For now, what we’ll hear on Thursday is still a guess, but expectations are high!
It’s just a very short announcement that the NANOGrav collaboration (see below) invites people to stick around Thursday, June 29th on their agenda. On that day, the consortium promises to make an “important announcement”. The message is causing a sensation – and not just among astrophysicists – and is already leading to great speculation.
Mark your calendars and get ready for an important announcement from the NANOGrav collaboration on June 29! #Nugraph
– NANOGrav PFC (NANOGrav) June 21, 2023
But no one outside the consortium knows exactly what it will announce. What we do know is that it has something to do with gravitational waves. Now we hear you: We’ve been receiving the necessary important announcements from that angle recently. And that’s right. It began in 2016 with the first-ever detection of gravitational waves, or ripples in space-time (see box) caused by extreme events, such as the merger of black holes. And in the years that followed, we were regularly surprised by new discoveries of gravitational waves, which were caused not only by the merger of black holes, but also by the merger of neutron stars and even by the collision between a neutron star and a black hole.
About spacetime (and the ripples in it)
Gravitational waves are ripples in space-time. It is best to imagine that space-time is a tightly stretched sheet of paper. Planets and stars lie like balls on this “plate”, as a result of which space-time is bent locally. In addition, waves also travel through space-time. These are gravitational waves and they arise when two heavy objects rotate or merge close to each other.
The mainly announced discoveries mentioned above were the work of the LIGO-Virgo collaboration. But the announcement ahead of us on Thursday comes from an entirely different partnership: NANOGrav. Within this partnership, radio telescopes are used to search for another type of gravitational wave, the low-frequency gravitational wave. “Like waves of light, gravitational waves can be emitted over a wide range of frequencies,” reads the NANOGrav website. “Just as we need different types of telescopes to observe the entire electromagnetic spectrum, we also need different types of gravitational wave detectors to detect the full spectrum of gravitational waves.”
In order to detect low-frequency gravitational waves, NANOGrav is focusing on pulsars. These are rapidly spinning neutron stars that, as they rotate, emit beams of radio waves that can be received on Earth. You can compare these broadcasts a bit with the light of the lighthouse shining over the beach. And just as you can use the light from a lighthouse—which hits the shore at the same intervals each time—to keep track of time, you can also use pulsars as a kind of “cosmic clock.” Because the bursts of radio waves, as we observe them here on Earth, also happen at the same intervals every time.
So pulsars can be used as a kind of clock. But how can you use it to detect gravitational waves? When gravitational waves move through space-time, ripples are created in space-time; Space is stretched in one direction and compressed perpendicular to it. “Ripples caused by gravitational waves traveling through our galaxy cause pulsars to arrive too early or too late,” reads the NANOGrav website. So deviations in the arrival time of the pulsars could indicate the passage of a gravitational wave. “These are gravitational waves with frequencies on the order of nanohertz, which correspond to a period ranging from months to decades,” Nanograph says. “These are much lower frequencies than ground-based interferometers like LIGO or space-based interferometers like LISA can monitor.”
And in this search for low-frequency gravitational waves, a potentially interesting discovery has now been made. But what could it be? As mentioned, there is already a lot of speculation about it. One possibility is that the researchers succeeded in detecting low-frequency gravitational waves in the manner described above – that is, by using pulsars. It could then be gravitational waves from a specific source, such as two spinning supermassive black holes in a distant galaxy. Or – and this would also be very exciting – a combination of all the gravitational waves coming together from all parts of the universe: the isotropic background of gravitational waves. Initial indications of its existence were found by NANOGrav earlier this year, so detecting such a gravitational-wave background is certainly not far off. It’s also something researchers have been pursuing for some time, not least because it could provide more knowledge about how frequently black holes (and thus the galaxies they reside in) merge. This is important if you want to get a better picture of galactic evolution in general. By the way, there are also wilder ideas to come up with in connection with the announcement of NANOGrav. For example, researchers can also report that they cannot find low-frequency gravitational waves. It’s not very plausible, but it would be big news, because it also shakes up the general theory of relativity.
For now, there is nothing else to do but wait until Thursday. Then — from the mouths of the scientists behind NANOGrav — you should hear the word redemption.
There’s also another good reason to sit on the edge of your seat later this week. Because in addition to NANOGrav, the IceCube Neutrino Observatory—which searches for neutrinos, or “ghost particles”—will also make an announcement next Thursday. In a tweet, the researchers promise us “exciting results.” The scientists over at NANOGrav have now confirmed that the IceCube announcement is separate from their big news, so we have at least two surprises waiting for us next Thursday.
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