How Do You Measure the Size of the Universe?

Popular science articles and internet discussion forums are full of conflicting reports of the size of the universe, ranging from 13 billion light years to numbers with too many zeroes to write down here.

Some of this confusion is because people are talking about slightly different things, while not being specific what they’re talking about. Some is because people genuinely don’t understand how big the universe is, and are just intuiting it from some statistic they have read, such as the age of the universe. So I put together this page to clear up some of the confusion.

First, let me say that everything that follows is derived from Big Bang cosmology. This is the currently accepted model of how the universe developed, and posits that that the universe began as a tremendous explosion from an infinitesimal point, which created space and ultimately everything that is in it. Space (and therefore the universe), have been expanding ever since. The discovery that space is expanding is credited to Edwin Hubble through his study of the red shifts of distant galaxies.

Before we talk about the size of the universe, we need to understand this concept of space expansion thoroughly, because it’s crucial to the discussion. So here is an abbreviated explanation. Hubble noted two things:

That very far objects such as distant galaxies are red-shifted in their spectra. Light coming from these galaxies appears to shift towards the red end of the spectrum. This is due to a common and well known phenomenon, known as the Doppler Effect, which causes the frequency of light (and sound) waves to appear to increase to the observer if the object emitting the waves is approaching the observer. Conversely, the frequency appears to decrease if the object is moving away from the observer.

That the farther an object is, the more its light is red-shifted. These two things in combination present the picture of an expanding universe. The red shift data indicates that objects that are far from us are moving away from us, no matter in which direction we look. And it shows that the farther these objects are, the faster they are moving away from us.

Now if you assume that the Earth is nothing special in the universe, that it has no special location, then it follows that no matter where you are in the universe, you would still observe the same thing – objects are moving away from you, and that the farther the objects, the faster they are moving away. If every object in the universe is moving away from every other object, it can only mean that the space between the objects is expanding.

It’s easy to understand why farther objects are moving away from us faster than nearer objects, if you think of space as expanding at a certain rate. Suppose that the rate of expansion is 1 inch per foot of space per year (purely arbitrary numbers, don’t take them seriously). Then every foot of space expands by 1 inch per year. If there is more space between two objects, obviously it will expand more, than if there is less space. If one of those objects is Earth, and you are standing on it, then the other object appears to move away from you as the space between you expands. The farther the object is, the more space there is between you to expand, and so the faster it appears to move away.

While quite controversial initially, most astrophysicists would agree today that the Universe is expanding.

Before we go much further with this idea, let’s clarify one more point. There are many cases where objects aren’t moving away from each other. For example, the Sun isn’t moving away from the Earth. While the moon is moving away from the Earth, it’s not because the space between the Earth and the moon is expanding, it’s because of the tides. If we look in our own neighborhood, yes there are stars that are moving away from us, but there are also stars moving towards us. Even in our galactic neighborhood, the Andromeda galaxy is moving towards us, not away, and is due to collide with the Milky Way in a few billion years. So what gives, is space expanding or not?

The short answer is that yes, space is expanding. However, at short ranges, gravity is a powerful force and can hold stuff together so it doesn’t drift apart with the expansion of space. It can even pull things closer to each other, regardless of the expansion of space between them. This is exactly what happens within the Solar System, or within our galaxy. The distances are just too short, and gravity predominates. While the space occupied by the Milky Way is expanding, just like the rest of the universe, objects are not drifting apart because they are held together by gravity.

From this we can conclude that the expansion of the universe becomes apparent only when we look at much larger scales. If you look at galaxies that are tens, or hundreds of millions of light years away from us, or even billions of light years, then the expansion of space is very evident. At such extreme distances, gravity plays no role, and the expansion becomes observable in the red shifts of objects.   Adapted: Essay Web

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