[DiogenesKynikos]

> Black body averages emission of trillions of trillions of atoms. Why it will not work for emission of trillions of trillions of galaxies? Can you prove that?

No, that's not what a blackbody is. A blackbody is an optically thick medium in thermal equilibrium. Galaxies are not blackbodies (not even close), and when you average a bunch of non-blackbody spectra, you don't get a blackbody. You'll get a spectrum with all sorts of atomic and molecular features. There is actually something called the "Cosmic Infrared Background," which is caused by distant galaxies, but it's not a blackbody and it has much larger amplitude variations than the CMB (because galaxies are distributed in a clumpy way).

> Space is flat in all directions.

Globally, spacetime is not flat (i.e., it is not Minkowski). Spacelike surfaces of constant coordinate time are flat, but the whole manifold is not flat. If this is all a bunch of gobbledygook to you, then you need to learn the basics of General Relativity.

[oneshtein]

> A blackbody is an optically thick medium in thermal equilibrium.

Black body can be simulated by a cavity with small hole, so incoming light will be scattered and fully absorbed, with zero reflections. In case of CMB, light from our Visible Universe will never return back to us, because it will be too weak and too stretched.

Moreover, this is really big journey for a photon, with very high probability to hit something on the way to us, so we may see a large portion of re-emitted EM radiation instead of the original light.

What is the difference between black sky and black body?

> Galaxies are not blackbodies (not even close), and when you average a bunch of non-blackbody spectra, you don't get a blackbody. You'll get a spectrum with all sorts of atomic and molecular features.

Emission from multiple random objects can be approximated as black body radiation, even when they are not in thermal equilibrium with their surroundings.

Moreover, we use statistic to distinguish between different emitters. In case of CMB, years may pass until we receive second photon from a same galaxy. Statistic doesn't work in such extreme cases, unless we will point an antenna in the same direction for a millennia or even longer.

> There is actually something called the "Cosmic Infrared Background," which is caused by distant galaxies, but it's not a blackbody and it has much larger amplitude variations than the CMB (because galaxies are distributed in a clumpy way).

CIB emitted mostly by stars and dust particles, which are hit by the star light, which are much closer to us than CMB emitters. We may get different picture from outside of our galaxy, or when we filter out local emitters.

> Spacelike surfaces of constant coordinate time are flat, but the whole manifold is not flat.

You are talking about model. Can you map your model back to physical reality, please? As I understand, you are trying to tell me that a point in the non-flat space-timecan have less or more neighbourhood points that in flat space time. In other words, wormholes or space-bubbles are possible in your imagination.

> then you need to learn the basics of General Relativity.

I'm too stupid to understand this great theory. I need simple explanations.

[DiogenesKynikos]

> Moreover, this is really big journey for a photon, with very high probability to hit something on the way to us

Wrong. The universe is remarkably empty, and photons can easily travel across the entire visible universe without hitting anything.

> Emission from multiple random objects can be approximated as black body radiation

Wrong. There are very specific conditions for blackbody radiation. Other conditions give rise to different types of spectra, such as synchrotron radiation, Bremsstrahlung, etc.

You're making a lot of claims about how physics works that are simply false. Before making up your own alternate theories of physics, you should learn physics as it is presently understood.

[oneshtein]

> The universe is remarkably empty, and photons can easily travel across the entire visible universe without hitting anything.

The universe is remarkably empty, but any small probability can be multiplied by a really big number, to get ~1.

For a simplified example, the lowest density of interstellar space is 100 molecules per m3. The number of water molecules in water is 3.3E28. If a photon travel 3.5E10 light years (35Bly), then it's roughly equivalent to passing a 1m3 of water (by density, regardless of optical properties of the medium). 4Tly is a rough equivalent of 113 meters of water for such space. Most of this mass will be hydrogen molecules, of course.

> There are very specific conditions for blackbody radiation. Other conditions give rise to different types of spectra, such as synchrotron radiation, Bremsstrahlung, etc.

Dark sky is the perfect absorber. Bremsstrahlung spectrum will approach black body spectrum anyway as density increases.

Gray body is not real, as I see.

[DiogenesKynikos]

> For a simplified example, the lowest density of interstellar space is 100 molecules per m3. The number of water molecules in water is 3.3E28. If a photon travel 3.5E10 light years (35Bly)

Galaxies are nowhere near 35 billion light years across. Large galaxies are a few tens of thousands of light years across. Once you get outside galaxies, density drops by further orders of magnitude. In other words, your "simplified example" is utter nonsense.

What's surprising to me is that you assume that physicists are complete ignoramuses who haven't even bothered to do the simplest calculations. Do you really think that no one has ever sat down and calculated the effect of foreground absorption on the CMB? There are entire PhD theses on this one subject.