Recycling 101

Illustration: NASA

Yours truly believes in recycling. Seems nature does too as she does this all the time as we are made of the remnants of stars and nothing goes to waste as nature's lazy and constantly repeats processes that work over and over again. Think spirals, sunflowers and galaxies as example number one. The principle of least action applies. Now, researchers are realizing black holes are recycled too based on signatures they are now seeing as we speak.

An international team of astrophysicists has found evidence that the universe recycles black holes, merging them to form even larger ones. Gravitational waves recorded in recent years show that some of the heaviest black holes within star clusters exhibit clear signs of being “second-generation” black holes—products of past collisions—and therefore could not have originated from the collapse of a massive star.

Impossible Black Holes

The evolutionary theory of stars explains that, at the end of the lives of the most massive stars, their cores compress until they form a point so dense that it curves space-time to infinity. This is the classic black hole, with masses 10 to 40 times that of the sun. There are also supermassive black holes, in the center of galaxies, with millions or billions of solar masses, whose origin is related to processes that occurred in the earliest moments of the universe.

Between these two extremes lies a contested category: black holes with masses between 40 and 100 solar masses. They are too heavy to be born after the death of a star, but they do not reach the necessary dimensions to emerge from the collapse of a gigantic cloud of matter. Conventional stellar physics considers them “impossible,” yet they appear frequently in detections.

Astrophysicists propose that these massive black holes could form by the merging of two or more smaller, ultradense objects. The idea was plausible, but it needed evidence. Until relatively recently, there was no way to obtain it.

Then gravitational wave detectors came on the scene. These instruments use lasers to measure the micro-distortion of space-time generated by the collision of extremely dense objects. The first detection, in 2015, confirmed a merger between black holes. Since then, each new signal has allowed for a better characterization of these structures and revealed that these collisions occur much more frequently than previously imagined.

A "normal" sized black hole, isolated in space.
Courtesy of Space Telescope Science Institute Office of Public Outreach

The Second-Generation Signature

The study, published this month in Nature Astronomy, analyzed a transient catalog of gravitational waves generated by the world's three leading observatories. The database included 153 reliable detections of black hole mergers. Among them, 34 corresponded to particularly heavy objects.

By comparing all the signals, the team identified two distinct populations. The lighter black holes, up to about 40 solar masses, showed small, aligned spins, as expected for objects born from the collapse of a star. But from a certain point, around 45 solar masses, a completely different population appeared: heavier black holes, spinning rapidly and in chaotic directions—a statistical signature that can arise only when the object has already participated in a previous merger.

“This is the exact signature you would expect if black holes repeatedly merged into dense stellar clusters,” said Isobel M. Romero-Shaw, coauthor of the research, in a statement from Cardiff University.

So far researchers have not directly observed any of these “impossible” black holes. They do not appear in x-rays or in the visible spectrum, unlike supermassive ones. However, their collisions vibrate space-time, and that vibration reveals masses that stellar physics cannot explain.

Combine this with the fact researchers may have seen a black hole explode and
the notion of recycling becomes something else altogether. 

Did we just see a black hole explode? UMass Amherst’s physicists think so. This artist’s concept takes a fanciful approach to imagining small primordial black holes. Credit: NASA’s Goddard Space Flight Center

In 2023, a neutrino telescope submerged deep in the Mediterranean Sea struck quantum gold, detecting a neutrino with 100,000 times more energy than even the Large Hadron Collider, Earth’s most powerful particle accelerator, can produce. Put simply, it seemed to be an impossible event. Now, physicists from the University of Massachusetts Amherst think this particle may be evidence of an explosion from a black hole that originated at the beginning of time. 

Somerthing's afoot - Sherlock Holmes.

 To be continued ...