In this video, we will learn how the universe began and how it has changed over the course of billions of years. Before we talk about its origins, it’s important to understand what we mean when we talk about the universe. We live on the planet Earth, which is one of eight planets orbiting the Sun. Also orbiting the Sun, we have other objects, like moons, comets, asteroids, and dwarf planets. All of this together is collectively known as the solar system. The Sun itself is a star and it’s just one of many stars that come in a wide range of sizes and colors.
If we could zoom out far enough, all of the stars in our night sky would make a shape, something like this. This is the Milky Way galaxy, a vast collection of around 100 billion stars. And the Milky Way itself is just one of many galaxies, at least 100 billion of them in total, in a wide range of shapes, sizes, and colors. And galaxies can sometimes grouped together to form even larger structures known as galaxy clusters. And if we take all of that together, every planet, every moon, every star, and every galaxy, all of this combined and everything in between makes up the universe. So the universe consists of everything that exists. All of the galaxies, the stars inside those galaxies, planets orbiting those stars, life on those planets, all of the gas and dust that lies in between, all of the light, and even time itself makes up the universe.
Now, let’s look in more detail at those galaxies. They’re positioned randomly throughout space. So if we look at how they move, we might expect them to be moving in random directions as well. But that turns out not to be the case. Very nearby galaxies, like our nearest large neighbor Andromeda, are moving towards us, attracted by the force of gravity between the two galaxies. But if we look further away, every galaxy is moving away from us. And it turns out the further away a galaxy is, the faster it’s moving away from us. It’s easy to misinterpret this to mean that we’re in some special place at the center of the universe. But this pattern isn’t true only for our Milky Way galaxy. If we lived instead in, say, this galaxy, we would still see all of the other galaxies moving away from us, with the ones the furthest away to start with moving the fastest.
So what’s really going on here? The easiest way to visualize this is to think of a cake. Baked into the cake, we have some blueberries. As the cake rises, the blueberries move further apart. If we imagine this from the point of view of any one of the blueberries, it looks as though the other blueberries are moving away from it and those that are the furthest away are moving away the fastest. But the blueberries aren’t really moving at all. What’s really happening is that the bits of cake in between are getting larger. This is also what’s happening in the universe. The universe itself is expanding, which means the space in between galaxies is getting larger, and the galaxies, then appear to be moving away from each other.
So, the more time passes, the larger the universe gets and the more the space in between galaxies increases. And this is going to help us think about where the universe came from, because if this is happening when we go forwards in time, we can think about what happens if we reverse time and go backwards. If we go backwards in time, we start with galaxies that are spaced out. And they gradually get closer together as space gets smaller. Now, let’s think about what would happen if we continue this even further back in time. The galaxies would get even closer together until they all ultimately merge into one point.
If we look at how fast galaxies appear to be moving away from us, we can work out how far back in time we would need to go for all of the galaxies to converge together at one point. This turns out to be 14 billion years, and so 14 billion years is the age of the universe. 14 billion years ago, the entire universe was condensed into a single point. That point inflated rapidly in an event we call the Big Bang. The name doesn’t really fit because there was no actual bang. Nonetheless, the Big Bang is the name we give to the initial expansion that started the universe. That single point 14 billion years ago contained every particle that would ever exist in the universe.
So let’s now start from the Big Bang and look at how the universe has evolved over time. Shortly after the Big Bang, the universe expanded very quickly, but it was still very small, which means it was very dense. At this point, it was a hot, high-pressure soup of particles. As the universe expanded further and cooled, the first atoms formed. These were just the very lightest elements, such as hydrogen and helium. Heavier elements like carbon and oxygen wouldn’t form until much later.
At this point, the matter was spread quite evenly throughout space. But as it moved around at random, occasionally lumps would form that were slightly more dense than elsewhere. And where those lumps formed, more matter would be attracted towards them under gravity. And so gradually, all of the matter in the universe fragmented into these clumps. Some of these became large enough to ignite, and the first stars were born. These stars were much larger than stars we have today like the Sun. So they wouldn’t have been around for very long, perhaps just a few million years.
These stars too grouped together under gravity forming star clusters and the first galaxies. As that first generation of stars died, their atoms remained within those galaxies and went on to form later generations of stars. Inside one of those galaxies, the Milky Way, the Sun and the solar system formed around 4.5 billion years ago. The universe has continued to expand ever since the Big Bang, causing galaxies to drift further and further apart. And this expansion is still continuing today. Using everything that we’ve learned about the Big Bang and the origins of the universe, let’s try some questions.
Which of the following occurred first? (a) The development of life on Earth, (b) the formation of the Milky Way, (c) the formation of the solar system, or (d) the Big Bang.
In order to answer this question, we need to recall how structures in the universe are related to each other. Earth is the planet we live on. And Earth is part of the solar system, which is just one of many planetary systems in the Milky Way galaxy. And the Milky Way galaxy is just one of many galaxies that make up the universe. So let’s look at our options. For there to be life on Earth, the Earth must already exist, which means the solar system of which it’s a part must already exist. So the development of life on Earth cannot have occurred first and, we can exclude option (a).
We also know that the solar system is part of the Milky Way, and we can recall that galaxies formed before the solar system. Therefore, the formation of the solar system cannot have occurred first. And we can eliminate option (c). Finally, we can recall that galaxies like the Milky Way were formed after the first stars, which were made of atoms that were created just after the Big Bang. The Big Bang is the origin of everything that there is, and so we can eliminate option (b). And the answer to which occurred first is (d) the Big Bang.
The following picture shows three galaxies with our own galaxy, the Milky Way, on the left. Which of the following pictures best shows how far apart these galaxies will be after the universe expands more?
Looking at the first picture, we can see that the three galaxies start off equally spaced in a line. We need to imagine how they will appear when the universe has expanded more. In option (a), the galaxies are still equally spaced, but the gaps between them have increased. In option (b), we have a large gap between the Milky Way and the first galaxy, but the other two galaxies are still close together.
Recall that as the universe expands, galaxies appear to be moving apart from one another. Both of these options show the other two galaxies moving away from the Milky Way. However, we should see galaxies moving away from us, whichever galaxy we’re observing from. We could, for example, imagine that we’re observing from the galaxy on the right. We can see that both the other galaxy and the Milky Way have moved away as expected. But in option (b), the middle galaxy has not moved away. We can therefore exclude option (b). And the best answer in this case is option (a), where all of the galaxies have moved further apart from one another.
Let’s now summarize what we’ve learned in this video. First, we saw that the universe is made up of everything there is. Next, we discussed how the universe began in the Big Bang 14 billion years ago. The universe started off hot and dense with high pressure. Next, we saw that the first atoms to form just after the Big Bang were hydrogen and helium. These atoms then clumped together under gravity to form the first stars and then the first galaxies. The solar system then formed about 4.5 billion years ago. And the expansion of the universe, which is still continuing today, causes galaxies to appear to be moving apart.