Somewhere in the darkness of space, two black holes, each at least 30 times the mass of our sun, orbited each other. They spun closer and closer until BOOM! After such a huge collision, what happened to the rest of the universe?
In the fraction of a second before their merge, these black holes sent strong vibrations, existing as gravity, that spread to the entire universe at the speed of light. For years gravitational waves stayed in the realm of theory, but in recent years, scientists have found proof that these vibrations of gravity do, in fact, exist.
To understand gravitational waves, one must first understand gravity: the force that causes objects to be attracted to other objects. Einstein was the first person to theorize gravitational waves through his theory of general relativity—how matter curves spacetime. He imagined gravity as a curve where mass creates depressions. The bigger the mass, the deeper the depression. As the mass moves, it sends ripples—gravitational waves (a simplified explanation).
Every time an object comes into contact with another object, it causes fluctuations in gravity. Gravitational pull has two factors—the mass of the two objects and the distance between the two objects—but no matter how small or how far, all objects are attracted to each other. Fluctuations in gravity from the merger are called gravitational waves—as the distance from the black hole collision increases, the strength of the waves decreases. These waves are so tiny that Einstein considered them impossible to detect.
Yet humanity didn’t back down. Scientists were determined to create an instrument to help us see these gravitational fluctuations. In the late 1900s, LIGO (short for Laser Interferometer Gravitational-Wave Observatory) was created, using lasers and mirrors to detect incredibly tiny distortions in distance caused by these waves. Of course, this initially came with many issues. Vibrations, even slight ones caused by a nearby car crash or seismic activity, would trigger the detection system. This is why in 2015 when LIGO detected gravitational movements, the research team was skeptical. After thorough analysis, however, they confirmed that it was not a fluke: they had detected the first-ever gravitational waves from the collision of two black holes.
Still, even with this complex process of detection, only waves of high frequency can be observed easily. Recently, a company called NANOGrav successfully detected low frequencies using a new technique: pulsar timing arrays. Pulsars, stars with a beam of light coming from each end, are rapidly rotating with an incredibly well-established period. If a gravitational wave passes through the pulsar, the light we receive changes its period. NanoGrav uses radio telescopes to measure these distortions in pulsars, helping us detect low-frequency gravitational waves. After measuring 68 pulsars, correlating them, and finding a pattern, NANOGrav found evidence of low-gravitational waves!
Discovering low-frequency gravitational waves is not just an accomplishment by itself as it is the basis for further scientific discoveries. These waves carry information from all over the cosmos and can help us understand even the most deeply relativistic concepts in our universe. These waves can also help us study phenomena in our cosmos that cannot be seen with light. Because light is only present in 0.01% of the universe, this is an incredibly useful tool for deeper research.
As you sit here reading this, know that black holes and other celestial bodies are actively causing our universe to stretch and contract, distorting the fabric of spacetime as well as the fabric of our world. As our understanding of the universe increases, we create other opportunities for further exploration that will change the way we view the world today.