Astronomers detected an ultra-low-frequency gravitational wave background via 15 years of pulsar observations, revealing a cosmic chorus twice as loud as expected.
Pulsar Timers Detect a Cosmic Chorus of Gravitational Waves
After diligently collecting data for 15 years in a galaxy-spanning experiment, a dedicated research team has achieved something truly extraordinary. Researchers working with the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, detected the continuous echo of gravitational waves passing through our universe by observing pulsars.
Unlike the high-frequency bursts generated by stellar-mass black hole and neutron star mergers picked up by terrestrial instruments like the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo, this newly measured background is composed of ultra-low-frequency waves. A single rise and fall of these massive waves can take years or even decades to pass by. Because gravitational waves travel at the speed of light, a single wavelength spans tens of light-years.
To spot them, scientists scrutinized pulsars—rapidly spinning remnants of massive stars that function like cosmic lighthouses by emitting beams of radio waves from their magnetic poles.
The findings indicate these powerful waves likely originate from pairs of supermassive black holes spiraling toward each other in cataclysmic cosmic collisions.
It’s like a choir, with all these supermassive black hole pairs chiming in at different frequencies.
According to Mingarelli, the gravitational wave background is about twice as loud as what I expected, pointing to potential explanations that range from more abundant and heavier supermassive black holes to phenomena predicted by string theory.
Subsolar Black Hole Candidates Spark Primordial Debates
While NANOGrav captured ultra-low-frequency signals, ground-based detectors have logged individual merger events that continue to push theoretical boundaries. On Nov. 12, the LIGO-Virgo-KAGRA collaboration issued an automated alert for a black hole merger designated S251112cm. The signal revealed an object with a mass far too small to be a standard stellar-mass black hole or a neutron star.
Standard stellar-mass black holes carry masses between 5 and 100 times that of the sun, formed when massive stars collapse. By contrast, this subsolar candidate carried an estimated chirp mass of roughly 0.1 to 0.87 solar masses. Theoretical physicists point out that such an event cannot be explained by conventional astrophysical processes.
Despite the excitement, researchers urge caution. Gravitational wave astronomer and LIGO team member Christopher Berry noted on Bluesky that there was still a significant chance of this being a false alarm, estimating a false alarm rate of 1 in 6.2 yr for this specific type of detection. If confirmed, however, the source could point to long-hypothesized primordial black holes born directly from overly dense pockets of plasma during the first few seconds after the Big Bang.
Theoretical Links Between Stochastic Waves and Dark Matter
A study published in Physical Review Letters by Professor Joachim Kopp of Johannes Gutenberg University Mainz and Dr. Azadeh Maleknejad of Swansea University introduces fresh calculations pointing to a previously unexplored process where stochastic gravitational waves gave rise to dark matter.
While visible matter makes up only about 4 percent of the cosmos, dark matter accounts for roughly 23 percent, with the remainder comprised of dark energy. The researchers investigated stochastic gravitational waves generated during early cosmic phases, such as cooling phase transitions or primordial magnetic fields.
In this article, we investigate the possibility of gravitational waves — which are believed to have been ubiquitous in the early universe — being partially converted into dark matter particles. This leads to a new mechanism of dark matter production that has not been researched before.
The study suggests that early background waves could have produced fermions—a class of particles including electrons, protons, and neutrons—that initially carried little or no mass before evolving into today’s dark matter particles. The team plans to follow up their analytical estimates with numerical calculations to refine these cosmological predictions.
