Scientists propose new method to search for dark matter using LIGO

A new study published in Physical Exam Letters (PRL) proposes to use gravitational wave detectors like LIGO to search for dark matter in a scalar field.

Dark matter, an elusive form of matter, makes up to 30% of the observable matter in the universe. It does not absorb, emit or reflect light, making it invisible to our eyes.

Its presence is inferred from its gravitational effects on visible matter, such as the motion of galaxy clusters and the rotation of galaxies. Due to its elusive nature, it has attracted widespread interest from scientists. But, despite extensive research, its nature remains unknown.

THE PRL The study, led by Dr. Alexandre Sébastien Göttel of Cardiff University, explores the search for a particular candidate for dark matter called scalar-field dark matter. Dr. Göttel spoke to Phys.org about the research.

“I recently changed fields from particle physics, focusing on solar neutrinos, to analyzing gravitational-wave data. The opportunity to search for dark matter with LIGO seemed like the perfect way to apply my expertise in both fields while learning more about interferometry,” said Dr. Göttel.

Gravitational wave detectors

Gravitational wave detectors are highly sensitive devices that detect tiny distortions (also called gravitational waves) in space-time.

The Laser Interferometer Gravitational-Wave Observatory, or LIGO, uses laser interferometers to detect gravitational waves. The device consists of two 4-kilometer-long arms, arranged at right angles. A laser beam is split in two and sent down each arm.

Gravitational waves stretch and compress spacetime itself, and since they are transverse in nature, they would cause the distance of one arm to stretch, while compressing the other. This means that the time taken by light would be different along each arm.

The two beams are then sent back to the center using a mirror and the interference patterns are measured. It is thanks to this modified interference pattern that LIGO detects the presence of a gravitational wave.

Using LIGO to detect dark matter

One hypothetical form of dark matter is scalar-field dark matter. These are ultralight scalar boson particles, meaning they have no intrinsic spin or directionality. Simply put, if they were to spin in space, their properties would remain unchanged.

Scalar field dark matter is thought to interact weakly with matter and light. This weak interaction, combined with its low mass, means that scalar field dark matter can exhibit wave-like structures, spreading out and overlapping to form wave-like patterns.

This allows them to create stable formations, like dark matter clouds that can move through space without breaking apart. This property of dark matter’s scalar field is essential for using gravitational wave detectors, like LIGO, to search for them.

“Some theories suggest that dark matter behaves more like a wave than a particle,” says Dr. Göttel. “These waves would cause tiny oscillations in normal matter, which can be detected by gravitational wave detectors,” he explains.

Testing mass effects

The research team used data from LIGO’s third round of observations and extended the search to lower frequencies (10 to 180 Hertz), improving sensitivity over previous work.

While previous studies accounted for the effect that dark matter from the scalar field would have on the beam splitter, similar to gravitational waves, the researchers also incorporated the effect on the mirrors of the interferometer arms.

“At the atomic level, one can imagine that the dark matter field fluctuates in parallel with the electromagnetic field. The oscillations of the dark matter field effectively change the fundamental constants, i.e. the fine structure constant and the electron mass, which govern the electromagnetic interactions,” said Dr. Göttel.

Since dark matter oscillations affect every atom in the universe, the research team had to crucially account for their effect on the test masses, or mirrors, in the interferometer’s arms.

Dr. Göttel said: “All matter would be affected by these oscillations, but oscillations from other parts of the instrument would have no or very little effect on the passing laser beam, which we can detect.”

Setting upper limits

The research team developed a theoretical model to understand how dark matter in the scalar field would interact with LIGO components, beam splitters and test masses.

They then used simulation software to understand how dark matter in the scalar field would affect LIGO’s output, if it were present. The simulation gives an idea of ​​what kind of signal or anomaly they should look for in LIGO’s data.

The research team then used the LIGO data and applied a method called logarithmic spectral analysis to identify patterns or signals corresponding to the predicted dark matter effects of the scalar field.

The team failed to find convincing evidence for the existence of scalar field dark matter in the LIGO data. However, they were able to set new upper limits on the strength of the interaction between dark matter and LIGO components.

This coupling strength is the threshold value beyond which the presence of scalar dark matter could be detected. The value of this coupling strength has been improved by a factor of 10,000 compared to previous work, in this particular frequency range.

“We are the first to account for additional differential effects in the test masses, which are significant at low frequencies. By combining this with a new analysis method that maximizes the statistical power of the data, we have achieved significantly improved results,” concludes Dr. Göttel.

The study presents methods for predicting the impact of changes in central optics, showing that small adjustments in mirror thickness could yield significant improvements. The research team also believes that future detectors will be able to outperform even indirect search methods and rule out entire classes of scalar dark matter theories.

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