When you hear about the farthest, youngest, or otherwise early in the universe object being discovered, exactly how it is described can be confusing. 

As the light from distant objects has traveled toward us, space has been expanding, and the distance between us and the object that the light has to travel from has been changing. I like to say how long the light traveled; that doesn’t tell you where the object is, but it reminds you that we’re seeing it as it was, not as it is. 

Distance is also a bit nonsensical. We know how far the light traveled – 13.5 billion light-years – but that distance isn’t the same as the distance between us and that object today or us and the object when it formed. It’s more like trying to say how far someone had to run when they ran up the down escalator. They might have ended up traveling one story, but they had to run up more than just one story of stairs, and it is all a bit messy to sort.

Making it more complicated, time is not constant, and space can change, too.

Gravitational waves are the space-time equivalent of crash alarms. When massive objects orbit closely, they radiate energy in the form of gravitational waves. Like soundwaves, these waves compress and expand what’s around them, but while sound waves change air, gravitational waves compress and expand space itself.

We can detect the waves from merging black holes and neutron stars with detectors on Earth, but just like there are sounds ears can’t hear, there are gravitational waves both too long and too short for us to detect with the size detectors we can put on our planet. Researchers want to detect all sizes of crashes though, so they get creative. Researchers started using pulsars to look for massive waves a while back. With radio telescopes, they looked for slight changes in pulsar timing that would indicate space expansions and contractions between us and pulsars. And this is awesome, except a lot of random things between us and pulsars can interact with radio light including things like random electrons.

To escape this issue, researchers have started monitoring pulsars in gamma-ray light — a high-energy color that pretty much just flies straight and true without interruption. After collecting five years of data, researcher and co-lead of this study, Aditya Parthasarathy, says: The Fermi results are already 30% as good as the radio pulsar timing arrays when it comes to potentially detecting the gravitational wave background. With another five years of pulsar data collection and analysis, it’ll be equally capable with the added bonus of not having to worry about all those stray electrons.

While nothing has been seen yet, the kinds of supermassive black hole mergers this may detect are rare, and the quality isn’t yet the best. But it will get there, and someday, we may see gravitational waves in the changing shape of space using Fermi.

This work appears in the journal Science and comes from the Fermi-LAT Collaboration.

More Information

MPIfR press release

NASA Goddard press release

“A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background,” The Fermi_LAT Collaboration, 2022 April 7, Science

VIDEO: Visualization of Merging Black Holes and Gravitational Waves (NASA)