Gravitational waves have transformed our understanding of the universe since scientists first anticipated their existence as predicted by Albert Einstein a century earlier. In 1905, Einstein introduced the concept of the fabric of space-time, showing mathematically that space and time are interwoven. By 1915, he added gravity through his theory of general relativity, explaining that gravity results from the way space-time curves in the presence of mass. General relativity predicted that massive accelerating objects, such as two heavy objects in a tight binary orbit, would create ripples in space-time that travel outward across the cosmos according to Astronomy Magazine.
Unlocking Cosmic Secrets Through Gravitational-Wave Astronomy
Those theoretical ripples became reality at 5:51 a.m. EDT on Sept. 14, 2015, when the Laser Interferometer Gravitational-wave Observatory (LIGO) registered a small chirp as a distortion passed. The signal, dubbed GW150914, originated some 1.3 billion years ago when two behemoth black holes smashed together in a far-flung galaxy. The Livingston, Louisiana LIGO detector registered it first, and seven milliseconds later the ripple reached the Hanford, Washington detector. The discovery opened up an entirely new branch of astronomy, shifting observatories from looking at space to listening to it.
Detecting Ultra-Low-Frequency Backgrounds With Pulsars
While terrestrial instruments like LIGO and Virgo measure high-frequency bursts from black hole and neutron star mergers—such as the GW170104 signal produced by black holes weighing 19 and 32 solar masses as reported by Astronomy Magazine—other projects target ultra-low-frequency waves. For the very first time, scientists heard the continuous echo of gravitational waves passing through our universe by observing pulsars after collecting data for 15 years in a galaxy-spanning experiment. This cosmic chorus, which is even louder than anticipated, was brought to fruition by the North American Nanohertz Observatory for Gravitational Waves (according to Earth).
NANOGrav researchers from the United States and Canada monitored radio wave pulses from dozens of millisecond pulsars using multiple observatories, culminating in an in-depth analysis of 67 pulsars. Pulsars maintain a consistent rhythm like cosmic metronomes, creating rhythmic pulses of radio waves. When a gravitational wave passes between Earth and a pulsar, it alters the timing of the radio waves because gravitational waves compress and stretch space, changing the distance the radio waves must travel. Chiara Mingarelli described the phenomenon like a choir with supermassive black hole pairs chiming in at different frequencies, providing the first-ever evidence for the gravitational wave background.
Investigating Dark Matter Origins Through Stochastic Waves
Beyond detecting existing signals, researchers are exploring how gravitational waves may have shaped the early universe. Gravitational waves may have played a key role in creating dark matter during the universe’s earliest moments, according to a study by Professor Joachim Kopp of Johannes Gutenberg University Mainz and the PRISMA++ Cluster of Excellence, working with Dr. Azadeh Maleknejad from Swansea University and published in Physical Review Letters.
While visible matter makes up only about four percent of the universe, dark matter represents roughly 23 percent, and dark energy comprises the rest. Stochastic gravitational waves, which arise from processes not involving massive objects, form part of the background signal filling the universe from its earliest phases after the Big Bang. Kopp explained that researchers investigated the possibility that gravitational waves ubiquitous in the early universe were partially converted into dark matter particles. The study suggests these early waves could have produced fermions—a class of particles including electrons, protons, and neutrons—that initially had little or no mass before evolving into today’s dark matter particles. Future research will involve numerical calculations to improve prediction accuracy and investigate other early universe effects.
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