Supermassive Black Holes on a Collision Course?

Yup, you heard that right! You may wonder what kind of impact this event would leave on the observable universe. If you did, then you are in the right place.

But first, What are Black Holes?

The answer is really simple. Black holes are simply put, celestial bodies with incredibly high mass density and high gravitational acceleration. This means that it influences so much gravitational force on bodies that even light, the fastest-known entity cannot escape its pull. But how this all came about? Now, that’s a question that needs some thinking to do.

The stellar black holes we observe today were formed due to a disturbance in their equilibrium between their internal pressure and their gravity when they were a Star. They are not called disturbance rather its just the end of their life cycle. There is a reason Black Holes are not that common in space...its because for a star to become a black hole, there is a really important condition that it has to meet. The condition is that the star has to be big and heavy and by heavy I mean heavy! Mathematically, a star with a mass of about 20 times our sun may produce a black at the end of its life. Just so you know, the mass of the sun is 1.989 × 10^30 kg. One can only imagine how big the value would be for a black hole ready star but these types of big values are not that big of a deal in Astronomy as space is vast and filled with mysterious objects. What I meant by a disturbance in the equilibrium between their internal pressure and their gravity was that stars are constantly under a tug of war between their internal pressure and their gravity/weight. Well they would be able to handle it if they are young and the nuclear reactions happening inside of them are enough to handle the gravitational inward force but this is not the case with really big and old stars. Stars have something called nuclear fuel. Throughout their lifetime, they constantly burn through these fuels to keep the nuclear reactions going and resist collapsing under their weight. Well, these nuclear fuels aren't unlimited and when a star runs out of this, the gravitational inward force immediately gets the upper hand and thus begins the journey of the end of that star. First, the star undergoes a phenomenon called a supernova in which it explodes and thrusts out all the outer parts of the star until just its core is left out. Furthermore, only if the core is about 2.5 times the mass of our sun can the process continue to form a black hole. The process may seem simple but the math involved in this is quite complex.

Now, coming back to the topic...

The first time we observed two black holes collide took place on 14th September 2015. Well, the collision took place around 1.3 billion years ago but we could only detect the gravitational waves produced by it 4 years ago. That's because the information contained to confirm the collision took 1.3 billion years to reach the Earth. The binary black hole consisted of black holes with estimated masses of around 36 and 29 times the mass of our sun. These two black holes spun around each other and merged to form a new more massive black hole of around 62 solar masses. Since then there have been 3 black hole merger observations with the most recent one observed on 14th August 2017. You may wonder why the resulting mass is only 62 and not 65 solar masses and what happened to the rest of the 3 solar masses? Well, it was emitted in the form of gravitational waves. LIGO(Laser Interferometer Gravitational-Wave Observatory) is a scientific observatory located at two places in the US. These are powerful Physics observatories established solely for detecting Gravitational Waves whose existence was predicted by Albert Einstein. LIGO is a very complex observatory which doesn't look like your typical observatory at all. It uses interferometers so powerful that they can measure a distance of 1/10,000th the width of a proton!

The basic phenomenon used here is Interferometry where when two similar oscillating waves in total sync are added up produce a new wave with amplitude twice as large as the original waves called Constructive Interference but when these two waves have a phase difference of 180 degrees, they end up producing null wave called Destructive Interference. This basic principle combined with the manipulation of gravitational wave's property to shrink and expands objects on the Earth is used to detect whenever a strong enough gravitational wave reaches us.

Even such powerful equipment can detect gravitational waves produced only by two kinds of events in the outer space. One being supernovae where a sudden pulse of gravitational waves are detected and the other one being moments before a black hole collision where the black holes are continuously spinning around each other and producing a continuous oscillating signal.

LIGO Livingston

Now that you know how gravitational waves are detected, let's get to the most recent black hole collision observed. To start, the two black holes set out on a collision course are super-massive black holes meaning, each of their masses reaches around 800 million times that of our sun. This event is over 2.5 billion old since the collision took place 2.5 billion light-years away. When they say, the black holes collide and the gravitational waves as a result of that collision is which we detect, it's not entirely correct. You see, before two black holes collide, they undergo a process where they gradually draw closer to each other in a death spiral and while all of this happens, they emit powerful gravitational waves and this spiral system is called Binary Black Hole(BBH). It is said that super-massive binary black holes produce the loudest gravitational waves in the universe. But, what happens at the end of the spiral in a BBH? Well, that's the problem astrophysicists are yet to solve. For now, it is speculated that black holes don't merge rather when they get very close to each other, they stall out for the rest of eternity. This close distance is so significant that it has it's own name, 1 Parsec and it is roughly around 3.2 light-years. Our particular pair of black holes are currently(i.e. 2.5 billion years ago) 430 parsecs apart. Currently, no one appears to know what happens after the pair cross the 1-parsec limit or if they do at all but if you imagine a universe without the final parsec problem, the universe would be filled with strong gravitational waves which would nullify the gravitational waves produced by other insignificant events.

After all these findings, there still exists a limitation on LIGO for detecting passing gravitational waves. You see, the previous findings of binary black holes were made up of a mere 20-30 solar masses but the current binary system under study is millions of solar masses so that means the residual energy released by the super-massive black holes would be millions of time stronger than the smaller ones. So, astrophysicists have used another technique which makes use of an array of pulsars close to the earth. A Pulsar is a fast-spinning neutron star. If you remember, I mentioned before that when a star runs out of nuclear fuel, they start collapsing inwards and if they are massive enough, they can even squeeze in all the atoms together with immense density creating a black hole but if the star is lucky enough, there is way for it to cheat death and live a little longer. This is made possible if the neutrons inside the atoms repel being squeezed in together. Astrophysicists have discovered over 2000 pulsars in the Milky Way galaxy till now and are expecting to find even more in the coming few years. A typical Pulsar has a rotation speed of 100 revolutions per second and shoots out radio waves as beams which can be detected through radio telescopes on Earth. Pulsars are very reliable sources of radio waves since their rhythm remains constant over a long period. Gravitational waves possess a unique property of shrinking and stretching space-time and this effect is proportional to the intensity of the gravitational waves. This property of gravitational waves combined with the property of pulsars to send out steady rhythms of radio waves are exploited to detect gravitational waves. When a gravitational wave passes through the space between a pulsar and Earth, it shrinks or stretches the space between them eventually increasing the distance between them which in turn disrupts the steady rhythm of the radio waves received. This disruption can be recorded through the radio telescopes placed on Earth. Although this procedure seems simple and straight forward compared to the working of LIGO, it takes massive patience to detect even the slightest disruption. A single pulsar's rhythm might be disrupted by only a few hundred nanoseconds over a decade.

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So what is the final conclusion after all these recordings?

A team of scientists have confirmed that currently there are over 112 super-massive black holes around us emitting strong gravitational waves but no one has conclusively proved that the black holes merge and this can be done only when the super-massive black holes overcome the final parsec problem. Once the black holes reduce the distance between them to less than 1 parsec( say 0.1-0.01 parsecs), they would emit immensely strong gravitational waves which would dwarf all the other gravitational waves from other sources. So, as long as the gravitational waves from the super-massive binary black holes are not discovered, the final parsec would be perceived as an unsolvable mystery in the Universe.