Physicists are coming up with ingenious new techniques to take use of the high sensitivity of gravitational wave detectors like LIGO. So yet, though, they have found no evidence of exotica.
Even the most powerful gravitational waves that flow through the globe, caused by distant black hole mergers, only stretch and compress each mile of Earth’s surface by one-thousandth the width of an atom. It’s difficult to comprehend how minute these ripples in the fabric of space-time are, much alone detect them. However, scientists received one in 2016, after spending decades creating and fine-tuning an equipment known as the Laser Interferometer Gravitational-Wave Observatory (LIGO).
The world of unseen black holes is unfolding, with roughly 100 gravitational waves already documented. However, it is just half of the tale.
Gravitational wave detectors are getting some extra work
“People have begun to wonder whether there is more to what we receive out of these equipment than simply gravitational waves?” Rana Adhikari, a physicist at the California Institute of Technology, explained his findings.
Researchers are developing methods to utilize similar detectors to seek for other elusive phenomena, most notably dark matter, the nonluminous substance that keeps galaxies together.
In December, a team headed by Hartmut Grote of Cardiff University published a paper in Nature describing how they utilized a gravitational wave detector to search for scalar-field dark matter, a lesser-known candidate for missing mass in and around galaxies. The researchers failed to detect a signal, ruling out a wide range of scalar-field dark matter theories. Now, the substance can only exist if it has a very weak effect on regular matter – at least a million times weaker than was previously considered feasible.
“It’s a pretty good finding,” said Keith Riles, a University of Michigan gravitational wave astronomer who was not involved in the study.
Until recently, the most likely candidate for dark matter was a slow-moving, weakly interacting particle akin to other fundamental particles — a kind of heavy neutrino. However, experimental searches for these so-called WIMPs continue to provide no results, leaving space for a plethora of options.
“In dark matter searches, we’ve sort of hit the point where we’re searching everywhere,” said Kathryn Zurek, a theoretical physicist at Caltech.
Three scientists argued in 1999 that dark matter may be formed of particles so light and abundant that they’re better conceived of as a field of energy that pervades the cosmos. This “scalar field” has a value at each point in space, and the value oscillates at a certain frequency.
The characteristics of other particles and basic forces would be gradually altered by scalar-field dark matter. The mass of the electron and the intensity of the electromagnetic force, for example, would fluctuate with the scalar field’s undulating amplitude.
For years, scientists have speculated about whether gravitational wave detectors might detect such a wobble. These detectors detect little disturbances using a technique known as interferometry. First, laser light enters a “beam splitter,” which separates the light and sends beams in opposite directions, like the arms of an L. The beams reflect off mirrors at the ends of both arms before returning to the L’s hinge and recombining. If the returning laser beams are thrown out of phase, as by a passing gravitational wave, which temporarily lengthens one arm of the interferometer while contracting the other, a distinct interference pattern of dark and light fringes appears.
Could scalar-field dark matter lead the beams to be out of sync, resulting in an interference pattern? “The prevailing assumption,” Grote said, “was that any distortions would effect both arms equally, canceling out.” But then, in 2019, Grote had an epiphany. “One morning when I woke up, I had an epiphany: the beam splitter is precisely what we need.”
The beam splitter is a glass block that behaves like a leaky mirror, reflecting half of the light that reaches its surface and allowing the other half to flow through. If scalar-field dark matter is present, the electromagnetic force lessens when the field reaches its maximum amplitude; Grote reasoned that this would cause atoms in the glass block to shrink. When the amplitude of the field decreases, the glass block expands. This wobbling will gently modify the distance traveled by the reflected light while having no effect on the transmitted light, resulting in an interference pattern.
Sander Vermeulen, Grote’s PhD student, used computers to trawl through data from the GEO600 gravitational wave detector in Germany for interference patterns caused by several million distinct frequencies of scalar-field dark matter. He didn’t see anything. “It’s disheartening because finding dark matter would be the discovery of a lifetime,” Vermeulen added.
However, the search was merely ever “a fishing expedition,” according to Zurek. The frequency of the scalar field and the degree of its influence on other particles (and hence the beam splitter) might be virtually anything. GEO600 can only detect a limited number of frequencies.
As a result, the inability of the GEO600 detector to detect scalar-field dark matter does not rule out its existence. “It’s more of a demonstration that we now have a new instrument to seek for dark matter,” Grote said. “We’ll keep looking.” He also intends to utilize interferometers to look for axions, another common candidate for dark matter.
Meanwhile, Riles and his colleagues have been looking for evidence of “black photons” in data from LIGO, which includes detectors in Livingston, Louisiana, and Hanford, Washington, as well as its Italian partner, the Virgo detector near Pisa. Dark photons are hypothesized light-like particles that would mostly interact with other dark matter particles but would sometimes collide with conventional atoms. If they’re all around us, they’ll press on one mirror in an interferometer more than the other at any one time, affecting the relative lengths of the arms. “There will be an imbalance in one way, simply a random fluctuation,” Riles said. “So you attempt to take advantage of it.”
Because dark photon wavelengths may be as long as the sun, any random fluctuations that upset the mirrors of the Hanford interferometer would have the same impact at the Livingston detector, over 5,000 kilometers distant, and linked effects in Pisa. However, the researchers discovered no such relationships in the data. Their findings, which were published last year, imply that dark photons, if they exist, must be at least 100 times weaker than previously thought.
According to Adhikari, gravitational wave detectors might detect “human-sized” dark matter particles weighing hundreds of kilos. These hefty particles would gravitationally attract LIGO’s mirrors and laser beams as they sped past the detector. “You’d witness a small blinking in the beam’s strength as the particle travels past,” Adhikari said. “The whole L-shape detector is a kind of net that can pick up these particles.”
What else may these sensitive devices pick up? At Caltech, Adhikari is working on an unique interferometer to hunt for evidence that space-time is pixelated, as certain quantum theories of gravity suggest. “That’s always physicists’ dream.” “Is it possible to quantify quantum gravity in the lab?” According to conventional opinion, a detector capable of probing such small distances would be so massive that it would collapse into a black hole under its own weight. Zurek, on the other hand, has been working on a concept that may detect quantum gravity using Adhikari’s setup or another experiment at Grote’s lab in Cardiff.
In alternative quantum gravity theories, space-time is not pixelated; rather, it is a 3D hologram that arises from a 2D quantum particle system. Zurek believes that gravitational wave detectors may detect this as well. Small quantum fluctuations in 2D space would be magnified when holographically projected into 3D space, perhaps creating waves in space-time large enough to be detected by an interferometer.
“When we first began working on this, folks asked, ‘What are you talking about?'” “You’re utterly insane,” Zurek said. “People are finally beginning to pay attention.”