Improved Parallel Plate Force Metrology

It is known that we can directly observe only 5% of the universe. The remaining 95% are invisible, and are therefore referred to as dark. We know that these dark 95 % exist, as without them the movement of the remaining mass in the universe could not be understood. Galaxies, such as our milky way, would be torn apart by centrifugal forces but dark matter holds the stars and gas together by its gravity. Dark energy is even more difficult to detect. It acts as a pressure causing the accelerated expansion of our universe and we can only observe that all objects move away from us with a speed proportional to their distance. We can imagine the situation as being on the surface of a balloon just being filled. While we can observe the effects of dark matter and dark energy, even after decades of intense research we don`t know what they are made of. A large number of theories has been devised to shed light on the issue but none could be proven experimentally so far. Some theories state a connection between dark energy and the zero-point energy. The latter is a property of quantum objects, which can never be truly at rest but keep fluctuating even at zero temperature. These vacuum fluctuations cause the so-called Casimir effect manifesting as a force between objects at close separation. Geckos, for instance, use this effect to stick to smooth surfaces more effectively. In the laboratory, we can measure Casimir forces but there still are open questions that cannot be answered with current experimental accuracy. CANNEX is the worldwide only experiment capable of performing highly accurate (metrological) force measurements between macroscopic plane-parallel plates. This geometry has the advantage of maximizing forces acting between its surfaces for which we can achieve higher precision at the same sensitivity. In addition, the comparison between experiment and theory is eased, as for most theories analytic solutions without extensive approximations exist. Technically, however, plane- parallel plates are a challenge. Parallelism has to be measured and corrected at the level of one part in a million of a degree. Perfectly flat and clean surfaces have to be used. Vibrations have to be attenuated, and thermal fluctuations have to be eliminated to less than 1/1000C. Once we pass all these hurdles, however, we will be able to measure the Casimir force with hundredfold higher accuracy and at larger distances than previously. Using the resulting data, we could answer questions being discussed in the field since decades. In CANNEX, we can also measure the gravitational attraction between the plates and any hypothetical effects predicted by dark matter or dark energy theories. If we don`t find such effects, we can exclude the corresponding theories within the experimental sensitivity. Using these results, we can potentially answer many open questions and expand our knowledge about the dark sector of the universe.

We can only observe 5% of the universe directly. The rest is not visible and thus named `dark'. Nonetheless we know that this dark sector exists, as otherwise the movement of visible matter could not be explained. Galaxies, such as the milky way, would be torn apart by centrifugal forces but dark matter keeps stars and gas together via its gravitational pull. Dark energy is even harder to observe. It acts as a negative pressure accelerating the expansion of our universe. While we can observe the effects of dark matter and dark energy in astronomy, even after decades of intense searches we still don't know what they are made of. Many theories have been devised to explain the dark sector but none could be confirmed experimentally. Some theories make a connection between dark energy and the so-called zero point energy. The latter is a property of quantum objects, which can never stand still, not even at zero temperature. These fluctuations give rise to the Casimir effect causing an attractive force between macroscopic objects at small separation. Geckos, for example, use this effect in order to stick to glass surfaces. In the corresponding theory, however, there are still some deep open questions that could not be answered completely by existing experiments. One way to better understand dark energy and dark matter, and to answer open questions regarding the Casimir effect is to perform very precise force measurements. For this purpose, we built CANNEX. It is the worldwide only experiment to perform metrological (very precise) force measurements between plane parallel plates. This geometry brings the advantage that any forces acting between the plates are maximized, which improves sensitivity. Another advantage is that theory and experiment can be compared more easily. If, eventually, we don't find any of the predicted effects within the experimental uncertainty, we can exclude the respective theories. In this way, we can answer many open questions and expand our knowledge about the dark sector. In just three years, we have built and tested one of the best seismic attenuation systems in Austria, a thermal control system keeping temperatures stable within 1/1000th of a degree, an optical measurement system keeping our 1 cm large plates parallel with a maximum deviation of one billionth of a meter, and several other technically complex systems. The experiment was transferred to the Conrad observatory of Geosphere Austria, where in isolation from the noise of civilisation it shall expand our knowledge of the dark part of the universe, the Casimir force and gravity at small separation at the interface between classical and quantum realms. Until first results are expected, however, we need to complete our testing phase, which will take until the end of 2026.