This week there were murmurs about an announcement from astrophysicists that would get us all riled up. That announcement was made on Wednesday and the short of it is that scientists have detected gravitational waves that are potentially caused by massive cosmic events.

Over the last 15 years, scientists from around the world have been watching a collection of pulsars in order to detect the gravitational waves passing through the universe.

Pulsars are, as described by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the ultra dense remnants of huge stars that died in a supernova. These remnants spin rapidly and appear as a pulse when observed from Earth. The fastest of these pulsars can spin hundreds of times per second and have been named millisecond pulsars. Importantly, these pulses are very stable and as such can be used as a precise measurement tool to observe say, whether gravitational waves affect the fabric of space time and disrupt the expected behaviour of the pulse.

By observing 68 of these millisecond pulsars using telescopes scattered around the Earth a group of 190 scientists from NANOGrav and Physics Frontiers Center (PFC) have found evidence of gravitational waves that have reverberated through the galaxy for years and even decades.

Of course, we have already detected gravitational waves with the help of instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO).

However, LIGO is very focussed in its abilities and detects the likes of black holes merging, the frequencies NANOGrav is looking for are far lower and stretch across light years, something ground based observatories like LIGO simply aren’t able to detect.

So, over 15 years scientists have been monitoring millisecond pulsars to see whether gravitational waves are affecting the time it takes for pulses to be observed and they found exactly that. Some pulses would arrive sooner, others later, consistent with gravitational waves affecting Earth as Ars Technica explains.

“In 2020, with just over twelve years of data, NANOGrav scientists began to see hints of a signal, an extra ‘hum’ that was common to the timing behavior of all pulsars in the array, and that careful consideration of possible alternative explanations could not eliminate. The collaboration felt confident that this signal was real, and becoming easier to detect as more observations were included. But it was still too faint to show the gravitational-wave signature predicted by general relativity. Now, their 15 years of pulsar observations are showing the first evidence for the presence of gravitational waves, with periods of years to decades,” NANOGrav wrote in a press release.

The source of this hum is an unknown quantity although there is a theory that these waves are generated by a pair of supermassive black holes orbiting each other. As these supermassive blackholes engage in a centuries long dance before they merge into a single entity, they emit gravitational waves that ripple through the universe and that is potentially what NANOGrav has detected.

Should this hum be coming from supermassive blackholes merging, there is evidence to suggest that there may be hundreds of thousands or even millions of these events happening across the universe. As such, the gravitational waves they produce overlap but also affect pulsar timing observations in unique patterns.

These patterns could be picked apart to track where they originate and that has some exciting possibilities.

Detecting our creation

So why is detecting supermassive blackholes merging so important? According to NANOGrav, these observations help us understand how the universe evolved to the scale it has. We can also understand how often galaxies collide, why black holes merge and more.

What is perhaps most exciting is that among this chorus of destruction and creation, we may be observing remnants of the ripples caused by the Big Bang. The event that is said to have kick started the universe happened billions of years ago but the nature of space and time means that we can still observe the after effects of the event. One such observation is the presence of cosmic microwave background which is simply described by the European Space Agency as “the cooled remnant of the first light that could ever travel freely throughout the Universe” and was released just after the Big Bang.

The idea that we’re still able to detect the gravitational waves from that event then isn’t beyond the realm of possibility.

As mentioned, the source of these gravitational waves is unknown at this stage. However, the collaboration, the patience and the fact that we turned dead stars into measurement tools is astounding. While humanity is capable of questionable things, we’re also able to do things that further our understanding of who we are and our wider universe.

All of the research regarding this discovery has been published in journals and you can read through it for free at the links below.

• The NANOGrav 15-year Data Set: Observations and Timing of 68 Millisecond Pulsars. DOI: 10.3847/2041-8213/acda9a
• The NANOGrav 15-Year Data Set: Detector Characterization and Noise Budget. DOI: 10.3847/2041-8213/acda88
• The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background. DOI: 10.3847/2041-8213/acdac6
• The NANOGrav 15-year Data Set: Constraints on Supermassive Black Hole Binaries from the Gravitational Wave Background. DOI: 10.3847/2041-8213/acdc91
• The NANOGrav 15-year Data Set: Search for Signals from New Physics. DOI: 10.3847/2041-8213/acdc91
• The NANOGrav 15-year Data Set: Bayesian Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries. DOI: arXiv:2306.16222
• Focus on NANOGrav’s 15 yr Data Set and the Gravitational Wave Background The Astrophysical Journal Letters.

[Image – Olena Shmahalo for NANOGrav]