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
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
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 takeslight
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 theuniverse
. We even have a distance scale called alight
year, which is the distance thatlight
travels in a year. In more familiar terms, alight
year is nine and a half trillion kilometers or just shy of six trillion miles. We use the unitlight
year because nothing travels faster thanlight
. It’s very convenient. But it is most convenient when we talk about the size of theuniverse
. Astronomers have determined the age of theuniverse
very precisely. It is 13.7billion
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 theuniverse
before it began because, duh- well, that’s what beginning means. When the...
universe
began, it was filled withlight
which then travelled through the cosmos. And, if theuniverse
began 13.7billion
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 theuniverse
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 theuniverse
, 13.7billion
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 13.7billion
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. Theuniverse
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 theuniverse
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
to get here. If we look at a sphere smaller than that special sphere, that
away. Naively, you’d expect that this
to get here, but it took 13.7
- and that’s with a B-
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
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 oflight
and it took 13.7billion
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 thislight
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 millionyears
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 wasonly
42 millionlight
...
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 9billion
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 highlight
s the confusion that arises from expanding space. We see thelight
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 thelight
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 46billion
...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 theuniverse
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 15billion
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 slight
ly later moment that we’ll never see. One day, the expansion of theuniverse
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
, even though it's
old. But we’re not seeing that distant point as it is now, but as it was shortly after the
