Scientists at the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have achieved a milestone in astrophysics, successfully ‘hearing’ gravitational waves from merging supermassive black holes for the first time. This discovery opens up a new window on the universe, offering fresh insights into long-standing questions, and shedding light on the composition and existence of the gravitational wave background.
What are Gravitational Waves?
Gravitational waves are ripples in the fabric of space-time, set in motion by cataclysmic events such as the collision of black holes or neutron stars. The gravitational waves detected by NANOGrav are ultra-low-frequency waves and are far more powerful than those previously recorded by experiments like LIGO and Virgo.
Challenges in Detecting Gravitational Waves
Unlike the high-frequency waves detected by earthbound instruments, these ultra-low-frequency waves could take years or even decades to pass by and could span tens of light-years. Due to their colossal size, no experiment on Earth could directly detect them. Therefore, the scientists had to look to the stars for their research.
Role of Pulsars in Detecting Gravitational Waves
The NANOGrav team achieved this milestone by observing pulsars, remnants of massive stars that emit beams of radio waves from their magnetic poles. As pulsars spin rapidly, they emit these radio waves rhythmically, akin to a perfectly timed metronome. Gravitational waves passing between us and a pulsar can disrupt this precise timing, offering a way to detect these otherwise elusive waves.
By closely timing the radio wave pulses from dozens of pulsars in our galaxy using various observatories, the NANOGrav scientists were able to detect the gravitational wave background. The findings are the result of an array of 67 pulsars analyzed over a 15-year data collection period.
Source of the Gravitational Waves
The main source of the detected gravitational wave background is thought to be pairs of supermassive black holes spiraling towards each other, eventually colliding in a catastrophic finale. These pairs of black holes form due to galaxy mergers, and their gravitational waves can help estimate the frequency of galaxy collisions throughout the universe’s history.
The research also suggests that there could be hundreds of thousands or even a million or more supermassive black hole binaries in the universe. However, not all the gravitational waves detected are necessarily from these pairs. Some theories predict that these waves could be generated by cosmic strings, a concept from string theory, or could be remnants of a ‘Big Bounce’ event, where a precursor universe collapsed in on itself before expanding back outward.
Accounting for unknown variability
However, the research also acknowledges the possibility of unknown variability in pulsars that could affect the detection of gravitational waves. Despite these uncertainties, the findings represent a significant advancement in our understanding of the universe.
The NANOGrav team plans to continue their research, exploring the potential contributors to the gravitational wave background. They aim to further analyze the background based on the frequency of the waves and their origin in the sky. This discovery marks a new era in astrophysics and promises to unravel more cosmic secrets in the years to come.
Source: Eureka Alert