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EI2GYB > ASTRO 23.11.25 17:43l 75 Lines 7496 Bytes #999 (0) @ WW
BID : 47854_EI2GYB
Read: GUEST
Subj: Is the Universe Infinite?
Path: IZ3LSV<IW8PGT<HB9CSR<IK6IHL<IK7NXU<HB9ON<DK0WUE<PI8ZTM<VK2IO<GB7BED<
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Sent: 251123/1243Z 47854@EI2GYB.DGL.IRL.EURO LinBPQ6.0.25
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Is the Universe Infinite?
The surface of the Earth is finite. We can measure it. If it was expanding, then its size would grow with time. And once again, good ol' Earth helps us understand what the universe might be doing beyond our observable horizon.
Our best assumption of what goes on beyond the horizon is that there's just more stuff. More stars, more galaxies, more AI-generated cat videos. Just like we assume that beyond our horizon on the Earth there's more.Earth.
So how big is the universe? Like, in total, even beyond what we can see? Well the truth is we'll likely never know. The observable limit is just that - a limit. It's not just a limit to what we can see. It's a limit to what we can know. There's a total amount of information contained in the universe, and a finite amount of information that we could ever hope to receive even in the infinite future.
All we can do is guess.
It's totally possible that the universe is infinite. It just goes and goes and goes without end, forever.
But it's also possible that it's finite. But how can a finite universe still not have an edge? Well, how can the surface of the Earth be finite and yet not have an edge?
Yes, it has an edge in the third dimension - we call it outer space. But again, that's cheating! The two-dimensional surface is both finite and borderless, and it accomplishes that seemingly paradoxical feat by being curved.
We know the surface of the Earth is curved. We can measure it without our feet ever leaving the ground. In mathematics, we can build a few tools to give us a clue as to the geometry of the Earth. One tool is triangles. On a perfectly flat plane, when you draw a triangle the interior angles add up to 180 degrees. Thank you, Euclid. But if you were to bust out a giant marker, pick three random cities, and draw giant lines connecting them, you would end up with a triangle with interior angles greater than 180 degrees.
The other is through parallel lines. On a flat plane, parallel lines never intersect. But on curved surfaces they do. If you and me start and the equator and follow straight lines moving north, we will eventually intersect at the north pole. Not because we turned, but because the Earth curved underneath us.
We can play the same games in the universe. We look at the light from the very early universe, from a special event when the cosmos cooled from a hot, dense plasma and released a flood of radiation, known as the cosmic microwave background, or CMB. The physics of that plasma is actually pretty straightforward (we have a decent understanding of plasmas here on Earth), and we know from our calculations that there should be slight variations in temperature from place to place across the CMB. And wouldn't you know it, there are!
Plus, we can calculate how big those splotches ought to be. If the universe is curved, then the path of light should have bent as it traveled all those billions of light-years. We then compare how big they are to how big we expect them to be. If they are different sizes, then we know the universe is curved.
They're exactly the size we expect them to be. And that's how we know the universe is flat.
Case closed? The universe is infinite? Not so fast.
If we were to attempt to measure the curvature of the Earth from, I don't know, your neighborhood, we wouldn't have much success. If your triangles are too small or parallel lines too short, then you won't be able to get a sense of the overall curvature. We are limited in our measurements to our observable bubble. And within that bubble everything seems as flat as flat can be.
So maybe the universe is curved, but on much, much larger scales than our tiny little observable patch (I know that tens of billions of light-years isn't exactly tiny, but it is compared to how big the universe COULD be).
It's entirely possible that the universe curves back on itself. That would mean you could travel in one direction long enough and eventually reach your starting point, just like on the Earth. But you would have to travel beyond the horizon, which in an expanding universe is impossible, so this is only possible as a theoretical exercise.
And you know what's really wild? I promise this is the last piece of forbidden chocolate. The universe could be flat and STILL be curved. Check it out. Take a flat piece of paper and draw some triangles and parallel lines on it. Now bend it into a cylinder. Those triangles are still triangles and those parallel lines are still parallel.
This is the difference between geometry and topology. The geometry of the universe appears to be flat. But one or more dimensions could be closed, meaning they wrap around while still maintain geometric flatness. And it can get weirder. A Mobius strip is just a cylinder with a rotation made before the ends connect up. A Klein bottle is just a donut with a rotation. A cylinder, a donut, a Mobius strip, and a Kelin bottle are all geometrically flat.
In three dimensions there are 17 known distinct topologies that are all geometrically flat. My favorite being, of course, Hansc-Wendt space, which involves hexagonal tilings of the same pattern.
We've searched for closed topologies. We've looked for intersection points in the cosmic microwave background, or galaxies that appear on opposite sides of the sky. As far as we can tell, the universe is both flat and simple, meaning none of the dimensions wrap around on themselves. But again, there's a limit to what we can see, so we may never know for sure.
And I haven't even gotten started on the multiverse, where our universe, even beyond the observable limit, is just one bubble amongst a potential infinity of other bubbles, all expanding away from each other and spawning new big bangs in the spaces between, but.I think that's enough for today.
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