One of the most exciting breakthroughs in astronomy over the past decade was the detection of gravitational waves. Since the days of Galileo Galilei, astronomy was about the detection of electromagnetic signals with telescopes. As it turns out, the main constituents of the Universe are not observable in that way.
Our current data indicates that 85% of the matter in the Universe is invisible electromagnetically, constituting dark matter. In addition, 70% of the energy budget of the Universe is dark energy. Cosmologists infer these constituents because they affect visible matter gravitationally. Can we build a detector of near-Earth objects that would sense the gravitational signal of passing dark objects?
If dark matter is made of asteroid-mass objects, like primordial black holes, our telescopes would not notice them even when they pass near Earth. In a recent paper, I showed that the LIGO-Virgo-KAGRA gravitational wave observatories could detect a dark object if it moves close to the speed of light and its mass is larger than a hundred million tons. Such an object would cross the radius of the Earth within two hundredths of a second and produce a gravitational tidal signal in the frequency band of LIGO-Virgo-KAGRA. Needless to say, no such object was detected so far.
Within a decade, the LISA space observatory will expand gravitational wave detection to the frequency range between milli- and micro-Hertz and a smaller spacetime strain. This will usher in a new era of sensitivity to dark near-Earth objects in the asteroid mass range. It could also open the door to the detection of Unidentified Anomalous Phenomena (UAP) gravitationally, which the Galileo Project observatories are attempting to detect electromagnetically. Pulsar Timing Arrays(PTAs) probe a frequency range of a few nano-Hertz, but so-far they were only sensitive to the cumulative gravitational wave background at these frequencies – which constitute the noise floor for the detection of individual sources.
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