INTO THE EVENT HORIZON: BLACK HOLES AND HOW THEY CAN END THE UNIVERSE

Introduction

Ultimate attraction, gravity at its peak, nothing and no one capable of escaping its inevitable pull; Nope, this is not an article about your crush but a deep dive into the mysterious gates into the unseen, the hunters of the universe: Black holes. While the idea of black holes brings up numerous paradoxes and unsolved problems that continually puzzle us, certain exciting observations and theories can simplify these extreme objects in the cosmos to determine their origin, behavior, and how they will possibly annihilate the universe, including themselves. Before being consumed by the notion of the end of the universe, let us try to focus on how these devices of "ultimate doom" actually come to exist.

How do black holes form?

First and foremost, it is essential to understand how black holes are born, which is when stars die! A star is a battlefront for the constant fight between two main forces: Gravity on the outer side and radiation resulting from the nuclear fusion reactions within the star from the inner layers. Gravity pulls the surface inwards while the heat, gas pressure, and radiation energy generated at the star's inner layers oppose gravity by pushing outwards against it, keeping the star in a stably sustained state called hydrostatic equilibrium.

Fig.1: hydrostatic equilibrium is what holds a star together.

The fusion inside the star occurs continuously, producing heavier elements and releasing vast amounts of energy after each consequent reaction. However, for stars much heavier than our sun, once the fusion of silicon into iron begins, the release of energy comes to a halt as this reaction consumes more energy than it gives away; at this point, the star cannot utilize iron to fuel further reactions and its build-up disrupts the hydrostatic equilibrium, bringing the star's life cycle closer and closer to the end.

Fig.2: inside a supergiant star; the iron core does not fuse into heavier elements.

Iron piles up inside the star and at some point the balance of hydrostatic equilibrium breaks. The gravitational force becomes dominant, causing the star to collapse into itself, imploding at nearly 25% the speed of light leading to an enormous explosion called a supernova. What is left behind is either a neutron star, unimaginably dense, spinning thousands of times per second, or if the original star was massive enough, a black hole.

Fig.3: A fireball in the supernova remnant W49B. with infrared data (red, green) and X-ray data (blue).

Lately, some astrophysicists have been suggesting that black holes of non-stellar origin may have formed by immense volumes of interstellar matter accumulating and collapsing in on themselves, creating supermassive black holes that are observed at the centers of large galaxies. Stephen Hawking has also suggested the existence of another type of black holes, known as primordial or mini black holes, which were formed during the big bang (Infoplease.com, n.d.). The idea is that in the early stages of the universe there existed sufficiently dense regions to form black holes; however, astronomers have not yet been able to gather enough evidence for their existence.

Structures of black holes

Black holes exhibit the most extreme embodiment of gravity; they bend light and deform spacetime. Send an astronaut towards it, and gravity would be millions of times more potent on a body part 1 centimeter closer to the black hole than another body part located 1 centimeter away from it. The astronaut would inevitably undergo “spaghettification” as they get pulled in; That is, the strong vertical stretching an object entering the powerful gravitational field of a black hole would experience. Such a strong gravitational tug causes the escape velocity to exceed the speed of light at a critical boundary known as the event horizon; in other words, not even light and other electromagnetic radiation can flee the black hole once they get too close. But how close is too close? In 1916, as he served the German army during the first world war, Karl Schwarzschild estimated the distance from the center of a black hole to the point of no escape; this is known as Schwarzschild radius, and it defines the very edges of the event horizon for us and is directly proportional to the mass of the black hole; for instance, the Schawzschild radius for the supermassive black hole at the center of the Milky Way galaxy is approximately 12 million kilometers! Past the event horizon, you are doomed to be swallowed into the depths of the black hole, the singularity. I am afraid it is impossible to say what lies down there with confidence. Perhaps a volumeless, infinitely dense point similar to the exact conditions in which the big bang occurred. There could lie a possible gateway into a parallel universe. The only thing we know for sure is that we do not know much about the center of a black hole. Gloomy, isn't it?

Fig.4: basic parts of a black hole

While the facts beyond the event horizon remain a mystery to us, we do know a number of things about black holes thanks to the objects in their proximity. Many black holes have matter orbiting them, the same way stars have planets circling them. In this disc of matter orbiting the black hole, the speed of particles can reach up to half the speed of light and the temperature up to a billion degrees.

Fig.5: distortion of light observable in this simulation of a complete revolution around a black hole and its accretion disk following a path perpendicular to the disk.

The black holes continually and violently influence these accretion disks and other sorts of matter (including stars) in their vicinity. These impacts on proximal matter and the way black holes affect the incoming rays of distant sources of light are the only evidence and tools that allow us to ascertain the existence of black holes and study them, since black holes do not reflect light after all.

Fig.6: The image of the galaxy in the background being distorted by the gravity of the black hole, a phenomenon called gravitational lensing.

Spinning supermassive black holes draw charged particles called plasma into their gravitational pull; these particles create a magnetic field that shoots away enormous amounts of energy at speeds close to that of light, travelling thousands of light years into space. This phenomenon creates a huge emission of white hot plasma into the cosmos known as the jet of the black hole.

Fig.7: simulation of the jet of a black hole, blasting away hot plasma into space

Black holes can rotate at a pace up to 90% the speed of light, leading to the development of a ring-shaped singularity. This powerful rotation of the ring singularity warps spacetime with itself, giving birth to a pumpkin-shaped region around the black hole known as the ergosphere. Motionlessness is impossible inside this pumpkin-shaped area surrounding the black hole, a cosmic whirlpool that entirely prevents immobility.

Fig.8: Ergosphere and ring singularity. It is impossible to stay still in the ergosphere!

Hawking radiation

In the 1970s, Stephen Hawking predicted that due to quantum fluctuations, there is a possibility that black holes emit radiation, meaning they slowly decay and lose their mass. Hawking (1975) estimated that this slow emission of radiation would ultimately cause the black hole to disappear by shrinking. These quantum fluctuations occur precisely at the event horizon. According to quantum mechanics, there exist particles in the universe that repeatedly pop into existence and are almost immediately annihilated by their counterparts, the antiparticles. At the event horizon, however, if one of these particles has already surpassed the point of no return, it would be sucked into the black hole, and its counterpart would remain in the universe, surviving the black hole’s pull. Therefore the black hole loses energy by emitting thermal radiation; thus slowly evaporating.

Fig.9: the member that escapes the black hole has no sister particle to be annihilated with; therefore it is emitted as thermal radiation by the black hole; the infalling particle essentially has negative energy, ultimately causing the black hole to lose mass.

According to one of the most fundamentally established laws of physics and science in general, the total amount of quantum information in the universe must be conserved; that is, quantum information, which can be thought of as a diary that contains the current state of a quantum particle (on the subatomic level) as well as the preceding states and their effects, cannot be destroyed. The apparent destructive behavior of black holes raises the following question: Are information and matter actually destroyed, leaving the universe in the process of being engulfed by the black hole? This question, better known as the Information paradox, is one of the many paradoxes that black holes have given rise to. Scientists have come up with several explanations which are outside the scope of this article; for a closer look into the information paradox, click here.

The closest black hole to Earth is 1500 light-years away from us with a mass three times that of the sun (Falk, 2021). This is considered a small black hole. The majority of black holes are heavier; there are supermassive black holes at the center of almost all large galaxies, each of them way more massive than our sun, taking the hawking radiation eons to bring them to their demise in a huge explosion that is equivalent to billions of nuclear bombs. This is a remarkably prolonged process; taking one googol year (that is 10 followed by 100 zeroes) or even more to entirely destroy the supermassive black hole.

A possible end to the universe

Einstein's theory of general relativity and our recent observations have revealed to us that the universe is not only expanding, but is doing so at a rate that is ever-increasing. As stated by prof. Robbert Dijkgraaf (2017) a theoretical physicist from the institute for advanced study in Princeton, it is anticipated that at some point in the future of the universe, this expansion rate will exceed the speed of light, meaning there will be specific celestial bodies we will never get to observe. This is the beginning of the end; neighboring galaxies are fading, and a lonely dark island-universe will be left for us (assuming we are still around). The stars in these isolated island-galaxies exhaust their fuel one by one, and the only remaining force in any galaxy will be gravity. Black holes consume every remaining bit of matter, obliterating the universe turning it into a kingdom of black holes! Hawking radiation will slowly demolish this kingdom, and absolute nullity, non-existence, ultimate darkness, abyss shall be the fate of the cosmos.

Conclusion

Black holes, the most extreme objects in the universe, may raise a myriad of paradoxes and scientifically hard-to-grasp phenomena, but they also bring about opportunities for new discoveries. Their birth is brought upon by the death of massive stars, the inescapable limit at their edge has been determined, and their eventual end will possibly be taking place after they kill the rest of the universe. These are a fraction of what we have discovered about these cosmic demons by exploring them through a scientific lens for decades. While a large number of mysteries lie unanswered inside black holes, the insatiable curiosity of humankind is more than sufficient to keep on the search to expand the boundaries of the acknowledged.