This is a re-run of a post that originally ran in July of 2018.
I have been reading A Little Book about the Big Bang,
by Tony Rothman (Harvard University Press, 2022).
He uses all the standard explanations of the expanding
universe, and it made me think to re-run this one.
“The universe is expanding.”
Since you are reading a science blog such as The Catholic Astronomer [now Sacred Space Astronomy], you probably are familiar with that statement. But do you know why astronomers say that the universe is expanding? What is the basis for that statement?
To address that question, let’s consider this: what would an expanding universe look like?
This is going to take some imagination. Let us imagine a universe. The diagram below will be our universe. It contains galaxies (the little colored ovals), but its size and shape is undefined. The galaxies extend out beyond what is seen below, but how much beyond we do not know. Furthermore, the galaxies may extend out farther to the right than to the left, or farther down than up. All we really know is that here is our universe, and it contains galaxies.
Now, let us expand this universe. We will expand it uniformly—that is, equally in all directions. We will color the expanded universe grey. Note that all the galaxies are the same in both the original and the expanded versions of our imaginary universe. If you wish, break out a ruler and measure; you will see that our expanded universe is indeed larger in all directions.
Now, let us focus on one particular galaxy. And, let us suppose that any observer living in that galaxy can only see so far out into the universe. Since our observer can only see so far, and since the galaxies extend out indefinitely, our observer will see himself or herself as being at the center of all that can be seen. Like this—
Now let’s do this same thing for the same galaxy, but with the universe expanded—
Finally, let’s overlay these two on top of each other, so that we see both from the point of view of the observer in our focus galaxy—
Note how, in the expansion of the universe, our focus galaxy appears to be at the center of the expansion. That makes sense, if you think about it: when we enlarged the universe, we made all the distances between galaxies bigger. So, the distance from our focus galaxy to each and every other galaxy in our imaginary universe is larger. So, each and every other galaxy is moved away from our focus galaxy. Note also that the farther galaxies are moved more than the nearer ones. A “zoomed in” picture shows this more clearly—
If we enlarge the entire overlay picture (you can click on any one of these images to enlarge it) and use a ruler to crudely measure both the distance from our central focus galaxy to every other galaxy, and the amount each galaxy moves away from our central focus galaxy, we find that this trend of farther galaxies moving more than nearer ones holds for all the galaxies, and that the pattern of increase is more or less linear—
Try this with a ruler for yourself—you will find you get the same results.
Now, let’s try this out on another galaxy. This time let’s chose the green galaxy that is just to the left of our previous focus galaxy—
Here is the overlay for this second galaxy—
As you can see, any observer who might be in the second galaxy would see exactly the same pattern as the observer in the first galaxy sees. Both observers see their galaxies as being at the center of an expanding universe, and both see that farther galaxies are moving away from them more, and that the trend is linear.
And this is exactly what we see in our universe. The first person to see it was Edwin Hubble in the late 1920’s. He measured the distances to different galaxies and the movement of those galaxies, and then made a graph of galaxy distance vs. galaxy movement. His graph showed a linear pattern of farther galaxies having more movement away from us. Indeed, graphs like this are now called “Hubble Plots”. Below is Hubble’s original graph, along with a more modern Hubble Plot.
Thus, our universe is behaving just like the expanding universe that we imagined above. This is why astronomers say “The universe is expanding.” The view from here, on our planet in our galaxy, is that we are at the center of this expansion. However, as we saw earlier, inhabitants of any other galaxy in the universe will see themselves at the center, too. (Who is really at the center, and whether a center even exists, is a matter for another day.)
It is important to keep in mind that this expansion only refers to the distances between galaxies. The distance between Venus and the sun, for example, is not getting bigger. The Earth is not getting bigger. Some moon orbiting some planet in some other star system is not getting bigger. Your hand is not getting bigger. When astronomers say “The universe is expanding”, they only mean the distances between galaxies is getting bigger.
However, there have been some astronomers who have not believed that the Hubble Plots are evidence for an expanding universe. One such astronomer was, interestingly enough, Edwin Hubble.
And that is the subject of my post for next week.