[On a typical sidewalk in Barzakh]

Socrates: One, two, three, four, five, six, but where is the seventh, Ishtar? Yesterday I was your guest among the seven of you, and in return, today you were supposed to be my guests.

Ishtar: Juno is late as usual.

Socrates: Without all seven, our conversation won’t be as lively.

Rabi: I heard Juno went to a Buddhist monastery between Manasarovar and Rakshastal this morning and hasn’t returned yet. How about we leave Barzakh and descend to Earth now? You could host us there.

Socrates: That’s not a bad idea. And the discussion I want to have with you would be most fitting on the shores of Manasarovar. Let’s go then.

[Between Manasarovar and Rakshastal]

Socrates: What’s the news, Juno? We missed you so much that we came all the way from the sky to Earth.

Juno: I was on my way to Barzakh, but I got distracted by the reflection of clouds on the lake and forgot about you all. You know how rare clouds are here.

Socrates: It’s not good for me to stare at clouds too much. I’ve already heard enough of Aristophanes’ insults without even looking at them.

Juno: Alright, no need to look at the clouds. Let’s go to the roof of the monastery, all of you come. From the roof, you can see Rakshastal, Manasarovar, and Mount Kailash all at once.

Socrates: With so much to see, will you still need the gift of my words?

Juno: That’s the test, Socrates. Yesterday we gave you the gift of a grand discussion on the constitution of an ideal state. In return, today you must offer us something so compelling that our eyes won’t wander elsewhere.

[On the roof of a Buddhist monastery by the shores of Lake Manasarovar.]

Socrates: I will talk about the fourteen billion years of history of the universe. Our European successors coined the word ‘universe’ by combining ‘uni’ and ‘verse’. ‘Uni’ means one, ‘verse’ means to transform, ‘universe’ means ‘transformed into one thing’. Yesterday you were talking about the nation, the institution created to form that nation is named ‘university’ because the meaning of ‘universe’ is found within it. Just as a university unites all students and teachers to create a national identity, the word universe brings everything in the cosmos into one single entity. I have many parts like hands, feet, nose, mouth, ears, heart, but all together I am one single person. Similarly, within the cosmic web, there are many galaxies, gases, stars, planets, satellites, asteroids, but all together the universe is one single entity.

The fourteen billion years of history of this universe can be divided into seven ages: Particle, Galactic, Stellar, Planetary, Chemical, Biological, and Cultural ages. The first three hundred thousand years are the Particle age, and the last three hundred thousand years are the Cultural age.

Rabi: The Brahmaputra river, born from many glaciers near Lake Manasarovar, also has seven stages.

Socrates: Exactly. And there is a metaphorical relationship with time as well. What do you think?

Rhea: Yes, like a river, our time only flows in one direction.

Socrates: Then we can elevate the comparison of the universe’s time with the Brahmaputra river from a metaphor to an allegory. The seven stages of the Brahmaputra (Angsi, Tsangpo, Siang, Brahmaputra, Jamuna, Padma, Meghna) can be compared to the seven ages of the universe. Shakespeare spoke of the seven ages of human life from childhood to old age. The universe has grown quite large after passing through seven ages since its birth. The Brahmaputra falls into the Bay of Bengal through the Meghna. Do you see any relationship between the sea and the time of the universe?

Rhea: If all the rivers from Angsi to Meghna are compared to the past and present, then the sea is undoubtedly a metaphor for the future.

Socrates: Exactly. Just as the river is narrow like the past, the sea is vast like the future. In the past, only one set of events happened in my life, like a river bound by two banks. But in the future, many things can happen, there are endless possibilities like the sea.

Rhea: Wonderful, Socrates, your introduction is excellent.

Socrates: Thank you. But remember Rabindranath’s words: ‘When I open my eyes to this light, your gaze will be fulfilled’. If you do not open the eyes of your mind, I will not be able to see. You must first take the responsibility of showing the relationship between each age of the universe and each stage of the Brahmaputra. Then I will talk in detail about each age over the next seven days.

Rhea: We agree. Who will talk about which?

Socrates: You can understand. Your names coincidentally match the seven days of the week or the seven planets of our time. Rabi with Sunday and the Sun, Shashi with Monday and the Moon, Mars with Tuesday; Hermes being another name for Mercury matches with Wednesday, Juno as Jupiter’s consort matches with Thursday, Ishtar as Venus’s predecessor matches with Friday, and Rhea as Saturn’s consort matches with Saturday. Rabi will start with the comparison of the Particle age with Angsi, then everyone will follow one by one, and Rhea will finish with the comparison of the Cultural age with Meghna. Rabi, start.

Ravi: If the particle age is the first 300,000 years of the universe’s history, then it is easy to match the river. The particle age is a time that humans have not been able to directly observe until now. As difficult as it is to find the origin of , it is equally difficult to find the origin of the universe. A conflict between science and metaphor can also be clarified by the Angsi River. The first stage of the Brahmaputra is actually not only the Angsi, but many other rivers join together to form Tsangpo at one point. You just took Angsi maybe to keep the total number at seven, or because the name sounds short and cute. The way brevity, beauty, metaphor, simile, allegory work in poetry, if you want to do the same in science, you will have to face a lot of trouble.

SOCRATES: Quite right, Ravi. We will use metaphors as Shakespeare did. When he calls Juliet the sun, it must always be borne in mind that Juliet does not actually become a ball of burning gas. Shashi, now it’s your turn.

Shashi: The galactic era is more naturally matched with Tsangpo. Seeing the whiteness of Tibet on both banks of Tsangpo, one can get the same feeling of vastness in a galaxy. Galaxies and their clusters are the largest structures in the Universe, the Tsangpo is also the longest part of the maputra. If we try to compare more than that, people will start calling us fundamentalists.

SOCRATES: That is enough. Even the comparison with the moon is not pleasant to hear for a long time. Mars, now begin your battle.

Mars: Siang in Arunachal is indeed a great battlefield, as violent as the birth of planets and stars from vast clouds of gas-dust during the Stellar Age. Yarlung-Tsangpo Canyon (Gorge) is the deepest canyon in the world, six kilometers deep. The Siang River has cut through the Hima millions of years to create this gorge in love with the Bay of Bengal. To those who have not heard the roar of this deep gorge (gorge), it is not possible to convey it with mere words of language. It is also not possible for any living person to witness the violent and beautiful scene of how a solar system is born from a swirling gas cloud.

SOCRATES: Perfect. Hermes, what do you think looking at Manas?

Hermes: I was thinking of the sudden great change in the cer of the river when the Siang cut through the mountains and entered the plains of Assam. This change can easily be compared to the first one or two billion years of the planetary era. After birth the Earth was hot as fire, bombarded by thousands of rocks from space. At the end of billions of years of war, when the ocean rises up on the earth’s surface, a nest of love, a factory of life, is created inside the ocean. If Arunachal’s Siang is a battlefield, Assam’s Brahmaputra is a garden of peace.

SOCRATES: I never thought of such a beautiful comparison before. , it’s your turn.

Juno: Brahmaputra becomes Jamuna after entering Bangladesh from Assam. Yamuna of India has no relation with this Jamuna. Yet since the word Jamuna brings to mind the love of Radha-Krishna and the Taj Mahal on the banks of the Yamuna in Agra, the comparison of the chemical age with the river Jamuna is natural. If in the Planetary Age hydrothermal vents were the home of love, then in the Chemical Age the first organisms that could mate and breed were born in this home. This confluence can be beautifully compared to the confluence of the Padma (Ganga) with the Jamuna at Rajbari near Dhaka.

SOCRATES: This meeting is indeed interesting. Consider that the Ganges and the Brahmaputra originate from almost the same place near , the Brahmaputra from the northern slopes of the Himalayas, the Ganges from the southern slopes. After traveling a long way, two rivers named Padma and Jamuna meet at a place called Rajbari. If the Jamuna is the symbol of chemistry, the Ganges can be the symbol of culture, as one of the oldest state cultures in the world grew on the banks of the Ganges. Well, Ishtar seems impatient, start.

Ishtar: It is very symbolic that the biological age begins at a place where the two largest rivers of South Asia meet. What flows from Rajbari to Chandpur as Padma is actually the result of the confluence of two big rivers (Brahmaputra named Jamuna, Ganga named Padma). As the Padma is one of the most revolutionary rivers in the world in terms of flow, the biological age comparison is apt. Because in this era, a planet filled with millions of species of plants, animals, fungi, bacteria is the biggest revolution in the history of the universe.

SOCRATES: Undoubtedly the greatest. Rhea, your duty is to finish now.

Rhea: But I think culture was a bigger revolution than life, and Meghna’s comparison with cultural era is the most natural. In the beginning the universe was like a single continuous gas, this gas gradually broke up into many different things, but these creations always wanted to merge with each other. Culture is the best way to bring different things together. Meghna river also does this. Many rivers of Bangladesh flow into the Meghna, and the Meghna runs together with everyone towards the Bay of Bengal, just as a culture wants to take many people together and runs toward the future, with many dreams in their hearts.

SOCRATES: Excellent conclusion, Rhea. Since the existence of time has come, I think we should think about it a little more.

Rhea: Yes, and we should get out of the metaphor and focus on a little real science.

Socrates: Why not? There are some mathematical laws of the universe whose ultimate source is unknown to humans. Moreover, even we the dead do not know it yet. But we know the objects on which these rules operate. Everything in the universe that obeys rules can be put into four categories: space, time, energy, matter (abbreviated as ‘STEM’). Einstein worked on these four things in the early 20th century. According to his special relativity, space and time are the same thing, and energy and matter are the same thing. General relativity (GR) describes the fundamental relationship between spacetime and energy-matter. I do not understand the mathematical basis of this superstructure, although understanding it is a major task in my future eternity.

Ravi: But if you don’t understand this mathematical basis, is there any point in discussing it? What I’ve learned from studying and researching relativity until I died is that people get it ninety percent wrong if they don’t know the math.

SOCRATES: Why ninety, I would say ninety-nine percent of the mistakes I make myself. My only knowledge is that I know nothing.

Ravi: Then the gift of your words will do us more harm than good.

SOCRATES: Oh, because my words harmed the youth, I had to drink hemlock and die. So now I am not afraid of harming anyone. But if you really want no one to be harmed, then please tell us yourself what is the best way to think of spacetime in the universe.

Ravi: Whatever I say in Bengali or English is bound to be wrong, only math can say it right. Since you can’t understand the math of spacetime, listen to the ninety-nine percent wrong version. After Einstein’s discovery of GR in 1915, he thought that the universe’s spacetime would be highly curved due to the large amount of energy-matter. The more matter there is, the more space bends. He also made a mathematical model of this four-dimensional curvature. Let alone humans, it is impossible for us to visualize the four-dimensional box of this model. But if we reduce the three dimensions of space to two dimensions, a three-dimensional model of space-time becomes very clear to the eye. You will understand if I draw on the tablet. Look at this:

The surface of this sphere, known as Einstein’s curveball, is two-dimensional space, and time is along the radius. In this model all the energy-matter of the universe is at a given time only on the surface of a sphere of a given radius. Radius increases with time, and the size of the universe increases with surface area. Einstein didn’t think of increasing the radius at first, he believed in a stable universe like Aristotle. He also added a constant called the cosmological constant to his equation so that the radius does not change. But later, around 1930, after receiving proof of the expansion of the universe, the constant was removed. In the 21st century, people brought this constant back not to stop the radius, but to control the rate at which the radius increases.

At present we know that the Universe is the same in all directions and has the same average density everywhere. This is also understood by the sphere of this three-dimensional spacetime. Think of the surface of the sphere as the surface of the Earth, and imagine that you are floating on a raft in the Pacific where everywhere you look is just water and water, the same in all directions, and the density of water is the same everywhere you go.

SOCRATES: But, Ravi, if I look up, I will not see the same.

Ravi: Hey, Socrates, you’re still as stubborn as ever. If we reduce the three dimensions of space to two dimensions in the beginning, then can there be anything called space other than the surface of the Pacific?

SOCRATES: No.

Ravi: And if there is no space without this surface, how can you look up? Looking up will require three dimensions of space which we have already reduced to two dimensions for the sake of this model.

SOCRATES: Well, I see.

Ravi: The fact that the universe looks the same in all directions is called the cosmological principle. Two more interesting phenomena of our universe can be explained using this three-dimensional model. The first is like this. Hubble found that all distant galaxies are moving away from us, and the farther away they are, the faster they are moving away. Actually galaxies are not moving, spacetime is expanding. If we think of the above sphere as a balloon, and if each galaxy is a spot on this balloon, then as the balloon gets bigger the radius or time will increase, the surface area of ​​the balloon will also increase and the spots (galaxies) will move away from each other. Each speck will feel that all the other specks are moving away from it, each will feel that it is at the center of the universe, since everything is moving away from it. But actually no one is the center. The surface of a sphere has no center, but if you stand anywhere on the surface and look around, you feel at the center of everything.

SOCRATES: Excellent. And what is the second?

Ravi: The second one is like this. Wherever we point our telescope from Earth, we can see up to the same distance in all directions. Since the speed of light is constant according to Einstein’s special relativity, our horizon is limited in both space and time. An example will make it clear. Let’s say we have three telescopes, named X, Y, Z, and let’s say all galaxies have the same absolute or true brightness. So the more sensitive the telescope, the more distant galaxies it can see. Now suppose X can see galaxies up to 1 billion lightyears away, Y can see up to 2 billion lightyears away, and Z up to 3 billion lightyears away. So the universe observed by X is a balloon with a radius of one billion lightyears, the balloon radius of Y is two billion lightyears, and the universe observed by Z is 3 billion lightyears in radius.

SOCRATES: But our three-dimensional spacetime balloon doesn’t match that. In the picture balloon the radius was time, in the observable universe of your three telescopes the radius is space, because its unit is light-years.

Ravi: This is where you have to think. Although the light-year is a unit of distance, it is related to time. Looking at a galaxy a billion light years away means seeing what it looked like a billion years ago, because it took a billion years for light to travel from there. The distance that light travels in one year at the speed of 300,000 km per second is one light year, or about 10 trillion km. So even in the balloon of the observable universe radius is actually time. X’s balloon has a radius of one billion years, because it can see a billion years into the past.

SOCRATES: I have two observations here. First of all, the picture of Einstein’s curveball, is it not a metaphor?

Ravi: Good point. It’s true that Curveball isn’t a real picture of the universe. Our world is four-dimensional, reducing it to three dimensions is mathematically trivial but psychologically illusory. But the curveball is still not a metaphor, its place is above metaphor, maybe analogy.

Socrates: I agree. Another observation is this. Looking at your map of the observable universe, I feel like I never see myself, never see the present, only see others, only see the past. If you can’t see yourself, is it possible to know yourself? I doubt my words now.

Ravi: If we can know about the present by looking at the past, can’t we know ourselves by looking at others?

SOCRATES: We have a better understanding of the universe’s present because we have a timeline of the past. In the case of society, it is not possible to understand the present without the past. However, I have doubts about whether it is possible to know oneself by looking at other people. But anyway, I thought I’d take this opportunity to show you the timeline I made.

This timeline has seven events from each era of the universe. So there are 49 total events. There is one slide for each event, and below the slides is a navigation that can be zoomed in and out and panned left and right. It will be useful for our meditation. Browsing the timeline can be a meditation on walking a tightrope into the history of the universe. I think it would be good for all of us if everyone, from Ravi onwards, looked at the seven events of the era and made some comments. Ravi?

Ravi: Yes, of course. At the outset, I must say again that studying history in this way without maths is not honorable for me. Still reading. The first event of the Particle Age is of course the Big Bang, which gave birth to our Universe approximately fourteen billion years ago. Ever since the universe was born, it has been expanding, sometimes fast, sometimes slowly. Along with the Big Bang, the Universe suddenly expanded in a phenomenon called inflation. After that all forces were born within one trillionth of a second. All the elementary particles, quarks, electrons, protons, etc. arrived within the first second. Within the first 15 minutes, multiple protons gathered to form the nucleus of an element like helium. At first energy was more dense than matter. Fifty thousand years after the Big Bang, matter defeated energy and increased in density. And after 300,000 years, electrons combine with proton nuclei to form atoms, and photons are freed from the tyranny of free electrons. We can still observe these free photons as microwaves. Through them we created the first image of the universe. This is where the particle age ends. Shashi?

Shashi: I saw the seven events of the galactic era on the timeline. The story goes like this. At the end of the particle age, the universe was essentially a single gas, with a density almost equal everywhere, but with some variation. Where the density is a little higher, in the dark age those areas tend to become denser under the influence of dark matter. One hundred million years after the Big Bang, the Dark Ages ended, two hundred million years later, galaxies and stars began to form from the accumulation of gas in areas of overdensity; Its name is Cosmic Dawn. In the first six hundred million years, the intergalactic medium formed, and in eight hundred million years, our galaxy, the Milky Way, was born. A billion years after the Big Bang, the Universe was filled with galaxies, and within two and a half billion years the quasars, the most massive and active galaxies, were born. The Galactic Era ended when clusters of galaxies formed three billion years after the Big Bang. Mars?

Mars: The start of the Stellar Age is estimated to be 10.4 billion years ago, three and a half billion years after the Big Bang. At that time, most stars were forming inside the galaxies of the Universe. Ten billion years ago, Population One stars were born, which are young, metal-rich, and reside in the galaxy’s disk. Nine billion years ago many clusters of stars formed and our galaxy got its thin disk. After a billion years, many voids were formed in the universe due to the formation of superclusters of galaxies. Six billion years ago, the repulsive dark energy won out over the attractive gravity, causing the expansion of the universe to accelerate. Five and a half billion years ago the Milky Way took on a spiral shape, and 4.6 billion years ago the Stellar Age ended with the birth of our Solar System. Hermes?

Hermes: The first important event in the planetary era was the birth of the Inner (closest to the Sun) planets 4.55 billion years ago. After 50 million years, the Sun attains the status of a main sequence, i.e. full-fledged star. About 4.4 billion years ago the hot Earth cooled and the oceans were born. But after that, for about 300 million years, there was a huge bombardment of rocks from space on the earth. At the end of this disaster, the continental crust was formed which is still moving, still changing the shape of the continents. 3.6 billion years ago, all the continents of the world joined together to form the Valbara supercontinent. This era ends here. Juno?

Juno: Socrates, you began the Chemical Age 3.6 billion years ago, when organic life probably first evolved from inert matter in hydrothermal vents on the ocean floor. The first fossils of organisms are found three and a half billion years ago. The first successful organisms, bacteria, left the sea and came to land about three billion years ago. A hundred million years later, large amounts of cyanobacteria in the oceans began releasing oxygen into the Earth’s atmosphere through photosynthesis. The first eukaryote to form a nucleus inside the cell appeared 2.7 billion years ago. The amount of oxygen in the atmosphere increased greatly two and a half billion years ago, three hundred million years later the ozone layer was formed. The chemical age ended with ozone. Ishtar?

Ishtar: Biodiversity began to increase during the Biological Era. The first complex cells were formed one and a half billion years ago. Many cells came together to bring about the multicellular revolution 600 million years ago. Two hundred million years later, animals emerged from water on land. Two hundred million years ago, warm-blooded animals appeared, and 65 million years ago, the asteroid impact ended with the extinction of the dinosaurs. Rhea?

Rhea: The Cultural Age begins with the birth of the first hominines approximately 7 million years ago. Animals of the genus Australopithecus and Homo appeared on earth between 4 and 1 million years ago. And the first modern humans walked the earth probably three hundred thousand years ago in Africa. Humans began to spread across the globe from Africa over a hundred thousand years ago. Fifty thousand years ago humans started a great revolution in religion, music and art. Agriculture began 10,000 years ago, followed by the first states 5,000 years later. Globalization began five hundred years ago, with its second wave beginning two hundred years ago with the Industrial Revolution. Here is the end of cultural age.

SOCRATES: It was really necessary to listen to everyone. You understand, I am not talking about the history of all places in the universe, I am talking about the history of the humans. The Universe came here as part of human history. This is why the Galactic Age focuses on the Milky Way, the Stellar Age focuses mainly on the stars of the Milky Way, the Planetary Age focuses on the Solar System, the Chemical and Biological Age focuses on Earth’s biosphere, and the Cultural Age focuses solely on human culture. It is our history, our meditation.