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Comment by arcastroe | Hacker News Reader

> In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us

Thank you, but I suppose I'm not really questioning the big bang piece. My question was mostly in regards to the continued acceleration piece. Feel free to disregard the "in random directions" part of my original post.

I'm picturing more of an explosion in empty space. A firework or granade of sorts. Any individual dust/shard of the explosion still sees all other objects moving away from it and the rest of my question stands. But I suppose this would imply a "center" to the explosion, which I've also heard is not the case.

Theres a few other comments offering more clarity to the acceleration piece. Thank you everyone!

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> I'm picturing more of an explosion in empty space.

No, that's not what the big bang is.

> this would imply a "center" to the explosion, which I've also heard is not the case

That's correct. The big bang does not work like anything ordinary that you are used to imagining. The math is straightforward and unambiguous, but there is no good ordinary language description that corresponds to the math.

I find helpful this analogy of the space-time (4-dimension) expansion from the big bang: the surface (2D) of an expanding bubble. YMMV.

As I understand it, before the big bang the whole observable universe was contained in a small sphere and then it started to expand metrically. Is this interpretation correct?

Another thing: suppose I point a laser beam to the space and by chance this laser beam never finds any kind of matter in its way, where is this laser going to? To an infinite void? Is it correct to say that stars radiate energy to the infinite then?

It's just an interpretation. Your interpretation is similar to the Big Bang model of visible Universe expansion. If you can convince us that your model is better than other models, then we will use your model, but nobody can prove than a model is correct, unless we will find a hidden recorder somewhere which was turned on for few dozens of billion years.

Photon will hit something, or will travel until it will be redshifted to obvilion, or will travel until end of the medium (photon is a wave, so it waves something).

> (photon is a wave, so it waves something)

That sounds suspiciously like postulating the 'ether'. Surely what a photon 'waves' is the electromagnetic field, which is not a medium, and which fills the whole of spacetime. There is no 'end of the medium'.

"Field" means 3d array of numbers. Spacetime means 4d array of numbers. You are talking about mathematical model of Universe, while I'm talking about physics. Mountain is not just an excitement in a height field. If photon is not waving something, then it's not a wave. Physicists prove that photon is a wave.

> If photon is not waving something, then it's not a wave.

No, you’ve simply hit the limits of needing/wanting to understand something in terms of something else similar or more familiar.

Makes the same point on a related matter: https://youtu.be/Q1lL-hXO27Q

> It's just an interpretation

No, it's not, it's our best current model's description of the actual physical reality of our universe.

> nobody can prove than a model is correct

That's true, but it's also true that we can show models to be incorrect, as in, falsified by the data. For example:

> Photon will hit something, or will travel until it will be redshifted to obvilion, or will travel until end of the medium

For the scenario that was posed, a laser beam that never hits anything, none of your statements here are true. The first is ruled out by the scenario; the second is known to be false because there is no "gravitational redshift" of light in the universe as a whole (because models in which there would be such a redshift are known not to correctly model our data on the universe as a whole), and there is no "end of the medium" (again, models in which there would be an "end of the medium", i.e., where the universe stopped containing matter and started being just vacuum, are known not to correctly model our data).

I have described what actually happens in my own response to the GP upthread.

> (photon is a wave, so it waves something).

Light is an electromagnetic wave; what "waves" is the electromagnetic field. (If you use a "photon" model, you are using the quantum electromagnetic field as opposed to its classical approximation.) There doesn't have to be any other "medium"; the electromagnetic field is present everywhere.

> No, it's not, it's our best current model's description of the actual physical reality of our universe.

OK, it's our best model, but it doesn't invalidate other models, less complete or less popular, it compete with them.

> That's true, but it's also true that we can show models to be incorrect, as in, falsified by the data.

Yep. The article is about the Huble Tension, which invalidates Big Bang model. We still use it.

> the second is known to be false because there is no "gravitational redshift" of light in the universe as a whole (because models in which there would be such a redshift are known not to correctly model our data on the universe as a whole)

The Big Bang model is incomplete too: galaxies with FTL speeds, different speeds of expansion, no center of bang, no flows, no source of energy, it stretches time and space, etc.

I assume that the only infinite thing in infinite Universe is Universe itselft. All other things are finite. Thus, a photon has finite life, like any other wave.

> there is no "end of the medium"

The right-hand rule in EM suggests that we are in north hemisphere of something, so south hemisphere will have symmetrical rule, unless you believe that God chose right-hand rule for the whole infinite universe. If we are in a sphere, then that sphere rotates and have a boundary.

> what "waves" is the electromagnetic field.

"Field" is an array of numbers. You are mixing model and reality.

> it doesn't invalidate other models

One model can't invalidate other models. Only data can invalidate a model.

What other models do you have in mind?

> the Huble Tension, which invalidates Big Bang model

No, it doesn't. It means we have more work to do, to figure out why two calculations of the Hubble constant, by different routes, give different answers.

Invalidating the Big Bang model would be finding evidence that there was no Big Bang at all. The Hubble tension is nothing of the sort.

> The Big Bang model is incomplete too: galaxies with FTL speeds, different speeds of expansion, no center of bang, no flows, no source of energy, it stretches time and space, etc.

None of these are issues at all. The model accounts for them all in a perfectly self-consistent fashion.

Also, your nomenclature is biased: for example, the "FTL speeds" you refer to are coordinate speeds, which have no physical meaning. "FTL" in General Relativity means "moving outside the light cones", and that does not happen.

> The right-hand rule in EM suggests that we are in north hemisphere of something

The right-hand rule is a human convention. It tells us nothing about physics.

> You are mixing model and reality.

No, you are incorrectly assuming that the word "field" can only refer to the model. That's not the case. Physicists commonly use the word "field" to refer to both the mathematical object in the model and the actual physical thing that is being modeled. Light is "waves of the electromagnetic field" in the latter sense.

> before the big bang the whole observable universe was contained in a small sphere and then it started to expand

We have no evidence of any time when the universe was not expanding. At the earliest times we have evidence of, the universe was already expanding (extremely rapidly--much, much, much more rapidly than it is now). At those times, our observable universe was indeed contained in a very small volume.

> suppose I point a laser beam to the space and by chance this laser beam never finds any kind of matter in its way, where is this laser going to?

Since the universe is spatially infinite in our best current model, the laser beam will just keep on going forever.

> To an infinite void?

According to our best current model, no, the laser beam will never stop passing by matter, of approximately the same average density as the matter we can see.

> Is it correct to say that stars radiate energy to the infinite then?

Yes, as long as you recognize that "the infinite" never becomes a "void".

>According to our best current model, no, the laser beam will never stop passing by matter, of approximately the same average density as the matter we can see.

I didn't understand this part, why would the laser beam would never stop passing by matter? Because of the metric expansion of the universe? Isn't it reasonable to assume that there is a skirt of the universe where matter keeps expanding into nothingness?

> why would the laser beam would never stop passing by matter?

Because there is matter everywhere in our universe. It never stops.

> Because of the metric expansion of the universe?

It is true that, in order for there to be matter everywhere in our universe, it must be either expanding or contracting. (There is an edge case, the Einstein static universe, but it is unstable against small perturbations, like a pencil balanced on its point, so it is not a viable option.)

> Isn't it reasonable to assume that there is a skirt of the universe where matter keeps expanding into nothingness?

No. Such a model cannot match our observed data.

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