“For the whole of human history, everything we’ve learned about the distant universe has come from light.”
These are the words of Ryan Lynch at a recent conference later posted on YouTube. He should know: Lynch is an astronomer at the Green Bank Observatory and a member of the International Pulsar Timing Array, or IPTA. The IPTA hunts for low-frequency gravitational waves. The goal, explains Lynch, is to carve ”a new way we can study the universe, using gravity instead of light.” Specifically, Lynch wants to use strange cosmic lighthouses called pulsars.
On his 143rd birthday, Inverse celebrates the world’s most iconic physicist — and interrogates the myth of his genius. Welcome to Einstein Week.
Pulsars are the leftover cores of exploded stars that spin and emit regular pulses of radio waves — like a lighthouse beaming light steadily across a dark sea. A pulsar timing array like the IPTA uses dozens of pulsars that spin thousands of times a second as gravitational wave detectors.
In January, Lynch revealed the latest data release from the IPTA, dubbed Data Release 2. The release compiles an analysis of decades-worth of pulsar observations from European, Australian, and North American radio observatories.
The Einstein Connection
Gravitational waves are ripples in spacetime that move at the speed of light. They were first predicted by Albert Einstein’s theory of general relativity, but they were first detected in 2015 — 100 years after Einstein developed the theory.
Gravitational waves distort how objects like pulsars experience space and time, making other objects seem closer or farther away than they otherwise would be.
The IPTA is trying to detect a gravitational wave background signal, a kind of static generated by all the gravitational waves emanating from binary supermassive black holes. The gravitational wave background should have a similar amplitude throughout the universe, but they aren’t as high-energy as the gravitational waves detected in the 2016 landmark discovery that first confirmed Einstein’s prediction. This makes the true gravitational wave background signal even more elusive.
“This is Einstein, so he probably doesn’t like that things go the predicted way.”
So far, the IPTA team has detected what seems to be a signature amplitude common to most pulsars, but they still don’t have a signature for the background, explains Siyuan Chen to Inverse. Chen is a postdoctoral researcher at the Kavli Institute for Astronomy and Astrophysics at Peking University in Beijing. Chen led the data analysis for the IPTA’s Data Release 2 while a researcher at the European wing of the IPTA in France. What they have so far, Chen says, is not enough to confirm that the signal they see comes from gravitational waves.
“The thing that we need to see is the specific amount of strength at different pulsars, and that's determined by the distance between two pulsars or the angle from us to the different pulsars. That's something we have not seen,” Chen says.
“We will need to observe more pulsars to detect that,” he adds.
Supermassive black hole binaries are a more likely culprit for the IPTA signal in part because astronomers already have evidence that they exist. But the source could also be from other exotic space phenomena like cosmic strings or even the gravitational waves formed directly after the Big Bang.
“There is observational evidence of supermassive black hole binaries,” says Chiara Caprini, a staff physicist at CERN. Caprini is a theorist of cosmology who studies models of gravity in the early universe and is not involved with the IPTA.
“All these other sources are valid in the context of theoretical models. We have no proof,” he explains to Inverse.
Chen agrees that the signal matches models that predict the gravitational wave background, but there are too many other potential sources to be sure. Only time – and a whole lot more data – will allow scientists to match the signal up with accuracy.
If the signal does turn out to be from the gravitational wave background, Chen says, then Einstein would probably have been pleased to see yet another result that confirms his theory.
“But then again, this is Einstein, so he probably doesn’t like that things go the predicted way, and he would rather see some new things outside his theory happening,” Chen adds.
A powerful collaboration
Collaboration is what makes the IPTA so powerful — and the quest for more data a little easier. The Indian wing of the IPTA has signed on for the next round of data releases, and data-sharing negotiations are underway with a Chinese PTA (FAST) and the South African PTA (MeerKAT). If all goes well, the next round of data – and stronger evidence for the GWB – could be compiled and released within two or three years, Chen says.
Pinpointing the source of the signal may allow astrophysicists to confirm theories like general relativity and to improve their models of the universe. These models could reveal how galaxies merge or trace the moments right after the Big Bang before there was light. And although he eventually came around, Einstein was initially skeptical that the universe was expanding.
Now, Caprini says, the results of experiments like the IPTA’s might allow scientists to probe the early days of that expansion using his theories.
“There has been a progress in these last 100 years, which is just amazing, and entirely, almost entirely, based on what he discovered,” Caprini says.
This research also leaves open the possibility of new discoveries beyond the bounds of our imagination. As Lynch puts it in his presentation, ”Whenever we look at the universe in a totally new way, we always end up finding things that were unexpected. And so serendipity and the unexpected is a big part of why we are doing this, as well.”
On his 143rd birthday, Inverse celebrates the world’s most iconic physicist — and interrogates the myth of his genius. Welcome to Einstein Week.
