University Post
University of Copenhagen
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Black holes link up in strange ways

Astrophysics — They can be much heavier than the scientists previously thought. And they move along unusual trajectories. The University Post went geek and dropped deep down into a black hole to understand what is going on.

In 2020, astrophysics has changed the whole narrative about how black holes work. For scientists, the breakthroughs are almost shocking.

»Black holes are created much heavier, and form into pairs in much more chaotic ways, than previously believed. This has really come as a surprise to many experts,« says Johan Samsing, assistant professor and astrophysicist at the Niels Bohr Institute (NBI) at the University of Copenhagen.

He is a specialist on how black holes interact with each other.

Since 1 September, Johan Samsing has partnered up with assistant professor Daniel D’Orazio, who is the world’s leading researcher in supermassive black holes, and they are expanding their research group over the coming year. Competition is fierce among researchers throughout the world who are interested in the same phenomena.

»You sit there every day and wonder if what you’re working on right now will be surpassed by other researchers somewhere else in the world in one hour’s time,” says Johan Samsing.«

A study of the period just after the Big Bang

Until a few years ago, black holes were almost impossible to do research on, as they did not emit any light and were therefore impossible to track with ground-based telescopes.

LIGO and Virgo

The observations of gravitational waves from 2015 until today come from two observatories: LIGO in the USA and Virgo in Italy. LIGO is a acronym for the Laser Interferometer Gravitational-Wave Observatory.

But by using new methods to measure gravitational waves, which it was only possible to register with two new observatories in 2015, the researchers can get closer to: Where, and how, do black holes arise, how do they meet up, and how do they collide?

The answers to these questions can give us a much better and deeper understanding of our universe – and ultimately the Earth’s history.

According to Johan Samsing, black holes are in many ways the ultimate manifestation and prediction of Einstein’s theory of gravity.

In a black hole, space and time are merged – according to Einstein – in one infinitely small point — a so-called singularity, where the black hole curves space and time so much that nothing can come out, and where time and space no longer exist — very similar to the earliest stages of our own universe right after the Big Bang.

»This is why research into black holes is a direct study of how our entire universe is interconnected,« says Johan Samsing.

They eat themselves up

The context of this research is observations of gravitational waves after black holes have collapsed, maybe after revolving around each other for millions to billions of years.

In 2020, more than 20 of these types of collisions were observed via gravitational waves, and according to Johan Samsing, this testifies to a rich population of black holes with masses that range from five to as much as 100 million solar mass units — astrophysicists measure the mass of black holes with reference to the mass of our own sun.


Albert Einstein predicted the theoretical existence of black holes in 1915 as a result of his general relativity theory. Indications of their actual existence came from observations early in the 1960s, and in 1965 the English mathematician Roger Penrose described how black holes are an inevitable consequence of Einstein’s general relativity theory. He received a (shared) Nobel prize for this in 2020.


The result of a collision between two black holes is a new black hole, which surprisingly has a smaller mass than the two original black holes combined.

The lost mass has been transformed into pure energy following Einstein’s formula E = mc2. You could say that the black holes consume part of their mass to release and emit huge amounts of pure energy when they collide, and this is done in the form of transmitted gravitational waves, which we are able to measure from here on Earth.

The observations can only reveal something about black holes by way of precise mathematical models, after the astrophysicists have analysed and compared them to the models of the evolution of the universe that they already have.

Johan Samsing and Daniel D’Orazio are experts in being able to devise exactly the mathematical models, which supercomputers subsequently can use to make calculations and thereby find out more about the black holes.

»It’s really difficult to come up with a simple, workable, and precise mathematical model when we have to combine mathematics with the physical models that describe our incredibly complicated universe,« says Johan Samsing.

Elliptical trajectories offer clues

One famous quote from a theoretical physicist John Wheeler is the following: ‘A black hole has no hair’.

And black holes are actually quite simple. In fact, physics can describe them solely by their mass and their rotation.

However, according to Johan Samsing, there is one bothersome little detail. It is the question of how circular their trajectories are. When two black holes’ trajectories around each other are not 100 per cent circular, they are more or less elliptic, and this question has turned out to be crucial for astrophysics.

And here Johan Samsing and Daniel D’Orazio have offered a key contribution to the science.

They have not just formulated a theory for what the mass of black holes is, and how they probably rotate, but they also have a theory for how elliptical their trajectories are – and this has turned out to be one of the most interesting things about the black holes.

It can be compared to the Earth’s orbit around the Sun. It is not circular, but elliptical, and the earth rotates around itself along an axis that is not perpendicular to the larger plane of the Earth’s orbit around the Sun, the ecliptic.

The holes’ culture of perfection

On Earth we feel these differences as summer/winter and day/night, but for black holes that revolve around each other, these differences show up as minor changes in their gravitational wave signal.

Black holes will constantly strive to move in perfect circular trajectories. Each time their orbit goes elliptical, they will try to rectify this, undesirable, state by emitting pure energy in the form of gravitational waves, thereby losing some of their kinetic energy.

The same applies to the shape of black holes: As soon as they start to become ellipsoid, they also emit gravitational waves – until they are perfectly round again.

»And it is precisely these changes that are the key to understanding the creation of the black holes,« says Johan Samsing.

By setting up mathematical models that can approximate the locations in our universe where black holes might arise, Johan Samsing and Daniel D’Orazio are now in the process of finding out how they have been created, along with their observed trajectory, rotation and mass.

»Are black holes, for example, created in very dense clusters of stars in a galaxy or every now and then, by chance, everywhere in a galaxy?« Johan Samsing asks. He and Daniel D’Orazio are looking for an answer to this.

Improbable black holes

Out of approximately 20 observations of gravitational waves that LIGO and Virgo have conducted since 2015, there is one in particular that stands out. And Johan Samsing is enthusiastic when he talks about a gravitational wave observation that took place on 2 September 2020.

On that day, one of the strangest collisions between two black holes was observed. It formed a new black hole of approximately 150 solar masses. It was named GW19052, and the collision appears to have happened in a way that the researchers had otherwise not considered probable.

Half of all the hundreds of billions of stars found in the universe are actually binary stars. Stars, in other words, in pairs, that revolve around each other.

According to the physicist’s theories, some of the binary star systems after billions of years burn out and collapse, becoming black holes, both circling and orbiting each other.

It is probable that two black holes end up colliding after millions or billions of years have passed, and this has been an accepted assumption in physics.

However, due to the laws of physics that apply to stars, they cannot be larger than 45-65 in solar mass, and the odd thing about GW190521 is that here two black holes collided with 66 and 85 solar masses, respectively.

This is far above the limit of how big black holes can be formed from stars according to the theory, and that’s why it has been a mystery where they came from.

Johan Samsing and Daniel D’Orazio have found a starting point in the clarification of this mystery.

When it comes to GW190521, the two black holes that came together and merged into one could well be the result of several clashes between smaller black holes before they gained the 66 and 85 solar mass weight.

»It is probable that this is what happened based on what we helped discover. This can actually take place in huge accretion disks of gas held together by a super-massive black hole with a mass of millions of suns in the centre,« says Johan Samsing.

Up until now, researchers have imagined that this will happen in the star cluster of the Milky Way. The Milky Way has 100 star clusters, each containing approximately 100,000 stars, and a cluster may contain 100 black holes.

»In these clusters, black holes can, in principle, collide several times, so they form into larger and heavier black holes. But in each collision, the new, composite black hole is accelerated so much by the strong gravitational waves emitted in the collision, that it leaves the star cluster. This is the theory. So the probability that more black holes have collided several times within the same star cluster before they are finally 66-85 solar masses, is very, very small,« says Johan Samsing.

Stars in threes

Now Johan Samsing has a possible explanation for why black hole courses are so elliptical when they should be more round.

He has recently showed how black holes can often collide through the help of a third black hole that affected two black hole circular trajectories thereby making them elliptic.

Johan Samsing

Got his PhD in 2014 in theoretical astrophysics from the Dark Cosmology Centre (DARK) at the Niels Bohr Institute. Went on to Princeton University in the USA, where he worked for five years, initially as an Einstein Fellow and then as a Spitzer Fellow. A member of the prestigious Princeton Society of Fellows.

Awarded a position late in 2019 at the Niels Bohr International Academy as a ‘Louis-Hansen Assistant Professor’ and was awarded at the same time a European Marie Curie Fellowship. In 2020 he was also awarded a Villum Young Investigator Grant of DKK 10 million.

Daniel d’Orazio

PhD in astrophysics in 2016 from Columbia University in New York, and before that a Fulbright Fellow at the University of Zurich, Switzerland. After his PhD he was an Einstein Fellow and an ITC Fellow at Harvard University.

Today, he is considered one of the world’s leading experts on super-massive black holes with billions in solar mass that are at the centre of galaxies. He was appointed assistant professor in astrophysics at NBIA at the Niels Bohr Institute on 1 September 2020.

This can result from a spectacular dance between three black holes, and Johan Samsing has created a simulation that illustrates how this takes place in a YouTube video, Black Hole GW Merger.

According to Johan Samsing, the result of the impact of a third black hole is often that two black holes that end up colliding, do not collide on a perfectly circular trajectory in accordance with the theory.

The theory assumes that two black holes stem from a binary star system that has not been affected by anything from the outside.

»Instead, they come together on an elliptical trajectory, when they are also affected by a third black hole. And now astrophysicists can measure how elliptical the trajectory is. And all this is done by means of the gravitational wave observations that have been made since 2015, in combination with advanced mathematics and supercomputers. It also allows us to say something about which areas of the universe the black holes collide in,« says Johan Samsing and continues to explain the theory:

»We can see that in a star cluster, the black holes are interacting in complicated trajectories, and in this way I found out that a large part of the collisions take place on elliptical trajectories. This points directly to the black hole collisions being created through chaotic interactions in very dense areas of our universe,« he says.

GW190521 was not just a collision between two very heavy black holes, as previously assumed, but they collided on an elliptical trajectory.

»To calculate under which conditions a non-circular collision can take place, is very, very difficult. But Daniel D’Orazio and I have been the first to do it,« says Johan Samsing.

And what can we learn from it?

»Binary stars that have collapsed into black holes will never be able to create trajectories for black holes that are not circular. First of all, we have figured out where GW190521 came from from its non-circular signal, or we have at least made a plausible theory about what happened,« says Johan Samsing.

Three black holes in a dance

The story of GW190521 does not stop here, because the spectacular collision contains even more surprises.

For some time after the observation of the gravitational waves from the collision at GW190521, a light signal was observed, and this is mysterious, because black holes do not emit any light.

But here the gas accretion disc theory offers an explanation.

In the impact between two black holes, gravitational waves have been observed. The new black hole is shot out through the accretion disc, resulting in a warming up of the gas that makes it light up.

»Our theory offers a complete ‘three-in-one’ package solution to the problems of heavy black holes, elliptic trajectories, and the subsequent observation of light, and it is now on its way to being published,« says Johan Samsing and Daniel D’Orazio.

It is worth mentioning that GW190521 is only one observation of a series of new, strange collisions observed in 2020.

This has given astrophysicists something to think about, and it has completely transformed the thinking about black holes.

»You simply do not know exactly how all the collisions have occurred and how they have been created, because there have been so many possibilities. Just a couple of years ago, they said you would never see collisions between black holes on non-circular trajectories, but Daniel D’Orazio and I have showed that they exist,« says Johan Samsing.