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Gravitational wave search provides insights into galaxy evolution and mergers

Given scientists’ current understanding of how often galaxies merge, limits point to fewer detectable pairs of supermassive black holes than previously expected.
Gravitational waves
Earth is constantly jostled by low-frequency gravitational waves from supermassive black hole binaries in distant galaxies. Astrophysicists are using pulsars as a galaxy-sized detector to measure Earth's motion from these waves.
B. Saxton (NRAO/AUI/NSF)
On the heels of their participation in the historic research that resulted in the detection of gravitational waves, West Virginia University (WVU) astrophysicists continue to plow new ground and build upon their work.

WVU scientists were members of the LIGO team that detected gravitational waves from merging pairs of black holes approximately 29 to 36 times the mass of the Sun, confirming that distortions in the fabric of space-time can be observed and measured.

WVU scientists are also continuing to make discoveries about the universe as members of North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which has spent the past decade searching for low-frequency gravitational waves emitted by pairs of black holes with masses many millions of times larger than those seen by LIGO.

Analysis of NANOGrav’s nine-year dataset provides constraining limits — estimates of the largest possible signal that could be in the data — on the prevalence of such supermassive black hole binaries throughout the universe.

Given scientists’ current understanding of how often galaxies merge, these limits point to fewer detectable pairs of supermassive black holes, referred to as black hole binaries, than previously expected. This result has a significant impact on scientists’ understanding of how galaxies and their central black holes co-evolve.

Detecting a wave
Low-frequency gravitational waves are extremely difficult to detect because their wavelengths span light-years and originate from black hole binaries in galaxies spread across the sky.

The combination of these giant binary black holes leads to a constant “hum” of gravitational waves that computer models predict should be detectable at Earth. Astrophysicists call this effect the “stochastic gravitational wave background” and detecting it requires special analysis techniques, such as the use of pulsars.

Pulsars are the cores of massive stars left behind after the stars go supernova and explode, emitting pulses of radio waves as they spin. The fastest pulsars rotate hundreds of times each second and emit a pulse every few milliseconds.

These “millisecond pulsars” are considered nature’s most precise clocks and are ideal for detecting the small signal from gravitational waves.

“We have the ability to detect very tiny deviations in the arrival time of pulses from pulsars that might be due to gravitational waves,” said Maura McLaughlin from WVU.

Astrophysicists use computer models to predict how often galaxies merge and form supermassive black hole binaries. Those models use several assumptions about how pairs of black holes evolve by predicting the strength of the hum.

“By using information about galaxy mergers and constraints on the background, we can predict the properties of the sources we might detect and even use a non-detection to better understand the physics of black hole binary evolution,” said Sean McWilliams, also from WVU.

According to Sarah Burke-Spolaor at the National Radio Astronomy Observatory in Socorro, New Mexico, there are two possible interpretations of this non-detection.

“Some supermassive black hole binaries may not be in circular orbits or are significantly interacting with gas or stars,” Burke-Spolaor said. “This would drive them to merge faster than simple models have assumed in the past.”

An alternate explanation is that many of these binaries inspiral too slowly to ever emit detectable gravitational waves.

NANOGrav is currently monitoring 54 pulsars using the National Science Foundation’s Green Bank Telescope in West Virginia and Arecibo Radio Observatory in Puerto Rico, the two most sensitive radio telescopes in the world at these frequencies. Their array of pulsars is continually growing as new millisecond pulsars are discovered.

The group also collaborates with radio astronomers in Europe and Australia as part of the International Pulsar Timing Array, giving them access to many more pulsar observations. This increase in sensitivity could lead to a detection in as little as five years.

In addition, this measurement helps constrain the properties of cosmic strings, very dense and thin cosmological objects, which many theorists believe evolved when the universe was just a fraction of a second old. These strings can form loops, which then decay through gravitational wave emission.

The most conservative NANOGrav limit on cosmic string tension is the most stringent limit to date and will continue to improve as NANOGrav continues operating.

“These new limits have the most astrophysically relevant implications for the gravitational wave background at these frequencies yet,” said Duncan Lorimer, also from WVU. “If we can keep our access to the Green Bank Telescope and the Arecibo Observatory in Puerto Rico, a detection is easily within reach, and we will soon have an entirely new way of understanding our universe and how galaxies form and evolve.”

NANOGrav is a collaboration of more than 60 scientists at more than a dozen institutions in the United States and Canada whose goal is detecting low-frequency gravitational waves to open a new window on the universe. The group uses radio pulsar timing observations to search for the ripples in the fabric of space-time.
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