Intermediate-Mass Black Holes

Over the years, astronomers have readily detected stellar black holes, which are relatively small and equal in mass to a few Suns or less, and supermassive black holes, which are equal in mass to millions of Suns. However, black holes of intermediate mass have notably eluded detection, prompting scientists to theorize why. New research suggests that intermediate-mass black holes might not have been discovered because they may not exist in our modern-day Universe for a very simple reason: the growth rate of black holes.

Scientists believe that stellar-mass black holes form when huge stars die and collapse inward. These are the “standard” black holes you might envision when you think of stars dying, or when you imagine the millions of black holes that dot our Universe. Supermassive black holes are those that form the hearts of large galaxies like our own Milky Way. To date, the oldest supermassive black holes found include a discovery from 2015 — a throwback to a much younger version of our Universe, when it was only about 875 million years old. The overall picture presented by our findings on supermassive black holes so far indicates that those early days of the Universe were friendlier for the formation of supermassive black holes, since matter was more concentrated in the much smaller Universe.

Image Credit: Ute Kraus/Wikimedia Commons

Notably absent in this picture of black holes in our Universe are intermediate-mass black holes of about 100 to 10,000 solar masses. Astronomers hope to be able to study these elusive creatures to better understand how supermassive black holes reach their incredible size and affect the Universe around them, but thus far they've been coming up with evidence that is mostly inconclusive. New research suggests that this may be an artifact of the growth rate of black holes.

Searching With Gravitational Waves

As scientists have continued to better understand the process of black holes engulfing stars, they've been able to observe the rate at which black holes grow. This has allowed them to estimate a growth rate of one solar mass per 10,000 years. While this is only an estimate, and they might grow even faster if they could consume dark matter or gas, assuming they consume solely stars and dense matter such as neutrons and white dwarfs, this speed should be fairly accurate. This means that even a small, stellar-mass black hole would grow far past the intermediate-mass stage within 10 billion years. Our Universe is approximately 13.8 billion years old, so if most black holes have had time to progress into the supermassive stage, they must also have started early in the Universe's life.

Researchers also say that intermediate-mass black holes that exist right now might be hard to identify, as they may be in dense clusters of stars. This would mean that light produced by objects they consume is not especially noticeable, and can easily be mistaken for other phenomena. The solution to this, according to the new research, is to search not for light, but for gravitational waves.

In fact, the researchers point out that among the discoveries that the Evolved Laser Interferometer Space Antenna (ELISA) mission planned for 2034 may make are gravitational waves generated by the collision of two intermediate-mass black holes. If ELISA can detect intermediate-mass black holes, we may be able to glean entirely new insights into the mysteries of their supermassive cousins, dark matter and energy, and how our Universe is expanding.

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