A new study sheds light on the astrophysically significant limits in the search for low-frequency gravitational waves using data from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), shedding light on the nature of galaxy evolution as well as the frequency of mergers.
The team analyzed NANOGrav's nine-year dataset, which stems from a campaign that was conducted by the Green Bank Telescope in West Virginia and the Arecibo Observatory in Puerto Rico, two of the most sensitive radio telescopes on Earth.
The findings revealed very constraining limits that exist on the prevalence of supermassive black hole binaries in the universe. These limits suggest that there are fewer detectable supermassive black hole binaries than we thought, a suggestion that could alter our perception of galaxy and black hole evolution.
Low-frequency gravitational waves stem from black hole binaries in galaxies that blanket the sky and are very difficult to detect. At the core of the massive stars that remain after stars go supernova and emit radio waves are pulsars. The fastest pulsars emit a pulse every few milliseconds - called "millisecond pulsars" (MSPs) - and are believed to be the most precise method of detecting small gravitational wave signals.
"This measurement is possible because the gravitational wave background imprints a unique signature onto the radio waves seen from a collection of MSPs," said Justin Ellis, co-author of the study and researcher from the California Institute of Technology in Pasadena, Calif.
Using this data, astrophysicists can use computer modeling to predict how often galaxies merge and form supermassive black hole binaries. These models are integral in our understanding of the evolution of these astrophysical phenomena.
"After nine years of observing a collection of MSPs, we haven't detected the stochastic background but we are beginning to rule out many predictions based on current models of galaxy evolution," Ellis said. "We are now at a point where the non-detection of gravitational waves is actually improving our understanding of black hole binary evolution."
"Pulsar timing arrays like NANOGrav are making novel observations of the evolution and nature of our universe," said Sarah Burke Spolaor, another co-author of the paper, from the National Radio Astronomy Observatory (NRAO) in Soccoro, N.M.
In addition to sheeding light on galaxy evolution and merger, the new findings also set strict constraints on cosmic strings, the dense, thin cosmological objects that many scientists believe evolved when the universe was just a fraction of a second old and have the ability to form loops that wither away through the emission of gravitational waves.
"These new results from NANOGrav have the most important astrophysical implications yet," said Scott Ransom, co-author of the study, from the NRAO in Charlottesville, Va. "As we improve our detection capabilities, we get closer and closer to that important threshold where the cosmic murmur begins to be heard. At that point, we'll be able to perform entirely new types of physics experiments on cosmic scales and open up a new window on the universe, just like LIGO just did for high-frequency gravitational waves."
The findings were published in the April 4 issue of The Astrophysical Journal.
