Question Video: Understanding the Expansion of the Universe | Nagwa Question Video: Understanding the Expansion of the Universe | Nagwa

Question Video: Understanding the Expansion of the Universe Science • Third Year of Preparatory School

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The diagram shows three galaxies, with our own galaxy, the Milky Way, on the left. Which of the following diagrams best shows how far apart these galaxies will be after the universe expands more? [A] Diagram A [B] Diagram B

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Video Transcript

The following diagram shows three galaxies, with our own galaxy, the Milky Way, on the left. Which of the following diagrams best shows how far apart these galaxies will be after the universe expands more?

To begin, let’s recall that the universe is expanding. And because of this, we observe distant galaxies in every direction to be moving farther away from our own galaxy, the Milky Way. But it’s important to note that our vantage point from within the Milky Way isn’t special in this regard, since this expansion can be observed throughout the entire universe. So, even if we lived in, say, this galaxy here, we would still see all other distant galaxies moving away from us.

Now, this isn’t caused by the galaxies pushing each other away somehow. After all, we know that any two objects with mass are attracted to each other gravitationally. Rather, we see distant galaxies apparently moving away from our galaxy because the space between the galaxies is itself expanding. This can be a tricky concept to grasp. So let’s think about another way to picture this sort of expansion.

Imagine we’re baking a loaf of raisin bread. Raisins are mixed into the dough. And we know that as the bread bakes, the dough will expand. So let’s now imagine the raisins’ movement as the loaf of bread gets bigger. The raisins themselves remain the same size. But as the bread enlarges, the raisins move away from one another, not because they’re swimming around in the dough, but simply because all the dough is expanding. If we look at any two raisins, we know that they’ll get farther apart from each other as the bread expands. And the farther apart they were to begin with, the farther they end up moving away from each other. No matter what happens, from the perspective of whatever raisin that we choose, we will always see the other raisins as moving away from it.

Now let’s apply this understanding to the concept of space to help us answer this question. In the first diagram, we see three galaxies equally spaced apart. As time passes and the universe expands, we know that these galaxies will seem to all be moving away from each other. This should be true no matter what galaxy we choose as a reference point.

Let’s look at answer option (A). Here, we see the two galaxies on the right as having moved farther from the Milky Way. But at the same time, those two galaxies have moved closer to each other. We know this isn’t right because we should see all the galaxies moving apart, no matter where we look from, not just from the Milky Way.

Answer option (B) shows the galaxies equally spaced apart and with more space between each galaxy than there was originally. So, even if we were inside either one of these galaxies on the right, we’d still see all the other ones as having moved farther away. We know that this is the correct way to think about the expansion of the universe, since our reference point from within it doesn’t matter. Therefore, option (B) is the correct answer.

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