Next-generation gravitational-wave detectors could pinpoint dark matter

Gravitational astronomy is a relatively new discipline that has opened many doors for astronomers to understand how the huge, violent end of the scale works. It has been used to map merging black holes and other extreme events across the universe. Now a team at Cal Tech’s Walter Burke Institute for Theoretical Physics thinks they have a new use for the new technology: limiting the properties of dark matter.

As we’ve reported many times in the past, dark matter is what makes up the vast majority of mass in the universe, but it’s invisible to ordinary electromagnetic waves, making it literally impossible for us to “see” in the way we normally would think about it. However, these particles, if that is, in fact, what they are, interact with another of the fundamental forces: gravity.

Which would make them a potential target for gravitational wave (GW) observatory studies. But there are some assumptions underlying that work. The first is that dark matter is a “macro” phenomenon, that is, it is not subject to the world of quantum mechanics. Gravitational waves will likely only work on what the authors call ultra-heavy dark matter, which in context is the mass of the papers themselves.

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UT video on what exactly gravitational waves are.

Interferometers designed to detect gravitational paths could potentially pick up signals affected by particles heavy enough to fall into this category. Specifically, those particles would affect three different characteristics of the gravitational wave, two of which the authors are calculating for the first time,

The first is the Doppler effect, which every high school physics student learns about, usually with an example of how ambulances sound different when they’re coming towards you versus when they’re heading away from you. The same phenomenon occurs with gravitational waves, as they affect space-time in a similar way depending on how their source moves relative to the GW observatory.

For a more nuanced look at the dark matter that can affect GWs, the authors look at Shapiro and Einstein’s lag. Shapiro delay is a change in the time it takes for a signal to travel from one end of an interferometer to the other. This can be changed depending on whether or not there is spacetime compaction somewhere along the arm of the interferometer. On the other hand, Einstein’s lag is an actual delay in the clock that the interferometer uses to measure gravitational waves. However, this effect cancels out in specific interferometer configurations.

UT video describing how gravitational astronomy fundamentally changes the way we understand the universe.

What the authors infer from all this is that modern GW observatories that should be online shortly, such as the Quantum Entanglement of Space-Time (GQuEST) experiment gravity at CalTech, should be able to detect passing dark matter. if it is large enough to be considered “ultra heavy”. But there is another nuance in the paper which is intriguing and points to a potentially deeper understanding of the underlying physics,

Physics students around the world are taught the fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. But there may be a fifth force, which has hitherto been invisible to our detection. This force, known as the Yukawa interaction, is a theoretical fifth fundamental force that operates between dark matter and the more traditional types of particles more familiar to students of classical physics — in theoretical physics, they’re known as baryons. So far, there has been no conclusive proof that this force exists, but some experiments have begun to work towards limiting it. If it exists, these same GW detectors may play a role in helping limit it further, according to the paper.

Finding a new fundamental force and solving a mystery that has plagued theoretical physics for decades is a heavy burden to place on a relatively new science. But that’s just how science itself moves forward, using new technologies to make further measurements and prove or disprove new theories. Now, after a long time, it’s time to shine for gravitational astronomy.

Learn more:
Du et al. – Macroscopic detection of dark matter with gravitational wave experiments
UT – We may soon detect gravitational waves from dying stars
UT – After decades of observations, astronomers have finally detected the pervasive background hum of merging supermassive black holes
UT – Gravitational wave detectors: how they work

Main picture:
Artist’s impression of Cosmic Explorer, one of the next generation gravitational wave detectors.
Credit – Matthew Evans / Provided

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