7 ms·

Even if there wasn’t a Big Bang, let’s say the universe was infinite, we would still have the CMB from the furtherest reaching photons from the edges of the vis

Even if there wasn’t a Big Bang, let’s say the universe was infinite, we would still have the CMB from the furtherest reaching photons from the edges of the visible universe that have been stretched to microwaves right? In other words eventually the nearest galaxies will one day become the CMB as the universe continues to expand until they are on the edge, until even they fall outside the visible universe and no photons/light outside our own galaxy will be capable of reaching us and the CMB disappears.

cyphar 5y ago
That's not what the CMB is. The CMB is an incredibly uniform pattern which appears to have been formed by a very hot plasma at some point in the past. It's difficult to imagine an explanation other than the Big Bang (or some kind of expansion of a very hot plasma) because the uniformity means it was almost certainly concentrated in space. But yes, in the distant future, it will not be possible to detect the CMB anymore (and it's entirely possible that alien civilisations that evolve at that time would probably never have a model of the Big Bang because there would be no remaining evidence of it).
- cheese_van 5y ago
  ...and the galaxies will have drifted so far apart, that nothing will appear to exist beyond their own galaxy, except dust.
- throwawaycities 5y ago
  > That's not what the CMB is. The CMB is an incredibly uniform pattern which appears to have been formed by a very hot plasma at some point in the past. I understood that the CMB is not photons from plasma but the first photons following recombination (when the universe cooled enough that electrons and protons formed first hydrogen atoms). In other words the universe was plasma just before the CMB.
  - cyphar 5y ago
    Yes, you're quite right -- sorry for not being precise enough. However my point was that the fact that we cannot see beyond the CMB and the uniformity of the pattern both point towards a Big Bang-esque explanation.
XorNot 5y ago
The problem in an infinite universe is every possible location in the sky eventually terminates on a stellar surface - a star. So the night sky, if the universe is infinite, wouldn't be dark - it would be as bright as the sun - in fact the universe would be full of omnidirectional radiation coming from all directions at all times. I suppose though, that if space is still expanding but the universe is infinite, this might temper it out but it doesn't seem like enough - it would have to be an expansion precisely tuned to on average send radiation to 4 kelvin so we only see a cosmic microwave background, and don't wind up being bathed in an infinite amount of whatever frequency of radiation. Infinity is funny like that.
- Gupie 5y ago
  > The problem in an infinite universe is every possible location in the sky eventually terminates on a stellar surface - a star. Isn't that only true if the universe is infinitely old as well as infinitely large?
  - XorNot 5y ago
    Sure, but postulating an infinitely sized universe (full of an infinite amount of mass) does ask the question as to why the universe would not be infinitely old? Particularly when you get into issues like entropy, which is the only real determinant of time even existing. A finite universe has a natural direction of entropy - whereas an infinite one does not, since there's an infinite amount of mass and energy and as such no possible lowest possible entropy state. It's actually worse then that though: an infinite massed universe by definition would contain every possible configuration of that mass somewhere within it. So you and I talking right now like this, our past and future conversations would also all be somewhere else in the infinite universe happening simultaneously. A universe with an infinite amount of mass and energy in infinite space doesn't really have any sensible notion of past, present or future - because all possible pasts, presents and future, exist at all times somewhere within it.
    - gilbetron 5y ago
      > A universe with an infinite amount of mass and energy in infinite space doesn't really have any sensible notion of past, present or future - because all possible pasts, presents and future, exist at all times somewhere within it. This is actually untrue for many reasons, but one is that there are different kinds of infinities. For instance, there are an infinite amount of real numbers between 0.0 and 1.0, and none of them are pi. Just because something is infinite, doesn't mean everything is possible within that infinity. Again, there are an infinite number of integers, but none of the are pi or 0.1 or 37.5

cyphar 5y ago

That's not what the CMB is. The CMB is an incredibly uniform pattern which appears to have been formed by a very hot plasma at some point in the past. It's difficult to imagine an explanation other than the Big Bang (or some kind of expansion of a very hot plasma) because the uniformity means it was almost certainly concentrated in space. But yes, in the distant future, it will not be possible to detect the CMB anymore (and it's entirely possible that alien civilisations that evolve at that time would probably never have a model of the Big Bang because there would be no remaining evidence of it).

cheese_van 5y ago
...and the galaxies will have drifted so far apart, that nothing will appear to exist beyond their own galaxy, except dust.
throwawaycities 5y ago
> That's not what the CMB is. The CMB is an incredibly uniform pattern which appears to have been formed by a very hot plasma at some point in the past. I understood that the CMB is not photons from plasma but the first photons following recombination (when the universe cooled enough that electrons and protons formed first hydrogen atoms). In other words the universe was plasma just before the CMB.
- cyphar 5y ago
  Yes, you're quite right -- sorry for not being precise enough. However my point was that the fact that we cannot see beyond the CMB and the uniformity of the pattern both point towards a Big Bang-esque explanation.

XorNot 5y ago

The problem in an infinite universe is every possible location in the sky eventually terminates on a stellar surface - a star. So the night sky, if the universe is infinite, wouldn't be dark - it would be as bright as the sun - in fact the universe would be full of omnidirectional radiation coming from all directions at all times. I suppose though, that if space is still expanding but the universe is infinite, this might temper it out but it doesn't seem like enough - it would have to be an expansion precisely tuned to on average send radiation to 4 kelvin so we only see a cosmic microwave background, and don't wind up being bathed in an infinite amount of whatever frequency of radiation. Infinity is funny like that.

Gupie 5y ago
> The problem in an infinite universe is every possible location in the sky eventually terminates on a stellar surface - a star. Isn't that only true if the universe is infinitely old as well as infinitely large?
- XorNot 5y ago
  Sure, but postulating an infinitely sized universe (full of an infinite amount of mass) does ask the question as to why the universe would not be infinitely old? Particularly when you get into issues like entropy, which is the only real determinant of time even existing. A finite universe has a natural direction of entropy - whereas an infinite one does not, since there's an infinite amount of mass and energy and as such no possible lowest possible entropy state. It's actually worse then that though: an infinite massed universe by definition would contain every possible configuration of that mass somewhere within it. So you and I talking right now like this, our past and future conversations would also all be somewhere else in the infinite universe happening simultaneously. A universe with an infinite amount of mass and energy in infinite space doesn't really have any sensible notion of past, present or future - because all possible pasts, presents and future, exist at all times somewhere within it.
  - gilbetron 5y ago
    > A universe with an infinite amount of mass and energy in infinite space doesn't really have any sensible notion of past, present or future - because all possible pasts, presents and future, exist at all times somewhere within it. This is actually untrue for many reasons, but one is that there are different kinds of infinities. For instance, there are an infinite amount of real numbers between 0.0 and 1.0, and none of them are pi. Just because something is infinite, doesn't mean everything is possible within that infinity. Again, there are an infinite number of integers, but none of the are pi or 0.1 or 37.5