If the universe is only 14 billion years old, how can it be 92 billion light years wide?

When we look into space, we are not

only

looking at things that are very far away, but we are looking at things that existed in the past. For example, the Sun is about 150 million kilometers away and it takes

light

about 8 minutes to get here. So, when we sit down in the morning with some toast and our first cup of coffee of the day and look at the Sun, we are not seeing it as it is now, but as it was when we put the bread in the toaster. . Time and space are inextricably intertwined when we talk about how far away things are.

This is especially true when we talk about large-scale structures of the

universe

. We even have a distance scale called a

light

year, which is the distance light travels in one year. In more familiar terms, a light year is equivalent to nine and a half

billion

kilometers, or just six

billion

miles. We use the unit of light year because nothing travels faster than light. It is very convenient. But it is more convenient when we talk about the size of the

universe

. Astronomers have very precisely determined the age of the universe. It is 13.7 billion

years

old. Exactly how we know this is perhaps the topic of another video, but for the moment, let's take that number as a fact.

If we do, we can ask a very simple question. How big is the visible universe? There are many complicated ways to think about this, but let's start with perhaps the most obvious. We can't see the universe before it began because, well, that's what beginning means. When the universe began, it was filled with light that then traveled through the cosmos. And, if the universe began 13.7 billion

years

ago and we are

only

now seeing it arrive, it had to have traveled 13.7 billion light years before colliding with Earth. And to confirm that point, astronomers can see light shortly after the universe began.

It's called cosmic microwave background radiation and it's the oldest thing we've ever seen. It is hitting the Earth from all directions and is, for all intents and purposes, a snapshot of the birth of the universe, 13.7 billion years ago. Consequently, it would be reasonable to say that the visible universe consists of a sphere, centered on the Earth, with a radius of 13.7 billion light years. Boom! Made. Except this is completely wrong. That's one of the tricky things about science. It can trick you if you think too fast. Sometimes you have to slow down and reflect on things. And, if you think about it for a moment, you'll realize that this way of thinking assumes that the universe is static and, at least on average, the distances between objects don't change.

But we know this is not true. To begin with, the Big Bang occurred. The universe is expanding and it used to expand much faster than it does now. And that means the simple answer is not enough. I don't want to delve too deeply into the details of the Big Bang, but it boils down to the idea that the universe was once smaller and hotter and has been expanding since the beginning. I made a video about it if you want more. And that expansion is key to understanding what is happening here. We start by imagining what the universe was like when this microwave background radiation was emitted.

It was hot everywhere. No place was special. Well, I guess one was, sort of. And that place is the current location of the Earth. But, at that moment, it seemed like anywhere else. Now there was a sphere centered around that point, and at that time, and that sphere is the origin of the cosmic microwave background that now reaches Earth. That radiation moved toward Earth at the speed of light and took 13.7 billion years to get here. If we look at a sphere smaller than that special sphere, that light has already passed through the Earth and we cannot see it.

The light from a larger sphere has not yet arrived, so we do not see it either. When this light was emitted shortly after the Big Bang, the radius of this sphere was about 42 million, that is, with a distance M of light years. You could naively expect that this light would have taken 42 million years to get here, but it took 13.7 billion (and that's with B) years to reach Earth. And the reason it took so long is because space was expanding and is expanding. Now, if you think about that, it means that the space between that sphere and the Earth's location had to expand pretty quickly.

Otherwise, the light would have passed the Earth a long time ago. I mean, it was only 42 million light years and it's been almost 14 billion years. So the sphere from which the microwaves were originally emitted also grew in size. And, in the simplest calculation, that sphere would now be 41 billion, that is, B, light years away. However, it turns out that the simplest calculation is not entirely correct. You see, about five billion years ago, an energy field we call dark energy became important. Dark energy is a repulsive form of gravity, meaning that the expansion of the universe is not slowing down, but rather accelerating.

That, of course, means that after 9 billion years of space expansion slowing, it is now accelerating. If the effect of dark energy is taken into account, the radius of the sphere from which the microwaves were emitted has increased from 42 million, with M, light years to 46 billion, with B, light years. And this highlights the confusion that arises from the expansion of space. We see the light shortly after the Big Bang. It was broadcast a short distance away and now the location it was broadcast from is about eleven hundred times further away than it was. Furthermore, when we see it, we see it as it was then and not as it is now.

In fact, remember that when the light was emitted, the conditions at the sphere's location were the same as here on Earth. But conditions here have changed. Instead of a bath of energy, we now have stars and galaxies. That is also true in that sphere. So if we could somehow see 46 billion light years away, presumably there are stars and galaxies too. But we don't see them there because the light from objects that are currently so far away has not yet had time to reach us. In fact, and this is mind-blowing, we will never see those stars and galaxies.

After all, they are moving away from us very quickly and space continues to expand. We saw that place when the universe was young, but we can't see it now. It gets stranger. Currently, around the Earth there is a sphere with a radius of about 15 billion light years. Objects that are now outside that sphere at this moment we will never see as they look now, no matter how long we wait. I'll repeat it so you can digest it. We can currently see objects 46 billion light years away, but we see them as they were in the distant past.

And any object that is currently within 15 billion light years will be able to be seen as it is now, although we will have to wait a long, long time for that light to reach us. And, due to expansion, the situation is getting worse. It means we are constantly losing stars that we can see. In fact, we lose about 20,000 stars per second. So there are stars that emitted photons at this time that we will eventually see, but the photons that they emitted at this time a little later we will never see. One day, the expansion of the universe will cause almost all the galaxies we see today in our telescopes, which I remind you we see now as they were in the distant past, will disappear from our sight.

Someday we will only be able to see galaxies from our local group, that is, the Milky Way, Andromeda and a few dozen smaller galaxies in the surrounding area. So that's the answer to the question. Our visible universe has a radius of about 46 billion light years, although it is only 13.7 billion years old. But we are not seeing that distant point as it is now, but as it was shortly after the universe began. And the ongoing expansion makes things even worse. The bottom line is that if we're ever going to try to explore other galaxies, we'd better get to work.

Well, that was kind of mind-blowing, but that's cosmology for you. If you liked what you heard, like, subscribe and share. And be sure to click the little bell icon to make sure you get notifications for every new video we post. Otherwise, you will only receive notifications about a few. And I'm sure you don't want to miss any new physics facts because, of course, physics is everything.