Announced today, the identification of a new type of gravitational waves using the whole Milky Way Galaxy as a detector will redefine our knowledge of the Universe and might lead to the detection of the mysterious cosmic strings. They could also further our understanding of the supermassive black holes
The instruments with which we researched gravitational waves for many years, like LIGO, are only 4 km long. Today’s announcement detected them using pulsars that are thousands of light-years apart, and this way, much longer wavelengths could be identified. It’s almost like using the whole Milky Way Galaxy as the gravitational waves detector, but the frequency it is sensitive to is on the scale of years or even decades. This meant that collected data from decades had to be used to find these signals, and it had to take so long; years or decades had to pass for just one wave to get through such a detector.
Supermassive black holes reside in the centers of galaxies and can be millions or even billions of times more massive than the Sun. They might merge when galaxies collide, and before that, they revolve around their common center of mass, the barycenter; they then should steadily produce gravitational waves of such long wavelengths. This mode of detecting them is not sensitive enough yet to point to individual sources, but instead, it catches many gravitational waves background hum that permeates the whole space.
Supermassive black holes are not the only candidates for producing these waves. The others are cosmic strings, one-dimensional topological defects in spacetime that originate just after the Big Bang. They are similar to cracks in ice which form during the phase transition from liquid water to a solid. If they exist, they would be like particles, but instead of being point-like, they would be stretched across the Universe and string-like. They would sometimes collide, which would cause loops in them to form, which could occasionally pinch and release gravitational waves.
With more extended observations of data from pulsars, the original individual sources of these long gravitational waves should be identified and point to supermassive black holes. However, if the sources cannot be singled out and we still get just constant hum, it will mean that it originates from cosmic strings; it would then need to lead to new physics. Because the idea of their existence is somewhat flamboyant, the leading theory of where the long gravitational waves come from is supermassive black holes.
We are currently unsure if they can merge, and they might rotate around their barycenter forever when they are around three light-years apart. This would mean that supermassive black holes in the centers of galaxies exist in multiples and not as a single huge one.