There's no point in arguing about this here. There's a very well defined, mathematical theory called General Relativity, which explains gravitational phenomena from Mercury's precession all the way to the expansion of the Universe.
If you take the time to learn General Relativity, and to learn how to apply it to cosmology, you will see that there are rigorous mathematical answers to the various questions you're raising.
I want to point out that this isn't esoteric stuff that only a few people understand. General Relativity and cosmology are part of a standard undergraduate physics curriculum. It only takes a few years of study, starting from Physics 101, to get to the point where you can derive the answers to all your questions from scratch.
Doesn't even need that much — their questions so far are at my level, and I keep messing up the much simpler special relatively questions on brilliant.org
"How" this specific expansion happens is an open question — not because nobody has any idea, but because we can't distinguish between three of them and a forth leads directly to the unsolved challenge of combining GR with quantum mechanics.
No, this is not an answer, because it breaks number of laws of physics, such conservation of energy. It looks like an excuse that an answer. It's just heavy stretching of the evidence until it fits the BB model of evolution of Universe.
Static Universe model of evolution doesn't requires such stretching: CMB is just light of distant galaxies. End of story.
>No, this is not an answer, because it breaks number of laws of physics, such conservation of energy.
THE WHOLE POINT of GR is that it explains things that "classical" physics did not, while also explaining everything that classical physics did. Nothing in GR "breaks the laws of physics" because GR largely IS the laws of physics now.
If you want to throw away GR by using a "Static Universe" theory, you have to re-derive a hundred different solutions to problems you bring back into physics by doing so. Einstein literally TRIED to put a static universe into GR because he thought it felt better, and turned out to be dead wrong!
In terms of "what drives the expansion", to us, within the universe, it's just what we see. It could very well be that it's a property of whatever "substrate" or "Stuff/emptyness" that a "Universe" exists in, if "exists" even makes sense in that context. It could be a completely unknowable to us thing. There are very likely phenomena and questions that we cannot ever answer, because we simply have no way of probing them.
All we know is that the way GR says to do the math works out really well for like 99.99% of things, and if you want to come up with a model that doesn't allow space to change "size", you have a shitload of math left to do at a minimum. If you want to understand how we got here, you have 400 years of physics history to read up on. None of this is about the "correctness" of GR either. It just makes the best predictions so far, and in science, all that matters is who makes the best predictions. Want to supersede the GR model? Just predict something correctly that GR cannot, while also predicting everything else correctly.
> If you want to throw away GR by using a "Static Universe" theory, you have to re-derive a hundred different solutions to problems you bring back into physics by doing so.
GR will be a special case in a new theory, which will explain laws of Universe better, which may join together GR and QM. If a formula does a good job, then it will be used anyway. We are not throwing away Newton physics just because GR does a better job in some cases.
> In terms of "what drives the expansion", to us, within the universe, it's just what we see.
Are you talking about a "light sail" effect? Yes, EM radiation creates pressure on dust particles, which pushes them away, but gravitation doesn't let it go. The same effect happens at size of galaxy. I'm not sure about superclusters, but it looks like we are falling into Great Attractor then into Shapley Attractor with all that dust.
So yes, this is possible, but EM radiation must be stronger than gravitation.
> It just makes the best predictions so far, and in science, all that matters is who makes the best predictions.
Predictions are very important, because they allow to prove or falsify a theory, but this is a game for theoretical physicists only. There is only one reality, which can be describer in many ways. Many different formulas can fit the same data. Many different techniques can be used to achieve the same result.
Moreover, every formula works in a range, then it doesn't work. Pi is an irrational number, which cannot be reproduced correctly in reality, thus every formula or path, which contains the irrational number, can be reproduced by physical reality with limited precision only. Multiply the error by many iterations, and new physics will emerge in the same place.
The only way to prove a theory, as I see it, is to make physical demonstrations at human scale, an analog, and then study it.
Hydrodynamic quantum analogs allows us to see pilot wave at work, so no mysteries in double slit experiment anymore: it just self-interference of the pilot wave. The same can be done for space effects.
It's easier to make computer model, to make predictions, but to make a correct model, we need to understand physics first. Egg and chicken. In case of a physical demonstration, nature performs all these calculations for free, automatically. Even when they are partially correct, they are still helpful.
> this is not an answer, because it breaks number of laws of physics, such conservation of energy
GR doesn't conserve energy. What follows is a bit beyond my level so I may be misremembering, but IIRC Noether's theorem is that conservation laws are always identical to some symmetries, and the symmetry for energy (time?) just isn't true in GR.
(I don't think it's even true in SR because space and time are observer dependent, but at least in SR you can get a different conserved quantity because all observers agree on a space-time interval; but as I implied in a different comment where I mentioned brilliant, this is my hobby not my profession).
> CMB is just light of distant galaxies. End of story.
The CMB is a perfect blackbody. Galaxies are far from a blackbody. Your explanation fails if one knows even a tiny amount about astronomy.
Before you criticize Big Bang cosmology, you should learn the theory. That means studying General Relativity, learning to derive the Friedmann Equations, learning about the (utterly overwhelming) observational evidence for the theory, etc. Then you'll be in a position to ask intelligent questions about the theory.
I promise you that if you learn the theory, you'll understand that the questions you're asking either don't make sense or have obvious answers. For example, conservation of energy does not hold in General Relativity. You keep saying that expansion is an ad hoc assumption that breaks physical laws. However, if you solve the Einstein Field Equations, you'll see that the universe must be either expanding or contracting. This fact bothered Einstein so much that he tried to modify General Relativity to get rid of it, something he regretted when observational evidence firmly established that the universe was indeed expanding. This was all the way back in the 1920s, and the evidence is so overwhelming now, a full century later, that it's impossible to deny.
> 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.
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).
> 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.
> 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.
> 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.
> 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.
> 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.
> 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.
(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.
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.
> 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.
> 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. :-/
If you take the time to learn General Relativity, and to learn how to apply it to cosmology, you will see that there are rigorous mathematical answers to the various questions you're raising.
I want to point out that this isn't esoteric stuff that only a few people understand. General Relativity and cosmology are part of a standard undergraduate physics curriculum. It only takes a few years of study, starting from Physics 101, to get to the point where you can derive the answers to all your questions from scratch.