A Roman Space Telescope will also find rogue black holes

How would microlensing work around a black hole. Credit: NASA’s Goddard Space Flight Center Conceptual Image Laboratory

In the past, we have reported on how the Roman Space Telescope may be able to detect hundreds of thousands of exoplanets using a technique known as microlensing. Exoplanets will not be the only thing it can find with this technique, however – it should be possible to find isolated black holes as well.

Isolated black holes are unique because most black holes that scientists have identified are those that directly interact with another object. However, those that are relatively small could wander around the galaxy on their own, which would be almost impossible to detect because they absorb all electromagnetic wavelengths.

Usually these small black holes weigh about 10 times as much as the sun. They form when a star dies and either a supernova goes or collapses directly into a black hole, depending on its weight. If the black hole is not surrounded by some absorbable gas or dust, then it would become essentially invisible to almost all instruments.

So far scientists have found 20 of these stellar black holes, but only because they are close to another astronomical object, evidencing their gravitational force in the movements of the companion object.

The neat thing about the microlensing technique that Roman will use to detect planets is that every large gravitational field will cause the microlensing effect. So if Roman sees what appears to be a microlensing effect without an obvious source of mass, it’s probably causing it to black hole.

To find the tiny perturbations that would cause the microlensing, Roman will have to look at hundreds of millions of stars for a very long time. But that is exactly what it aims to do. With this additional data, scientists will be able to answer questions, such as why isolated black holes only appear to have a mass about 10 times that of the sun, or exactly how many star-shaped black holes there are in the galaxy. The current estimate is about 100 million.

How to use gravitational lensing to detect black holes. Credit: NASA

No matter the answers to these questions, Roman will give more data to inform conclusions on these questions and many others when it launches around 2025.

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