Gravitational Waves: R&D for Gravitational Wave Detectors

The Gravity Lab’s main focus is research and development for improved gravitational wave detectors. Gravitational waves are distortions of spacetime traveling at the speed of light, caused by accelerated motion of masses. Their existence had been predicted by Einstein’s theory of general relativity, but the first successful direct detection did not happen until 2015, with the first confirmed observation of a gravitational wave by LIGO, the Laser Interferometer Gravitational-Wave Observatory. Gravitational waves are incredibly challenging to detect – the ones we are currently able to measure produce deflections much smaller than the diameter of an atomic nucleus in km-scale interferometers. That is despite them originating from some of the most violent events in the universe – mergers of binary black-hole systems or binary neutron stars. With its first direct detection of gravitational waves, LIGO opened up a whole new window into the universe and laid the foundation for the new field of gravitational wave astronomy. With increasing sensitivity and more and more refined data analysis, we can now begin detailed studies of the astronomical events producing gravitational waves, their distribution, etc. There are ongoing efforts to extend our detection and analysis capabilities to include new sources of gravitational waves, including for example core-collapse supernovae or primordial gravitational waves. It is truly an exciting time for gravitational wave research!

We are contributing to this endeavor by supporting the development of future, third-generation gravitational wave detectors with greatly improved sensitivity. These new detectors are expected to operate at cryogenic temperatures to decrease the background noise – for LIGO Voyager, for example, currently an operating temperature of 123K is considered. Achieving the goal of a cryogenic gravitational wave interferometer comes with many technological challenges. To realize the proposed third-generation gravitational wave detectors like LIGO Voyager or the European Einstein Telescope (ET), a great number of open questions needs to be addressed, starting with benchtop explorations and continuing via larger-scale prototypes to the final instruments.

In particular, we are developing an optical cryostat to study ice formation on cold optical surfaces and material properties at low temperature. The buildup of thin ice layers on the test masses in a cryogenic gravitational wave interferometer can drastically change their carefully optimized optical and mechanical properties. As the first use of the setup, we are planning to characterize the properties of such ice layers and test possible mitigation strategies.

If you are a student interested in research opportunities in this field, or if you are looking for more information on specific possible projects, please contact Dr. Svenja Fleischer!

Precision Lab Experiments Testing Gravity

We also have some minor involvement with small-scale lab experiments studying gravity in collaboration with the Eöt-Wash Group at the University of Washington. These experiments use torsion balances, which are much refined, modern versions of the kind of instrument used in the famous Cavendish experiment. Shown on the right is a detail photo of part of a cryogenic torsion balance operating at a temperature of ~10K. Modern torsion balances are very sensitive instruments used to test various aspects of gravity or to search for hypothetical new interactions weaker than gravity. Two examples of projects pursued by the Eöt-Wash Group are:

• tests of the weak equivalence principle, which – when stated as the universality of free fall – predicts that any object, independent of its mass or composition, will experience the same acceleration in a given uniform gravitational field
• tests of the gravitational inverse-square law at sub-millimeter distances, providing stringent limits on various beyond-the-standard model particle physics theories

Recent Publications

• The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration,
  A.G. Abac et al., Ultralight vector dark matter search using data from the KAGRA O3GK run. arXiv:2403.03004 [astro-ph.CO] (2024).
• The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration:
  A.G. Abac et al., Search for eccentric black hole coalescenses during the third observing run of LIGO and Virgo. arXiv:2308.03822 [astro-ph.HE] (2023).
• R. Abbott et al., LIGO Scientific Collaboration, Virgo Collaboration, KAGRA Collaboration. Open data from the third observing run of LIGO, Virgo, KAGRA, and GEO. Astrophys. J. Suppl. Ser. 267, 29 (2023).
• S. M. Fleischer, M. P. Ross, K. Venkateswara, C. A. Hagedorn, E. A. Shaw, E. Swanson, B. R. Heckel, and J. H. Gundlach, A cryogenic torsion balance using a liquid-cryogen free, ultra-low vibration cryostat. Rev. Sci. Instrum. 93, 064505 (2022).
• J. G. Lee, E. G. Adelberger, T. S. Cook, S. M. Fleischer, and B. R. Heckel. A Fourier-Bessel test of gravity at separations down to 52μm. Phys. Rev. Lett. 124, 101101 (2020).