The very fabric of spacetime is constantly warping, on unimaginably tiny scales, as ripples from past cataclysms wash over us. Astrophysicists have now detected a background sea of gravitational waves, using a galaxy-scale detector made up of dead stars.
Gravitational waves are distortions in spacetime itself, which were first predicted by Albert Einstein over a century ago. But it wasn’t until 2015 that they were directly detected for the first time, as the purpose-built LIGO facility picked up gravitational waves from a collision between black holes. Around 100 detections have been made in the years since.
These have all been higher-frequency signals, short and sharp “shouts” coming from collisions between dense objects like black holes and neutron stars. But it’s long been predicted that there should be a kind of background hum of gravitational waves as well, with much lower frequency signals that pervade the universe from slower events. Now, astronomers have detected these whispers.
The key to the breakthrough was using a different type of detector. Previous observations were made using LIGO, Virgo and KAGRA, underground facilities that beam lasers down long tunnels and watch for tiny distortions that can indicate a gravitational wave has rolled past and bent the beam ever so slightly.
But the new detection works on a galactic scale, using an array of pulsars. These remnants of collapsed stars have strong magnetic fields and fire off beams of electromagnetic signals that rotate like lighthouses. The recurring signal is so precise it’s predictable right down to fractions of a nanosecond, making them a kind of cosmic atomic clock. When gravitational waves wash past – even very low frequency ones – they will physically stretch or shrink the distance to these pulsars, making the signals arrive ever-so-slightly early or late.
The NANOGrav project observed a network of 68 pulsars across the Milky Way for 15 years, watching for tiny disruptions to their routines. These slow waves can take years or even decades to pass by, so it’s only by gathering data over that long a period that the researchers were able to detect a consistent background signal of low frequency gravitational waves, as predicted.
While the chirps that LIGO and other detectors hear come from colliding objects with masses of a few Suns, this background signal is thought to originate from the much bigger, slower mergers between supermassive black holes, with masses millions of times larger.
The team presented the first tentative results a few years ago, but with more work have now announced the detection more confidently. That’s after accounting for a staggering amount of other factors.
“To tease out the gravitational-wave background, we had to nail down a multitude of confusing effects, such as the motion of the pulsars, the perturbations due to the free electrons in our galaxy, the instabilities of the reference clocks at the radio observatories, and even the precise location of the center of the solar system, which we determined with help from NASA's Juno and Cassini missions,” said Michele Vallisneri, a team member on NANOGrav.
The detections will only get more precise, the team says, as additional telescopes come online to join the project in the coming years.
The research was published in a series of papers in The Astrophysical Journal. The team describes the work in the video below.
Source: Caltech