Article by: Andacs Robert Eugen, on 05 July 2023, at 07:58 am PDT

In a remarkable feat, a global alliance of astronomers has intercepted a delicate reverberation of gravitational waves traversing the vast expanse of the universe. Employing lifeless stars as a colossal network of gravitational wave detectors, the collaborative effort known as NANOGrav has successfully measured a low-frequency hum, akin to a melodious chorus of ripples in the fabric of spacetime.

While the team responsible for this ground-breaking discovery remains cautious, their prevailing belief is that the detected background hum of gravitational waves stems from myriad ancient events involving the merging of supermassive black holes.

Gravitational waves are undulations in spacetime that arise from the accelerated motion of massive objects. These waves were originally postulated by Albert Einstein in his general theory of relativity, wherein he hypothesized that as gravitational waves traverse space, they cause periodic contractions and expansions, akin to the rhythmic ebb and flow of tides.

In 2015, researchers achieved the momentous feat of directly detecting gravitational waves when the Laser Interferometer Gravitational-Wave Observatory (LIGO) captured signals emanating from the merger of a pair of black holes that had journeyed a staggering 1.3 billion light-years to reach Earth.

The NANOGrav collaboration endeavors to detect these spacetime ripples on an interstellar scale. To this end, the team harnesses the power of pulsars—rapidly rotating remnants of dead stars that emit beams of radio waves. Conceptually, pulsars can be likened to cosmic lighthouses, emitting periodic emissions that sweep across the Earth at regular intervals.

The NANOGrav team focuses on pulsars that exhibit extraordinarily rapid rotations, reaching speeds of up to 1,000 revolutions per second. The pulsar emissions can be measured with the precision of an exquisitely accurate cosmic clock. As gravitational waves traverse the space between a pulsar and Earth at the speed of light, they minutely stretch and compress this distance, subtly altering the intervals between the pulsar's ticks.

Pulsars are so remarkably precise in their ticking that their pulses can be measured with an accuracy of within 100 nanoseconds. Consequently, astronomers can calculate the distance between a pulsar and Earth with a margin of error within 100 feet (30 meters). Given that gravitational waves cause variations in the distance between pulsars and Earth spanning tens of miles, pulsars prove exquisitely sensitive to detecting these effects.

The initial challenge for the NANOGrav team was to overcome the noise inherent in their cosmic gravitational wave detector. This entailed accounting for noise originating from radio receivers and subtle astrophysical phenomena influencing pulsar behavior. Despite these complexities, the team's methodology proved sensitive enough to detect the cumulative impact of all massive black hole mergers transpiring throughout the universe since the Big Bang—an astounding multitude of potentially overlapping signals, reaching up to a million in number.

To illustrate this concept, imagine standing amidst the bustling cacophony of a downtown district and perceiving a faint symphony emanating from a distance. Amidst the clamor of cars and people, it becomes impossible to isolate the sound of a single instrument, but the collective hum of a hundred instruments becomes discernible. Similarly, the NANOGrav team had to disentangle the unique signature of this gravitational wave "background" from other competing signals