| Since 2016, the gravitational waves discoveries of LIGO/VIRGO have thrilled astrophysicists. This array of three observatories has detected the cataclysmic mergers of: (1) pairs of very massive black holes, (2) pairs of neutron stars, and (3) neutron stars with black holes. This sketch suggests gravitational waves created by two black holes in a tight mutual orbit. LIGO/VIRGO opened a new window to the cosmos and will, no doubt, continue to make thrilling discoveries for many years to come. The black hole masses in the detected mergers range from 8 to 85 Msun (Msun is the mass of our Sun). This means, at merger, the colliding bodies were orbiting one another several hundred times per second (several 100 Hz), and the peak gravitational wave frequencies were twice that. As the masses of merging black holes increase, so do their event horizon radii. Hence, they “collide” when their centers are farther apart, reducing the peak gravitational wave frequency. Indeed, peak frequency is inversely proportional to mass in these events. The next new frontier is searching for gravitational waves of very low frequency generated by supermassive black hole binaries. Supermassive black holes, with masses up to 60 billion Msun, typically occupy the centers of major galaxies. Galaxies grow by merger and acquisition that gradually evolve on a time scale of billions of years. We see countless examples of these mergers at various stages. When two large galaxies merge, their central supermassive black holes eventually merge as well, perhaps over 100 of millions of years or more. As these supermassives approach one another, they become a binary system in a mutual orbit of great size, emitting gravitational waves of very low frequency. No one is patient enough to search for waves that oscillate once per century, but astrophysicists are patient enough to search for waves that oscillate once per decade, a frequency of 3 nHz (3 billionths of 1 Hz). However, we can’t do that on Earth — we have to look to the heavens.
Pulsars meet this need. As, sketched here, pulsars are rapidly rotating neutron stars with strong magnetic fields that sweep radiation across the cosmos, like lighthouse beacons. Their very large masses provide enormous inertia, making their rotation rate extremely stable. As gravitational waves pass between a pulsar and Earth, the space between us stretches and compresses, changing the arrival time of the pulsar beam. Astrophysicists have established, and are aggressively expanding, the capability of monitoring large numbers of pulsars throughout the sky. The IPTA (International Pulsar Timing Array) now regularly monitors 100 pulsars, each of which spins hundreds of times per second and whose pulse arrival time jitter is less than 300 ns. Based on 11 years of pulsar timing observations, the NANOGrav (North American) portion of IPTA recently reported detecting random, oscillating, spatial distortions coming from all directions in the sky, a so-called isotropic stochastic background. The observed distortions occurred at frequencies between 3 and 700 nHz, with a median strain of 2 parts in 1000 trillion. (Strain is the fractional length change.) NANOGrav was unable to confirm that these spatial distortions had gravitational waves’ distinctive quadrupole signature (stretch and compression in orthogonal transverse directions). If NANOGrav’s effect is due to gravitational waves, the most probable sources are binary supermassive black holes. Another group proposes testing this hypothesis by targeting 13 known supermassive black hole binaries with measured masses and orbital periods. Their orbital periods range from 1 to 11 years, their masses range from 1.2 to 18 billion Msun, and the strains they generate here are between 1 and 72 parts in 10,000 trillion. (Fortunately for life on Earth, there are no black holes that massive in our galaxy, binary or otherwise.) Hunting for signals from a known direction and frequency greatly improves the odds of detection, as will more years of observation, and the addition of more monitored pulsars. This latter group estimates that, by 2025, IPTA will observe oscillating, spatial distortions from 3 of the 13 targets with signal to noise (S/N) ratios exceeding 3, corresponding to about 99% statistical confidence. By the early 2030’s, IPTA could have S/N > 3, and possibly > 6, for all 13 targets. This would firmly establish these signals being gravitational waves from supermassive black hole binaries. An essential advantage of detecting this type of gravitational waves is that they will continue and grow ever stronger for millions of years, as the supermassive binaries get closer and orbit faster. This will enable very precise measurements and much greater insight into the physics of the most extreme creations of nature. By comparison, the LIGO/VIRGO events last tenths of a second. We just need to be patient. Best Wishes, Robert September 2021 Note: Previous newsletters can be found on my website. | 