Searching for clues beneath the Earth's surface

Using a network of gravimeters, Mainz-based scientists are attempting to track down weakly interacting matter in the interior of the Earth

15.07.2020

An interdisciplinary research team led by Prof. Dmitry Budker and Nataniel Figueroa of the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) and the Helmholtz Institute Mainz (HIM) has adopted a new approach in the search for dark matter. They evaluated data from a global network of gravimeters of the International Geodynamics and Earth Tide Service (IGETS), hunting for previously unknown signals that could be attributed to a new form of matter. So far, their search has not yet found any such signals, but they already have plans to refine their approach in the next stage of the project and to join together with other groups that are following a similar trail.

"Although all sorts of attempts have been made, it has not yet been possible to detect dark matter, although we know that it makes up a large proportion of the matter in the universe," says Dmitry Budker. "That is why we must leave no stone unturned and also adopt unconventional perspectives and pursue unusual ideas." Specifically, the researchers have outlined a proposed new scenario in their recent article in the European Physical Journal. This assumes that hypothetical 'hidden internal objects' trapped inside the Earth near its center will oscillate back and forth at characteristic frequencies.

To illustrate this, Budker describes an object falling towards the Earth. Assuming there was a tunnel from one side of the Earth's surface to the other that passed through the Earth's center, the object would fall towards the center, constantly accelerating due to gravity as it did so, and when it reached the other side it would move toward the surface of the Earth once more, slowing down again in the process and eventually falling back. In other words, it would oscillate at a specific frequency. "Such a scenario would be impossible in the absence of a tunnel for masses of ordinary matter because of non-gravitational interactions. But very weakly interacting objects, such as the particles we are looking for, could perform these oscillations without the need for a tunnel," Budker points out. "Rather in the way that X-rays and the light from a flashlight can sometimes penetrate matter almost unimpeded," he adds. 

The situation described above could therefore theoretically occur if the trapped object were to consist of a small mass of some kind of ‘hidden material’ - what the authors call a 'hidden internal object' or 'HIO' - that has only feeble, if any, non-gravitational interactions with normal matter. And it is gravity that primarily causes even the elusive dark matter to interact with ordinary matter. So it is feasible that such potential HIOs could be particles of dark matter that are being confined inside the Earth by the force of gravity.

All in all, this suggests a tantalizing prospect. "Our idea is that it might be possible to detect the presence of such HIOs by means of sensitive measurements of the acceleration due to gravity on the surface of the Earth," explains Nataniel Figueroa, a co-author of the article and fellowship holder at the Mainz Physics Academy, which coordinates all activities relating to graduate training and the promotion of young research talents within the PRISMA+ Cluster of Excellence. "We don't need to construct our own experiment to obtain this data because it is already available and is publicly accessible. The data have been collected by the International Geodynamics and Earth Tide Service (IGETS) global network of gravimeters." Distributed around the Earth, superconducting gravimeters (SGs) continuously measure temporal gravity fluctuations with high precision and long-term stability.

In their current paper, the researchers explain how they used Fourier analysis to search the IGETS data sets for characteristic spectral lines that could be an indication of the existence of HIOs – and thus perhaps even the presence of dark matter. In this case, each gravimeter in the network would detect a weak periodic signal at a characteristic frequency determined by the geometry of the orbit of the HIO and the location of the gravimeter. In their initial analysis, the researchers were unable to find any evidence for HIOs and, by extension, dark matter. However, they consider it may be possible to improve the sensitivity of their search by several orders of magnitude – for example, through better understanding of the other terrestrial noise sources and more advanced data analysis. Two other research groups are also currently using a similar approach to search for dark matter inside the Earth. "We have made contact with our colleagues to discuss future cooperation," explains Dmitry Budker, "because the project would certainly benefit from pooling our resources and ideas."

The interdisciplinary research team includes: Wenxiang Hu from the Peking University (Beijing), who came to Mainz as a visiting student and worked on the theory and modeling of HIO orbits and the likely gravimeter signals, Matthew Lawson, a postdoc at Stockholm University, who undertook the analysis of the data along with Nataniel L. Figueroa, Derek F. Jackson Kimball, who is a professor at California State University East Bay and scientific coordinator of the CASPER and GNOME experiments – which aim to detect and analyze dark matter using nuclear magnetic resonance and a global network of optical magnetometers; like Dmitry Budker, he was involved in various aspects of the project, Prof. Allen P. Mills Jr. from UC Riverside, who proposed the idea and participated in various phases of the project, and Dr. Christian Voigt of the GFZ German Research Center for Geosciences in Potsdam, who contributed his expertise in the field of geophysics.