The Moon’s orbit proposed as a gravitational wave detector

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Researchers from UAB, IFAE and University College London propose to use variations in the distance between the Earth and the Moon, measurable with sub-centimetre precision, as a new detector of gravitational waves in a range of frequencies that current devices cannot detect. The research, which could pave the way to detecting signals from the early universe, was published recently in Physical examination letters.

Gravitational waves, predicted by Albert Einstein at the beginning of the 20th century and first detected in 2015, are the new messengers of the most violent processes taking place in the universe. Gravitational wave detectors scan different frequency ranges, like moving a dial when tuning into a radio station. Nevertheless, there are frequencies impossible to cover with current devices and which can harbor fundamental signals for understanding the cosmos. A particular example can be seen in microhertz waves, which could have been produced at the dawn of our universe, and are virtually invisible to even the most advanced technology available today.

In a recent article published in the journal Physical examination letters, researchers Diego Blas from the Department of Physics at the Universitat Autònoma de Barcelona (UAB) and the Institut de Física d’Altes Energies (IFAE), and Alexander Jenkins from University College London (UCL), point out that a natural gravitational wave detector exists in our immediate environment: the Earth-Moon system. The gravitational waves constantly hitting this system generate tiny deviations in the Moon’s orbit. Although these deviations are minute, Blas and Jenkins plan to take advantage of the fact that the exact position of the Moon is known with an error of at most one centimeter, thanks to the use of lasers sent from different observatories which reflect continuously. on mirrors left on the surface of the Moon by the Apollo space mission and others. This incredible precision, with an error of at most a billionth of a part, is what can detect a small disturbance caused by ancient gravitational waves. The Moon’s orbit lasts about 28 days, which translates to particularly relevant sensitivity when it comes to microhertz, the frequency range that researchers are interested in.

Similarly, they also propose to use the information that other binary systems in the universe can provide as gravitational wave detectors. This is the case of the binary systems of pulsars distributed throughout the galaxy, systems in which the beam of radiation from the pulsar makes it possible to obtain the orbit of these stars with incredible precision (with precision to a millionth). Since these orbits last about 20 days, the passage of gravitational waves in the microhertz frequency range particularly affects them. Blas and Jenkins concluded that these systems could also be potential detectors of these types of gravitational waves.

With these “natural detectors” in the microhertz frequency range, Blas and Jenkins were able to propose a new form of study of gravitational waves emitted by the distant universe. More specifically, those produced by the possible presence of transitions in highly energetic phases of the early universe, commonly observed in many models.

“Perhaps most interestingly, this method complements future ESA/NASA missions, such as LISA, and observatories participating in the Square Kilometer Array (SKA) project, to achieve near-total gravitational-wave coverage. from nanohertz (SKA) down to centihertz (LIGO/VIRGO) frequency ranges. the microhertz frequency range is a challenge, which can now be achieved without the need to build new detectors, and by observing only the orbits of systems we already know. This connection between fundamental aspects of the universe and more mundane objects is particularly fascinating and may eventually lead to the detection of the first signals we have ever seen, and thus change what we know about the cosmos,” he concludes.

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Material provided by Autonomous University of Barcelona. Note: Content may be edited for style and length.

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