In 2015, the iconic Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever tangible detection of gravitational waves. The waves were the result of two black holes colliding far out in the universe; since then, plenty of such signals have been seen from merging black holes, neutron stars, and even a few mixed mergers between the two.
Yet despite the success of LIGO—located at two American sites and supported by the Virgo detector located in Italy and Japan’s Kamioka Gravitational-Wave Detector (KAGRA)—astronomers have been able to confirm only one of these gravitational-wave-generating events using “traditional” light-based astronomy. This event was the merger of two neutron stars that produced the GW170817 gravitational wave signal.
Now, a team of scientists at the University of Minnesota has developed software upgrades that can help astronomers warn of merger events just 30 seconds after gravitational waves are captured on Earth. This early warning system should allow more merger events to be tracked with light astronomy.
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“With this software, we can detect the gravitational wave from neutron star collisions, which are normally too faint to see unless we know exactly where to look,” team member Andrew Toivonen and Ph.D. student at the University of Minnesota Twin Cities School of Physics and Astronomy, said in a statement. “The first detection of gravitational waves will help localize the collision and help astronomers and astrophysicists complete further studies.”
What are gravitational waves?
Gravitational waves are tiny ripples in the fabric of space and time, the two of which are united into a four-dimensional entity called “space-time.” Such waves were first predicted by Albert Einstein in his 1915 theory of gravity, general relativity.
General relativity predicts that gravity arises from objects with mass that distort the very fabric of spacetime. The greater the mass, the more extreme the curve, which explains why stars have a greater gravitational influence than planets.
Einstein also theorized that when objects accelerate, they cause spacetime to oscillate. These waves are only noticeable when really massive objects are being accelerated — objects like neutron stars and black holes that orbit each other in binary systems and emit gravitational waves as they do so. This continuous emission of gravitational waves, Einstein said, would carry away angular momentum and cause the ultradense objects to converge and eventually merge, a collision that sends out a high-pitched “squeal” of gravitational waves.
However, Einstein believed that even gravitational waves from objects significant enough to generate them would be too weak to be detected here on Earth.
Fortunately, he was wrong.
Still, spotting gravitational waves is still no mean feat. After all, binary systems of neutron stars and black holes are millions (sometimes even billions) of light-years away, and gravitational waves lose energy as they travel through space.
In order for LIGO to detect gravitational waves from these events, this massive laser interferometer consists of two L-shaped arms, each 2.5 miles (4 kilometers) long. When in phase, the laser light shines down each of these arms. This means that when the beams meet, the peaks and troughs of their waves line up and the laser light is amplified, something called “constructive interference”.
However, if a gravitational wave passes through one of these lasers and space is compressed and stretched, then the laser passing over that part of space will be out of phase, meaning that troughs meet peaks, and vice versa, resulting in ” destructive interference’ and therefore no amplification.
The changes that LIGO picks up to “hear” gravitational waves are 0.0001 times wider than a proton, particles that sit at the heart of atomic nuclei. In “standard” astronomical terms, this is equivalent to measuring the distance to the nearest star, Proxima Centauri, about 4.2 light-years away, with a quantitative accuracy equal to the width of a human hair.
LIGO, Virgo and KAGRA are currently in their fourth operational cycle, which began on 24 May 2023 and is scheduled to last until February 2025. Between each of the previous operational cycles, scientists in the LIGO/Virgo/KAGRA collaboration upgraded the software used to detecting the shape of gravitational wave signals, tracing the evolution of the signal, and then estimating the masses of the neutron stars or black holes that crashed to create the signal. This software also sends an alert to other scientists.
Thanks to simulations created using data collected from observing periods one through three, as well as artificially generated gravitational wave signals, the team now knows that upgrades can be made to the observing software that allow warnings to be issued in within 30 seconds of detecting a gravitational wave during observation. Such improvements will affect observation period four.
This should help astronomers track the locations of these events in the sky with light astronomy, something no gravitational wave detector can currently do, and determine how collisions between the most exotic and mysterious objects in space play out. over time.
This is unlikely to be the end of gravitational wave detection warning upgrades. At the end of this current operational cycle, LIGO/Virgo/KAGRA scientists will use data collected over nearly two years of “listening” to a universal symphony of colliding black holes and neutron stars to further improve the warning rate.
The team’s research is published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).
