Focal Plane November ’24 – Searching for Gravitational Waves from Supermassive Black Hole Mergers

            The October 3, 2024 NASA Universe of Learning featured discussion on gravity.  One of the topics covered was searching for gravitational waves from supermassive black hole mergers. Chiara Mingarelli, PhD – assistant professor of Physics at Yale University, shared her research on supermassive black hole mergers.  Supermassive black holes are over a billion times the Sun’s mass.  Astronomers study these black holes to determine how they were formed.

            There are two possible methods that researchers consider. The first method is that they formed directly from a giant molecular cloud that collapsed immediately into a massive black hole.  This method would imply that supermassive black holes would be found from the early days of the Universe. The James Webb Space Telescope has found supermassive black holes in the early universe to support the feasibility of this formation theory.

The other possible formation process involves mergers.  As galaxies got bigger by merger,, central black holes have also merged and grew over time. From the early Universe, and continuing today, galaxies have merged. And through the process of mergers, galaxies have grown in mass.  This process also involves massive and super massive black holes. We have found supermassive black holes at the center of large galaxies.  These massive black holes may have formed by mergers; as separate smaller galaxy merger with each their own center black holes also merged and grew in mass.  

            How can researchers find evidence of these past actions in today’s Universe.  Fortunately, mergers of massive gravitational objects create gravitational waves. Gravitational waves change the fabric of the Universe. The wave distort time by 100 nanoseconds over a decade. These are low frequency waves that equal 1 nano-hertz in a 30 year orbit.

Mergers take millions of years, and over this time gravitational waves are created. Now, in our Universe there could be many mergers happening at the same time creating a choppy gravitational sea of waves. Just like researchers found the cosmic microwave background to learn about the early Universe, in June 2023 researchers had found a gravitational wave background from all the mergers over the life of the Universe.       

            How can researchers track a wave to its origin to determine if black holes were involved in creating a supermassive black hole? Fortunately, the Universe contains lighthouses to help researchers. Pulsars are cosmic lighthouses. They are almost perfect clocks with their rotations. Gravitational waves change space by shrinking or expanding the fabric of the Universe as they pass. Since gravitational waves change the distance of space  between Earth and an observer pulsar we can measure the change to determine the source. 

            How can researchers measure such a short wave distortion among this choppy sea of waves? Researchers search for millisecond pulsars, since they are stable down to 100’s of nanoseconds over a decade.  These stable characteristics help researchers search for particular waves in the gravitational wave background. As researchers find these identified sources a map is made to find origins, similar to how the CMB mapped the early Universe’s structure. The map will allow researchers to find hotspots, luminous gas in galactic disks, and trace the cosmic web to find a larger structure of space.  The goal: 1) locate luminous gas in galactic disks 2) find the light – locate galaxies 3) search for individual merger systems. The more found the better the map to determine the origin of summer massive black holes.