Black Holes

What is a Black Hole?

Black holes are among the most fascinating and mysterious objects in the universe. They are dark and impenetrable giants that conceal secrets that could revolutionise our view of the cosmos. Predicted by Einstein’s theory of general relativity in the early 20th century, their existence has long been debated, leaving us with many questions about their nature.

From how they form to what lies within their depths, what are black holes? Is it possible to escape their relentless grasp? What secrets do they hide, and, most importantly, what can they reveal about our origins and the fate of the universe?

The search for answers pushes us to explore the boundaries of our knowledge ever deeper into these mysterious cosmic monsters.

a dark gravitational monster?

In gravity’s fierce clutch, enigma forms,
Matter compressed to void, in darkness swarms,
Abyss devours light, and spacetime warps.

Imagine a region of space with the most intense gravity known, so fierce that nothing can escape it. Stephen Hawking defined black holes as a collection of events from which it is impossible to escape to a great distance.
They are characterised by two main components: the event horizon and the singularity.

The event horizon is the boundary separating the interior of the black hole from the rest of the universe. It is an imaginary surface that marks the ultimate limit not to be crossed. Beyond the event horizon, gravity is so intense that it becomes an eternal prison, a place from which even light can’t return.

As predicted by Einstein’s theory of general relativity, at the heart of a black hole lies its core: the gravitational singularity. It is a region where the curvature of spacetime reaches infinity. The singular region has zero volume but contains all the mass of the black hole. One can imagine this singularity as a place of infinite density, where gravitational force intensifies beyond any known limit. It would appear as a mathematical point, losing physical meaning. The boundaries of our understanding dissolve within the hearts of black holes, and their mystery deepens.

Although some physicists propose that the singularity may have physical dimensions and a length equal to the Planck length (1.6 \cdot 10^{-35} m), the most petite possible size, the truth is that nothing can be said about the singularity because we cannot observe it, as it lies within the area of the black hole beyond the horizon. It is as if the cosmos wants to censor its nakedness, preventing us from discovering its true nature.

An almost eternal prison?

Once the event horizon is crossed,
the fate will be to reach the singularity at the centreof the black hole.

Black holes can have different shapes and characteristics depending on the presence or absence of rotation, charge, and density. However, in discussions, the simplest and easiest to visualise case is preferred: the Schwarzschild black hole, which has a spherical event horizon, a point-like singularity, and no charge.

The event horizon is a distance from the centre of the black hole, called the “Schwarzschild radius.” This distance depends on the mass of the black hole itself, so the more significant the mass, the larger the spherical zone from which we can no longer return.
The equation that describes this important physical phenomenon is:

R_S = \frac{2GM}{c^2}

This radius, if crossed, determines irreversible imprisonment by the black hole: both matter and electromagnetic radiation will fall victim to it. Once the event horizon is crossed, the fate will be to reach the singularity at the centre of the black hole.

Like any other massive body, black holes also have an escape velocity. But this does not comfort us since this velocity is greater than the speed of light. A rate that, according to physics, is insurmountable. Therefore, nothing can escape its captivity.

Black holes and Gravity

Whether it’s electromagnetic radiation or matter,
for the black hole, it makes no difference.

Fortunately, a black hole does not indiscriminately eat everything nearby. Objects outside the event horizon and moving at a sufficient speed with an appropriate trajectory can avoid being sucked in.

If we were to replace a star or a planet with a black hole of the same mass, the gravitational force at a specific distance from the substitute object would remain unchanged. It means the objects orbiting the original star or planet would continue to circle the substitute black hole.
The reason is related to Newton’s law of universal gravitation, in which the gravitational force between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. Therefore, if a black hole has the same mass as a star or a planet, the gravitational attraction it exerts on objects nearby will be the same.
Objects orbiting around the black hole can continue to do so, provided they do not get too close to the black hole or cross the event horizon.

And what about light? If black holes gravitationally attract massive objects, why do they trap light with no mass?
In the General Theory of Relativity, gravity is not described as a force acting between objects with mass but rather as a curvature of space-time caused by the presence of mass. According to Einstein’s theory, a massive body “deforms” space-time around it, creating a curvature that drives the motion of nearby objects as if the body’s mass attracted them. It is why light travelling near massive objects undergoes deviation, while trapped light cannot escape. This phenomenon of light deviation is called Gravitational Lensing and is often used to detect black holes.
In addition, light contributes to “weighing down” the black hole as if it had mass.
Consider the most famous equation formulated by Einstein:
The simplified form of the complete relativistic energy equation, which also takes into account the motion of the object.

This equation describes the relationship between energy and mass, stating that mass converts into energy and vice versa. The equation tells us that a small amount of mass can transform into a large amount of energy, and this principle is the basis for the processes that release vast amounts of energy in nuclear reactions.
In the black hole, the energy of photons is converted into mass and contributes to feeding the core, just like a massive object would do.

Whether it’s electromagnetic radiation or matter, for the black hole, it makes no difference. It will voraciously and tirelessly devour any form of energy to convert it into its mass and compress it into the singularity. But it is precisely thanks to its incredible size and during its violent meals that it allows us to detect it through indirect effects.
Observing a black hole can be challenging, as they neither emit nor reflect light. However, scientists have revealed their presence by studying the surrounding matter and the reactions that occur when a black hole is active. These phenomena include extremely bright jets, streaming stars that shine intensely before being swallowed, and nearby stars’ orbits that reveal the black hole’s mass and position.

The photo of M87* by the Event Horizon Telescope Collaboration
is experimental confirmation of the existence of black holes.

Since the prediction of black holes, many have been sceptical about their existence, and scientists thought we could hardly observe one directly. So far, our progress in observing black holes and gravitational waves has led to extraordinary discoveries, such as the photograph of M87*, a black hole with a mass of 6.6 billion suns. However, the enigma of the singularity remains unresolved. To penetrate this mystery, we need a more comprehensive theory capable of explaining how gravity would behave in the singularity, that is, on microscopic scales: quantum scales.

Only then can we hope to unveil the secrets hidden in the hearts of these majestic objects?

Types of black holes

In an observable universe that is vast and majestic, with a diameter of about 90 billion light-years, there are approximately 40 billion billion black holes. These enigmatic and abyssal entities roam the cosmic depths, concealing secrets waiting to be unveiled.

These black holes differ from one another based on their intrinsic physical characteristics of electric charge, rotation, size, and activity. These peculiarities give rise to various black holes, each with its history and behaviour within the vast theatre of the universe.

Black holes categorised by rotation and charge

“Only a few know, how much one must know
how little one knows”

Black holes can be distinguished by their shape due to rotation or having a charge. Singularity and the event horizon change shape depending on the rotation characteristics of the black hole.

A non-rotating black hole, also called a Schwarzschild black hole, is characterised by a singularity that manifests as a unique and infinitesimal point at the centre and a spherical event horizon. In contrast, a rotating black hole has a singularity that extends to form a ring located in the plane of rotation, and the event horizon has an ellipsoidal shape.

Rotating black holes, called Kerr black holes, thus have a bulge in the equatorial region of the event horizon surface, similar to that observed in stars, planets, and other rotating celestial bodies. The ellipsoidal shape is due to the centrifugal force generated by their rotation.
When a non-rotating spherical black hole has a charge, it is called a Reissner-Nordström black hole. Similarly, rotating black holes with charge are called Kerr-Newman black holes.

The charge of a black hole is relevant because it affects the behaviour of the electromagnetic field around the black hole and, consequently, the way the black hole interacts with the charged matter and radiation in its vicinity. However, in astrophysical reality, the charge of black holes is generally considered negligible compared to other properties, such as mass and angular momentum.

Black Hole Type Description Characteristics Shape
Schwarzschild No Angular Momentum
No Charge
J = 0
Q = 0
Kerr Angular Momentum
No Charge
J ≠ 0
Q = 0
Reissner–Nordström No Angular Momentum
J = 0
Q ≠ 0
Kerr–Newman Angular Momentum

J ≠ 0
Q ≠ 0





Black holes categorised by Density

Stellar Black Holes, Supermassive Black Holes,
Mini-Black Holes and Intermediate Mass Black Holes.

In this vast and mysterious universe, black holes of unthinkable sizes and powers are measured in solar masses: from the smallest ones, with only two or three solar masses, to colossal ones like Abell 1201, with an astonishing 30 billion solar masses.

A celestial body’s gravitational force depends on its mass’s compression. To transform Earth into a black hole, we would have to compress its mass into a sphere the size of a coin. If, on the other hand, we wanted to turn the Sun into a black hole, its mass would have to be compressed into a sphere with a diameter of just 3 km before it collapsed into a singularity.

Considering these aspects, we can divide black holes into the following categories:

  • Stellar black holes
  • Supermassive black holes
  • Mini black holes (purely hypothetical)
  • Intermediate-mass black holes

We will only discuss stellar and supermassive black holes, as intermediate-mass ones can be considered stellar black holes. Mini-black holes require special treatment and are purely theoretical objects.

stellar black holes

Beyond a brilliant light, lies a vast shadow

Stellar black holes are born from the gravitational collapse of massive stars at the end of their lives, and their masses range from 3 to 20 times the mass of the Sun. Usually, stellar black holes do not increase in size, although this can happen if celestial bodies or nearby stars are captured by their gravitational field.
Most stellar black holes form when a star with an initial mass of about 20-30 times that of the Sun exhausts its nuclear fuel and collapses under its gravity, generating a supernova. At this point, if the remaining mass exceeds a specific mass limit, the object will implode on itself, creating a black hole. Once formed, the black hole can grow by incorporating mass from the surrounding environment.

In addition to this more common mechanism, there are alternative theories regarding the formation of stellar black holes, such as the direct collapse of a massive star directly into a black hole without going through the supernova phase: the merger of neutron stars when the combined mass exceeds a specific limit, or the union of two or more primordial black holes.

Despite studying these alternative theories, the collapse of massive stars remains the most accepted and understood process in the scientific community for forming stellar black holes.


supermassive black holes

Forged through the sacrifices of myriad stars, it reigns from the center of galaxies

Lost Voices Soundscape

by AllenGrey | (Hiroshi Meshup)

In addition to stellar black holes, there are many more massive and frightening ones. Supermassive black holes are the most enormous among black holes, located at the centres of galaxies. Their mass ranges from a million to several billion solar masses, compressed in sizes between 15 times that of the Sun and the entire Solar System.

Sagittarius A*, the supermassive black hole at the heart of the Milky Way, has a mass equivalent to 4 million suns, concentrated in a radius of just 17 Suns. In the centres of galaxies, nearby stars and vast gaseous zones allow supermassive black holes to overgrow.

Despite their age, the formation of supermassive black holes is not yet entirely clear. Two of the most significant hypotheses are direct chain collapse and generation from dark matter.

Direct chain collapse suggests the supermassive bodies originated from a stellar one. Such stellar black holes, located in galactic centres, would have begun to feed on nearby stars, involving other black holes and giving rise to the monsters observed in active galactic nuclei.

A subsequent hypothesis suggests a supermassive black hole generated from dark matter or the quasi-star Hypothesis. According to this theory, in a non-homogeneous universe, halos of dark matter would have formed soon after the Big Bang. In these zones, concentrated, very dense areas would have collapsed into quasi-stellar objects, giving rise to black holes without going through the massive star phase, with tens of thousands of solar masses from the beginning.

Regardless of their origin, once at the centre of a galaxy, a black hole can grow by accretion, devouring matter and merging with other black holes, influencing the galaxy’s shape and finding around it the material needed to reach the sizes observed today. This theory would explain the existence of intermediate-mass black holes, which would be a phase from normal-sized black holes to supermassive ones.

Black holes categorised by activity

Quiet or lively black holes: swirling matter, dazzling quasars, and the dynamic cores of galaxies.

An additional classification is related to the activity state of black holes: active and inactive black holes.

Active black holes

Active black holes are far from black; in some cases, they are even very bright, the most bright thing in the universe. These black holes interact with the surrounding matter. Therefore, they are surrounded by an accretion disk consisting of matter (gas, dust, or stars) drawn toward the black hole by its intense gravitational force at very high speeds. As the matter in the accretion disk approaches the black hole, it heats up due to friction forces and gravitational compression, emitting considerable amounts of electromagnetic radiation across the visible spectrum.

Inactive black holes

On the other hand, inactive black holes, like enigmatic and silent creatures of the universe, do not attract matter toward them and therefore do not have accretion disks. Consequently, they do not emit any radiation. They represent the essence of black holes, hidden in darkness and mystery. These are the most challenging black holes to detect, and the only way we have to sense their presence is by observing the gravitational effects on nearby stars.

Quasar and Galactic Nuclei

It is precisely the activity state of a black hole related to quasars.
There was a time when astronomers believed that quasars, objects with extreme luminosity, were the most distant and mysterious objects in the universe, located at the farthest reaches of the cosmos. These mysterious lights seemed elusive, and their secrets were hidden in time and space.

After several studies, their true nature emerged. Quasars are supermassive black holes at the centres of some galaxies, acting as active galactic nuclei. Their brightness is so extraordinary that it completely overshadows the light of the billions of stars contained in the galaxies in which they reside. These cosmic giants voraciously swallow the surrounding matter, generating incredible luminosity and energy jets that extend for millions of light-years.

We can therefore say that the quasar is a temporary state of the black hole and the active galactic nucleus. A supermassive black hole at the heart of a galaxy remains invisible until it begins to consume matter. At that point, the accretion disk lights up and produces energy jets from the magnetic poles, transforming the black hole into a quasar and the nucleus into an active galactic nucleus. When the black hole’s “meal” ends, the quasar becomes a dark and inactive monster again .

Approaching a black hole

What lies inside a black hole? What would happen if we could approach one of them?
To fully understand this, we need to perform a mental experiment. Let’s imagine we are on a spacecraft, ready to explore the unknown beyond the event horizon of the supermassive black hole M87*, with a radius of 19 billion kilometres.


As we near the event horizon, the river of time begins to meander, ever more slowly.

Equipped with a hypothetical suit that completely protects us from radiation, we would cautiously approach the horizon while sending a visible light signal, such as a green light, to our spaceship. Someone on the ship will send us alerts of the same type. The first symptom of the solid gravitational intensity is that time begins to dilate. Light signals start to reach the ship more slowly because time would not flow the same way in the spaceship area and near the event horizon.
Our light signals would arrive at the ship with a colour increasingly closer to red as the waves lose energy during their journey to escape the gravitational attraction: this phenomenon is called “gravitational redshift”. On the other hand, the signals sent by the ship would become increasingly closer to blue, as would the light coming from all the stars around, because the gravitational attraction gives these rays more energy. These energy changes cause the wavelength of the light waves to shorten and become blue or lengthen and become red.
As we gradually approach the event horizon, the signals we send would arrive slower and less energetic until they are no longer visible light but infrared. An instant before entering the event horizon, the last light signal sent will be the last one the ship can receive. The one sent when we are precisely at the event horizon would take infinite time to be sent.


At the event horizon, time stands still;
and as we move and gaze behind,
the universe’s future unfolds before our eyes.

We would not notice a significant increase in time dilation because all our senses and biology would slow down the same way, while on the spacecraft and Earth, time would flow much faster.

At 20 billion kilometres from the event horizon of M27*, time flows 30% slower than Earth. At one billion kilometres, it flows 88% slower. One year here is eight years on Earth. Let’s get closer: time is 99% slower at one million kilometres, and one year is 100 years on Earth. At 1000 kilometres from the horizon, one year is 5000 years on Earth, while at the event horizon, the time finally stops.
Moreover, crossing the horizon would have no perceptible effect because the horizon is an imaginary surface, not a physical one, and it is just a boundary area.

If we turned around, we could see the entire future of the universe in an instant, although it would present itself as a vast and blinding blue wall. Furthermore, inside the event horizon, everything would become particularly bizarre. Time would become a spatial dimension, and space a temporal dimension. It means that advancing in time means moving spatially and radially towards the singularity. Here things get dangerous.

At this point, gravity forces us to fall into the singularity. We would fall into the centre quickly and only a few seconds in the case of M87*.
According to what we know from general relativity, during this short journey toward singularity, we could experience spaghettification and tidal forces.

The gravitational field would stretch our body in the direction of the fall, i.e., the temporal direction, and flatten it in the other directions, spaghettifying us into very thin filaments. The gravitational forces would exert tidal forces as if that were not enough. When the gravitational force of another influences a very large object, the gravity force can vary considerably from one side to the other of the object. Gravity tends to distort its shape without changing the volume. In our case, the effects would be devastating when we are close to the singularity.

Assuming our heads face the singularity and our legs point towards the horizon, the tidal forces would differ at the two ends. And would break us in two parts. As with the two broken parts, we would break them in half until we reached a subatomic decomposition.
We are still studying what might happen once we reach the singularity. At this point, the laws of classical physics and general relativity cease to make sense, and our body, whose matter has been spaghettified and pulverised at the subatomic level, finds itself in a completely unknown and incomprehensible environment.

the defeat of general relativity

Should general relativity and quantum mechanics entwine in a symphony,
the universe’s secrets would unfold in our hands

This is where General Relativity breaks down and can no longer provide answers or mathematically describe the phenomenon. Here, at the singularity, the scales are microscopic, and we would need a theory capable of describing quantum phenomena but in the context of gravity. General relativity cannot describe quantum phenomena, and quantum mechanics cannot describe gravity. Since the two theories do not communicate, we need to find a solution: a theory of everything capable of solving this great mystery that could open the door to new technologies and a deeper understanding of the very nature of reality.

While we are waiting to solve the problem of the theory of everything, some physicists have hypothesised that black holes could function as portals to other universes or dimensions. These hypotheses are based on young and unverified theories that seek to unite Einstein’s general relativity with quantum mechanics, such as string theory and loop quantum gravity.

A popular hypothesis is that of wormholes, which are theoretical solutions to the equations of general relativity. Space-time tunnels connect two distant points in space-time, allowing instant passage between them, leading to remote regions of the universe or even parallel universes.
Another hypothesis is related to the theory of eternal inflation, which suggests that our universe may be just one of an infinite number of universes that make up the multiverse. According to this theory, black holes could give rise to new universes within the multiverse through the “nucleation of baby universes” process. In this scenario, the singularity at the centre of a black hole could be the starting point for creating a new universe with independent physical laws and constants from our own.

Finally, a recent hypothesis suggests that black holes could be two-dimensional holograms projected into a three-dimensional space. According to the black hole holography theory, information about the matter and energy entering a black hole is preserved on its two-dimensional surface, called the event horizon. In this scenario, the interior of the black hole could be just an illusion, and passing through a black hole could reveal a new universe or hidden dimension behind the event horizon.

Death of a black hole

Are black holes eternal, or can they die? Will they eventually release the matter trapped within?
These are some of the most fascinating questions that science seeks to answer.

Hawking Radiation

Should general relativity and quantum mechanics entwine in a symphony,
the universe’s secrets would unfold in our hands

In 1974, legendary physicist Stephen Hawking proposed a revolutionary idea: black holes are not entirely black but emit faint radiation, now known as Hawking radiation.

Hawking radiation can be considered a phenomenon involving virtual particles just outside the event horizon of the black hole. These particles do not come directly from the black hole but materialise due to quantum fluctuations of the cosmic vacuum in particle-antiparticle pairs.

Near the event horizon, the black hole captures one particle of the pair while the other escapes and propagates into outer space. Because of the Law of Energy Conservation, the trapped particle must have negative energy, while the escaping particle has positive energy. In this way, the black hole loses mass and appears to emit particles to an outside observer – the Hawking radiation.

However, this description, while intuitive, is greatly simplified. In the theory of fields in curved spacetime, where gravity is involved, the definition of a particle becomes less clear, and other elements need to be considered.

The search for Hawking radiation challenges the human mind and technology. In laboratories worldwide, scientists are trying to recreate conditions similar to those of black holes, hoping to capture traces of phenomena analogous to Hawking radiation.


Someday, we may unveil black holes’ secrets,
reaching their heart to uncover untold realms.

And so, black holes continue to capture our imagination and challenge the frontiers of science. From the depths of stellar black holes to the supermassive colossi that rule the hearts of galaxies, these spacetime abysses fascinate and frighten us.

With each discovery, scientists shed new light on these wonders of nature, studying the extraordinary forces that create them and the laws that govern their behaviour.

Perhaps one day, we will unravel all the mysteries of black holes, reaching their very core, where we might discover anything, even new universes.

FAQ About Study motivation

All the frequent questions students are aking online

What are black holes and how do they form?

Black holes are incredibly dense and compact celestial objects that typically form when a massive star exhausts its fuel and collapses. During the collapse, gravity becomes so strong that it compresses matter into an infinitely small space, creating a singularity where the laws of physics lose meaning. The area around the singularity, where gravity is so strong that even light cannot escape, extends to the event horizon boundary. Black holes can also form from the collision of two stars or the merger of galaxies.

What happens when something falls into the Black Hole?

When matter, such as a star or a cloud of gas, falls into a black hole, it is crushed and compressed into an infinitely small point called the “singularity.” Once past the event horizon, time and space switch places due to gravity, and the matter is forced to go in only one direction – toward the center of the black hole. The falling object is stretched and broken into half as it approaches the singularity until it disappears. Even light cannot escape the gravity of a black hole, so once something enters the event horizon, it is inevitably drawn toward the singularity at the center of the black hole.

What is the event horizon of a black hole?

The event horizon of a black hole is the boundary surrounding the black hole beyond which the gravitational attraction is so strong that nothing can escape, not even light. Once an object crosses the event horizon, it is inevitably drawn toward the singularity at the center of the black hole. The event horizon is a critical feature of black holes because it marks the point of no return, beyond which the laws of physics as we know them break down. The size of the event horizon depends on the mass of the black hole.

What is the singularity and why is it so important to understand Black Holes?

The singularity is a mathematical point of infinite density and zero volume, where the laws of physics become uncertain and lose meaning. Inside a black hole, the singularity represents where all the matter that falls into the black hole is compressed into an infinitely small space and where gravity becomes so strong that escape is impossible.

Black holes can be destroyed or dissipate?

According to Einstein’s theory of general relativity, black holes are permanent objects and cannot be destroyed. However, the theory of quantum mechanics suggests that black holes emit a radiation called “Hawking radiation,” which takes away energy from the black hole and causes it to evaporate over an extremely long time slowly. This process is prolonged and cannot be directly observed. Still, it is considered one of the possible solutions to the “black hole information paradox,” which concerns the conservation of information inside the black hole.

Can we use Black Holes as a source of Energy to use on Earth?

There is no known method for directly utilizing the energy produced by a black hole. However, some scientists have theorized the possibility of using the rotation of a black hole to generate energy. It is supposed that an accretion disk around the black hole could produce a large amount of energy, although we are not sure about its efficiency. However, currently, this theory remains only an idea, and there are no known practical methods for harnessing the energy produced by a black hole.

What is the information paradox?

The black hole information paradox concerns the conservation of information that is swallowed by a black hole. According to general relativity, all information about the matter that falls into a black hole should be destroyed and lost due to the singularity inside. However, quantum mechanics suggests that information cannot be destroyed, contradicting the two theories. This paradox has been the subject of much discussion and research by scientists, and various solutions have been proposed, including the theory of Hawking radiation and quantum information theory. According to some of these ideas, information could be emitted as radiation, sent to other universes through wormholes, or arranged holographically on a two-dimensional surface.

Can black holes be used for time travel or to travel to other dimensions?

Currently, no experimental or theoretical evidence suggests that black holes can be used to travel to the past or other dimensions. However, it is possible to travel to the future, even using gravity near a black hole. Additionally, some physical theories suggest the existence of space-time bridges or “wormholes” that could theoretically connect two points in the universe or two different universes. I have written an article and produced a video that explains in detail how the physics of time travel works. You can find both by clicking here.

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Bibliographic sources and publications

Black Holes
Thorne, K. S. (1994). Black Holes and Time Warps: Einstein’s Outrageous Legacy. W. W. Norton & Company.
Shapiro, S. L., & Teukolsky, S. A. (1983). Black Holes, White Dwarfs, and Neutron Stars: The Physics of Compact Objects. Wiley.

Stephen Hawking and Black Holes
Hawking, S. W. (1974). Black hole explosions? Nature, 248(5443), 30-31.
Hawking, S. (1988). A Brief History of Time. Bantam Books.

Einstein and Black Holes
Einstein, A., & Rosen, N. (1935). The Particle Problem in the General Theory of Relativity. Physical Review, 48(1), 73-77.
Einstein, A. (1916). Die Grundlage der allgemeinen Relativitätstheorie. Annalen der Physik, 354(7), 769-822.

Solutions of Einstein’s Equations & Black Holes
Kerr, R. P. (1963). Gravitational field of a spinning mass as an example of algebraically special metrics. Physical Review Letters, 11(5), 237-238.
Newman, E. T., Couch, E., Chinnapared, K., Exton, A., Prakash, A., & Torrence, R. (1965). Metric of a Rotating, Charged Mass. Journal of Mathematical Physics, 6(6), 918-919.

Quasars and Galactic Nuclei
Schmidt, M. (1963). 3C 273: A star-like object with large red-shift. Nature, 197, 1040-1040.
Lynden-Bell, D. (1969). Galactic nuclei as collapsed old quasars. Nature, 223(5207), 690-694.

Black Hole activity
Blandford, R. D., & Znajek, R. L. (1977). Electromagnetic extraction of energy from Kerr black holes. Monthly Notices of the Royal Astronomical Society, 179(3), 433-456.
Fabian, A. C. (2012). Observational Evidence of Active Galactic Nuclei Feedback. Annual Review of Astronomy and Astrophysics, 50, 455-489.

Sagittarius A*
Genzel, R., Eckart, A., Ott, T., & Eisenhauer, F. (1997). On the nature of the dark mass in the centre of the Milky Way. Monthly Notices of the Royal Astronomical Society, 291(1), 219-234.
Ghez, A. M., Klein, B. L., Morris, M., & Becklin, E. E. (1998). High Proper‐Motion Stars in the Vicinity of Sagittarius A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy. The Astrophysical Journal, 509(2), 678-686.

Ford, H. C., Harms, R. J., Tsvetanov, Z. I., Hartig, G. F., Dressel, L. L., Kriss, G. A., … & Golombek, D. (1994). Detection of Nuclear Spiral Arms in M87 and Evidence for the Anisotropic Distribution of Ionized Gas around a Massive Black Hole. The Astrophysical Journal Letters, 435(1), L27-L30.
Event Horizon Telescope Collaboration. (2019). First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole. The Astrophysical Journal Letters, 875(1), L1.

What happens inside a Black Hole
Penrose, R. (1965). Gravitational collapse and space-time singularities. Physical Review Letters, 14(3), 57-59.
Wald, R. M. (1984). General Relativity. University of Chicago Press.

Morris, M. S., & Thorne, K. S. (1988). Wormholes in spacetime and their use for interstellar travel: A tool for teaching general relativity. American Journal of Physics, 56(5), 395-412.
Visser, M. (1995). Lorentzian Wormholes: From Einstein to Hawking. Springer Science & Business Media.

Black Holes formed by Dark Energy
Chapline, G. (2005). Dark energy stars. Proceedings of the Texas Symposium on Relativistic Astrophysics.
Frampton, P. H., Ludwick, K. J., & Scherrer, R. J. (2009). Pseudo-black-hole model as applied to the galactic center black hole candidates and quasars. Physical Review D, 80(6), 063003.

Eternal Inflation and Black Holes
Guth, A. H. (1981). Inflationary universe: A possible solution to the horizon and flatness problems. Physical Review D, 23(2), 347-356.
Vilenkin, A. (1983). Birth of Inflationary Universes. Physical Review D, 27(12), 2848-2855.

Hawking radiation
Hawking, S. W. (1974). Black hole explosions? Nature, 248(5443), 30-31.
Hawking, S. W. (1975). Particle creation by black holes. Communications in Mathematical Physics, 43(3), 199-220.

Quantum Mechanics and Black Holes
Bekenstein, J. D. (1973). Black holes and entropy. Physical Review D, 7(8), 2333-2346.
Page, D. N. (1993). Information in black hole radiation. Physical Review Letters, 71(23), 3743-3746.