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.
