2. Galactic age

Socrates: Yesterday, Ravi gave us an overview of the Particle Age and mentioned that his explanation wasn’t nearly enough to truly “understand” it. To really grasp it, there’s no way around the math. Today, Shashi is supposed to start discussing the Galactic age, and this will continue as long as we’re on the banks of the Tsangpo river. So, how do you want to begin?

Shashi: Since it’s already late at night, we’re by the Tsangpo’s bank, and the sky is clear, we could start by taking a picture of a galaxy with the telescope.

Ravi: Good idea. Shashi, then, why don’t you handle Ashvin-1?

Socrates: Ashvin-1? What does that mean?

Ravi: We have two telescopes, both named after the twin stars, Ashvin 1 and 2, known as the twin brothers in the Gemini constellation.

Shashi: After mounting the telescope, I’ll connect it to the Unistellar app from my phone—see here. Now, I’ll go into the app’s catalog and select a galaxy; once I tap on “GoTo,” Ashvin will start moving. I’ve joined as the operator using my phone, and if you all connect to the same app as observers, you’ll be able to see on your phones what the telescope is viewing.

Juno: Yes, I can see it. I think we should target the Whirlpool Galaxy.

Shashi: Alright, tapping on it now. Everyone can see Ashvin-1 moving towards the Whirlpool Galaxy. It’s there now. The galaxy isn’t visible yet because we’re in live mode, not accumulating photons. Once I tap on “Enhanced Vision,” Ashvin will start collecting light. Here we go! You can see the exposure time ticking below; it’s already at 7 seconds. The Whirlpool Galaxy is already faintly visible. The more light we accumulate, the clearer the galaxy will become.

Socrates: I see—this is actually a merging of two galaxies.

Shashi: Up front is the Whirlpool, known as Messier 51, which spans about 75,000 light-years. Just behind it is a small dwarf galaxy, NGC 5195, also called M51b, about 15,000 light-years in size. Both are around 30 million light-years away. The bluish light comes from young stars, while the reddish glow comes from older stars. Our universe now contains roughly a trillion galaxies, all of which formed within the first four billion years of the universe’s 14-billion-year history.

Socrates: So, if the universe’s first 300,000 years were the Particle Age, then from then until around four billion years of age was the Galactic Age. But I don’t see any resemblance between this vast structure of gas, stars, and dust and the universe at 300,000 years old. Let me clarify. Yesterday, Ravi showed us an image of the universe at 300,000 years old. He demonstrated that the universe was then a single, boring cloud of gas with almost uniform temperature throughout. There were slight temperature variations, but they averaged only around 300 microkelvin. How did such enormous galaxies emerge from such a bland gas cloud in just four billion years? And not just a few galaxies—around a trillion, or perhaps even more.

Shashi: Scientists are still trying to understand this. On one hand, observational astronomers are making observations, and on the other, theoretical astrophysicists are doing calculations. Observations and calculations have not yet fully aligned because the massive cosmic “birth event” that began after the universe was 300,000 years old and continued until it was about a billion years old has not yet been directly observed through telescopes. The first 200 million years were the “Dark Ages,” followed by the “Cosmic Dawn,” a 200-million-year period during which the first stars and galaxies formed. The following 600 million years are known as the “Reionization Epoch,” because during this time, the ultraviolet light from the first stars reionized all the primordial neutral hydrogen in the intergalactic medium, stripping electrons from atoms.

Socrates: At first, you said scientists are still trying to understand, but then you told such a grand story that it sounds like they understand a lot. Which one is correct?

Shashi: Watch this video and decide for yourself. At the beginning of the 21st century, theoretical astrophysicists created a massive computer simulation called the “Millennium Run.” They instructed a supercomputer to use all the fundamental cosmological and physical theories to create a simulation showing the entire history of a small section of the universe from 400,000 years after the Big Bang onward. In other words, they made a time-lapse movie of the universe—not a real movie, but a simulated one.

Socrates: So, something like the time-lapse we see in Google Earth’s historical imagery?

Shashi: Good point. Let’s try to understand it using Google Earth. In this map, you can see the “face” of Dhaka in the 2022 image, as the Buriganga River creates a shape on Dhaka’s western boundary that people like to see as a human face. But in 1984, this face was not there because, without the gray buildings along the river’s edges, the river’s shape was not visible in satellite images. The closer we get to 2022, the more buildings grow, overtaking the greenery like with gray. You could say that as Dhaka becomes more “human” from 1984 to 2022, greenery is increasingly destroyed.

Socrates: Are you about to compare this to the entire universe?

Shashi: Why not? Just as you see buildings overtaking greenery in Dhaka’s time-lapse, if we could create a time-lapse of the universe, we’d see how the ultraviolet light from galaxy structures spread out, ionizing and “destroying” the innocent neutral hydrogen gas in the intergalactic medium by stripping electrons from their atoms. If buildings represent galaxies, then the hydrogen gas outside galaxies is like the greenery.

Socrates: But the Millennium Simulation doesn’t show this.

Shashi: Correct. We haven’t yet created such a time-lapse movie of the universe. Perhaps in the next few decades, with more advanced telescopes, it may become possible. For now, this simulation is our best resource. In the movie, you’ll see the last 13.5 billion years of history—not of the entire universe, but of a section 600 million light-years across both horizontally and vertically. If we assume an average distance of about 1 million light-years between galaxies, we can imagine around 600 galaxies lined up across the frame, which helps us understand the scale.

Socrates: And the “$z$” displayed above—what is that? I see it going from 20 to 0 as the movie plays, similar to how Google Earth moves from 1984 to 2022.

Shashi: It’s called redshift, but explaining it would take us off track. For now, it’s enough to remember that redshift here is a proxy for time. A redshift of $z=20$ means the universe was 400 million years old, and $z=0$ means the present. If the universe’s current age is 13.9 billion years, then this time-lapse movie actually shows us the entire history of the last 13.5 billion years.

Socrates: But what does it show? The first frame looks familiar, like the 300,000-year-old gas from which the CMB came. But after that, this vast web-like structure that forms from the gas, like a neural network in the human brain—what is that?

Shashi: Socrates, no one can come up with metaphors like you. The comparison to the brain is indeed interesting. But first, let’s go over the general process of galaxy formation. At the end of the Particle Age, the gas we saw at 300,000 years old had areas where the temperature was slightly lower than average, leading to regions with higher gas density. Even as the universe expanded over time, these dense regions didn’t expand. Instead, due to gravity, they grew denser and denser. When these clumps of gas became dense enough, they started forming stars, marking the beginning of galaxy formation. Initially, galaxies were irregular and unstructured but gradually became more organized. The Millennium Simulation shows us this process. In this movie, each bright dot represents the position of a galaxy. Just as neurons are the structural units of the human brain, galaxies are the structural units of the universe. The cosmic web, consisting of around a trillion galaxies, resembles a web. The part of the cosmic web you see in this simulation gives an idea of the whole. Regions with many galaxies and clusters are called nodes, while the regions connecting these nodes, with fewer galaxies, are called filaments.

Socrates: You said each bright dot represents a galaxy’s position. Why the position and not the galaxy itself?

Shashi: We can’t call these dots galaxies exactly, as the Millennium Run was created using dark matter.

Socrates: What’s that?

Shashi: Scientists believe that of the total energy-matter content in our universe, only about 5% is the ordinary energy-matter we’re familiar with (like what we discussed in the Particle Age), 25% is dark matter, and the remaining 70% is dark energy. Any matter or energy in the universe that we cannot directly detect has been given the prefix “dark.” Dark matter, therefore, is about five times more abundant than visible matter. Since gravity is proportional to mass and dark matter is more massive, galaxy structures should form based on dark matter. By simulating dark matter, we can infer the distribution of visible matter. Thus, the Millennium Simulation models 10 billion “particles” of dark matter, with each particle having a mass equivalent to a billion suns. This means that several dots in the time-lapse movie combine to create a galaxy’s scaffolding, where visible matter might form hundreds of billions of stars.

Socrates: It seems astronomers have fallen in love with the dark. Perhaps, in the future, theoretical physicists’ theories will all lean toward the dark side. People know far less than they think they do, yet claim to know far more.

Mars: Within the next 20 years, we might begin to see real evidence of the Millennium Run.

Socrates: Good. I have another question. You mentioned all galaxies formed within the first four billion years. Why can’t new galaxies form after that or now or in the future?

Shashi: Because, Socrates, dense gas gradually condenses due to gravity until it becomes dense enough to form a galaxy. But if there isn’t enough dense gas, galaxy formation can’t even begin. By four billion years after the Big Bang, the universe had expanded so much that there wasn’t enough dense gas left to form galaxies. Most of the gas never became galaxies and instead formed the intergalactic medium, the sparse space between galaxies with very low gas density. The cosmic web contains even emptier regions than the intergalactic medium, called cosmic voids, where gas density is even lower. Opposing these voids in the cosmic web, you’ll find clusters and superclusters of galaxies in the densest regions called nodes.

Hermes: Watching your Millennium Run makes me want to run too. How much longer within Earth’s gravity? We’re celestial beings after all. Just looking at the Whirlpool through a telescope doesn’t satisfy me. Let’s go straight to the Whirlpool Galaxy, everyone.

[In the Millennium Simulation’s stimulation, everyone takes off into the void. After passing the Milky Way, Shashi stops everyone.]

Shashi: Hold on, hold on. The Andromeda Galaxy is clearly visible, but the Whirlpool is still 30 million light-years away. Just like Voyager 1 once looked back at Earth on its way out of the solar system to capture the famous “Pale Blue Dot” image, let’s also take a moment to look back at the Milky Way.

[Everyone gazes back at the Milky Way in contemplation.]

Socrates: Seeing the face of the Milky Way from this face-on view is truly remarkable. It’s even more beautiful than what we could see in the NASA Eyes animation. Explain, Shashi.

Shashi: Our spiral galaxy is lovingly called Akashganga in Indian tradition, as if the river Ganges has descended from the sky. Of course, no human could ever see the Milky Way in this form. This galaxy spans 100,000 light-years, which would take 100,000 years to cross at the speed of light. And if we traveled at Voyager’s average speed (17 km/sec), it would take 2 billion years. We’re lucky we’re already dead—otherwise, we’d never experience this fortune.

Socrates: Why is the center of the galaxy so bright?

Shashi: In a spiral galaxy, if the galaxy were a country, the center would be its capital, where the stars are most densely packed. There are so many stars because right at the center, there’s a supermassive black hole, which might remind us of mortality—a giant mass grave sits at the heart of our galaxy.

Socrates: A supermassive black hole is a mass grave?

Shashi: A black hole is the corpse of a massive star, one that was many times larger than the Sun. When these stars die, they become so dense that gravity pulls in everything around them, not even allowing light to escape. Since light cannot escape, they are invisible, hence the name black hole. In the central region of the Milky Way, where stars are densely packed, the concentration of stellar corpses is also high, similar to how urban areas have more cemeteries than villages. These stellar corpses merge over time, growing in size, and eventually forming a supermassive black hole at the galaxy’s core.

Socrates: But why only at the center? Couldn’t a supermassive black hole exist elsewhere?

Shashi: Forget Einstein, if you even understood Newtonian gravity, you wouldn’t be asking this. In any gravitational system, the most massive object naturally resides closest to the center of mass. For instance, in a seesaw, the heavier child sits closer to the fulcrum. Similarly, our black hole, being the most massive object, lies near the galactic center. Extending from either side of it is a bar-shaped region with so many stars that it appears as a single structure from this distance. This bar extends from both ends into the two main spiral arms: from our viewpoint, the Perseus Arm above and the Scutum-Centaurus Arm below. You won’t see the Sun from here, but you can see its orbit in this image. In this orbit around the center, the Sun completes a revolution around our galaxy every 250 million years (1 galactic year).

Hermes: If we take off again, we could see the Milky Way’s edge-on view too, and understand how thin its disk is.

Socrates: Hermes is always eager to take off.

[In an instant, everyone moves 500,000 light-years away from the Milky Way, aligned with the disk.]

Shashi: The thin disk is clearly visible—it’s only about 1,000 light-years thick. One could think of flying out like a drone to photograph it, but that would still take millions of years. On either side of the thin disk lies the thick disk, about 10,000 light-years thick. Within the disk, Population I (younger) stars orbit nearly circularly around the galactic center. Where we previously saw the bar, we now see a bulge—a central, swollen region with a high density of stars. Surrounding this disk is a spherical stellar halo, where Population II (older) stars roam in elliptical orbits, occasionally passing above and below the disk. In the halo, dark matter exerts more influence than visible matter. But the most interesting feature of the halo is the scattered globular clusters, each containing 100,000 to 500,000 stars, all very ancient.

Socrates: How ancient?

Shashi: As old as our galaxy itself—around 13 billion years. Thirteen billion years ago, our galaxy was probably quite irregular, and among the first stars formed, only those smaller than the Sun have survived in the halo or globular clusters above and below the disk. The disk has no place for these older stars; it’s reserved for the young. Nowhere else can you see a better example of how a new generation displaces the old.

Socrates: But how did the galaxy go from an irregular shape to first forming a disk and then spiral arms within that disk?

Shashi: We still don’t fully understand the detailed process. However, when we explain how flat solar systems like ours formed from large, irregular gas clouds during the Stellar Age, this process will become clearer. It’s best to discuss it then, as spiral galaxies are flat like our solar system. For now, I’ll just mention that our galaxy’s thin disk formed 9 billion years ago, and spiral arms appeared only 5.5 billion years ago, meaning truly during the Stellar Age.

Hermes: Then let’s head toward the Whirlpool Galaxy. As we travel, let’s learn about different types of galaxies.

Shashi: Edwin Hubble was the first to conclusively prove that there are many galaxies beyond our own. In 1924, he identified that the fuzzy object known as the Andromeda “Nebula” was actually a separate galaxy outside of the Milky Way. After discovering many more galaxies, he organized them into a “tuning fork” classification, like this image.

Socrates: It looks like there are three panels here, each representing how different types of galaxies appeared across three eras. The panel on the left is the present. The other two show 4 and 11 billion years ago. It seems like galaxies primarily fall into three categories: elliptical, lenticular, and spiral, with barred spiral as a distinct type within spirals.

Shashi: There’s also an “irregular” category, which isn’t shown here, as it doesn’t conform to any specific shape. You can see elliptical galaxies classified from E0 to E7, with E0 being the most circular and E7 the most elongated. Lenticular galaxies are marked S0, as they’re in between ellipticals and spirals—they have a disk like spirals but lack spiral arms, and they have a large oval bulge like ellipticals around the disk. Spiral galaxies are classified from Sa to Sd based on their bulge and arm types: Sa galaxies have the largest bulge and smoothest arms, while Sd galaxies have the smallest bulge and the most spread-out arms. Barred spirals follow the same classification, but with a bar extending from the bulge.

Socrates: It seems to me that a galaxy begins as an elliptical, slowly becomes lenticular, and eventually turns into a spiral.

Shashi: Hubble himself thought the same. That’s why he called reddish elliptical galaxies “early-type” and bluish spirals “late-type.” But in reality, it seems that all these types of galaxies have existed over the past 11 billion years. Only, the further back we go, the more the structure of each type of galaxy appears less defined and less organized. An elliptical galaxy might evolve into a spiral over time, but the reverse can also happen.

Socrates: Why does this transformation happen?

Shashi: Mainly due to interactions with other galaxies, such as merging or environmental effects in galaxy clusters. Speaking of merging, we’re approaching the Whirlpool Galaxy. See it for yourself—this is what a merger looks like, though it can take hundreds of millions of years for two galaxies to completely merge. The Whirlpool appears 10 degrees across in our sky, about 20 times larger than the Sun’s apparent size in Earth’s sky. The smaller, background galaxy is actually irregular or a dwarf galaxy, but this interaction is distorting the shape of the larger galaxy too. A large spiral galaxy can become irregular due to interactions like this. The Antennae Galaxies are another good example. Let’s take a close look at both of these galaxies.

Socrates: Why are the two tails of the Antennae Galaxies so long? Are they made of gas?

Shashi: No, they’re made of stars. As the two galaxies spiral around each other and merge, their stars don’t actually collide due to the vast distances between them within each galaxy. Instead, the gravitational pull of one galaxy strips stars from the other, ejecting them into space to form the two long tails.

Socrates: The center of the merging galaxies seems brighter than a regular galaxy.

Shashi: That’s because of the collision of interstellar gas. In addition to stars, galaxies contain a lot of interstellar gas. Unlike stars, gas clouds are large, so they collide during a merger. This heats and compresses the gas, triggering intense star formation. This phenomenon is called a starburst, and galaxies where it occurs are known as starburst galaxies.

Socrates: Isn’t there a ring visible at the center of this galaxy?

Shashi: Yes, that’s also due to a merger. In the case of the Antennae pair, the galaxies didn’t collide head-on, but here one galaxy crashed straight into another. This head-on collision in Messier 95 generates a longitudinal wave that propagates outward from the center. The wave compresses and expands in cycles, creating new stars in the compressed regions. That’s the compression front you’re seeing, Socrates.

Socrates: What happens to the two supermassive black holes of each galaxy during such a merger?

Shashi: Of course, the two black holes eventually merge, but it takes a long time. By the time the two galaxies have fully combined into one, the two black holes may have also merged. However, black holes have a more interesting fate. Just as merging can trigger starbursts in galaxies, it can also activate them. When the supermassive black hole at the center of a galaxy actively consumes stars, gas, and other material from its accretion disk at the equator and emits jets from its poles, that galaxy is called an Active Galactic Nucleus (AGN) because only the nucleus is active.

Hermes: To see one firsthand, let’s go directly to the Centaurus A galaxy.

Socrates: All I see is a dusty, messy disk.

Shashi: That’s because, Socrates, the jets from the center are visible only in radio wavelengths, and the hot gas around the jets can only be seen in X-rays. Since we’re only seeing visible light, we’ll see nothing but the dust in the disk.

Ishtar: Then, here you go—I’m granting you the ability to see in all frequencies of light. Look now in three lights at once.

Socrates: I’ve never seen a more beautiful galaxy in my life. No telescope image could capture this beauty. But, Ishtar, for radio and X-ray, you didn’t create strange colors; you’re showing everything with familiar visible colors.

Ishtar: I don’t have the power to imagine new colors. Since it’s currently impossible to visualize beyond visible light, consider all other colors depicted here in terms of visible light. Even in false color images, you can enjoy them more than in true color images, you know.

Socrates: I didn’t know.

Shashi: Anyway, let me clarify AGNs a bit further with this model. All AGNs are essentially the same, but from Earth, people see them at different angles and call them by different names. When viewed from the pole, or jet-aligned, it’s called a blazar; from the equator or disk-aligned, it’s a Seyfert-2 or narrow-line galaxy; and at an intermediate angle, it’s called a quasar, Seyfert-1, or broad-line galaxy. The story is the same in each case: there’s a supermassive black hole at the center with a thin accretion disk around it, where gas and stars are being drawn in, all surrounded by a donut-shaped dusty torus. The gas closer to the accretion disk moves faster, and the gas farther out moves slower; I’ll explain later why that results in broad lines (and what is meant by ‘lines’) nearby and narrow lines from afar.

Socrates: It looks like the accretion disk is the black hole’s dinner table, and the poles are where the waste is expelled.

Shashi: You could say that. When the disk’s gas and stars are exhausted, the black hole becomes inactive, and the AGN ceases to exist. Our galaxy’s central black hole is currently inactive. However, due to a merger or interaction with surrounding things, a galaxy’s black hole can become active again. In many cases, black holes alternate between active and inactive states over millions of years.

Socrates: Then AGNs must be terrifying places to be. Let’s not get too close to Centaurus A.

Shashi: The danger isn’t limited to just active galaxies. Studies have shown that the central region of any galaxy, active or inactive, is perilous due to the high density of stars. Where there are more stars, there are more deaths, and thus more supernova explosions. Consequently, a solar system in a galaxy’s central region can’t remain safe for extended periods. The time required for intelligent life to evolve, like humans, may not be available there. Based on this concept, scientists have calculated a region in galaxies where life could develop, called the Galactic Habitable Zone. Humans live in the Milky Way’s safer rural area, so the risk is much lower.

Hermes: Let’s return to Earth now. This time, let’s take a boat and travel along the Tsangpo River.

Shashi: A boat is actually the perfect setting to explain what I want to talk about now. At the start of the 20th century, we didn’t even know there were galaxies outside the Milky Way. Everyone thought the Milky Way was the entire universe. In the 18th century, the French astronomer Charles Messier cataloged many nebulae, which were actually galaxies, but back then, they were thought to be gas clouds within our galaxy. However, not everyone dismissed the idea of other galaxies; for example, the German philosopher Immanuel Kant suggested that these known nebulae could be separate “island universes.” But without knowing the distances, it was impossible to prove.

Socrates: It’s not just distance; we’d also need to know the size of our galaxy.

Shashi: Right. If we know the size of our galaxy and the distance of a nebula, and we find that the nebula is much farther away than the size of our galaxy, then we’d have to assume that the nebula is actually an independent galaxy. We had a general idea of the Milky Way’s size by the 19th century. But the first solid method to measure distance was introduced by Henrietta Swan Leavitt in 1912. She discovered a relationship between the period of brightness variation in Cepheid variable stars and their actual brightness.

Socrates: What do you mean by “actual” brightness?

Shashi: We never really know the true brightness of a star. For instance, here on the riverbank at night, we see lights inside various houses. Every house might be using the same wattage bulb, but the farther a house is from us, the dimmer its light appears. What we see is the “apparent” brightness. If I somehow know the “true” brightness of one of these bulbs, I can calculate the house’s distance by comparing the apparent brightness with the actual one. For example, if a 20-watt bulb appears to be 10 watts, the house would be at a certain distance; if it appears to be only 5 watts, the house must be farther. But to know the true brightness, we’d need to visit the house and check. Leavitt found an alternative way to determine this—her “house” was the Cepheid variable stars. If we know the true brightness of a Cepheid in another galaxy, we can use it to measure that galaxy’s distance.

Socrates: So, did Leavitt calculate the distance to the first galaxy?

Shashi: Leavitt developed the method, but Edwin Hubble was the first to apply it successfully in the 1920s. While measuring the distances and speeds of about thirty galaxies, Hubble noticed a strange phenomenon: the farther a galaxy is from us, the faster it’s moving away. This relationship between distance and speed is shown in the inset of this diagram.

Socrates: How did he measure the speed?

Shashi: That part is simple—through the Doppler effect. Let me explain. Look over there, a boat is approaching us, and someone on it is playing a dungchen. Do you notice any change in the sound as the boat gets closer?

Socrates: Yes, the sound gets sharper as it approaches.

Shashi: So, the frequency of the sound increases, and the wavelength decreases. Now the boat is passing us and moving away. Listen—the sound becomes less sharp, the frequency decreases, and the wavelength increases. The diagram illustrates this phenomenon on a cosmic scale. Since the universe is expanding, all galaxies are moving away from each other, just like points drawn on a balloon would move apart as it’s inflated. The galaxies themselves aren’t moving; rather, the space (the balloon) is expanding, increasing the distances between them. If a galaxy is moving away from us, its light frequency will appear lower, similar to the boat’s sound as it moved away. Since red has a lower frequency than blue, this shift in frequency is called redshift. We observe that the light from all galaxies is redshifted, moving toward the red end of the spectrum. The degree of this shift indicates the galaxy’s speed of recession. Just as the dungchen on the boat would sound even lower if the boat were moving faster away, the light from a galaxy appears more redshifted the faster it’s moving away. Hubble used this relationship to create his famous diagram, with the speed of recession on the x-axis, distance on the y-axis, and a straight line indicating the relationship.

Socrates: But what does this mean?

Shashi: What do you think?

Socrates: It seems that all galaxies were once closer together, like the balloon was once smaller.

Shashi: Exactly. This idea was something Einstein couldn’t initially accept in the early 1920s. He believed the universe was static. But it turned out that all galaxies were moving away from each other, meaning everything was once in a smaller space. If we go far enough back in time, we reach a point when all the matter and energy in all galaxies were contained in a single point. That’s the Big Bang. So, through studying galaxies in the 20th century, we discovered our cosmic history.

Socrates: Can the speed of galaxies really tell us the age of the universe?

Shashi: Yes, we can estimate the universe’s age using the Hubble constant. The recession speed of galaxies located 1 million light-years (Mly) from us essentially defines the Hubble constant. Based on the best measurements from modern telescopes, its value is about 21 km/s/Mly. This means that for every 1 Mly of distance, galaxies appear to be receding 21 km/s faster. A galaxy 2 Mly away would thus recede at 42 km/s, a galaxy 3 Mly away at 63 km/s, and so on. The inverse of the Hubble constant provides an approximation of the universe’s age. By dividing 1 by 21 km/s/Mly (remembering that 1 Mly equals \(9.5 \times 10^{14}\) km), we obtain an estimated age of around 14 billion years, which is considered the approximate age of our universe.