[oneshtein]

> The CMB is a perfect blackbody. Galaxies are far from a blackbody.

CMB is not emitted by a single galaxy or even group of galaxies. It's light of trillions of supeclusters, like our Visible Universe, averaged. I expect that almost any local unevenness should be polished out when averaged over such large area and distance. We are not seeing stream of photons from individual emitters, we see random photons from extremely huge range of emitters at extremely huge range from us.

If clump together all radiation from all our Visible Universe into single stream of photons, then we will see something very similar.

> For example, conservation of energy does not hold in General Relativity.

Then something is wrong.

[DiogenesKynikos]

If you average a bunch of different types of galaxies, you do not get a blackbody.

Do you know what does give you a blackbody? An optically thick medium with a uniform temperature, which is what the CMB "last scattering surface" is.

I just have one question for you: do you think that physicists are all a bunch of dunces? You're doing extremely simple questions. Do you think that physicists haven't worked out the basics of the theory? Again, instead of raising extremely simple objections, your time would be better spent understanding the theory first.

>> For example, conservation of energy does not hold in General Relativity.

> Then something is wrong.

Energy conservation only holds locally, when space is nearly flat. The true conservation law in General Relativity is more complicated (energy-momentum conservation).

[oneshtein]

> If you average a bunch of different types of galaxies, you do not get a blackbody.

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?

> Energy conservation only holds locally, when space is nearly flat.

Space is flat in all directions.

[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.

[ben_w]

> Then something is wrong.

Yes, you.

(I suspect also GR, but not for any reason you give — the maths presumes no singularities from what I've been told, and yet they happen anyway with easy initial conditions).

For the broader point, if there were galaxies trillion of light years away whose light had time to reach us, they'd be trillions of years old by now, and therefore we'd expect a lot more galaxies near us to be that age too.

We don't see any evidence of nearby galaxies that old; denying the conclusion means falsifying the hypothesis.

Also, they'd have to go on forever to not look clumpy, and then we would still need a source of red-shift to stop them being as bright as the surface of a star in all directions.

[oneshtein]

> Yes, you.

I know that. I'm heretic. Moreover, I'm too stupid to understand all these great theories. I need simple explanations.

> For the broader point, if there were galaxies trillion of light years away whose light had time to reach us, they'd be trillions of years old by now, and therefore we'd expect a lot more galaxies near us to be that age too.

Of course, not. Space is mostly empty. If elementary particles are generated constantly from pure energy (which doesn't violate laws of conservation) just of pure luck at cosmic scale, then light from distant neighbors slowly pushed this newborn dust into the center of a gigantic void, where it started to concentrate. In such case, we will have huge gap of void between our region of space and our neighbors.

> Also, they'd have to go on forever to not look clumpy, and then we would still need a source of red-shift to stop them being as bright as the surface of a star in all directions.

Surface area of a distant object reduces at r^2, while brightness of the distant object diminishes at r^3. Moreover, the probability of hitting something grows with d^1, so total brightness diminishes with (d^3*d)/d^2 = d^2. The number of objects in the sky increases with area = d^2. So, d^2/d^2 = const. I see no infinity. At average, the brightness of sky must be very similar in all directions. The larger the distance - the closer to average brightness must be. CMB must be almost ideal.

[ben_w]

> If elementary particles are generated constantly from pure energy (which doesn't violate laws of conservation) just of pure luck at cosmic scale, then light from distant neighbors slowly pushed this newborn dust into the center of a gigantic void, where it started to concentrate. In such case, we will have huge gap of void between our region of space and our neighbors.

Requires simultaneous behaviour from all directions at great distances while also not having that behaviour here, and also having us being really close to the physical center of this phenomenon rather than off to one side — even a fraction of a percent would be easily noticeable given the CMB is so close to the same in all directions; we see a red/blue-shift dipole from us moving at 370-ish km/s relative to it's comoving rest frame, so that's the scale of fractional away-from-perfect-centre you'd have to explain.

> Surface area of a distant object reduces at r^2, while brightness of the distant object diminishes at r^3.

If space was flat, which is your presumption, those would both be 1/r^2.

> Moreover, the probability of hitting something grows with d^1

You should be able to tell that's wrong by it being an unbounded function, when probability stops at 1.

You should look up Olber's paradox.

[oneshtein]

> If space was flat, which is your presumption, those would both be 1/r^2.

You forgot about red shift, which also diminishes the source, so, very very roughly, it's 1/r^3.

> Requires simultaneous behaviour from all directions at great distances while also not having that behaviour here, and also having us being really close to the physical center of this phenomenon rather than off to one side — even a fraction of a percent would be easily noticeable given the CMB is so close to the same in all directions; we see a red/blue-shift dipole from us moving at 370-ish km/s relative to it's comoving rest frame, so that's the scale of fractional away-from-perfect-centre you'd have to explain.

When we are in a fog, we always in the center of the visible area. With such larger distances, the probability of hitting something for a photon is very near to 1, even when interstellar space is extremely clear (hard to calculate exact numbers for me).

> You should be able to tell that's wrong by it being an unbounded function, when probability stops at 1.

When we see direct light, then probability is below 1. When don't, then it's 1. :-/

> You should look up Olber's paradox.

You should look at the picture of the darkest spot on the sky: it's full of stars. :-/

https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field