Black holes are a central phenomenon of astrophysics

Black holes are a mysterious phenomenon of particular interest to astrophysicists. In the past 10 years in particular, knowledge about them has increased a great deal, and black hole research has experienced a revival. Current levels of enthusiasm were last seen in the 1960s. There still remains plenty to investigate.

The fundamental characteristics of black holes were already predicted by the general theory of relativity developed by Albert Einstein. Black holes are extremely dense concentrations of mass in spacetime. For astrophysicists, black holes are fairly simple, as only three quantities are needed to describe their properties: mass, rotation and electric charge. Black holes can help examine key theories in physics.

– Black holes are formed in the death of extremely massive stars (those with at least 15–20 solar masses), says Professor Peter Johansson.

In the final stages of a massive star, the fusion of various elements occurs in layers (hydrogen in the outermost layer), with the fusion of heavier elements progressing deeper inside the star until the core fuses into iron. Anything heavier than iron cannot be made by fusion without losing energy at the same time. This results in the sudden halting of the fusion process in the core of the star, which collapses under the influence of gravity.

– If the mass of the core exceeds roughly three times the mass of the Sun, the result can be a black hole, whose enormous gravitational pull does not allow even light to escape, Johansson says.

A phenomenon of cosmic scale

Our Milky Way is estimated to have some 100 million stellar-mass black holes. Not one of them is anywhere near the solar system, but much further away in space. As there are roughly 100 to 200 billion stars in the Milky Way, it is possible to calculate that only about one in every thousand stars will become a black hole.

– Most stars, like our Sun, shrink into small white dwarfs at the end of their lifespan, Johansson says.

Black holes are surrounded by an event horizon, or a boundary that functions like a unidirectional membrane. Through it, matter, light and information flow to the hole, but they can never escape.

– The mass of supermassive black holes can exceed 10 billion solar masses. In such cases, the event horizon is larger than our solar system, Johansson says.

Radiation originating close to the event horizon of black holes has been observed by synchronising radio observatories around Earth. These observations of the Event Horizon telescope have depicted the shadows of two supermassive black holes, one in the supergiant elliptical galaxy M87 and one in our Milky Way.

– Most black holes are much smaller stellar-mass black holes, whose mutual collisions can be investigated by measuring gravitational waves, says Johansson.

For the time being, there has been no success in measuring the gravitational waves of distant supermassive black holes.

– While the images taken by the Event Horizon telescope directly prove that the black holes are actually there, they are fairly open to interpretation. By studying gravitational waves, we gain direct knowledge on the characteristics of colliding black holes, Johansson says.

This information is more relevant for research than the images.

The effect of black holes on galaxies

Most of the black holes in the Milky Way have not yet been observed. A black hole isolated in empty space emits no radiation, and gravitational waves can only be observed in conjunction with colliding black holes.

– Black holes can help us explore fundamental physics: for example, whether the validity of the general theory of relativity varies close to black holes. So far, the theory has remained very accurate based on observations, Johansson notes.

Should variation be found, it would constitute a cataclysmic revolution in physics. Other matters to investigate include how gas and radiation behave close to the event horizon, what processes cause gas to heat up in the accretion disks of black holes, and how supermassive black holes affect galaxies.

– It may be possible, for instance, that supermassive black holes are setting the upper limit to the collective mass of stars in a galaxy. In other words, when the mass of a black hole grows sufficiently large, the radiation originating in its hot accretion disk drives gas away from the galaxy, stopping the formation of stars. This way, supermassive black holes could directly influence the creation of stars in the universe.

There appears to be only one supermassive black hole in the centre of each galaxy, and their mass correlates with the stellar mass of the galaxy. Consequently, the largest black holes are found in the largest galaxies, establishing a clear connection between black holes and galactic evolution.

A threat to Earth?

Do black holes pose a threat? Even though a black hole could easily swallow Earth and the entire solar system, such a scenario is chiefly a thought experiment. According to Johansson, it is extremely unlikely that a black hole would threaten Earth.

– In the last five billion years, no stars, let alone black holes, have drifted anywhere near the inner solar system. You can always speculate, but statistically speaking collisions between a black hole and the solar system would take place, on average, once every 1,000 billion billion years. This is a period 100 billion times longer than the age of our universe in its current state, in addition to which the Sun has only roughly 5 billion years left of its lifespan. To put it another way, you don’t have to worry, Johansson says.