In 1915, Albert Einstein published his theory of General Relativity, which describes gravity as the curvature of space (technically, four-dimensional spacetime) by mass. The theory predicts that orbiting masses oscillate space, generating minute gravitational waves. In 2016, the LIGO observatory directly confirmed the prediction, detecting kilohertz frequency waves by measuring timing differences in laser signals bounced across several kilometers (a method called laser interferometry). Subsequently, numerous emissions have been found, mostly from one-off collisions of massive black holes. Recently, scientists announced detection of a persistent gravitational wave background (GWB), continually rippling the cosmos like a calm ocean breeze.
To discover the GWB, pulsar timing arrays (PTAs) were employed. Millisecond pulsars are highly compact objects within stellar binary systems that spin extraordinarily fast. They emit light from their magnetic poles, which revolve around the pulsar’s spin axis. The light exhibits an extremely stable, precise period, like a cosmic beacon. A PTA leverages a network of these pulsars to create a galaxy-scale interferometer, capable of detecting gravitational waves in the nanohertz band, nearly a trillion times lower in frequency than LIGO’s signals.
Nanohertz waves gradually oscillate over ten to twenty years, requiring decades-long PTA observation by multiple radio telescopes to gather sufficient data. Two scientific collaborations identified the GWB, NANOGrav (comprising North American observatories in Puerto Rico, West Virginia, and New Mexico) and EPTA (comprising European observatories in the United Kingdom, Germany, France, Italy, and the Netherlands). NANOGrav studied 68 pulsars over a 15-year period, while EPTA studied 25 pulsars over 25 years. The groups also incorporated observations from Australian, Chinese, and Indian observatories.
The project generated colossal volumes of data that had to be meticulously refined, analyzed, and cross-checked to discern the GWB. In statistics, sigma (σ) is a measure of data confidence. While 5σ is desired (a 0.00006% chance of being random), the NANOGrav/EPTA findings are about 3σ (a 0.3% chance), a statistically huge difference. While merging supermassive black holes within galactic cores are a currently favored cause, the data can support other merger scenarios, as well as some currently hypothetical phenomena such as cosmic strings, inflatons, and ultralight dark matter. Attaining 5σ data confidence will help pare down the possibilities. This will require further study with larger PTAs, more observatories, and more sophisticated data analysis techniques.
But the prospects for astrophysics and cosmology are promising. Aside from potentially confirming new phenomena, GWB study may deepen our understanding of how matter and energy evolved after the Big Bang. Conversely, it may offer compelling insights into the universe’s ultimate fate. Time will tell what secrets gravitational waves will echo across the cosmic ocean.
You’ve been listening to Western Slopes Skies, produced by the Black Canyon Astronomical Society and KVNF Community Radio. This feature was written and voiced by Michael T. Williams.
