Our universe is a single thing, a single thing created from a single infinitesimally small point in a gigantic ‘explosion’ called the big bang almost 14 billion years ago.

THE MOST IMPORTANT fact in modern astronomy is that we can see and observe the past. Except for the first 500,000 years of our history, we can observe all past periods of the 14 billion years of history of the universe. I am not saying this in a figurative way; it is actually true that we can see the past.

But how is that possible?

It is possible because we see using light and the speed of light is finite. Light travels 300 thousand (300k) kilometers in 1 second. That actually means light travels around 1 foot in 1 nano-second.

One nano-second is a billion times smaller than 1 second. During the class, some of you were sitting at a distance of 10 feet from me. That meant, I was seeing you as you were 10 nano-seconds ago. The light from the light-bulbs in the classroom was reflected from your body and took 10 nano-seconds to reach my eyes after the reflection.

This actually means, it is impossible to see the present using our naked eyes. Even my hand is a foot away from my eyes. Light takes around 1 nano-second to reach my eyes after getting reflected from my hand. So, I see my hands as they were 1 nano-second ago.

Seeing the past is not just possible, it is actually the only we can see. We are almost blind to the present and the future. Our vision is a vision of the past.

Now, some of you took a picture of Bode’s galaxy from IUB rooftop. Bode’s galaxy is around 12 million light-years away.

1 light year = the distance light travels in 1 year = 10 trillion km.

That means it took light 12 million years to reach us from Bode’s galaxy. The light that left this galaxy 12 million years ago is reaching us only now. We are seeing this galaxy as it was 12 million years ago. At the moment, it is impossible to see what happened to this galaxy in the last 12 million years, for that we have to wait another 12 million years ourselves. The light that left this galaxy today will reach us after 12 million years. Read this paragraph again and again and try to grasp the immensity of this fact.

Have a look at this picture taken using a normal camera from the site of the Very Large Telescope, the largest Optical Telescope in the world located in the Atacama Desert of Chile. In the sky, you can clearly see the Andromeda galaxy, the galaxy closest to us, around 2.5 million light years away. That means you are seeing the Andromeda galaxy as it was 2.5 million years ago.

We have talked a lot about the Orion constellation, its famous nebula, and its famous stars Betelgeuse. Betelgeuse is around 550 light years away. Whenever you feel like looking at the past, look up, identify the Orion constellation, then identify the reddish bright Betelgeuse in the constellation, and it will immediately take you back almost 500 years. The light of Betelgeuse entering your eyes right now left Betelgeuse when Akbar’s grandfather Babur invaded Northern India.

Now think about the famous Orion Nebula, identified as Messier 42 or M 42. It is almost 1400 light years away. When you see it through the eye of our Ashvin-2 (eQuinox telescope), you go back to the times when the prophet Muhammad was still living.

The farther you look in space the further you see in time.

Light is like a letter sent you in the good old times when there was no fast mode of transportation. Let us say you live in the 19th century and you have written a letter to your loved one in China and it took a month for the letter to reach him. When he reads the letter, he will learn what you were thinking one month ago. He has no way of knowing what happened to you in that one month after writing the letter.

Enough with the poetry of time. Let us go back to prosaic science. Using light as a letter, we can study the past of our universe and uncover everything about its history.

If we want to know what the universe looked like 5 billion years ago, we just need to find a galaxy 5 billion light years away and keep observing it closely.

Want to see the universe when it was just 2 billion years old? Just find a galaxy 12 billion light years away and it will show what it meant to exist in a universe only 2 billion years old.

Light is literally a letter. A light coming to you from a galaxy will not only tell you how far the galaxy is, but also what the galaxy is made of.

This is the method using which we have uncovered the history of the universe as laid out in the timeline below. The events related to earth after 5 billion years ago, of course, were not known using light as letter.

Credit: Pasachoff & Filippenko 2019, Cambridge Univ. Press.

Let us list the key events shown above up to the extinction of dinosaurs:

• 14 billion years ago: big bang, beginning of the universe from an unknown past.
• 12 billion years ago: almost all galaxies in the universe were formed.
• 5 billion years ago: our solar system formed.
• 4 billion years ago: age of the oldest rocks found on earth.
• 3.5 billion years ago: primordial atmosphere of earth and the signs of first life.
• 2 billion years ago: plants begin producing oxygen on earth.
• 1 billion year ago: earth’s atmosphere takes the present form.
• 250 million years ago: our sun began the last revolution around the galactic center.
• 65 million years ago: dinosaurs went extinct due to the collision of an asteroid.

This last event has some astronomical significance; at least if the ‘asteroid impact’ hypothesis is correct. A 10-km asteroid hit the earth around 65 million years ago. The impact was so violent that it sent a huge amount of rocks into the atmosphere and the earth was shrouded in a dense cloud of dust and mist for almost ten thousand years. During that time, earth was dark due the lack of sunlight. The largest animals went extinct due to their higher demand of food and nutrition, small animals like our dear ancestors survived.

It is not easy to fathom 14 billion years, to feel the immensity of this span of time. A cosmic calendar comes in handy where the whole 14 billion years is fit into the span of 1 year. Imagine 14 billion years is equal to 1 year. Then how much would be 1 billion year? A little more than 1 month (a year has 12 months).

In this cosmic calendar, big bang happened on 1 January, 12 am, midnight. And the current time is 31 December, 12 am, midnight.

Let us see when the key events of the universe happened in this calendar of imagination:

• 1 January: big bang.
• May: formation of the disk of our galaxy, the Milky Way.
• September: formation of our solar system.
• October: plants create oxygen via photosynthesis.
• November: first life.
• 15 December: the most ancient fossils of life.
• 18 December: animals with backbone arise.
• 30 December: dinosaurs are extinct.
• 31 December, 11:52 pm: modern humans evolve in Africa.
• 31 December, 11:59 pm: modern humans migrate to the whole world from their African homeland.
• 1.2 seconds ago: Columbus arrives in America and begins the violent process of destruction of the native American cultures.

This is still not very easy to fathom. This imaginary calendar is so compressed that a 530-year-old event (Columbus’ inhuman invasion of the Americas) is just 1.2 seconds old.

So much about time. Time is tiring. Let us rather delve into space.

Zero time is unthinkable, we started our temporal journey from a time that is after zero time.

Zero space is also unthinkable. But there are things that are so tiny that for all practical purposes we can think of them as the neighbors of zero.

In time, we traveled from almost zero to the present time, a journey of almost 14 billion years. In space, we will begin from a length of 1 meter which is very close to the average height of a woman of earth (1.6 meter), and first go toward smaller things all the way up to the tiniest thing found by earthlings so far, and then draw ourselves back to large scale, come to the scale of 1 meter, and then start an opposite journey toward big things all the way up to the size of the whole observable universe.

Before going into this journey, we have to understand how to number things. One, two, three are rather easy. But how do we call things that are trillions of times bigger or smaller? Using powers of 10.

$$ 10^3 = \text{ three zeros after 1} = 1,000 = 1 \text{ thousand} = 1k $$

$$ 10^6 = \text{ six zeros after 1} = 1,000,000 = 1 \text{ million} $$

$$ 10^9 = \text{ nine zeros after 1} = 1,000,000,000 = 1 \text{ billion} $$

$$ 10^{12} = \text{ twelve zeros after 1} = 1,000,000,000,000 = 1 \text{ trillion} $$

Similarly, if there is a minus sign before the power of 10, that will mean how many times the number is smaller than one.

$$ 10^{-3} = \text{ 1,000 (3 zeros) times smaller than 1} $$

$$ 10^{-6} = \text{ 1 million (6 zeros) times smaller than 1} $$

$$ 10^{-9} = \text{ 1 billion (9 zeros) times smaller than 1} $$

$$ 10^{-12} = \text{ 1 trillion (12 zeros) times smaller than 1} $$

Do not memorize any of these. Just try to feel the numbers and then delve into the journey in space using this webpage: https://scaleofuniverse.com.

Here, you can zoom in to go toward smaller known objects. The size of each object is shown using a negative power of 10. That means it tells you how many times smaller than 1 meter an object is.

And, you can zoom out in this webpage to go toward bigger objects. Here the size of each object is given in positive powers of 10. So here you see how many times bigger than 1 meter an object is.

I have made the the following video of this journey using this webpage.

A sunflower can be as big as a human. We start from there. I start by zooming in and then come back to the human size of around 1 meter. Then I start going toward big things from 1:45 minutes of the video. I am listing some key timesteps in the video where you can pause for a moment and think.

• 0:43: a virus is almost 1 million times smaller ($10^{-6}$) than 1 meter.
• 0:52: a molecule of water is almost 1 billion times smaller ($10^{-9}$) than 1 meter.
• 1:09: the length of gamma ray wave is almost 1 trillion times smaller ($10^{-12}$) than 1 meter.
• 2:20: earth is almost 1 million ($10^6$) times bigger than 1 meter. The star Sirius B is almost as small as earth.
• 3:00: the sun is almost 1 billion times bigger than 1 meter. Basically the sun is a billion meter (or million kilometer) in diameter. Pause and Think.
• 3:34: the star Antares is almost 1 trillion meter in diameter (a thousand times bigger than the sun).
• 3:46: the Orion Nebula almost 1 million times bigger than the star Antares. You have to gather a billion stars like the sun to get to the size of the Great Orion Nebula.
• 4:08: our galaxy the Milky Way is almost a thousand times bigger than the Orion Nebula. To get its size in meter, you have to add 21 zeros after one, or 100 thousand light years.
• 4:20: our galaxy is part of a group of galaxies called the Local Group. The size of this group is almost 100 times bigger than the size of the Milky Way. Aside from the Milky Way, Andromeda is the other big galaxy in this group.
• 4:23: there are even bigger clusters and superclusters of galaxies in the universe.
• 4:32: in order to get to the size of the whole observable universe, you have to add 27 zeros after 1.

But that is not the end. This is only the portion of the universe we can observe. The universe in reality is much bigger, could be even infinite. We do not know.

Again, do not memorize any of these. Try to feel. Feeling the universal space and time is like the emotion of love, you cannot memorize your way into it or out of it.