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

When we look out into space, we are not

only

looking at things that are far away, we're looking at things that existed in the past. For instance, the Sun is about 93 million miles away and it takes

light

about 8 minutes to get here. Thus, when we sit down in the morning with some toast and our first cup of coffee for the day and look at the Sun, we’re seeing it not 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 that

light

travels in a year. In more familiar terms, a

light

year is nine and a half trillion kilometers or just shy of six trillion miles. We use the unit

light

year because nothing travels faster than

light

. It’s very convenient. But it is most convenient when we talk about the size of the

universe

.

Astronomers have determined the age of the

universe

very precisely. It is 13.7

billion

years

old. Exactly how we know this is perhaps the subject of another video, but for the moment, let’s take that number as a given. If we do, we can ask a very simple question. How big is the visible

universe

? There are lots of tricky ways to think about that, but let’s start with perhaps the most obvious. We can’t see the

universe

before it began because, duh- well, that’s what beginning means. When

the

universe

began, it was filled with

light

which then travelled through the cosmos. And, if the

universe

began 13.7

billion

years

ago and we’re just now seeing it arrive, it had to have traveled 13.7

billion

light

years

before it hit Earth. And to make that point solid, astronomers can actually see

light

from shortly after the

universe

began. It’s called the Cosmic Microwave Background radiation and it’s the oldest thing we’ve ever seen. It is hitting the Earth from every direction and

it is, to all intents and purposes, a photograph of the birth of the

universe

, 13.7

billion

years

ago. Accordingly, 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! Done. Except that this is completely wrong. That’s one of the sneaky things about science. It can fool you if you think too fast. Sometimes you have to slow down and mull things over. 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, distances between objects aren’t changing. But we know this isn’t true. To begin with, the Big Bang happened. The

universe

is expanding and it used to be expanding much faster than it is now. And that means that the simple answer isn’t good enough. I don’t want to get 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 going on here. We start by imagining what the

universe

looked like when this microwave background radiation was emitted. It was hot everywhere. No place was special. Well, I guess one was, kind of. And that place is the current location of the Earth. But, at that moment, it looked like everywhere 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 arriving at the Earth now. That radiation moved toward the Earth at the speed of

light

and it took 13.7

billion

years

to get here. If we look at a sphere smaller than that special sphere, that

light

has already passed the Earth and we can’t see it.

Light

from a bigger sphere isn’t here yet, so we don’t see it either. When this

light

was emitted shortly after the Big Bang, this sphere’s radius was about 42

million- that’s with an M-

light

years

away. Naively, you’d expect that this

light

would have taken 42 million

years

to get here, but it took 13.7

billion

- and that’s with a B-

years

to travel to Earth. And the reason that it took so long is that space was and is expanding. Now, if you think about that, it means that space between that sphere and the location of the Earth had to be expanding pretty fast. Otherwise the

light

would have passed by the Earth long ago. I mean, it was

only

42

million

light

years

and it has been nearly 14

billion

years

. So that 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’s with a B-

light

years

away. However, it turns out that the simplest calculation isn’t quite right. You see, about five

billion

years

ago, an energy field that we call dark energy became important. Dark energy is a repulsive form of gravity, which means that the expansion of

the

universe

isn’t slowing down, it’s accelerating. That, of course, means that after 9

billion

years

of the expansion of space slowing down, it’s now speeding up. When you take into account the effect of dark energy, that radius of the sphere from which the microwaves were emitted has grown from 42 million, with an M,

light

years

to 46

billion

, with a B,

light

years

. And this high

light

s the confusion that arises from expanding space. We see the

light

from shortly after the Big Bang. It was

emitted a short distance away and now the location from which it was emitted is now about eleven hundred times farther than it was. Further, when we see it, we see it as it was then and not as it is now. Indeed, remember that when the

light

was emitted, the conditions at the location of the sphere were the same as here on Earth. But the conditions here have changed. Instead of a bath of energy, we now have stars and galaxies. That’s also true on that sphere. So, if we could somehow see 46

billion

light

years

away, presumably there are also stars and galaxies. But we don’t see them there because

light

from objects that are currently that far away now hasn’t had time to reach us yet. In fact- and this is a mind blower- we’ll never see those stars and galaxies. After all, they are moving away from us very fast and space is still expanding. We saw that location when the

universe

was young, but we can’t see them now. It gets weirder. Currently, there is a sphere around the

Earth with a radius of about 15

billion

light

years

. Objects that are now outside that sphere at this moment we’ll never see as they look now, no matter how long we wait. I’ll repeat that so it can sink in. We can currently see objects 46

billion

light

years

away, but we see them as they were in the distant past. And any objects that are currently within 15

billion

light

years

we will be able to as they are now, although we’ll have to wait a long, long time for that

light

to get to us. And,

because of the expansion, it gets worse. It means that we constantly lose stars that we can see. Indeed, we lose about 20,000 stars per second. So there are stars that emitted photons at this moment that we will eventually see, but the photons that they emitted at this s

light

ly later moment that we’ll never see. One day, the expansion of the

universe

will make it so that almost all of the galaxies we see in our telescopes today, which I remind you now we’re seeing as they were in the distant

past, will slip from our view. We will one day

only

be able to see galaxies from our local group, meaning the Milky Way, Andromeda, and a few dozen minor galaxies in the vicinity. So that’s the answer to the question. Our visible

universe

has a radius of about 46

billion

light

years

, even though it's

only

13.7

billion

years

old. But we’re 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 better get cracking. Okay, so that one was something of a mindblower, but that’s cosmology for you. If you liked what you heard, please like, subscribe and share. And be sure to click on the little bell icon to make sure that you get notified of every new video we post. Otherwise you

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get notified about a few. And I’m sure you wouldn’t want to miss some new tidbit about physics because, of course, physics is

everything.