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Professor Brian Cox is an English physicist and Professor of Particle Physics in the School of Physics and Astronomy at the University of Manchester in the UK.

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Now why is it that we think that the known universe is larger than we can observe? Well, one point is that it's expanding and we always see the same radiation out there, so the glow of the Big Bang. But there are some deeper reasons. The one from the theory of inflation, the best way to explain the universe, the properties that we see, is that it's very much bigger than the piece we can see. So for example, we measure space to be what's called flat. I don't even have to say what's called flat. It is flat. So if you imagine slices of space, let's imagine slices of them at different times. So you just slice the universe and say there's a big sheet, like this table. There's a sheet of space, there's another sheet, another sheet. And it can have a geometry, right? It can be flat like a tabletop, or it could be curved like a sphere, or it could be curved in the opposite direction, sort of like a saddle or a bowl. And we can measure that. And when we measure it, we see it's absolutely flat. And that's a very unusual thing for it to be like. It requires, because what Einstein's theory says is that the shape of space, that the curvature of space is determined by the stuff that's in it. That's basically Einstein's theory of general relativity. Put stuff in space and it curves it and bends it and warps it and stretches it and so on. And what we find is that there's precisely the right amount of stuff in the universe to have a completely flat universe. And the explanation, the most favored explanation for that is the universe is way bigger than the piece we can see. And so it's like looking at a piece of the earth. If you look at a little one mile square of the earth, then it's flat. You have to look at big distances, kind of a vaudevoir to the radius of the earth, or not, you know, bigger than one kilometer anyway, or one mile, to see that actually you're on a curved surface. And that's one of the ideas about the universe and why it appears to be the way that it is, because it's way, way bigger. So we're just looking at a little piece, and that's why it looks flat. And that's one of the ideas. Now, when you say flat, like that, my brain doesn't understand this, because from our perspective, when you look up at the Milky Way, you see all these stars all over the place. So if you're saying flat, like how much height and what are you saying? So in terms of like the way to measure it. The best way to think about it is not to think of three dimensions of space, because then we can't picture it. But you can think of two, like this tabletop, and that's all right, we just forget the other one for now. And so you know what flat is on this table. I mean, you could define it. So you could say, for example, that if I draw a triangle on the top of the table, then all the angles add up to 180 degrees. So that actually defines flat. If you did that on the surface of the earth with a big triangle, then the angles wouldn't add up to 180 degrees. Or you could draw a circle and say, what's pi? So pi is the ratio of the circumference of a circle to its diameter. That's only true on a flat surface. It's different if the surface is curved. So you can define flatness. So when you're saying flatness, what is the height and what is the width? Like if you're talking about it as if it's a table, there must be some sort of a, there's a dimension to it, correct? Oh, yeah, there's a third dimension of space. But the same applies. It's just a generalization of geometry then. So you can pick the point is we can picture it in two dimensions. But you can you can draw, you can quite literally you can imagine sending light beams out. And we do this measurement, actually, we can look at the the the most distant light we can see, which is something called the cosmic microwave background radiation, which is, if you imagine looking out, if you look at the Andromeda galaxy, which we can see with the naked eye here in LA, you can see that it's the most distant object you can see with the naked eye. And it's about two, 2 million light years away or so, which means the light took 2 million years to get to us. So it's a long way away, but it's very big. So as you look further out into the universe, the more and more distant galaxies, you're looking further back in time, because you look at something that's a billion light years away, then the light took a billion years to get to us. So you see as it was a billion years in the past. And we can actually look so far out that we can see almost back to 13.8 billion years ago, which is very close to the Big Bang. So we can look to light that began its journey before there were galaxies. And that's the oldest light in the universe, which is, by the way, one of the one of the pieces of evidence from people say I don't believe in the Big Bang. The answer is, well, you can see it. So it's just there. You can see it. We have pictures of it. And that light, it turns out that there are structures or ripples in that light, which we can use as a ruler. So quite literally as a ruler on the sky. And then because that light has been traveling through the universe, we can see how that rule has been distorted as the light has traveled through space. And so we can infer where the space is flat or curved or how it's warps, if you like, just from that measurement.