Black hole ripples can help describe the expansion of the universe


July 05, 2023

(Nanowerk News) The light echoes from jets launched from black holes offer a new way to determine the distances to these exotic objects and to study the largely unobserved population at the center of galaxies. It could also help determine the expansion rate of the universe. The technique, developed by a team at the University of Newcastle and tested on the black hole archetype Cygnus X-1, was presented by graduate researcher and team member Patrick O’Neill at the National Astronomy Meeting in Cardiff.

Most black holes are the remnants of stars that ended their lives in supernova explosions. They have such strong gravitational fields that not even light can escape their grasp, which is why they are described as black. Even so, the effect on its environment can be quite obvious, as matter orbiting the black hole is concentrated into the disk, and can become very hot. This means they are powerful X-ray sources, and many also have associated jets that spew gas and dust over great distances.

The calculated distances to most black holes are based on the brightness of the X-rays and related measurements of their mass, which can be inferred from how fast material is rotating around them. O’Neill and the rest of the team took a different approach. Schematic of the reflection process in the Cygnus X-1 black hole Schematic of the reflection process in the Cygnus X-1 black hole. The light emitted from the jet is shown in blue, which we say comes from a point source some distance above the black hole. X-rays follow a path directly observed by our telescopes. Light that follows a path (labeled b & c), reflects off the disc and follows a green path to reach us. Light traveling along a) follows the shortest path to reach us, whereas light traveling along c) takes the longest path, and therefore takes the longest to reach us. (Image: Patrick O’Neill. Background image: NASA/JPL-Caltech)

The light from the black hole is emitted in all directions, so that it reaches the disk. Just like a mirror, the disk then reflects some of the incoming light. Starting from the innermost part of the disk, the reflected light will ripple outwards because the light emitted in the beam takes longer to reach the outer part of the disc. This light ‘echo’ was akin to a sound echo.

This effectively means we see the light coming from the jet in two ways: light that travels directly at us, and light that is reflected by the disk. By simultaneously monitoring the brightness of light traveling directly at us and reflected light, it is possible to deduce how far the jet is above the disk. It also tells astronomers how close the disk’s inner boundary is to the black hole itself. Closer in the gravitational field the black hole disrupts the disk shape.

Monitoring the light emitted from the jet black hole and the disk around it simultaneously allowed the team to calculate the size of the disk, and the fraction of light it reflects. It provides an absolute measurement of the disk’s brightness, and hence the distance to the black hole disk system.

Dense clouds of gas and dust usually block infrared, visible, and ultraviolet light emitted from galactic centers (including our own), limiting our view. In contrast, X-rays can traverse this region unhindered, so the distance to a supermassive black hole must be measurable. If it could be done, it would provide a new way to determine how fast the universe is expanding, something that remains unresolved 94 years after the discovery of expansion itself.

It is also a powerful tool for investigating black hole populations at the center of galaxies. Until now, astronomers tended to observe black holes that were relatively light, and far from the plane of the Galaxy where most stars are found (Our Galaxy has spiral arms in a flat disk that extends outward from a central bar).

Sometimes black holes and massive stars orbit each other in binary systems. If a massive star explodes as a supernova, the black hole can be ejected from the galactic plane. The heavier a black hole, the smaller its acceleration, so heavier mass black holes will be found closer to the galactic plane and in the center of the galaxy.

O’Neill said: “Often we are limited to observations of distant galaxies to make inferences about the Milky Way. This cutting-edge technique offers a method for probing previously hidden galactic centers, offering new insights into the evolution of our own Galaxy and how black holes accumulate matter (MOU1). It’s also exciting to think that we can help set the pace of the Universe’s expansion – and gain a better understanding of its future.”

The team now wanted to build a picture of the population of black holes at the center of galaxies. This could help find objects such as intermediate-mass black holes, objects thought to originate from black hole mergers from single stars, and a step toward forming the monster-sized supermassive black holes found at the centers of most galaxies.


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