From every corner of the universe, planets, stars, stellar debris, and other massive objects begin a complex but inherently unstable gravitational dance. Each mass bends the fabric of spacetime around it, and all other masses move along trajectories determined by this curved spacetime. However, this simple process of one mass moving through space curved by another mass is inherently unstable because the gravitational mass moving through the gravitational field itself emits gravitational radiation, or gravitational waves.
One hundred years after the development of general relativity, the LIGO scientific collaboration discovered that these gravitational waves emanate from low-mass black holes (less than a few hundred solar masses) during the final stages of inspiration and merger. It was not detected until Since the first discovery in 2015, about 100 more gravitational wave signals have been detected, all in the same final stage of spiraling into each other and merging.
For the first time, a new class of gravitational wave signals has been observed in a completely different way. Scientists monitored the behavior of millisecond pulsars, the most accurate natural clocks in the universe. In the NANOGrav series of papers, this collaboration provides compelling evidence that a gravitational wave background exists, detectable on timescales approximately 10 billion times longer than LIGO. This is the first time such a cosmic gravitational wave background has been directly detected, and the next steps will be even more interesting.
First of all, it cannot be overstated how big of an accomplishment it is to observe these gravitational waves. One notable prediction of general relativity was that, unlike Newtonian gravity, systems bound by gravity cannot be stable forever. According to Newton’s laws, two masses orbiting each other in the universe are in the shape of a closed ellipse, always returning to the same point with each rotation, and this rotation never stops, remaining stable forever.
This is not the case in general relativity. According to Einstein’s theory of gravity, his two masses orbiting each other cannot orbit forever because the curvature of spacetime strictly forbids this. These masses radiate energy in the form of gravitational waves, which gradually converge as their orbits decay. If you wait long enough, these masses will eventually move closer together and into narrower orbits, where they will travel even faster and emit gravitational waves of higher frequency (shorter period) and larger amplitude. And so on until they finally combine.
In Einstein’s universe, which as far as we know best represents our universe, all systems are unstable in this way. Even if the Sun and Earth were to live forever in exactly the same state as they are now, in about 1026 years the Earth and Sun would converge and merge.
Evidence suggests that this type of orbital collapse and the consequent emission of gravitational waves was already occurring before we directly measured the first gravitational waves. These clues came from a type of celestial object known as a millisecond pulsar, the most accurate natural clock in the universe. Pulsars are neutron stars with incredibly strong magnetic fields. At the surface of a neutron star, the magnetic field is billions to trillions of times more powerful than the magnetic field at the surface of Earth.
Not all neutron stars are pulsars, but we wonder if that’s because not all neutron stars pulsate, or just that most neutron stars rotate in such a way that their magnetic axes point in our direction. I don’t know yet if it’s because there isn’t one. Most of the observed pulsars are young or rotate very slowly. However, it is known to rotate with age. Therefore, there is a population of very old pulsars that rotate with a period of 1 to 10 milliseconds and pulse more than 100 times per second. These millisecond pulsars are the most accurate natural clocks in the universe, capable of maintaining time accurate to up to 1 microsecond for decades.