# How Big is the Universe?

The Universe is big, but how big is it? That all depends on whether the Universe is finite or infinite. Even the word “big” is tough to get clear. Are we talking about the size of the Universe we can see, or the Universe’s actual size right now?

The Universe is big, but how big is it? And what the heck kind of question is that? Are elephants big? Trucks? Dinosaurs? Cheese? Is cheese big? How big is cheese? How big is big?

The word “big” is tough to get clear. Are we talking about the size of the Universe we can see, or the Universe’s actual size right now? This becomes even more complicated when we are trying to work under assumptions of either the Universe is finite or the Universe is infinite.

One difficulty with talking about the size, is that the Universe is expanding. Light takes time to travel from distant galaxies, and while that light travels, the Universe continues to expand. So our problem with talking about how big it is, is that there is no single meaning to distance when it comes to the universe. For this reason, astronomers usually don’t worry about the distance to galaxies at all, and instead focus on redshift, which is measured by z. The bigger the z, the more redshift, and the more distant the galaxy.

As an example, consider one of the most distant galaxies we’ve observed, which has a redshift of 7.5. Using this, we can determine distance by calculating how long the light has traveled to reach us. With a redshift of 7.5, that comes out to be about 13 billion years. You might think that means it’s 13 billion light years away, but 13 billion years ago the universe was smaller, so it was actually closer at the time the light left that galaxy. Using this, if you calculate that distance, it was only a short 3.4 billion light years away.

Now the galaxy is much farther than that. After the light left the galaxy, the galaxy continued to move away from us. It is now about 29 billion light years away. Which is definitely more than 13, and quite a bit more than its original 3.4.

Usually it is this big distance that people mean when they ask for the size of the universe. This is known as the co-moving distance. Of course, we can only see so far. So, how far can we see? The most distant light we are able to observe is from the cosmic microwave background, which has a redshift of about z = 1,000.

This means the co-moving distance of the cosmic background is about 46 billion light years. Sticking us at the center of a massive sphere, the currently observable universe has a diameter of about 92 billion light years. Even with this observed distance, we know that it extends much further than that. If what we could see was all there is, we would see galaxies tend to gravitate towards us, which we don’t observe.

In fact we don’t see any kind of galaxy clumping to a particular point at all. So as far as we know the universe could extend forever. It could be even stranger than that. Despite some media controversy, if the BICEP2 detection of early inflation is correct, it is likely the Universe undergoes a type of inflation with the intimidating moniker of “eternal inflation”. If it is the case, our observable universe is merely one bubble within an endless sea of other bubble universes. This is otherwise referred to as… the multiverse.

So, in the immortal words of Douglas Adams, “Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space”

What do you think? Does the Universe go on for ever? Tell us in the comments below. And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

## 4 Replies to “How Big is the Universe?”

1. forj says:

a couple of things that i wanted to get clarification on.

“In fact we don’t see any kind of galaxy clumping to a particular point at all”

– dont we see galaxies clumping in gigantic galaxy clusters where filaments of galaxies intersect? I do understand that at extremely large scales the universe appears homogeneous. but we do see galaxies clumping at particular points.. the center of massive galaxy clusters where filaments intersect. right?

“our observable universe is merely one bubble within an endless sea of other bubble universes”

– wouldnt an object that was at the farthest distance in our observable universe – or approx. 46 bly comoving distance, then have its own observable universe that includes our galaxy at one end, and then 46 bly of galaxies in the opposite direction that are outside our observable universe? to me that doesnt equate to a separate bubble universe or the idea of a multiverse. it’s just another part of the universe that happenes to be beyond our observation limit. so calling the bordering portions of space outside our observation limit separate bubble universes seems a bit off.. as those spaces are observable from the edge of our horizon and to me seem analogous to what is over the horizon on earth, sortof

1. fractal says:

The observational horizon spheres that you are talking about are not what is meant by bubble universes or multiverse. Brian most likely means to say the following:
If this inflationary theory is right, then theoretical physics implies that inflation could very well occur many times, apart from the universe that we know and are part of. So in that case, probably many more universes with different properties (will) exist(ed) outside our universe.

2. john kulick says:

A string of light clocks are end to end across the universe. ( A light clock is simply two reflective surfaces separated by a distance with a pulse of light traveling back and fourth between the mirrors, thereby establishing a relative measure of distance and an interval of time. Such light clocks, and the structure they define in space, are critical to special relativity and hence general relativity).

Since these “mirrors” are apart of the fabric of space, they are “carried” by the expansion of spacetime, just as the galaxies are carried by the expansion of spacetime.

Now lets keep the model consistent with special relativity. The light clocks keep their local measure of an interval of time and distance. If each light clock was 1 million light years long, they would remain locally measured to be 1 million light years long since the light clocks are apart of the fabric of space, despite the expansion of spacetime increasing the “absolute” distance between the light clocks.
Because we use local rulers and local clocks to measure intervals, (and not absolute measures), this results in the present local measure of distance to equal the measure in the past. If we look back 10 x 10^9 years, we see measures of distance and time as they were and as they are within the observable Universe.

These descriptions of a universe that is actually much bigger in the present but we are unaware of the size since we can only see the size of the Universe as it was in the past do not fully incorporate the structure of Observable space and its relationship to measures of distance and time

3. ZorpFundCEO says:

It may be false to assume the universe is expanding just because the objects in it appear to be flying away from each other. Who’s to say what is happening outside of the observable space…