On Earth, acceleration is defined as g(t)=9.8m/s, to which the gravimeter can accurately observe variations to the 12th decimal. In theory, gravity from CDOs would change the g value slightly as they move closer to or further away from the gravimeter. In this experiment, the researchers looked for time-dependence g(t) that changes within the 55-minute period of CDOs orbit inside the Earth.

Data Ruled Out CDOs at Earth’s Core

In order to calculate the orbital period of a CDO traveling inside of the earth’s core, Horowitz and Widmer-Schnidrig used data on the distribution of mass inside of the earth and the earth’s gravitational field. A near earth satellite orbits the earth in 90 minutes, if that satellite could orbit inside of the earth’s core, its orbital period would be 55 minutes.

The data collected ruled out the presence of CDOs inside the earth’s core, unless they have an extremely low mass or small orbital radii. While this particular experiment failed to observe CDOs, the researchers hope to replicate the experiment with more sensitive gravimeters and expand the study to other celestial bodies.

“One can use the very high sensitivity of gravimeters to search for concentrations of dark matter down to much smaller masses than in previous studies,” summarized Horowitz. “This both limits some theories of dark matter and should lead to even more sensitive dark matter searches in the future.”