Ten years after the groundbreaking discovery of gravitational waves, physicists are emphasizing that this is just the beginning of their journey into the cosmos. On September 14, 2015, the twin facilities of the Laser Interferometer Gravitational-wave Observatory (LIGO), located in Hanford, Washington, and Livingston, Louisiana, detected ripples in space-time that originated over a billion years ago from the catastrophic merger of two black holes in distant galaxies. This milestone was the result of more than four decades of significant breakthroughs and advancements in experimental techniques.
Since the initial detection, observing black hole binaries has become a routine occurrence. The sensitivity of the LIGO detectors, along with their sister observatories—Virgo near Pisa, Italy, and KAGRA located under Mount Ikenoyama, Japan—has approximately doubled over the past decade. This enhancement allows scientists to monitor a broader region of the Universe that contains about eight times as many galaxies as before. “We see binary black holes every three days on average now, which is pretty amazing to me,” remarks David Reitze, a physicist at the California Institute of Technology and a longtime director of the LIGO observatories. “It’s only going to get better.”
Looking ahead, researchers in both the United States and Europe aim to construct larger observatories capable of detecting gravitational waves from anywhere within the observable Universe. One ambitious project is the Cosmic Explorer (CE), which proposes to be an interferometer similar to LIGO but with arms that are ten times longer, extending up to 40 kilometers. If realized, the CE could identify 100,000 black-hole mergers annually, allowing scientists to observe events that occurred over ten billion years ago during an era when galaxies were actively forming and merging stars and black holes.
In addition to black-hole mergers, the Cosmic Explorer would also detect more than one million mergers of lighter objects known as neutron stars, translating to approximately one detection every few seconds, as noted by Stefan Ballmer, a physicist at Syracuse University in New York. However, constructing the CE poses challenges due to its extensive arms, which will encounter the curvature of the Earth. To mitigate this, physicists are scouting for naturally bowl-shaped locations in the United States to minimize the need for extensive digging.
Meanwhile, a series of enhancements named LIGO A (A-sharp) is planned, which could more than double the sensitivity of the existing observatory by the early 2030s. These upgrades will test cutting-edge technology that may eventually be incorporated into the Cosmic Explorer. Key improvements will include increasing the power of the lasers used in the interferometer arms and installing heavier, more stable, and highly reflective suspended mirrors at the ends of the arms. These advancements are crucial since interferometers detect gravitational waves by measuring minuscule changes in the time it takes for laser light to bounce between mirrors.
Despite the promising future, the continued funding for maintaining LIGO and any upgrades—let alone new facilities—could be jeopardized if President Donald Trump successfully implements his plans to significantly reduce the budget of the US National Science Foundation, which has been the primary source of funding for LIGO's construction and operations. However, Congress has indicated a preference for less drastic cuts, leaving researchers cautiously optimistic. “I think we just have to wait and see,” Reitze concludes.
As scientists continue to explore the universe through gravitational wave detection, the potential for future projects, such as the Einstein Telescope, could further revolutionize our understanding of cosmic phenomena. This next generation of gravitational-wave detectors promises to unlock new secrets of the universe, expanding our knowledge of black holes, neutron stars, and the very fabric of space-time itself.
