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Also, despite the age of the universe (13.8B years), the diameter of the observable universe is 93 billion light years. That's because the universe expanded (an
by namirez 7y ago
Also, despite the age of the universe (13.8B years), the diameter of the observable universe is 93 billion light years. That's because the universe expanded (and probably still is expanding) faster than light.
- NetOpWibby 7y agoThat’s fascinating to think about.
- calvertdw 7y agoAs observed, it's only 13.8, though, right?
- kijin 7y agoNo, it's observed as 93 billion light years. That's why we call it the "observable" universe. There is no meaningful spatial boundary at a point that is currently at a distance of 13.8 billion light years from us. We can detect (severely red-shifted) photons from beyond that point perfectly fine! The actual boundary is 46.5 billion light years away in both directions, hence the "observable" diameter of 93 billion. But of course the meaning of "observed" is kinda strange here, since we're not directly measuring the distance but estimating it based on from other observations.
- Udik 7y agoWhere I supposed that "observed as <distance>" just means how much red-shifted its light is- we don't really have any other way to measure such distances. This in turn only measures how much the space has expanded between us and the original starting point, much closer than 13 billion light years away. So we can say that we're seeing a star that is "now" 46 billion ly from us, but must have been only a few billion ly away when it emitted the light we're receiving. Correct?
- kijin 7y agoYou're right. The star could not have been more than a few billion light years away when it emitted the photons that we're seeing. Otherwise, not only would the photons have not reached us yet, they will probably never reach us because the universe is expanding faster than the speed of light.
- pkofod 7y agoI feel so stupid when talking and hearing about physics... but we are not in any meaningful way "at the center" of the universe are we? Or is every point in some sense at the center (a point of reference thing)? I'm asking because why would it be 46.5 "each way"?
- spc476 7y agoImagine we exist in a 2D universe, but one that happens to be the surface of a sphere. Any point you pick on that surface is "at the center of the universe."
- eru 7y agoHmm, but it looks like the universe is flat, doesn't it? (Or is it flat like a torus, so doesn't need any curvature to go loop on itself?)
- kijin 7y agoCosmologists talk about "horizons" a lot, and the analogy of standing on a sphere actually works quite well. Remember that horizons only make sense on curved surfaces. You can't see beyond the point where certain features of spacetime (black holes, expansion, or sheer distance) prevent signals from reaching you, just as you can't see beyond a mountain range or the curvature of Earth itself. Of course you'll need to extrapolate the analogy to three, four, or more dimensions, but the basic idea is the same.
- eru 7y agoI'm not talking about our own light cone. I'm talking about https://en.wikipedia.org/wiki/Shape_of_the_universe https://en.wikipedia.org/wiki/Shape_of_the_universe Basically, ignoring wormholes and black holes and assuming that spacetime is locally flat everywhere and it's mathematically a manifold, my question is: what's the shape of the (global) universe? Global as opposed to observable. So we might have a hard time answering that question. How would you be able to distinguish between the (n-dimensional equivalent of) a torus vs a flat infinite space, if you can't see the repetition? You'd even have a hard time distinguishing a hypersphere from a flat infinite space, if the hypesphere was big enough so that we can't tell it's curvature apart from no curvature. Or the universe might be weirdly shaped, and we just happen to live in the flat part. So I guess the question comes down to: * assuming no edges * assume Copernicus at least for space (we might have a special position in time) * What's the simplest theory about the shape of the global universe that satisfies our observations? I suspect general relativity toys around with such questions, because I know that they sometimes look at cosmological (toy) models for the whole universe, and not just what's in the light cone of one particular observer.
- laurencei 7y agoCan you ELI5 - how can we observe something 93 billion light years away, if the light has only been traveling for 13.8B years? Shouldnt the "observable" distance be the the age of the universe?
- devnulloverflow 7y agoYour intuition that there is an limited observable distance because is correct. But because of the expansion, that limit is bigger than "speed_of_light * age of universe" . I don't know enough General Relativity to give a solid explanation. But here is a rough over-simplification: Imagine you've spent the last 5 seconds blowing up a balloon, but there is an ant walking on that balloon, starting from a marked spot. In the 1st second, the ant moved 10mm. But in the next four seconds you blew up the balloon, so that 10mm of rubber is now 70mm long. The total distance from the ant's starting mark to her end-point can now be well over than 70mm, even though she only walks at less than 10mm/second. As the ant, so the photon.
- laurencei 7y agoYep - I kind of understand that. Thanks.
- sytelus 7y agoI'm bit confused. Regardless of expansion, these objects are right now > 13.8B ly away. So the light from them still needs that much time to get here. If light from them has arrived sooner, we would see less redshift and perceive them as a closer. No?
- rokalakt1337 7y agoSo does this mean speed faster than light is possible?
- zaarn 7y agoTwo objects can move away from eachother faster than the speed of light as seen by a third observer. The space between objects can also expand faster than light, the object itself can still not move faster than light. If the space between two objects is expanding faster than light then for all intents and purposes, the other objects no longer exists to another. There is no way to contact or observe that object other than the light that was send your way before space went FTL.
- eru 7y agoTo give another example: If you have a laser pointer and flick your wrist fast enough, the spot on the ground that your cat chases can move faster than light without violating any laws of physics. That's perhaps easier to see, if you think of a giant laserpointer aimed at the moon. Similarly, a shadow on the wall can move faster than the speed of light.
- Aenyn 7y agoWouldn't there be a delay between the time you flicked your wrist and the time the spot actually aligns, due to the limited speed of light?
- doliveira 7y agoYeah, and observing this is a good way to see that information isn't actually being transferred faster than light.
- eru 7y agoYep, definitely. An outside observer that just sees the spot on the moon (but has no clue that it's produced by you flicking your wrists), just sees a spot that moves insanely fast. No information, energy nor matter travels faster than light here, of course.
- eecc 7y agoAah of course... somehow we should believe that at some stage something accelerated to an impossible - according to all current observation - speed and then decelerated, somehow dissipating this energy somewhere. Just to fit a bunch of observations that wouldn't otherwise make sense for the current model. I remember the story of a chap from Pisa having a hard time trying to budge a bunch of clerics to consider his observations proof that their model was broken. ;)