Gravitational Waves from Supermassive Black Holes Observed

Recent findings from NANOGrav have revealed a gravitational-wave signal that provides new insights into the early universe and the nature of supermassive black holes. According to a paper submitted to arXiv, the signal is a nHz-band gravitational-wave background resulting from the merger of binary supermassive black holes, each with masses around one billion solar masses.

These black holes are theorized to have formed from primordial black holes, which are believed to have been created shortly after the Big Bang. The research indicates that when a primordial black hole accretes matter at a high rate, it emits a significant number of high-energy photons.

This emission heats the surrounding plasma and is associated with high-redshift cosmological 21cm line emissions. However, since such emissions have yet to be detected, researchers have established a strict upper limit on the accretion rate for these primordial black holes.

The study finds that a primordial black hole abundance in the range of 10 to the negative 14 to 10 to the negative 12, with masses between one solar mass and one thousand solar masses, successfully fits the observed nHz band gravitational-wave background while remaining consistent with the limits on 21cm line emissions.

The authors propose that future observations could further test this scenario by examining both the gravitational wave background and the cosmological 21cm line. This research underscores the complexity of understanding the formation of supermassive black holes and their potential origins linked to primordial black holes, emphasizing how gravitational waves can illuminate aspects of the universe's formative years.