Every video until
now, we've been working from the assumption
that the observable universe is smaller than the
entire universe. And if you go by the
cosmic inflation theory-- and it was founded by Alan Guth. And I have almost personal
connection to Alan Guth. When I was at MIT, I always
used to go to this Chinese food truck. And I always used to show up at
the food truck like two seconds before Alan Guth. Like he was always one or
two people behind me in line. But anyway, he was the founder
of the cosmic inflation theory, which is basically this idea
that in the very early moments or the very early period
after the Big Bang, we went through
this major inflation in the expansion of space. But anyway, based on the theory
of cosmic inflation, then the observable universe is
on the order of-- or maybe another way to say it,
the entire universe is on the order of--
10 to the 23 times the size of the
observable universe. So that would mean that this
is just a tiny, tiny fraction. I mean, this is an
unimaginable large number. In fact, it is unimaginable. So already everything
we've talked about, this itself is a huge--
this is an incomprehensible amount of space. But this is an
incomprehensible multiple of this incomprehensible
amount of space. And that's just
based on that theory. But it is possible-- we cannot
rule out even the idea-- that the actual universe is
smaller than the observable universe. And that one is
in some ways even more mind blowing than the
idea that the universe is this big, the fact that
what we're observing is actually larger than
the actual universe. And so you might say, well,
Sal, that's impossible. But just think about
it a little bit. This is the observable universe. And the way we've
depicted it, it's based on how long the light
has taken to reach us. We've already covered before
that this point in space is now 46 billion light
years away, not 13.7, the way it looks
right over here. It just took 13.7 billion
years to reach us. If there's any photon that
would take longer than 13.7 billion years to reach us,
it hasn't reached just yet. Because it could have only
started 13.7 billion years ago. So they're on their way. And they started at
some point outside of our observable universe. So our observable universe
will grow over time. But with that
said, let's imagine that the actual
universe is a subset of this observable universe. Let's say it's roughly
half the diameter. So let's say it looks like this. Maybe I'll make it a
little bit of an oval. Maybe the actual
universe-- and this is just to be a little
bit provocative. And it's not impossible. Let's say that this is
the actual universe. And the way I drew
it, it makes it look like Earth is the center. That we're the center of it. But remember, this is very
likely to be the surface-- or it is curved. It has a slight curvature. But it could very
well be the surface of a four-dimensional object. And maybe the simplest
one to visualize is four-dimensional sphere. So if you really
wanted to visualize this right, this whole
volume-- and remember this whole picture, it keeps
looking two-dimensional. But it has depth. It is a volume of space,
an incredibly vast volume of space. And so what I've
done here is this is an ellipsoid right here. It's elliptical volume
of space that I've bubbled out right over here. But if this was really
the entire universe, and if the entire
universe really were the surface of a
four-dimensional sphere, then the reality is
that this entire space could be represented like this. It could be represented
as the surface. If this was a four-dimensional
sphere-- obviously, I can only draw
three-dimensional spheres. But let me show you that
it's not just a circle, that it actually has
some depth to it. And I can even shade
it right over here. And so you can imagine
that this point over here is actually the same thing
as that point over there. That they have wrapped
around and that they're connected right at
the back over here-- let me go behind-- that they're
connected right over there. And that this point
and this point are actually the same point,
that they've wrapped around. Maybe they've wrapped around. Actually, the way I've
drawn it right here, they would actually all
wrap around right back there at that point, if I'm
visualizing properly. But if you go in
any one direction, you would come back on the
other side of the surface. Let's say that
Earth is right here. The way we depicted it,
Earth is the center. But we see that when
you look I like this, there is no center to the
surface of a sphere, even a four-dimensional sphere. So in this sense, if you
go in any one direction, you'll come back
out the other side. So if you start from Earth
and you go in that direction, once you get there,
you're really here again. And then you would
come back to Earth. And so if this were the
case, if the actual volume of the true universe was
smaller than what it looks like, the observable
universe, then what's all this stuff on the outside? And to think about
it, think about what would happen if 13.7
billion years ago, when we were in that primitive
state, where that background radiation, those
photons are being-- those electromagnetic
waves are being released, let's say they get released. And those photons on
their first pass-- and I think you know where this
is going-- on their first pass, they would get to us in about--
this looks like a distance of about, I don't
know, this looks like about 6 billion years. Then they would pass us up. And then they would
get back to this point again in another
6 billion years. And then they would
come back here. And so that very
first past photon are going to be right over here. And from our point
of view, we're not going to see them for
a couple of billion years. And so when we do
see them, we're going to perceive them--
we're going to say, wow, it took 15, 16 billion
years for that photon to get to me. That must be from
something out here. But the reality is it's
a photon from something within a smaller physical
universe, within a smaller actual universe, that's just
taken several passes by us. And we're just seeing a
pass after 14 billion years. We just think it's from
something further out. Now the other thing
is you say, well, if this was the case,
if we could just go in one direction
of the universe and then come out of the
other side, and if all of that was within the
observable universe, wouldn't we be able to tell? Wouldn't we be able to
look in two directions and see the same thing from
a different perspective? And the answer there is to
think about what happens-- or actually, wouldn't we even
be able to see ourselves? Because we emit some light,
and it would take maybe-- I don't know how far that is. Let's say that's 6 or
7 billion light years to get right over here, which
would be right over there. And then it would take another
6 or 7 billion light years to get over there. So maybe that background
radiation we're seeing is actually background radiation
emitted from that exact point in space that we are right now,
or from a very similar point in space, to where
we are right now. Or part of the
background radiation is from a similar point in
space that we are right now. So how come we can't
just see ourselves? Well, I kind of just
answered the question. That second pass, if you're
observing the same point in space, if you're
observing light from the same point in
space on a previous pass, that light was emitted a
long, long, long time ago, maybe 13 billion years ago. And so it would
be unrecognizable. This region of space,
the region of space that we are in right now, if
we saw the same region of space 13 billion years ago, we
just wouldn't recognize it. Now, there are some
people attempting to see if there
are some patterns, see if you can model how
the universe would change and if you see patterns. And maybe the actual universe
is a subset of the observable. We just haven't seen it yet. But it's completely
a possibility. Hopefully, I didn't confuse you. I actually find this kind
of an interesting idea. That this light that
has taken 13-- let's say the light that's taken us
8 billion years to reach us. We think it's from something,
based on this scale, 8 billion light years out. It's actually further because
the universe is expanding. So it would have actually
traversed more space than that. But we think it's from
something like that. But it could've been
something further in, if the actual
universe is smaller. And it's just on
its second pass. It's actually coming back again. And that's why it took 8
billion years to reach us. And we don't even
recognize it because it looks very different than that
region of space right now. Or that region of space
after 4 billion years looks completely different than
it did when it first released. Anyway, hopefully I didn't
confuse you too much. But I think this is a
fascinating, fascinating topic.