Featured image credit: Olena Shmahalo for NANOGrav
Featured image description: Artist’s impression of gravitational waves formed by black hole binaries.
After 15 years of observations, physicists at the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have found evidence for gravitational waves that pass Earth over time periods that can span decades. As the paper itself states: “The era of nHz GW astronomy is upon us.”
This discovery comes 8 years after the first observation of gravitational waves, which confirmed yet another aspect of Albert Einstein’s theory of general relativity. This measurement, made at the Laser Interferometer Gravitational-Wave Observatory (LIGO), collected data of high frequency, short lifetime gravitational waves created during large astronomical events. LIGO is a ground-based observatory, so can only detect waves with relatively high amplitudes. For measurement of these larger, continuous waves, researchers needed to somehow convert the entire galaxy into an observatory.
But what are they? Gravitational waves are periodic distortions in spacetime, which are often visualised as ripples in spacetime’s so-called fabric. General relativity teaches us that masses warp spacetime, meaning that if large masses interact with each other, (for example two stars orbiting each other in close proximity) we would expect to see ripples that travel across the universe. These ripples look like the stretching and squeezing of space between objects as the wave passes through. In other words, distances themselves change.
Gravitational forces are weak compared to other forces in our universe, such as electromagnetism, so gravitational wave detectors need to be extremely sensitive. Detectors at LIGO measure motion at a scale ten thousand times smaller than an atomic nucleus. Given that LIGO, with its immense sensitivity, is only able to detect the very largest and most sudden gravitational disturbances, then in order to detect these gentler waves, physicists needed a much larger observatory which is better able to appreciate the full frequency range of gravitational waves.
The observation method used by NANOGrav is centred on pulsars: relatively small, dense stars which spin at extremely high speeds, emitting radio signals all the while. This makes them excellent galactic timekeepers. These stars, spread throughout the galaxy, form the network that the NANOGrav collaboration turned into their “observatory”. Since gravitational waves stretch and squeeze spacetime, they can influence the times these pulsar signals reach Earth in small but measurable ways, given a large enough observation time. The observations were carried out using arrays of radio telescopes, which detect signals from the pulsars. If the changes to the signal timing fit the predictions from Einstein’s theory, then scientists could confidently say that large-scale gravitational waves permeate the galaxy. Over a period of 15 years (beginning 2004), measurements were taken of pulsars spread out across the galaxy. Observations confirmed prediction, and on June 29 NANOGrav’s findings were released in The Astrophysical Journal Letters.
We can now think of the universe as having a background field of slowly varying gravitational waves. This finding is of great interest to black hole physicists, as it confirms models about the time evolution of black hole populations. Black holes gain mass by consuming surrounding matter such as stars and dust but can also grow by merging with one another. These black hole merger events produce large gravitational waves, the nature of which was confirmed in this recent discovery.
In merging events, black holes orbit each other, becoming closer and closer together and generating gravitational waves with each rotation. Black hole merging events can take millions of years to occur, which in cosmological timescales can still seem like a very short period. With possibly hundreds of millions of these events happening all over the universe, these waves overlap together to form a distinctive “hum”. Each individual wave affects the timings of the pulsar signals, which is why the observation at NANOGrav required such a large dataset for such a long time. The gravitational wave background, being a superposition (sum) of many waves from many sources, appears mostly like a “noise” in the signal detection — hence the huge effort required to unpick the cause from the data available to astronomers.
NANOGrav is a collaboration of over 170 members at more than 70 institutions, mostly in North America. Hot on the heels of discoveries from CERN and LIGO, we are now firmly in the age of large-scale collaboration in science.