Today is Valentine’s day! So I’m going to tell you an unforgettable, romantic and passionate love story, which has marked a before and after in our way of understanding the Universe. It’s the story of two black holes thirty times more massive than the Sun.

Milky Way.
Photo by Alex Staudinger.

AltruPhysics returns after the end of the year break. I hope you’re ticking off all those new year’s resolutions. And if you haven’t started yet (or if you’ve already done all of them), I recommend that you watch my last video on how to start the year by making the world a little bit better.

But today is a day to celebrate love, so I’ve decided to start a series of videos about something I hold very close to my heart. It’s the subject on which I did my PhD: Black holes.

Specifically, in this episode, I am going to explain how stellar-mass black hole binaries form. And I’m going to explain it all with a love story that began a billion years ago, in a galaxy far, far away…

The greatest love story in the Universe

Ana and Bruno are children of a catastrophe. They are born orphans. Nobody can see them. Nobody can hear them. They are ghosts in the vastness of space.

But the whims of the Universe lead Bruno to Ana, and Ana to Bruno. And so, over the years, an invisible force brings them closer to each other, until they end up dancing together. And not just any dance, but a celestial choreography that reveals the most intimate secrets of gravity.

This is how the story of Ana and Bruno begins. Does it seem too poetic to be true? Well, in the remainder of the episode, I’m going to show you that everything I just said is based on scientific facts. Actually, it’s one of the greatest discoveries in history, which led to the 2017 Nobel Prize in Physics.

But first, let’s put ourselves in context.

Stellar-mass black holes

Black holes are classified into three types, depending on their mass:

  • Those with masses similar to the Sun, or less than about 100 solar masses, are called “stellar-mass black holes”.
  • Those with masses in the millions or even tens of billions of solar masses are called “supermassive black holes”.
  • And those with an intermediate mass, between 100 and a million, are called… “intermediate-mass black holes”.

Ana and Bruno had masses 29 and 36 times the mass of the Sun respectively. So they fall into the category of stellar-mass black holes. So how are these monsters formed?

Black holes of this mass are usually the result of a huge supernova explosion, a truly catastrophic event. And that exploding star is called the “progenitor”, since it’s the mother of the future black hole. So, indeed, Ana and Bruno were sadly born orphans.

Right now I’m not going to explain what a supernova is or what a black hole is (subjects which, of course, deserve their own episode). But I will simply mention that black holes, when they’re alone, do not emit or reflect light or any other type of radiation. In other words, they are, indeed, black. The only thing a black hole can emit is the so-called Hawking radiation, which has a negligible effect on black holes of this mass.

Therefore no one could ever see Ana and Bruno. And, of course, in space no one can hear you scream. So, indeed, Ana and Bruno couldn’t be heard either.

However, when black holes are not alone, then a lot of interesting things can happen. For example, if an innocent star passes by close enough to a black hole, it can get sucked into it. And in doing so, the stellar material accelerates so much that it emits enormous amounts of radiation. So black holes can actually be very luminous if there is material around them.

But imagine that there is no star around. Imagine that the only thing near a black hole is… another black hole. Let me tell you something that will seem contradictory to you. And it’s that when black holes are alone, they’re invisible. But when they’re in a pair, they can be more luminous than all the stars in the Universe combined.

Now, what they emit is not normal light, that is, electromagnetic radiation, like that emitted by stars. No, what a pair of black holes emit are gravitational waves (which, again, deserves its own episode).

So how are these pairs formed?

Stellar-mass black hole binaries

A stellar-mass black hole binary is nothing but a system composed of two stellar-mass black holes, orbiting around one other. In other words, it’s like the Earth and the Moon… but with two black holes instead of the Earth and the Moon.

To this day, it’s not entirely clear how stellar-mass black hole binaries form. Broadly speaking, there are two possibilities:

1. Isolated binary

One option is that the two progenitor stars were already paired before giving rise to the two black holes. At first, this option seems reasonable, but it has some problems.

Earlier, I said that for a black hole to form, the progenitor star has to die, exploding in a supernova. These explosions are quite chaotic and are usually not quite symmetrical. So the explosion may be a little more powerful in one direction than another. And this causes the resulting black hole to feel “a kick” in a certain direction.

So, if the two progenitor stars were already together from the start, in order to become a black hole binary the orbit needs to survive not one, but two supernova explosions, with their respective kicks. This can be tricky.

2. Binary in a star cluster

Another option is that the two black holes form separately and meet later to form the binary.

But of course, as I already explained in another episode, the Universe is huge, so if two black holes were wandering around through space, it would be unlikely that they would cross paths by chance and form an orbit.

So this casual meeting could only happen in really packed places. For example, in star clusters. In such dense places, coincidental encounters are more likely, and a black hole can interact with other objects around until it eventually locks with another black hole, forming a close enough orbit.

Thus, when I said that “the whims of the Universe led Bruno to Ana and Ana to Bruno” I was actually referring to these complex mechanisms that make two holes form a binary.

Gravitational waves

In the story of Ana and Bruno, I mentioned an “invisible force that brings them closer to each other”. Here, of course, I mean gravity. Yes, saying that gravity is a force is a very Newtonian thing to say, but hey, for the sake of poetry let’s just let it slide.

In the end, indeed, when the two components of the binary are close enough, they start to dance. They draw an inward spiral until they end up becoming one.

And you may wonder: But what about the conservation of energy? Good question. If we go all Newtonian again, a binary should be circulating for an infinite time, to preserve the energy of the orbit. But Einstein predicted a little over a hundred years ago that the binary does, in fact, slowly lose energy, as it emits gravitational waves.

These waves are not actually emitted by black holes, but by the space-time around them. They are vibrations that move at the same speed as light, and have been one of the greatest mysteries in physics until my former colleagues from the LIGO collaboration observed them for the first time, on September 14, 2015. That’s why, when merging together, Ana and Bruno revealed one of the most intimate secrets of gravity.

Laser Interferometer Gravitational-Wave Observatory at Hanford
Aerial views of LIGO Hanford Observatory.

This discovery not only received a Nobel Prize, among many other scientific awards, but also opened a new window into the Universe, allowing us to see things that could never be seen before.

But if you want to know more about black holes, gravitational waves and their discovery, as well as the end of Ana and Bruno’s story, be sure to subscribe, since I’ll talk about all this and more in the next episodes of AltruPhysics.