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.