# What If The Universe Had a "Scattering Layer"?

Suppose the universe is closed, and its edges are bounded by a layer of plasma that reflects light perfectly. Would the universe then appear (to light and radio wave astronomers) as infinite? What would the (reflected ) light from receding galaxies look like?

I think it would appear infinite. The reflected light would look like additional stars. You can see the same effect if you’ve ever looked into a mirror that has another mirror behind it. What you see is infinite copies of yourself receeding into the distance.

In a similar vein, I’ve sometimes imagined that the Universe wraps around, as if all of space was projected onto the surface of a sphere. If you travelled away from Earth (or any other starting point) for long enough, you’d eventually come back to Earth again from the opposite direction. To any observer inside the Universe, the Universe would appear to be infinite.

I think it would depend on the shape of the universe (i.e. the shape of the reflector we’re inside) and our position within it. If we were very close to one side, I think we’d be able to determine that we were looking at objects over here, and their reflections over there.

It wouldn’t look like anything.
We can’t see the “edge” of the Universe, so it doesn’t matter what’s out there.
Me, I’m voting for an energy field that give you ESP.

If the universe had a “skin,” it might well reflect like that. Like glass, if there’s light on one side and not on the other, you have a mirror from the lighted side. Hell, being absolute void, it might reflect all energy/matter. This is giving me a headache!

1. The mirror surface has to be moving outward. Else the nearby galaxies, due to their relative motions away from each other, would hit it. People in those galaxies would notice that.

2. Okay, so it’s moving away as part of the expansion of the universe. Note that the relative expansions speeds are not constrained by the speed of light. One reason it’s so hard to see object over 10 billion light years away is that they are moving away at phenomenal rates and their light is so red shifted it’s really hard to detect.

Galaxies more than ~14 billion light years away we’ll never see since those galaxies are moving away from us (from our point of view) at greater than the speed of light. Total red shift and then some. Nothing will reach us.

1. Ditto the scattering layer of the OP. We’d have to be fairly close. Too far, and the photon will never come bounce back. Fairly far and they will bounce and come back but be red shifted a lot. So, you want to be fairly close: a “mere” few billion light years. But again, there will be a Doppler shift. (Einstein apparently included some stuff about Doppler shift off moving mirrors in one of his 1905 papers.)

Would this amount of Doppler shift match the Doppler shift of the virtual galaxies seen in the scattering layer image? Don’t know off the top of my head.

1. Another wrinkle: Light reflected off certain types of mirrors can get polarized. But I guess the OP’s “perfectly” criteria would rule this out. Else, Astronomers might notice a lot of galaxies “over there” are producing polarized light for some odd reason.

Ditto the layer can’t be absorbing any energy else it would warm up and that would stick out like a sore thumb from the cosmic background radiation.

Hmm, I’m rethinking the Doppler idea again here. A photon hitting a mirror imparts momentum to the mirror. How does this scattering layer act like a mirror in reflecting light like a moving mirror and not acquire energy? (A wizard did it?)

1. But will this reflect gravity? Would Astronomers on a galaxy near the layer notice that the remarkably similar galaxy just over yonder doesn’t seem to be perturbed/perturbing their respective motions.