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Jens
2011-May-05, 09:07 AM
I may have asked this before, but I don't remember, so here goes. It's a hypothetical question, and I'm not sure how to figure out the answer. In an infinite and eternal universe, where would the "surface of last scattering" be? I assume it would still exist, but I'm not even absolutely sure of that.

astromark
2011-May-05, 09:35 AM
I am a little ( Yes I am Little...)Oops. Jens., Your question confuses me a little.
When talking of the Universe the cosmic microwave background would be the only remnants of the last 'scattering'
Is this what you are asking ?
On a very different level planet earth has many last scatterings... Live down wind of any active volcano and you can see the last scatterings neatly layered in the exposed ground of roading deviations.
Is that what you are asking ?
Your question is a little vague for me as yet...

Jens
2011-May-05, 11:38 AM
Astromark, yes, I mean the cosmic microwave background. I mean to ask, in an infinitely old and vast universe, would there be a CMB, and at what distance?

StupendousMan
2011-May-05, 12:07 PM
In an infinitely old and infinitely large distance, the CMB would be at a distance of infinity, and the radiation from it would have infinitely small flux. In other words, it wouldn't be visible.

ngc3314
2011-May-05, 12:28 PM
But in a sense this is Olber's paradx - the "last scattering surface" in any particular direction would be the photosphere of the first star along that line of sight. For conditions like those in our Universe, I vaguely recall that the typical distance is something absurd-sounding like a quadrillion light-years (and differing hugely from point to point on the celestial sphere).

George
2011-May-05, 12:57 PM
But what dynamics are assumed for this universe? If it is similarly isotropic as ours, wouldn't some regions of last scattering be local -- say from some old red dwarfs? If we are in an oscillating "Honey I Shrunk the Universe, But It Exploded" universe, then we have our normal CMBR.

WayneFrancis
2011-May-05, 03:12 PM
I may have asked this before, but I don't remember, so here goes. It's a hypothetical question, and I'm not sure how to figure out the answer. In an infinite and eternal universe, where would the "surface of last scattering" be? I assume it would still exist, but I'm not even absolutely sure of that.

In space... well its about ~48 billion light years away in all directions. Doesn't really matter if the universe is infinite or finite as, as I understand it, the universe seems to be orders of magnitude large then that.

WayneFrancis
2011-May-05, 03:21 PM
Astromark, yes, I mean the cosmic microwave background. I mean to ask, in an infinitely old and vast universe, would there be a CMB, and at what distance?

Oh I didn't get that. Well in a infinitely old universe if the universe was cyclic then it would appear just as it does. If the universe wasn't cycling from expanding and contracting then I'd say there wouldn't be one. IE If the universe is infinitely old and not cyclic then its anyone guess because at that point it conflicts with observations so then you'd have to define your fictional universe before anyone could give an answer.

caveman1917
2011-May-05, 05:31 PM
Well, if it is infinitely old in the sense of not expanding, then its energy density was always the same as now, so there is no surface of last scattering, no CMBR. There would have never been any scattering in the first place as the relevant photon-baryon plasma requires a high density/temperature.

Jens
2011-May-06, 11:56 AM
The thing that puzzles me is, I've heard a comparison where there is a "surface of last scattering" of sound on the earth, where you can't make out individual sounds because the sounds interfere with one another. But the earth isn't expanding. Is there something fundamentally different between sound and light, where light would not make such a surface even at an infinite distance?

Strange
2011-May-06, 02:22 PM
I have heard an analogy to explain the surface of last scattering using sound. It went something like: imagine a very large crowd of people all shouting; at some point everyone stops but the sound continues (gradually dieing away) as you hear the noise from further and further away. (I was quite impressed with the image at the time, but can barely remember it now!)

Jeff Root
2011-May-06, 06:01 PM
My impression is that there are some similarities between the
Universe's surface of last scattering and this surface of last
scattering of sound that you describe, but the origins have
nothing in common. Your original question seems to be based
on the assumption that the two also have similar origins.

The origin of the surface of last scattering for sound is the
distance that different sounds can travel through air without
becoming indistinguishable from one another, while the origin
of the surface of last scattering of the CBR is the change
from an opaque plasma to transparent gas as hydrogen
atoms formed and lost their ionization. Without the change
from a dense, hot, opaque, ionized plasma to a thin, cold,
transparent, neutral gas, there would be no cosmic surface
of last scattering.

As for your question about a difference between sound and
light, sound consists of molecules bumping into each other
to pass on kinetic energy from one molecule to the next, so
the energy gets spread around and mixed up, while light goes
in straight lines and doesn't bump into other light. It will, of
course, get spread around and mixed up when it bumps into
molecules, which it was doing when the Universe was filled
with plasma.

-- Jeff, in Minneapolis