Albert Einstein envisioned the universe as something like the wave pool at an excessively-popular amusement park. On Thursday, researchers announced a major new step to proving this is true.
About a century ago, the theoretical physicist envisioned that the universe’s biggest structures could bend and warp space-time. On smaller scales, the gravity of planets like Earth can retain large-ish objects like the Moon. But gnarly structures like black holes are massive and common enough to produce a series of gravitational imprints throughout the space-time fabric, rippling continuously through time. Einstein theorized that the lump sum of these strong gravitational impressions creates a series of ripples that echo throughout the universe.
The new work, published Thursday in the journal Science, is the next major step to proving the collective existence of these undulations — called the gravitational wave background. The new technique described in the study could help astrophysicists write a vibrant new chapter of astronomy.
What’s new — Just detecting the gravitational wave background would be incredible. But beyond yet again proving Einstein right, this technique would help answer questions about the universe’s dynamics, like how big black holes are when they crash into one another, as well as bolstering what we know about how galaxies grow.
“Gravitational waves are funny because they have a tremendous amount of energy, but they basically do nothing,” Matthew Kerr, an astrophysicist currently based at the U.S. Naval Research Laboratory, tells Inverse. Kerr has been a part of the Fermi gamma-ray space telescope research community since the late 2000s and is one of the study’s authors.
Gravitational waves come from powerful objects like black holes and pulsars located far away, but they don’t announce themselves to us as major disturbances. By the time these ripples reach Earth, the waves are faint, and detecting them requires smart techniques and sensitive instruments. That’s why the ingenuity of LIGO — short for Laser Interferometer Gravitational-Wave Observatory — excited the astrophysics community back in 2017 when it detected a wriggle of space-time that stretched through our planet by a very tiny amount, created by two highly-dense objects violently careening into one other millions of light years away.
The new study comes from a project using Fermi, a telescope roughly the size of two refrigerators that launched in 2008. In the paper, scientists announced they had pooled 12 years of Fermi data to use pulsar-timing arrays to form a new gravitational-wave hunting technique.
How it works — Fermi is tuned for gamma rays, which are the highest-energy form of light. Kerr and his colleagues are looking for low-frequency gravitational waves from supermassive black holes that have been merging throughout the universe and filling space with their ripples — ones too subtle for LIGO to currently detect.
So far, scientists have used radio waves to hunt for proof of the gravitational wave background. But Fermi has several unique perks.
For one, it is “looking at all the stuff all the time,” Kerr said, thanks to a large field of view capable of seeing one-sixth of the sky at a given time. “This is really useful for us because we can see a large number of pulsars at any given time, and that’s key to doing this work where we are looking for gravitational waves.”
Pulsars are celestial gifts, Kerr said. “They’re like little lighthouses and every time they point at the Earth, we see a pulse from them.”
They are what happens when a large star, upon its demise, collapses into itself. This afterlife material eventually spins with high precision because of the tight clasp, producing jets from the stellar corpse’s poles. When they sweep the sky in our general direction, we can observe them as evenly-timed intervals of energy.
Why it matters — Pulsars are the steady clocks of space. For that reason, astrophysicists use pulsars in the Milky Way galaxy as precisely-timed beacons. Their pulse timing could hypothetically change if something shortens or lengthens the distance between the pulsar and Earth. Scientists have already detected slight delays and speeding using radio wave arrays.
Now, they are working on definitively proving that these disruptions are caused by black holes twisting and bending the fabric of space-time. This would turn space-time into something like a loose bed sheet, warp the distance it takes the pulsar signal to reach Earth.
Fermi’s gamma-ray detections are valuable because they provide hearty data to corroborate radio waves. Gamma rays are less dirty than radio waves, according to Kerr.
“When light is coming from pulsars to Earth, it's affected by the space that’s between us and the pulsar,” Kerr says. “Space is mostly empty, but there is some stuff out there — hydrogen atoms and electrons and pieces of dust — and when radio waves go through that, they are actually refracted much like light is when it goes through a prism.”
“People who’ve been doing this work have an idea of how to do that, and include that in their analyses, but there’s still some unknowns as to what kind of residual effects there might be from this propagation of radio waves through space,” he adds.
But Fermi will come to the rescue. “Gamma rays won’t have that problem at all. They are high-energy light, they just go straight through from the pulsar and arrive here at Earth at the Fermi space telescope without any additional bending.”
This is a separate way to make the measurement; by comparing the two, researchers can see if radio waves are pointing to a gravitational wave background, or if the models need to be fixed.
The new study announces that the Fermi pulsar-timing array technique has now “reached a sensitivity to gravitational waves that’s about 30 percent as good as what we are currently able to achieve with radio telescopes,” according to Kerr.
This means Fermi is already getting close to being as sensitive as football-field swathes of radio telescope arrays, which cover much more real estate than that of a double-refrigerator-sized space telescope. This efficiency also excites astrophysicists.
What’s next — Fermi is now on par with the radio studies that are currently being done, Kerr said. And Fermi’s gravitational-wave detection capacity may increase in the next three to four years.
Alternatively, Fermi might just show that these changes to the pulsar signals are caused by something else. Maybe black holes aren’t merging as frequently as astrophysicists think. Perhaps they are less massive, and then ruling-out the background could be a way of checking other theories about how galaxies grow.
Nevertheless, this project will have everlasting effects on the gravitational-wave subfield of astronomy one way or another.
