Cosmological principle about space and time

[elcaro]

TL;DR Summary

The known cosmological principle states that the universe is (at sufficient large scales) homogeneous and isotropic. Are there also cosmological principles about space and time itself, for instance that spacetime is an interconnected whole, that there are no gaps and no edges or boundaries to spacetime?

AFAIK there is no cosmological principle formulated about space and time. If it would be formulated, it would more or less state that spacetime is an interconnected whole, and has no gaps, edges or boundaries. It doesn't need to state wether spacetime is finite or infinite, that is an open question.

But would this be a reasonable assumption about spacetime, or does our current understanding of the big bang or black holes would likely break these assumptions about spacetime?

Some further thoughts on this:
There are physical ideas out there in which the universe could have emerged from a literal nothing (or: a small space with radius r, having r go to zero), but this would incorporate the idea that spacetime somehow had an edge or boundary. The idea of a begin of time (or: emergence of time) seems to me a contradiction, since a beginning is already a temporal concept (pre assumes the existence of time and the concept of change, since without time, nothing can change) while "at the same time" time is said to have a begin (ie did not yet exist), which in any cases involves a change, which - without being there time - could not occur.

 

[PeterDonis]

There are physical ideas out there in which the universe could have emerged from a literal nothing (or: a small space with radius r, having r go to zero), but this would incorporate the idea that spacetime somehow had an edge or boundary.

Not necessarily. The universe could have the spatial topology of a 3-sphere, which would have a finite volume but no boundary.

Rather than just make general statements, I strongly suggest that you find a specific model and give a reference to it as a basis for discussion.

 

[PeterDonis]

The idea of a begin of time (or: emergence of time) seems to me a contradiction, since a beginning is already a temporal concept (pre assumes the existence of time and the concept of change, since without time, nothing can change) while "at the same time" time is said to have a begin (ie did not yet exist), which in any cases involves a change, which - without being there time - could not occur.

Again, I strongly suggest that you spend some time looking at references instead of making general speculations. You will find plenty of spacetime models, including all of the standard FRW spacetimes, in which the phrase "a beginning of time" has a perfectly well-defined meaning and does not raise any of the issues you suggest.

 

[Drakkith]

There are physical ideas out there in which the universe could have emerged from a literal nothing

I can't think of any serious scientific hypotheses that say this. There is always something before that transitions into the universe we live in today.

The idea of a begin of time (or: emergence of time) seems to me a contradiction, since a beginning is already a temporal concept (pre assumes the existence of time and the concept of change, since without time, nothing can change) while "at the same time" time is said to have a begin (ie did not yet exist), which in any cases involves a change, which - without being there time - could not occur.

Again, I can't think of any serious hypotheses or models that say that time 'comes into being'.

This may be what mainstream media (i.e. movies and tv) says, or it might be a comment by a scientist during some interview or something, but I don't think any of the actual science makes either of these claims.

 

[elcaro]

Again, I strongly suggest that you spend some time looking at references instead of making general speculations. You will find plenty of spacetime models, including all of the standard FRW spacetimes, in which the phrase "a beginning of time" has a perfectly well-defined meaning and does not raise any of the issues you suggest.

Creation of universes from nothing

 

[elcaro]

Not necessarily. The universe could have the spatial topology of a 3-sphere, which would have a finite volume but no boundary.

In models were the universe starts from literally nothing and quantum tunnels into De Sitterspace, there is a spacetime boundary, since it implies some earliest moment in time before which no time is defined.

 

[Drakkith]

Vilenkin- creation of the universe from nothing

Creation of universes from nothing

Despite what the titles of these say, neither of these involve the universe coming from literally nothing. Check around the 7:00 timeframe in the video where Vilenkin agrees with this.

 

[elcaro]

Despite what the titles of these say, neither of these involve the universe coming from literally nothing. Check around the 7:00 timeframe in the video where Vilenkin agrees with this.

What do you mean? The abstract of the paper states:

Abstract​

A cosmological model is proposed in which the universe is created by quantum tunneling from literally nothing into a de Sitter space. After the tunneling, the model evolves along the lines of the inflationary scenario. This model does not have a big-bang singularity and does not require any initial or boundary conditions.

How can you state this theory is not about the universe emerging from LITERALLY NOTHING, if that is what the abstract of the paper LITERALLY STATES?

 

[PeroK]

What precisely is the difference between "nothing", "absolutely nothing", "literally nothing" and "absolutely, literally nothing"?

 

[elcaro]

What precisely is the difference between "nothing", "absolutely nothing", "literally nothing" and "absolutely, literally nothing"?

I don't know, I just use the terminology used in the paper itself, I did not use the term "absolute".

But please answer the question, is or isn't the paper about the emerging of universes from literally nothing, like the abstract says, or not?

 

[PeroK]

I don't know, I just use the terminology used in the paper itself, I did not use the term "absolute".

But please answer the question, is or isn't the paper about the emerging of universes from literally nothing, like the abstract says, or not?

From what I understand it's nothing except the laws of physics.

 

[elcaro]

From what I understand it's nothing except the laws of physics.

That is what he says, indeed. But how could "laws of physics" exist apart from and causally prior to the existence of space, time and matter/energy itself? It is in fact alluding to the "alknowing creator" that creates the universe from nothing, which thus is a religious idea in disguise. It is not a physical theory as far as I know. And why does a universe not emerge in my living room every day? I have a lot of nothing stored between the atoms of my room and the quantum fields that fill my room, but nothing ever happened. Or does it need a special kind of "nothing"?

 

[PeroK]

That is what he says, indeed. But how could "laws of physics" exist apart from and causally prior to the existence of space, time and matter/energy itself?

You can't and never could do physics by pure thought. Why not only the laws of physics to begin with? In some sense, the laws of mathematics (and logic) don't appear to need a physical universe.

And why does a universe not emerge in my living room every day? I have a lot of nothing stored between the atoms of my room and the quantum fields that fill my room, but nothing ever happened. Or does it need a special kind of "nothing"?

You don't have nothing in your room: you have matter and/or quantum fields or whatever.

 

[elcaro]

You can't and never could do physics by pure thought. Why not only the laws of physics to begin with? In some sense, the laws of mathematics (and logic) don't appear to need a physical universe.

Mathematics and also physics is a mental discipline, so they need a mental subject in order to exist. Mental subjects normally require a physical universe to exist.

You don't have nothing in your room: you have matter and/or quantum fields or whatever.

I am pretty damn sure I have a lot of nothing in my room, virtual particles coming in and out of existence from that nothing.

 

[PeroK]

Mathematics and also physics is a mental discipline, so they need a mental subject in order to exist. Mental subjects normally require a physical universe to exist.I am pretty damn sure I have a lot of nothing in my room, virtual particles coming in and out of existence from that nothing.

This is your personal opinion and personal philosophy. I'm not convinced that what you believe must necessarily dictate what physical theories may ultimately govern our universe. Why should I? Do I believe in the principle of the infallibility of elcaro's neurons?

That applies equally to anyone's neurons. What should anyone's neurons be infallible?

 

[elcaro]

This is your personal opinion and personal philosophy. I'm not convinced that what you believe must necessarily dictate what physical theories may ultimately govern our universe. Why should I? Do I believe in the principle of the infallibility of elcaro's neurons?

That applies equally to anyone's neurons. What should anyone's neurons be infallible?

Neither do we believe in the infallibility of Einsteins neurons, or whatever scientist, we just apply rules of logic and apply them to physical phenomena, make theories about physical reality that can make predictions that can be tested. I don't have a personal theory about the universe. We are discussing this in the context of cosmological principles about space and time. Wether or not such principles (like the one I formulated) can be said to apply to the universe. We just check if the logic makes sense, since in the case of spacetime and events outside (or predating) the observable universe or behind a black hole horizon, we can not make direct observations to test our ideas.

 

[Drakkith]

How can you state this theory is not about the universe emerging from LITERALLY NOTHING, if that is what the abstract of the paper LITERALLY STATES?

Because he contradicts himself in the video, and because of the highly ambiguous nature of discussing 'nothing'.

 

[PeroK]

We just check if the logic makes sense, since in the case of spacetime and events outside (or predating) the observable universe or behind a black hole horizon, we can not make direct observations to test our ideas.

There may be no question of logic. The question is about which premises we apply that logic to.

For example:

1) Premise: the laws of physics do not require a physical universe

Conclusion: the universe could have emerged from nothing except the laws of physics.

2) Premise: the laws of physics require a physical universe

Conclusion: the universe could not have emerged from nothing except the laws of physics

Both are perfectly logical. It's the initial premise that is the critical factor.

 

[PeterDonis]

Vilenkin- creation of the universe from nothing

A video is not a valid reference. We need to see the peer-reviewed paper.

Creation of universes from nothing

Unfortunately this is behind a paywall and I can't find a preprint on arxiv.

 

[PeterDonis]

How can you state this theory is not about the universe emerging from LITERALLY NOTHING, if that is what the abstract of the paper LITERALLY STATES?

The abstract does not give the underlying mathematical model. Without the underlying mathematical model, we have no way of telling what the author means by "literally nothing". You can't just wave your hands and conjure a meaning for that phrase out of thin air.

Unfortunately, as I noted in a previous post, the paper you linked to is behind a paywall and it doesn't look like a preprint is on arxiv.

 

[PeterDonis]

Since we have no valid basis for discussion (paper is paywalled), this thread is closed.