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- lightyears 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, lightyears to 46 billion, with a B, lightyears. And this highlights 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 lightyears
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 lightyears
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 slightly
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 lightyears, 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,
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some new tidbit about physics because, of course, physics is everything.
Observable universe. Not universe. For all we know, the universe is infinite. It's certainly much larger than we can observe.
And we can observe further than the amount of time that has passed because the universe is growing, and the furthest observable galaxies are moving about four times the speed of light away from us.
(spacetime itself can expand at any speed. This doesn't violate that objects in spacetime can't travel faster than light. In fact, the galaxies are pretty much stationary relative to the cmb, as are we.)
Edit: I thought it was a question - I didn't realise there was a video answering it, opps.
We are seeing the most distant objects as they looked around 13 Billion years ago (or whenever they formed). Some of their light has already passed us (in the past) and more will reach us in the future, though gradually redshifted.
Because space is expanding it took light a long time to cross the distance between the emitting object and earth. The objects were much closer to earth back then so the light is able to reach us. But a light beam shined towards us now, from 90 Billion light years will never reach us, because the space between us will expand faster than light can cross.
Inflation.