The Milky Way the sky over rocks in Namibia

Our galaxy, the Milky Way, is believed to have a supermassive black hole at its centre. Image © ArCaLu/Shutterstock

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The universe could contain 40 trillion black holes

There are trillions of black holes in the universe which could make up as much as 1% of all known matter. 

The findings of an Italian-led team suggest that collapsing stars could provide the building blocks for supermassive black holes, some of the largest and most powerful objects in the entire universe. 

There may be as many as 40 trillion black holes in the universe. 

A team of European researchers estimated that black holes formed from collapsing stars could make up as much as 1% of the observable universe's baryonic matter, which includes everything made up of atoms, but excludes particles such as electrons and most dark matter. 

Most of these black holes are likely to be smaller than average, and scientists believe they could be a starting point, or 'light seed', for supermassive versions to form as they merge together. 

Dr Lumen Boco, a co-author of the paper, says, 'Our work provides a robust theory for the generation of light seeds for massive and supermassive black holes at high redshift, and can constitute a starting point to investigate the origin of heavy seeds that we will pursue in the future.' 

The findings of the research, led by the Scuola Internazionale Superiore di Studi Avanzati, were published in The Astrophysical Journal.

A colourful composite image of the Cassisopeia A supernova

Supernovas of the largest stars are believed to begin the process of a star becoming a black hole, if conditions are right. Image © NASA/DOE/Fermi LAT Collaboration, CXC/SAO/JPL-Caltech/Steward/O. Krause et al., and NRAO/AUI; licensed under CC BY 2.0 via Flickr.

What are black holes? 

A black hole is the name given to an area of space which is so dense, it warps the fabric of the universe itself. A large amount of matter packed into a relatively small space generates such a strong gravitational force that even light is unable to escape. 

The boundary that separates the area from which nothing can escape from the rest of space is known as the event horizon. As light cannot escape from inside this area black holes are invisible, but can be observed from the way their gravity affects space around them, as well as sometimes being caught in the act of ingesting matter. 

The source of black holes' power rests in the way they're formed. Those forming today are stellar black holes, which are created when some of the largest stars reach the end of their lives and collapse in on themselves.  

This occurs after large stars weighing at least 25 times the mass of the Sun run out of fuel and undergo a supernova, as their outer layers explode into space. If the remaining star fragment is heavy enough and contains enough heavy elements, then it is thought to collapse under its own weight into a black hole. 

It is thought that while today only stars that are large enough go on to form black holes, billions of years ago the early universe had more opportunities for the formation of these stellar objects.  

Some scientists hypothesise that primordial black holes formed as a result of the early universe having some incredibly dense areas, which would have meant matter would be more likely to collapse. These primordial black holes were probably smaller than average, due to the lower threshold of matter needed for them to form, but would still weigh many times the mass of our entire planet.  

Black holes can also shrink over time, with noted scientist Stephen Hawking calculating that quantum effects acting on a black hole will cause it to create and emit particles as radiation. Over time, this would cause it to eventually fade away to nothing if matter is not ingested. 

However, none of these theories explain the existence of supermassive black holes, which are millions to billions of times larger than the mass of own Sun.

An image of Messier 87 and the shadow of the black hole at its centre

The first ever image of a black hole and its shadow, created from the observations of many different telescopes. Image © EHT Collaboration

Grow your own black hole 

Supermassive black holes remain an open question in astronomy, with many different ideas of how they could have come about. One theory suggests that they are formed of small black holes, termed 'light seeds', merged together to create even denser objects. 

Large clouds of gas formed by the Big Bang may also have collapsed in on themselves in the early universe to form black holes. As these objects did not form after a supernova which dissipates most of its mass, the resulting black holes can be many times larger and are therefore termed 'heavy seeds'. 

The merging of both light and heavy seeds and the subsequent ingestion of more matter may then lead to a supermassive black hole. It is believed that one may exist at the centre of the Milky Way, as well as at the heart of many other galaxies. 

To investigate one potential source for these enormous objects, the researchers wanted to assess how many stellar black holes were in existence.  

They estimated that stellar black holes contain around 1% of the universe that we can see, which is the same mass as billions of galaxies. 

In future, researchers hope to find out more about black holes through the introduction of new technologies, such as the James Webb Space Telescope (JWST). Its predecessor, the Hubble Space Telescope, revealed in 2022 how some black holes can contribute to star formation by the matter they leave behind. 

It is hoped that JWST's sensors will allow the identification of small black holes that are yet to be detected, and peer back into the early universe to find out more about how the galaxy as we see it formed.