*Researchers investigate the effects of dark matter particles on the growth of black hole shadows*

Black holes have caught the attention of physicists over the years, being the best natural laboratories for testing the predictions of Einstein’s theory of gravity. While we cannot obtain an image of a black hole —since no light escapes its interiors owing to its extreme gravity — we can capture an image of its surroundings because of an interesting phenomenon. Its strong gravity interacts with light to cast a black hole “shadow” — a dark interior enveloped by a bright ring. Last year, a network of extremely powerful telescopes called the Event Horizon Telescope captured the very first image of a black hole shadow. It provided direct confirmation of the existence of black holes, in line with the predictions of Einstein’s theory.

But what more can we know from these shadows? In a recent study published in the journal *Physics Letters B*, Rittick Roy and Prof. Urjit Yajnik from the Indian Institute of Technology Bombay (IIT Bombay) set out to consider this question. The answer lies in the interaction of black holes with another enigma of physics — dark matter.

Many theorists believe that dark matter is made up of particles called axions, which have a much smaller mass compared to other fundamental particles found in nature. In the current study, by a careful investigation of these elusive axions around the black holes, the researchers have concluded that black hole shadows could increase in size over time. Their study suggests that this growth of the black hole shadow can be directly observed.

The ability to observe the growth depends on an exciting process lying in the realm of quantum mechanics. Particles created spontaneously out of vacuum under the effect of the strong gravity of a black hole escape the pull of this gravity in a phenomenon known as the ‘Hawking Radiation’, first predicted by Stephen Hawking.

Here, the authors focus on a similar effect, which they are calling the ‘quasi-Hawking effect’, in which the spontaneously created particles form a cloud near the black hole instead of escaping. Their conglomeration into a cloud decreases the spin of the black hole — which is a measure of how fast a black hole is rotating — leading to the growth of the black hole shadow.

The authors derived mathematical relationships between the properties of a black hole and the axions to estimate the timescale of the growth. The estimation of this timescale is essential to guide the future observations of black hole shadows that will be made by our telescopes. They found that the timescale depends on the properties of the black hole and the axions, and the most suitable candidate to realistically observe this growth is the black hole at the centre of our very own galaxy, called Sagittarius *A** (Sgr *A**).

The authors then used this black hole as the prototype for their further study. Using a combination of numerical simulation and the previously derived mathematical equations, they inferred that the timescale of the shadow growth depends on the properties of the dark matter and the resolution of the telescope used to observe the growth. These predictions will serve as templates for observations which can attempt to study this growth.

But, there could be other processes that could cause the growth of the shadow, warn the authors.

“However, the quasi-Hawking effect is a quantum process. It is slow but steady, and this slow growth would be the smoking gun signature of this quantum process,” says Dr Yajnik, about the process they are analyzing.

The observation of the growth of black hole shadows will also provide evidence of the presence of axions, which have escaped direct detection till now.

“These particles are crucial missing links to key theoretical puzzles in particle physics,” adds Dr Yajnik. Their confirmation can open a whole new arena towards the theoretical understanding of axions. “Moreover, the quasi-Hawking effect is also our best chance of observing the Hawking effect,” says Rittick.

This study is the first to underscore that black hole spin may decrease due to the effect of the axions, and this effect can be observed realistically with improvements in current techniques. Although the Event Horizon Telescope is currently incapable of observing such slow changes of the black hole shadow, manifold improvements to its observational capabilities have already been earmarked.

The theoretical physicists have predicted a new effect; now it’s up to the observational astronomers to tune their techniques, feels Dr Yajnik.

“Once the challenge is thrown, the experimental world comes around to match the required precision,” he says, citing the trend in the annals of physics.

Incidentally, researchers have recently proposed a breakthrough in observational techniques. It may not be long before black hole shadows throw some light on dark matter.

*This article has been run past the researchers, whose work is covered, to ensure accuracy.*