New gravitational waves detected by the NANOGrav observatory originate from supermassive black holes billions of times larger than the Sun, potentially revealing the nature of the universe.
Astronomers have for the first time discovered massive ripples in the fabric of spacetime. These enormous gravitational waves are believed to come from merging supermassive black holes, each one billions of times more massive than the Sun, as reported by National Geographic on June 28.
Simulation of two supermassive black holes orbiting each other. (Photo: National Geographic).
By studying the timing fluctuations in radio signals from rotating stellar remnants known as pulsars, a team of scientists from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) was able to capture the aforementioned gravitational waves. Future research on these waves could provide clues about the moments immediately following the Big Bang, helping to unravel mysteries such as the nature of dark matter, which makes up 5/6 of the total matter in the universe.
Whenever an object with mass accelerates, it creates distortions known as gravitational waves that travel at the speed of light, stretching and compressing spacetime along their path. Gravitational waves were predicted by Albert Einstein in 1916. Scientists first detected evidence of gravitational waves using the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015.
Just like light, which comes in various wavelengths and frequencies—ranging from high-frequency gamma rays to low-frequency radio waves—gravitational waves also vary. LIGO detected high-frequency gravitational waves with wavelengths around 2,896 km/h. Currently, the NANOGrav research team has found low-frequency gravitational waves with wavelengths so long that light takes years or even decades to travel between their peaks. They detail their findings in five studies published in the Astrophysical Journal Letters.
To identify these massive ripples, scientists needed to assemble sensors larger than Earth. Over the past 15 years, NANOGrav has been analyzing dead stars within the Milky Way to create a galaxy-sized gravitational wave detector. They focused on millisecond pulsars, which are produced when a massive star dies in a supernova explosion, leaving behind a rapidly spinning stellar remnant. These highly dense remnants emit beams of radio waves from their magnetic poles, flashing like a lighthouse. Each time the beam sweeps across Earth, a radio telescope detects a pulse. Hundreds of radio pulses appear every second with extreme precision.
The research team monitors 68 pulsars located within a few thousand light-years from Earth. As gravitational waves cause spacetime to stretch and compress, they alter the timing between radio pulses, causing some pulses to slow down while others speed up in a predictably unique manner. This variability has also been observed in a pair of pulsars, depending on the distance between the two stars in the system, demonstrating that gravitational waves affect both.
Pulsars generate very weak radio signals; therefore, to conduct this research, scientists spend thousands of hours each year observing with the world’s largest radio telescopes, including the Arecibo Observatory in Puerto Rico, the Green Bank Observatory in West Virginia, and the Very Large Array in New Mexico. This allows them to detect the timing of radio pulses to within a millionth of a second.
It is likely that the newly detected source of radio waves is a pair of supermassive black holes that are 100 million to 10 billion times more massive than the Sun. In contrast, the gravitational waves detected by LIGO came from collisions between smaller black holes or neutron stars with masses only a few tens of solar masses. Astronomers believe that supermassive black holes are located at the centers of the largest galaxies in the universe. When two galaxies merge, the black holes at their centers move into the core of the new galaxy, forming a binary system that generates gravitational waves as the two supermassive black holes slowly orbit each other.
As NANOGrav continues to collect more data over time, the research team hopes that their instruments will be sensitive enough to identify gravitational waves from specific binary black hole systems. This would allow them to combine their findings with other observatories to analyze targets using both light and gravitational forces.
