2 ms·

Flat, as far as WMAP can tell! https://wmap.gsfc.nasa.gov/universe/uni_shape.html https://wmap.gsfc.nasa.gov/universe/uni_shape.html Of course the curvature c

Flat, as far as WMAP can tell! https://wmap.gsfc.nasa.gov/universe/uni_shape.html https://wmap.gsfc.nasa.gov/universe/uni_shape.html Of course the curvature could maybe be arbitrarily small, but... it's definitely very close to flat.

jazzyjackson 8y ago
Hold up, so the flat-earthers just need to think a little bigger? (Legit tho I've been trying to understand the 'shape' of the universe for a while now -- in the sense of, when I look at stars and galaxies in the sky above me, what direction am I looking as it relates to where things are in the universe relative to each other? As a kid I took Bill Nyes word for it: everything is on the surface of a balloon, expanding from the center. But do I ever see the far side of the balloon? Or is everywhere I look somehow constrained to the surface in every direction away from me, such that the 'other side of the balloon' is infinitely far away... but if I could see far enough, my own spot on the surface of this sphere would be visible to me, minus a few billion years... I want to understand this but I am very confused! This looks like an informative website so I'll keep reading ... thank you)
- roywiggins 8y ago
  The balloon-surface model works if the universe is actually positively curved. It bends around in on itself and comes back. So you would be able to see yourself (eventually) after light made its way around the surface and back to you. As far as anyone can tell, though, the universe is flat. So photons traveling outwards from you will never come back around- they'll just keep going. I'm partial to the raisin bread model of the universe: https://www.loc.gov/rr/scitech/mysteries/universe.html https://www.loc.gov/rr/scitech/mysteries/universe.html It's a tad misleading: the raisin bread is, probably, infinite in extent. As a practical matter, we can only see a finite portion of the raisin bread. As we gaze more deeply into the sky, we're looking further into the raisin bread. Because light only travels at a finite speed, the further something is away, the older the image we see of it is. At a certain point, all we can see is the goopy bread batter that the raisin bread used to be: that's (sort of, kinda) the Cosmic Microwave Background: it's the oldest light in the universe, and we can't see anything older than it. So the universe looks like a bunch of galaxies, with us at the center, and a sphere of microwave radiation at the edge. But every single observer sees a similar sphere around their exact location, so it's a kind of illusion.
  - eridius 8y ago
    Does the expansion of the universe mean that galaxies that are too far away from us are seemingly moving faster than the speed of light, and this is why we can't see any galaxies past a given point (because that's where they "accelerate" faster than light)?
    - roywiggins 8y ago
      A lot of the visible universe actually appears to be receding faster than the speed of light: https://phys.org/news/2015-10-galaxies-faster.html https://phys.org/news/2015-10-galaxies-faster.html It's because the space inbetween is getting bigger. But light keeps moving towards us from those galaxies, so we see those photons eventually anyway. You can still hear things that are going above the speed of sound, as long as they're headed away from you- if they're headed toward you, you'll not hear them (probably?). But the waves they throw off will expand behind them just fine.
      - acqq 8y ago
        > if they're headed toward you, you'll not hear them (probably?) Actually, you would not have much time to hear, but what you would hear (among the other effects) would be the sound “in reverse”(!) https://physics.stackexchange.com/questions/14880/hearing-a-sound-backwards-because-of-doppler-effect https://physics.stackexchange.com/questions/14880/hearing-a-...
        rcxdude 8y ago
        For a practical (if dangerous!) example, see supersonic rounds: if one is shot towards you, you hear the bullet whizzing by before you hear the gunshot.
        LgWoodenBadger 8y ago
        Well, a supersonic round wouldn't whizz until it drops down to subsonic speeds. You'd hear the crack of the round then the crack from the original shot.

jazzyjackson 8y ago

Hold up, so the flat-earthers just need to think a little bigger? (Legit tho I've been trying to understand the 'shape' of the universe for a while now -- in the sense of, when I look at stars and galaxies in the sky above me, what direction am I looking as it relates to where things are in the universe relative to each other? As a kid I took Bill Nyes word for it: everything is on the surface of a balloon, expanding from the center. But do I ever see the far side of the balloon? Or is everywhere I look somehow constrained to the surface in every direction away from me, such that the 'other side of the balloon' is infinitely far away... but if I could see far enough, my own spot on the surface of this sphere would be visible to me, minus a few billion years... I want to understand this but I am very confused! This looks like an informative website so I'll keep reading ... thank you)

roywiggins 8y ago
The balloon-surface model works if the universe is actually positively curved. It bends around in on itself and comes back. So you would be able to see yourself (eventually) after light made its way around the surface and back to you. As far as anyone can tell, though, the universe is flat. So photons traveling outwards from you will never come back around- they'll just keep going. I'm partial to the raisin bread model of the universe: https://www.loc.gov/rr/scitech/mysteries/universe.html https://www.loc.gov/rr/scitech/mysteries/universe.html It's a tad misleading: the raisin bread is, probably, infinite in extent. As a practical matter, we can only see a finite portion of the raisin bread. As we gaze more deeply into the sky, we're looking further into the raisin bread. Because light only travels at a finite speed, the further something is away, the older the image we see of it is. At a certain point, all we can see is the goopy bread batter that the raisin bread used to be: that's (sort of, kinda) the Cosmic Microwave Background: it's the oldest light in the universe, and we can't see anything older than it. So the universe looks like a bunch of galaxies, with us at the center, and a sphere of microwave radiation at the edge. But every single observer sees a similar sphere around their exact location, so it's a kind of illusion.
- eridius 8y ago
  Does the expansion of the universe mean that galaxies that are too far away from us are seemingly moving faster than the speed of light, and this is why we can't see any galaxies past a given point (because that's where they "accelerate" faster than light)?
  - roywiggins 8y ago
    A lot of the visible universe actually appears to be receding faster than the speed of light: https://phys.org/news/2015-10-galaxies-faster.html https://phys.org/news/2015-10-galaxies-faster.html It's because the space inbetween is getting bigger. But light keeps moving towards us from those galaxies, so we see those photons eventually anyway. You can still hear things that are going above the speed of sound, as long as they're headed away from you- if they're headed toward you, you'll not hear them (probably?). But the waves they throw off will expand behind them just fine.
    - acqq 8y ago
      > if they're headed toward you, you'll not hear them (probably?) Actually, you would not have much time to hear, but what you would hear (among the other effects) would be the sound “in reverse”(!) https://physics.stackexchange.com/questions/14880/hearing-a-sound-backwards-because-of-doppler-effect https://physics.stackexchange.com/questions/14880/hearing-a-...
      - rcxdude 8y ago
        For a practical (if dangerous!) example, see supersonic rounds: if one is shot towards you, you hear the bullet whizzing by before you hear the gunshot.
        LgWoodenBadger 8y ago
        Well, a supersonic round wouldn't whizz until it drops down to subsonic speeds. You'd hear the crack of the round then the crack from the original shot.