Dark Matter Could Have Caused Massive Black Holes at the Beginning of Time

Artist's impression of an early Universe supermassive black hole. (ESA/Hubble, NASA, M. Kornmesser)

As our ability to peek farther and deeper into the Universe grows, we've discovered something quite surprising: Before the Universe was even 10% of its current age, there were supermassive black holes with masses millions to billions of times that of the Sun.

This is a real cosmological puzzle. Given our understanding of black hole growth rates, there shouldn't have been enough time since the Big Bang for them to become so large. However, their existence is obvious, implying that something weird is afoot.

According to new study, that item could be dark matter, one of the strangest objects in the Universe.

According to physicist and astronomer Hai-Bo Yu of the University of California Riverside, "we can think of two reasons why the early Universe black holes are so enormous."

The seed – or'baby' – black hole is either considerably larger than we anticipated, or it is growing far quicker than we thought, or both. What are the physical mechanics for developing a huge enough seed black hole or reaching a fast enough growth rate?

One of the Universe's greatest mysteries is dark matter. We have no idea what it is or what material it is composed of. The only way it interacts with ordinary baryonic matter in the Universe – the substance that makes up everything we can perceive – is through gravity.

We can see gravitational effects in the Universe, such as galaxies rotating and light curving through a strong gravitational field, and remove the gravitational influence of normal matter to measure dark matter composition, because it interacts gravitationally. There's a lot of it, too. Dark matter accounts for approximately 85% of the matter in the Uerse.

A halo of dark matter surrounds most galaxies, and it's thought to be crucial to their development. The direct collapse of a dense cloud of gas is one concept for the genesis of supermassive black holes. Yu and his colleagues questioned if there was anything else they could do.

According to Yu, this technique cannot produce a huge enough seed black hole to accommodate newly discovered supermassive black holes unless the seed black hole grows at an exceptionally fast rate.

An alternative explanation is provided by our research: Gravothermal instability causes a self-interacting dark matter halo's centre portion to collapse into a seed black hole.

Dark matter, as far as we know, only interacts gravitationally with baryonic matter, but it may be able to interact with itself.

The team's scenario begins with the development of a dark matter halo in the early Universe, which comes together gravitationally. Because non-self-interacting dark matter particles are unable to transfer their energy to other particles, the inward pull of gravity would compete with the outward push of heat and pressure. Particles condensing towards the centre of the halo would speed up under the increasing gravity and recoil under the higher pressure.

However, self-interacting dark matter particles could transfer energy to neighbouring particles, causing friction in the revolving dark matter fluid. The particles would slow down, diminishing angular momentum and shrinking the central halo until it eventually collapsed under its own mass, forming the seed of a black hole.

The seed might then grow by absorbing baryonic materials, according to the researchers. While the dark matter'seed' can have a large enough mass to allow the black hole to expand rapidly, both types of matter are necessary.

According to Yu, stars and gas dominate the core portions of many galaxies.

As a result, it's only natural to wonder how this baryonic stuff influences the collapse process. We demonstrate that it will hasten the commencement of the collapse. This trait is exactly what we need to explain the early universe's birth of supermassive black holes. The self-interactions also cause viscosity, which helps the collapse process by dissipating the central halo's angular momentum.

The team thinks that future equipment, which will be even more sensitive, may be able to locate early Universe galaxies with brightnesses that are above our current observatories' capabilities.

This should help them validate their concept, which would not only answer the conundrum of early Universe supermassive black holes, but also shed light on the mysterious nature of dark matter.

The research has been published in The Astrophysical Journal Letters.

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