Within one second of the big bang, the first newborn black holes may have announced their formation with gravitational waves that stretched and squeezed the fabric of existence as they rippled outward into the expanding universe. Now researchers at Northwestern University have begun planning a tabletop-size sensor that could detect these primordial howls for the first time.
The gigantic $1-billion Laser Interferometer Gravitational-Wave Observatory (LIGO) first measured the spacetime ripples known as gravitational waves in 2016; these phenomena came from the collision and merging of distant supermassive black holes. Since then, massive detectors have also recorded gravitational waves from merging neutron stars. Northwestern's proposed mini detector, which received an influx of funding in July, could measure higher-frequency waves from objects that have never been measured before—such as black holes in the earliest universe.
Current gravitational-wave detectors such as U.S.-based LIGO and Europe's Virgo use a sprawling system of mirrors and laser “arms” that stretch for kilometers to measure tiny changes in distance caused by passing gravitational waves. Northwestern's Levitated Sensor Detector would use lasers to suspend a glass bead inside a vacuum chamber, creating an extremely force-sensitive sensor with arms just a meter long. It would listen for echoes from the formation of primordial black holes and the activity of theoretical particles called axions, both of which are candidates for mysterious dark matter—hidden materials that may constitute much of the universe's mass and are invisible except for their gravitational presence.
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“I think there is more interest in expanding the frequency range in the search for gravitational waves, particularly after the recent exciting LIGO discoveries,” says Andrew Geraci, a physicist at Northwestern and principal investigator on the new detector project. “These sources that are dark matter–related are a bit more speculative—the sources that LIGO found were pretty much expected to exist.”
To try detecting waves from such sources, the Northwestern project will use $1 million from the W. M. Keck Foundation, a U.S. charitable foundation based in Los Angeles, and additional support from the university. After two years of development, the meter-long prototype would operate for a preliminary year and potentially pave the way for a larger detector that could reach 10 meters in length.
Many researchers question whether anything has the energy to be a strong gravitational-wave source at such high frequencies—above 10 kilohertz—says Rana Adhikari, an experimental physicist at the California Institute of Technology, who is not involved in the levitated sensor project. But he adds that the hypothetical sources linked to dark matter could prove the exception: “We may be surprised by all of the exotica the universe produces in the ultrasonic gravitational-wave regime.”