Strange celestial bodies are beginning to reveal themselves. We have not yet fully understood how they are formed. If we think more extremely, it is possible that even the narrow fabric of space-time could be torn apart. In that case, our galaxy would simply scatter in all directions. This would have a tremendously fatal impact on the fate of the entire galaxy. 

Hello, I am an astronomer who studies galaxies, and my name is Ji Woong-bae. Nice to meet you. I think it would be good to go back to the moment when quasars were first revealed to astronomers. Until then, astronomers had only observed the universe using visible light, which can be seen with the naked eye, or slightly longer infrared waves, or slightly shorter ultraviolet waves, staying largely within the range of visible light. However, as we moved into the early 1960s...

There are large dish-shaped antennas, and now we have started to observe various light coming from space using these radio antennas. This field is called radio astronomy, and with the start of radio observations, strange celestial bodies that were previously unknown began to reveal themselves. There are dark stars that do not appear very bright when viewed with the naked eye or in visible light, but when we observe them in the radio wavelength, they seem to emit a very bright radio signal. So, what is the reason for this strange difference? Why are these celestial bodies emitting an enormous amount of radio waves in such a narrow range? This led us to ponder these questions. Eventually, we began to observe the light spectrum of these celestial bodies, and the spectrum itself turned out to be significantly redshifted.

We measure the degree of change that we call redshift, and it turns out that the light is coming from a very distant source. When we calculate it, we find that it is about 4 to 5 billion light-years away, which is an incredibly long distance. But what is even more surprising is that, despite this immense distance, when we observed it through radio waves, it emitted an incredibly bright energy. Considering that distance, it indicates that this celestial body is actually emitting a tremendous amount of energy. Thus, the existence of this unknown celestial body, which is releasing an astonishing amount of energy, has been recognized. For a long time, astronomers referred to such celestial bodies simply as ordinary small objects when viewed through optical observation.

It appears as a point, so it looks like a star, but it is something that is not a star. This is why we refer to it as a quasar, or in English, a quasar stellar radio source or simply a celestial object. Because the name is a bit long, it is often shortened to the acronym QSO. The first type of object known as a quasar was discovered in 1962, and since then, as we have conducted radio observations throughout the universe, we have come to understand that there exists an unknown, extremely powerful source of radiation. 

Now, the important question is: what is this mysterious source that is emitting such immense energy, precisely directed at a small point from such vast distances? According to current estimates by astronomers, the mechanism that could allow for this is likely the activity of a very, very massive black hole located at the center of a galaxy.

Our galaxy has a very massive black hole. Similarly, there has probably been an extremely heavy supermassive black hole at the center of this galaxy for billions of years, dating back to a very distant past. This black hole, with its immense gravity, will swallow up various stars and gas materials around it. The materials that get pulled in will spiral around the black hole at a very high speed, causing them to heat up to extremely high temperatures. This high temperature corresponds to various forms of light, such as X-rays, gamma rays, and radio waves, which are then emitted. From a distance, we observe this and wonder, What is that? as it appears as a small dot. This is how the identity of quasars began to be revealed. In fact, quasars are...

It's a very difficult and good question, but we still don't have a precise understanding of what is being created. However, as I mentioned earlier, particularly in the centers of galaxies, quasars that existed in the early universe likely represent the remnants of black holes that were explosively consuming matter in the centers of young galaxies that had just formed. Today, we more commonly refer to cases where black holes are actively growing in the centers of galaxies as active galaxies. The central part of these galaxies that contains the black hole is called an active galactic nucleus, abbreviated as AGN. The activity of these objects has already been occurring for billions of years.

It has existed since the very distant universe, and it has continued to exist until now, which is why we are now able to observe it. There is a very unique characteristic when observing these quasars. Interestingly, quasars can only be observed in the distant universe. When we look at the relatively close universe, around ten million light-years from us, we find that these quasars are hardly visible. If we map out how frequently quasars are observed in the universe, we see that they were most commonly found around five to six billion years ago. They were discovered with the highest frequency during that time. However, as we move into modern times, their numbers have drastically decreased. Therefore, these quasars likely existed when the universe, which has a total history of about 13.8 billion years, was reaching its mid-life.

Around this time, as dawn was breaking and the morning was becoming clear, we can consider this period in the context of cosmic history. From this point on, the formation of quasars became significantly rare. We can propose a few scenarios regarding the formation process of quasars. First, black holes explosively consume a large amount of gas and matter, producing a spectacular display. At that time, they would also be expelling energy. The fact that there were many quasars in the early universe suggests that there was more food available for them to consume back then. This is quite a logical conclusion because, as time passes in the universe, galaxies begin to deplete the fresh gas and matter they initially possessed to form stars.

Eventually, after a long time, there will be almost no materials left to create new stars in the galaxy. Therefore, in the early universe, when young galaxies were formed, there was still plenty of food for black holes, which is why their activity was more frequent. Another important point is that the universe is expanding. As the scale of the universe continues to widen, the density of stars and galaxies in space is gradually decreasing. Conversely, if we go back in time, we can see that galaxies existed at a much higher density, more closely packed together. When the density between galaxies becomes very high, neighboring galaxies interact gravitationally, leading to frequent collisions. About 5 to 6 billion years ago, galaxies were colliding with each other quite often, and in that process, other...

The gas material that the galaxy held is now being drawn into the center of the galaxy in large amounts, allowing us to more easily see the traces of black holes that have just finished a grand meal and are now burping. Interestingly, quasars are extremely bright celestial bodies; if they existed in the universe, they would certainly be visible. However, quasars are only observed in the distant universe, and no matter how hard we look in the nearby universe, we cannot see them. This suggests that the bright quasars are likely becoming less frequent as we move further into the universe. This understanding is quite fascinating. It means that the universe is not only expanding but that this expansion is actually accelerating, which we currently understand. Therefore, the galaxies filling the universe and the interactions between them are also changing.

As the distance between galaxies continues to widen, the chances of collisions between them will gradually decrease. In the very distant future, considering what will happen to the universe, it is still true that the gravitational effects between neighboring galaxies are stronger than the expansion of the universe that tries to separate them. Therefore, our galaxy and the Andromeda Galaxy, which are adjacent to each other, are on a collision course. However, as time goes on, there will eventually be no neighboring galaxies left that could collide with each other. At that point, the galaxies will have already merged into a round shape, resembling elliptical galaxies, and will be spaced apart from each other. Even then, each of those galaxies will still have supermassive black holes that will intermittently consume matter.

Although one might be trying to flaunt their presence, by that time, for example, if several trillion years have passed, there will hardly be any real matter left in the galaxy. The replenishment of new gaseous substances will become extremely rare, and the opportunity to suddenly devour another small galaxy and steal the gaseous material it contains will also disappear. Ultimately, at the center of the galaxy, there will only be dead black holes that have ceased to be active. If we accept the current model of accelerated expansion, the universe will continue to expand excessively, causing neighboring stars and galaxies to drift apart so much that they won't be able to collide or form a single group due to gravity. Furthermore, if we think even more extremely, neighboring atoms and molecules may not be able to bond due to the vast distances between them.

It is possible that space-time could be torn apart. If that happens, we may find it difficult to discover any structures that can even be called galaxies, let alone black holes. For example, our galaxy currently revolves around the supermassive black hole at the center, Sagittarius A*, with stars captured by the gravity of the surrounding galaxies forming a cohesive group. However, if the expansion of the universe accelerates to the point where even the gravitational interactions between stars cannot withstand it, our galaxy would simply disperse in all directions. Consequently, stars would also separate from one another. Therefore, if the current trend continues, our universe will ultimately reach a state where no new stars are formed, no new galaxies exist, and it will remain in the peak of its beauty, like it is now.

I think the stars and black holes that were created will slowly cool down and die, turning the universe into a very gloomy space of death, far away from everything. It's a very sharp question, but if we consider the definition of entropy in physics, the reason we can perceive the passage of time is that entropy continues to flow, and little by little, changes are occurring in the universe. We can observe that something is changing, which allows us to recognize that time is flowing in one direction. However, as you mentioned, if the universe reaches maximum entropy and there is no further increase in entropy, everything in the universe will just be scattered.

If we were to become a being that is spread out in all directions, then literally, we would have no way to feel that time is passing in that universe. This is because all molecules and all atoms would be split into individual parts and frozen in place. For example, temperature is determined by the average kinetic energy of the molecules occupying a space, which reflects how quickly each molecule is moving. However, if the universe were to perfectly expand endlessly to the point where not a single particle in the cosmic space is moving, creating a state that is close to absolute zero, then, as I mentioned earlier, temperature would simply be referred to as absolute zero, where the entire universe has frozen solid. The accelerated expansion I mentioned earlier would continue indefinitely.

In astronomy, there is a possibility of the universe's end being referred to as heat death. This is a very interesting fact, but strictly speaking, even though the black hole at the center of a galaxy may be very large, it occupies a relatively small part of the entire galaxy. Typically, we describe the stars in the galactic center as forming a spherical structure, which is usually called the galactic bulge. However, the mass of the black hole in the tightly packed bulge at the center is only one-thousandth of the total mass of the galaxy's center. In percentage terms, that's only about 0.1%. Nevertheless, this small seed of a black hole has a profoundly critical impact on the fate of the entire galaxy. This is because, as mentioned earlier, black holes possess an enormous amount of energy.

As it swallows matter, it forms an accretion disk that becomes extremely hot. Then, that disk's temperature rises significantly, emitting high-energy particles and high-energy light in all directions. At the same time, since a black hole is created from something that collapses into a very small size, it spins at a very high speed. This rotation creates a powerful magnetic field along its axis. Consequently, charged particles, like electrons, that are drifting around the accretion disk are accelerated to high speeds, almost close to the speed of light, creating a powerful energy jet. This energy jet, expelled from the center of the black hole, will pull in fresh gas and materials from all around.

It's a feeling, but let's imagine that I pour a whole bag of flour onto a tray and I'm making bread and cookies. If there's a lot of flour piled up, I could keep making cookies without stopping for a long time. But let's say suddenly the flour causes my nose to itch, and I have to sneeze. In that moment, all the flour that's piled up would fly away. Then, since the ingredients left on the tray have drastically decreased, the amount of cookies I can make would also be significantly reduced. Similarly, as the galaxy's center continues to expel energy jets from black holes, it blows away all the gas and materials the galaxy contains, which means the galaxy's opportunity to create new stars quickly diminishes. So, sometimes when I talk about studying the evolution of galaxies, I find that quite a few people...

People sometimes ask questions like, How does a galaxy die? In the case of stars, we say that when their evolution is complete and their lifespan ends, they explode. So, if we think about galaxies evolving, it follows that galaxies would also have a lifespan. When a galaxy reaches the end of its lifespan, one might wonder if it would explode like a star. However, while individual stars do explode at the end of their lifespans, the entire galaxy does not explode. That said, if we consider the phenomenon of explosions from a different perspective, as I mentioned earlier, the supermassive black hole at the center of a galaxy, such as a quasar, is very active and emits energy in all directions. It radiates various wavelengths of light, which could be defined as an explosion. Therefore, we could say that galaxies do experience explosions in that sense.

In the field of galaxies, this black hole is considered very important because it prevents the galaxy from creating more and more stars, thereby diminishing its vitality. In astronomy, this is referred to as the quasar-induced change, which is known as quasar mode, or, as I mentioned earlier, feedback from active galactic nuclei (AGN) residing at the center of galaxies, commonly called AGN feedback. Nowadays, in addition to observations, we are also using supercomputers to conduct highly detailed, ultra-high-resolution simulations. These recent simulation data illustrate the entire process from the early stages of the Big Bang, where matter gathers, galaxies form, and stars are created within those galaxies. It is evident that the mechanisms of quasar activity are consistently depicted in these simulations. Therefore, when we examine the simulation data, we find that throughout the process, the vast structures of the universe are intricately involved.

At the point where the knots are connected, there is now a galaxy. From the position of that galaxy, we can discover traces of something spewing energy in a long, bifurcated manner all around. Recently, astronomers made a very interesting observation. Typically, no matter how active a quasar is, it is still thought to affect only a scale slightly beyond that of the galaxy. So, no matter how large it is, the energy jets from the black hole at the center of the quasar would influence only a few thousand to tens of thousands of light-years. This was the general understanding. However, surprisingly, a recent discovery revealed that there is a vast cosmic structure where galaxies are intertwined like a web. In this cosmic structure, galaxies are lined up in a single row, forming an overall filament scale. It’s roughly the size of about 120 ordinary galaxies, similar to our own, grouped together in a very long formation.

Recent discoveries have shown that the activity of supermassive black holes can have an impact on their surrounding environments. This means that the activity of these black holes does not only affect the local environment around the galaxy they inhabit but can occasionally influence large-scale structures that connect multiple galaxies. This leads us to consider the possibility that such activities could have effects on a very large scale. Quasars are indeed very dangerous entities. However, fortunately, we are quite far from them, so there is no need to worry about Earth facing a mass extinction due to their influence right now. Now, let's consider a hypothetical scenario where a quasar is located very close, astronomically speaking, just a few hundred to a few thousand light-years away.

Let's imagine that there is a quasar. In that case, various high-energy particles, which are accelerated to nearly the speed of light, will be flying towards Earth. These particles will then pour down onto our planet. Of course, Earth has its own magnetic field, which protects our terrestrial life from various cosmic radiation and solar wind particles emitted by the Sun. However, if we were to encounter particles coming from a black hole that is millions or even billions of times heavier than the Sun, traveling at the speed of light, it would likely be incredibly difficult to withstand. For instance, if the ozone layer in the stratosphere, located about 20 to 30 kilometers above the Earth's surface, were exposed to these high-energy particles, the ozone layer would almost instantly suffer catastrophic damage, with about 80 to 90% of it being destroyed. Then...

Of course, thanks to the ozone layer, sunlight that wasn't reaching Earth will seep in, and naturally, high-energy particles emitted by black holes will also come to Earth. Then, living organisms on the surface will be anesthetized, just like when we put dishes in a sterilizer, and all the germs on them are completely killed. Similarly, all life forms on Earth, including humans, will be sterilized. Consequently, our Earth will end up being a clean, barren planet with neither oceans nor water, just a rocky planet. 

That's a very good question, but it's still a somewhat controversial topic. This is because, as we observe the various structures surrounding our galaxy, including the dark matter and gas that form a halo around it, we are uncovering some intriguing structures.

발견하게 되었어요. 이제 세르미 감마선 우주 망원경이라고 해서 우리가 눈으로 보는 가시광선 말고 훨씬 더 에너지에 해당하는 감마선 빛을 관측하는 우주 망원경이 있습니다. 페르미로 우리 은하를 관측하다 보니까, 우리가 살고 있는 우리 은하 원반의 정 중앙으로부터 수직하게 위로 하나, 아래로 하나 이렇게 동그랗게 굉장히 뜨거운 고에너지 입자가 동그랗게 불려 나간 것처럼 보이는 전파 거품, 감마선 거품을 발견했습니다. 그래서 이거를 우리가 페르미가 발견한 거품이라고 해서 페르미 버블이라고 부르는데, 이 페르미 버블이 아마도 수억 년 전까지는 우리 은하에서 나름 활동적으로 활동하고 있던 우리 은하 중심 블랙홀의 어떤 에너지 제트의 흔적일 수도 있다, 이런 가설들이 나온 적이 있습니다. 재밌는 게 지금 우리...

The galaxy is not actively emitting energy jets. However, when we look at the Fermi bubbles, which are gas clouds extending above and below the disk, it appears that something in the center of the galaxy was once spewing energy up and down. But this doesn't necessarily mean that our galaxy was a quasar, and there are possible hypotheses regarding this. Our galaxy is about 10 billion years old, and when it was newly formed, it went through a phase where a lot of young stars were explosively being born, akin to a baby boom era of stars. During that time, an enormous number of stars were born simultaneously, and the stars that were born around the same time likely died around the same time as well. This means that all those similarly aged stars would have undergone a series of supernova explosions, and the remnants of those stellar explosions would have spread far above and below the galactic disk.

There may be an eruption. The structure of the Fermi bubbles, observed at the center of our galaxy, may tell the story of the active past of our galaxy's center, or it may simply show the traces of the baby boom era stars that were born densely at the galactic center without a black hole. The conclusion is still uncertain. It's a very good question. Currently, our galaxy and Andromeda are about 2.5 million light-years apart, and they are approaching each other at a speed of about 110 kilometers per second. So, if we calculate roughly, it is expected that the two galaxies will collide within about 6 billion years. Naturally, both our galaxy and Andromeda have similar sizes and are of very similar scale, so when the two galaxies collide, the black holes that each galaxy harbors will also...

We will take away all the matter. Naturally, both our galaxy and Andromeda currently harbor somewhat dormant black holes. However, both black holes will eventually wake up, and we can imagine a future where something like quasar mode is activated again. But I want to add an interesting point regarding the possibility of a collision between Andromeda and our galaxy. For a long time, I, too, have studied galactic collisions, and it has been widely thought that a collision between our galaxy and Andromeda is almost certain. However, there is a bit of a variable here. Whether the two galaxies will collide or pass by each other will depend on the masses of our galaxy and Andromeda. But, frustratingly,

The mass of our galaxy and the Andromeda galaxy still has quite a bit of uncertainty. Sometimes it appears that Andromeda is heavier than our galaxy, and other times it suggests that our galaxy is actually heavier. As of now, there is still ambiguity about which of the two galaxies is the dominant one. Because of this uncertainty, the mass assigned to each galaxy significantly affects whether they will collide or not. Recently, a very interesting paper was published that provides updated estimates of the masses of the two galaxies, as well as the many small galaxies that exist between them, often referred to as satellite galaxies. By applying a more refined approach to their interactions, it was initially believed that a collision would occur with 100% certainty.

I thought that according to this new result, there is about a 50% chance that a collision may or may not occur. Even if a collision does happen, it won't be within the 6 billion years I previously thought; instead, it suggests that we will have to wait at least 8 billion years for a collision to occur. Interestingly, the possibility of a collision between the Andromeda Galaxy and our galaxy is still somewhat open. I believe that galaxies are very important in two major aspects of galactic astronomy. First, as I briefly introduced earlier, quasars are crucial indicators of the aging of galaxies, as they rapidly deplete their gas and materials, losing their vitality. Furthermore, the activity of these quasars affects not only the galaxy that hosts them but also other nearby galaxies.

There is a statistic that is quite interesting. For example, around galaxies where quasars are active, there are also dwarf galaxies living nearby. If we compare the dwarf galaxies that are in the direction of the quasar's energy jets with those that are not, we find that the dwarf galaxies directly exposed to these energy jets are significantly less capable of forming stars. Therefore, the activity of quasars can be seen as a very important factor in determining how well the entire universe can create stars in the future. Additionally, as I mentioned earlier, quasars are relatively easy to observe only in the distant universe. In the nearby universe, they are not visible, so when we discover a quasar, we can infer with high probability that it is located in the distant universe.

There must be a distribution of quasars, and by mapping how and when they have existed, we can gain a more detailed understanding of the early history of the universe, particularly the first half, from shortly after the Big Bang to the present. Another advantage of quasars is that, although they are very distant, they are also very bright celestial objects, making them relatively easy to observe. We can monitor a significant period of the universe's early history, approximately from when it was about 5 to 6 billion years old, which corresponds to the middle age of the universe. 

When the Big Bang theory was first proposed, there were astronomers who accepted it, but initially, many astronomers were skeptical of the Big Bang theory. During that time, alternative theories about the universe emerged.

The expansion is an undeniable fact that is observed. The universe is expanding, and it is an undeniable truth that the surrounding galaxies are moving further away. However, the astronomers, including Lemaitre, who proposed the Big Bang theory, reversed this expanding universe. In the past, the universe must have been at a narrower scale, boiling at high density and high temperature. They suggested that this hot early universe expanded to become the universe we see today. On the other hand, some astronomers thought it was absurd to believe that the early universe boiled at an infinitely high temperature and density. The problem is that, even if it is hard to believe, the observational fact that the universe is expanding cannot be denied. So, what are the alternatives?

If we present a hypothesis, it's okay. The universe is expanding, but just because it is expanding, if the density of the universe is gradually decreasing as empty space is added, new matter is created in that empty space, resulting in the density of the universe remaining the same from the beginning until now. This means that the universe has never experienced extremely high temperatures or extremely high densities at any point in the past. From ancient times until now, the universe has simply been filled with matter at a consistently low density. The density has been maintained while the universe space has steadily grown. This is how the argument goes. To counter the Big Bang theory, a hypothesis was proposed that the density of the universe always remains constant, maintaining a steady state, which is referred to as the steady state cosmology. The term steady state does not mean that it is a fixed normal state, but rather that it has maintained a static condition.

This is also referred to as the steady state theory, but this is not a correct hypothesis. This is evident from the evidence presented in the meetings. If the steady state theory were correct, the universe would have had the same density from the past to the present, and the components filling the universe would also be similar. However, what about quasars? They are only observed in the distant past of the universe, and in the relatively recent past, quasars are hardly visible. This indicates that the appearance of the universe in the past and the present is very different. This is what we are discussing. Therefore, it has been proven that the universe has not maintained the same appearance from the past to the present as suggested by the steady state theory. This serves as very important evidence that the universe is continuously changing and that it was formed through the Big Bang theory.

우주 최강 천체 퀘이사가 우주 진화의 증거인 이유 (스치기만 해도 지구 멸망)

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