Lesson Explainer: The Big Bang Science

In this explainer, 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, we first need to understand what we mean by “the universe.”

We live on planet Earth, which orbits the Sun. Besides Earth, there are seven other planets orbiting the Sun, as well as a host of other objects including moons, comets, and asteroids. The Sun and everything that orbits it are known collectively as the solar system.

If we look outside the solar system, we see many other stars like the Sun. From an area on Earth with a very dark sky, we can see about 6‎ ‎000 other stars, and each of these stars has its own planetary system of objects like planets, asteroids, and comets. These are part of a larger structure called the Milky Way, which consists of every star we can see, plus many more that are too far away or too faint for our eyes to see. We cannot travel far enough away to take a picture of the Milky Way from outside it, but if we could, it would look something like the artist’s impression below.

Our galaxy milky way

Every speck in that image is a star, surrounded by its own solar system, just one of which is the Sun. In total, there are at least 100 billion (i.e., 100‎ ‎000‎ ‎000‎ ‎000) stars in the Milky Way. It is a galaxy, a structure containing stars and their solar systems, and also all of the gas and dust in between them. The Milky Way is just one of at least 100 billion galaxies that make up the universe. The universe is the name given to everything that exists: all the galaxies, and the stars, planets, and life inside them, all the gas and dust in between them, all the light, and even time.

The universe is unimaginably vast. We cannot say exactly how large, because there is no edge to it that we can see. In fact, there may not even be an edge.

Example 1: Understanding the Universe

Which of the following statements is true?

  1. The universe contains our galaxy, but not other galaxies.
  2. The universe contains the eight planets of the solar system and the Sun, but not other stars or galaxies.
  3. The universe contains all of the planets, stars, and galaxies that exist.


This question asks us to consider what we mean when we talk about the universe. We know that the universe includes everything there is, so let’s look at the options available and see which one is the best answer.

Option (A) states that the universe contains our galaxy, which is correct, but not other galaxies. We know that the universe includes every galaxy, so we can eliminate this option.

Option (B) states that the universe contains the eight planets of the solar system and the Sun—which it does—but not other stars or galaxies. There are many other stars in our galaxy, which we know is part of the universe, and we have already determined that the universe includes other galaxies, so this cannot be correct.

The final option, (C), states that the universe contains all of the planets, stars, and galaxies that exist. Although these are not the only things in the universe, it is true that all of these things are part it. Therefore, (C) is the correct answer.

If we look at how galaxies are moving through space, we will see something interesting. We might expect that galaxies would be moving at random, with some drifting toward us and some away. Or perhaps the force of gravity should be pulling everything closer together. Instead, we see that only our very closest neighbors seem to be influenced by gravity: a few small galaxies that orbit the Milky Way, much as the planets orbit the Sun, and our nearest large neighbor galaxy, Andromeda, which is hurtling toward us. If we look further away than Andromeda, every single galaxy is moving away from us. And the further away we look, the faster the galaxies are moving away.

At first glance, it is easy to misinterpret this to mean that we occupy some special place in the universe, if everything else is moving away from the Milky Way. In fact, what is happening is more like the movement of raisins baked into a loaf of bread as it rises.

As the bread becomes larger, the raisins move away from each other. And the further apart they were to begin with, the further apart they move. It does not matter which raisin we choose as a starting point, it will always see the others moving away from it. This is because the raisins themselves are not really moving; the bread in between them is getting larger.

Similarly, what we see in space is that the galaxies themselves are not really moving away from each other, but rather the space in between them is getting larger. In fact, the whole universe is expanding all the time. This does not mean that individual constituents of the galaxy are expanding: planets, stars, and galaxies stay the same size. It is only the space in between them that grows as the universe expands.

Example 2: Understanding the Expansion of the Universe

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?


In this example, we have three galaxies spaced equally apart in a line, and we need to imagine how they will look once the universe expands more.

Option (A) shows the three galaxies still equally spaced, but with the gap in between them having increased. In option (B), we have the two other galaxies close together, with a larger gap to the Milky Way.

Both options show the other galaxies moving away from the Milky Way, as expected. However, as the universe expands, we know that all galaxies appear to move away from all other galaxies. In option (B), the middle and right-hand galaxies end up closer together than they were to start with, so this cannot be right. The best option is therefore (A), in which all of the galaxies have moved away from each other.

The universe may be infinitely large, but it has not existed forever. To understand where it came from, let’s consider what happens if we run the expansion backward in time. All of the galaxies are currently moving apart as the universe expands, so if we go backward in time, they will get closer together and the universe will shrink. As we go back further, everything will get more densely packed, and the universe will get smaller. If we go back far enough, about 14 billion years, we will find that the galaxies are so close together that they merged together at a single point. This point is the big bang.

The big bang is what we call the initial expansion that started the universe. From a single point, every particle that would ever exist emerged. From that tiny point, the universe stretched and expanded until it reached the size it is now. That expansion is still continuing, and it is why we can still see galaxies spreading out today.

In its initial stages, when everything was packed together, the universe was dense and hot. There were no stars and galaxies; there were not even any atoms. The entire universe was a dense, hot soup of particles at high pressure. As it expanded, the universe cooled down, and some of the particles joined together to form atoms, the building blocks that make up all of the elements we have today. Only the very lightest elements, hydrogen and helium, were created in the short time after the big bang; heavier elements such as the oxygen we breathe and the carbon in our bodies would not be created until much later.

Example 3: Understanding the Big Bang

Which of the following statements is true?

  1. At the very beginning, the universe had a very high temperature, high pressure, and high density.
  2. At the very beginning, the universe had a very low temperature, low pressure, and low density.


This question asks what the universe was like in its very early stages, shortly after the big bang. Recall that the universe has been expanding ever since the big bang, so in the beginning it was much smaller than it is today but had the same amount of matter. This means it must have had much higher density in the past. It was also very hot and had high pressure. The correct answer is therefore (A).

The universe in its early stages was quite smooth, like a soup, but it had small lumps where the matter was slightly more dense than in other places. Since gravity pulls matter together, these regions that were slightly more dense than others attracted more matter toward them and became more dense, meaning they pulled even more matter toward them. In this way, the lumps grew until almost all of the matter in the universe was clumped into separate pockets, with vast expanses of nothing in between. In these clumps, the first stars formed. These first stars were much bigger than the Sun and clustered into groups. Gradually, these groups were drawn together by gravity and merged to form structures like the galaxies we see today.

That first generation of very massive stars only lived for a few million years—a very short lifespan compared to the age of the universe. At the end of their lives, their atoms stayed within the galaxies that had formed around them and condensed again to form a later generation of stars. These later generations included smaller stars like the Sun along with our solar system, which formed together about 4.5 billion years ago.

The timeline of the formation of the universe is summarized in the diagram below.

These final examples will give some practice with understanding the order in which structures formed in the universe.

Example 4: Understanding the Age of the Universe

Approximately how long ago did the big bang occur?

  1. 6‎ ‎000 years ago
  2. 1 million years ago
  3. 14 million years ago
  4. 4 billion years ago
  5. 14 billion years ago


The big bang was the origin of the universe, so this question is asking how old the universe is. Let’s consider each of the options in turn.

The first option is 6‎ ‎000 years ago. This is roughly how far back recorded human history goes, so if humans already existed 6‎ ‎000 years ago, then the universe must be older than this. The next option is 1 million years ago. Some early ancestors of humans have been found to be from this period, so this is also too recent. We also have lots of evidence of life from 14 million years ago, including orangutans spreading through southern Asia. Next, we get to 4 billion years, which is close to the age of Earth and the solar system, but remember that these formed a long time after the big bang. Finally, we have option (E), 14 billion years ago, which is the correct answer.

Example 5: Understanding the Origin of the Universe

Which of the following occurred first?

  1. The development of life on Earth
  2. The formation of the Milky Way
  3. The formation of the solar system
  4. The big bang


In order to answer this question, we need to understand the order in which structures formed in the universe. The first option is (A), the development of life on Earth. In order for there to be life on Earth, planet Earth itself must form first, which means we need the solar system to form before there can be life on Earth. This means we can eliminate (A). We can also recall that the solar system formed after the formation of galaxies, and our galaxy is the Milky Way, so (B) must have happened before (C), which means we can eliminate (C). Finally, we can recall that galaxies formed after the first stars, which formed from atoms that were created after the big bang. The big bang was the origin of everything in the universe, so this must have occurred first. Therefore, the answer is (D), the big bang.

Key Points

  • The solar system is just one of at least 100 billion planetary systems that make up the Milky Way galaxy.
  • There are at least 100 billion galaxies in the universe.
  • The universe is made up of everything there is: planets, stars, galaxies, time, and space.
  • The universe began in the big bang, approximately 14 billion years ago.
  • In the beginning, the universe was hot and dense and had high pressure.
  • As the universe expanded and cooled, atoms of hydrogen and helium formed.
  • Over time, atoms clumped together under gravity to form the first stars and then the first galaxies.
  • The solar system formed about 4.5 billion years ago.
  • The expansion of the universe that started with the big bang still continues today and causes galaxies to appear to be moving apart from one another.

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