In September 2015, humanity captured "the tremor of the universe" for the first time. A hundred years after Einstein predicted it, gravitational waves reached Earth. The final ripple left as two black holes orbited and merged was recorded by the detectors at the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States. Ten years on, gravitational waves have become a new "eye" for viewing the universe.
In an interview with ChosunBiz, Lee Hyungmok, professor of astronomy at Seoul National University (head of the Gravitational-Wave Universe Research Group), said, "There have been about 300 gravitational-wave events detected so far," adding, "At first, even a single event was amazing, but now we have reached a stage where we can discuss the distribution and evolution of black holes using gravitational-wave statistics."
◇Research now possible on the evolution of stars and black holes
Gravitational-wave observations are handled by four detectors: two LIGO units in the United States, Virgo in Italy, and KAGRA in Japan. More than 2,200 researchers worldwide form the joint observing network LVK (LIGO-Virgo-KAGRA), and Korean researchers are also participating. Lee is one of the 1,600 members of the LIGO collaboration. Around 40 Korean researchers belong to LVK.
Lee explained, "Gravitational waves are produced when two massive celestial bodies orbit close together and then merge," adding, "You can think of spacetime sloshing so that the distance between two points minutely increases and decreases." LIGO detected gravitational waves by measuring tiny changes in the distance that a laser travels back and forth between two mirrors.
The massive celestial bodies that produce gravitational waves are black holes. They are objects formed when very massive stars collapse extremely, with gravity so strong that they suck in matter and even light, hence the name black hole. The first gravitational waves, which began from a binary black hole system 1.3 billion light-years away (1 light-year is the distance light travels in one year, about 946 billion kilometers), subtly rippled spacetime as they reached Earth. Recalling the first detection in 2015, Lee said, "I felt a shiver when we confirmed that a real signal had been captured."
When LIGO first went online, it captured three gravitational-wave events in three months, but now about two are found each week, bringing the cumulative number of observations to more than 300. It also turned out that there are more black holes with larger-than-expected masses. Lee said, "At first it was just, 'So this exists in the universe,' but now we can calculate mass, distance, and spin axis to study black hole evolution," adding, "We may have to revise existing models for stellar evolution and black hole merger mechanisms."
◇"Korea to lead research on cosmic expansion and dark energy"
Gravitational-wave research is still in its infancy. There are only four detectors worldwide. That is because building them takes decades, and operating them costs hundreds of billions of won. Lee said, "While it is difficult for Korea to build a detector right away, we can expand our influence within the global network in data analysis and electromagnetic follow-up observations."
In the past, the universe could only be observed through light emitted by celestial bodies, but now we can capture multiple signals at once—gravitational waves and electromagnetic waves and particles from celestial collisions—and approach it from multiple angles. This is the dawn of the era of multi-messenger astronomy. Since its launch in 2021, the Gravitational-Wave Universe Research Group, led by Director General Lee, has been capturing multiple messengers and analyzing them with artificial intelligence (AI) to precisely measure the Hubble constant, which indicates the degree of cosmic expansion.
Lee said, "We can reconstruct the expansion history of the universe with gravitational waves," adding, "This is directly connected to research on dark energy." Scientists believe that only 5% of the universe consists of matter that emits light and is observable, while 70% is dark energy that drives the expansion of the universe. The remaining 25% is called dark matter, which does not emit light but attracts objects.
Seoul National University has installed a multiwavelength optical telescope at the El Sauce Observatory in the Andes, about 480 kilometers from Santiago, Chile. The Gravitational-Wave Universe Research Group aims to be the fastest in the world to capture a kilonova—the light that emerges immediately as gravitational waves are generated—with this telescope. A kilonova is a phenomenon in which colliding neutron stars at the end of their lives emit intense light, and it has been known as the source of how elements heavier than iron spread throughout the universe.
Lee explained, "We observe in seven dimensions by adding time, wavelength, brightness, and line-of-sight velocity as one dimension each to the three-dimensional space we live in," adding, "This enables not only kilonova detection but also measuring the mass of supermassive black holes, calculating the expansion rate of the universe, and creating an all-sky spectral map." He also said, "When gravitational waves are detected, we need to scan the entire sky quickly," adding, "Our telescope can simultaneously measure brightness changes at multiple wavelengths to swiftly determine whether it is a kilonova, so its speed in spectral measurements is a major advantage."